franc3d/ng reference manual: version 0 · 1 table of contents: table of contents:

Reference

Manual

Version 6

Fracture Analysis Consultants, Inc

www.fracanalysis.com

Revised: November 2011

http://www.fracanalysis.com/

1

Table of Contents:

Table of Contents: ............................................................................................................... 1 1. Introduction .................................................................................................................... 5 2. General and 3D View Manipulation .............................................................................. 5

2.1 View Controls .......................................................................................................... 8

3. Menus ............................................................................................................................. 9 3.1 Help .......................................................................................................................... 9

3.1.1 About Dialog ................................................................................................... 10 3.2 File ......................................................................................................................... 11 3.3 Edit ......................................................................................................................... 12

3.4 Cracks .................................................................................................................... 12 3.5 Materials ................................................................................................................ 13

3.6 Loads ...................................................................................................................... 14 3.7 Analysis.................................................................................................................. 14 3.8 Fatigue.................................................................................................................... 15 3.9 Fretting ................................................................................................................... 15

3.10 Advanced ............................................................................................................. 16 4. File Menu Wizards and Dialog Boxes ......................................................................... 17

4.1 Open…Ctrl-O ........................................................................................................ 17 4.1.1 FRANC3D Restart Files ................................................................................. 18 4.1.2 FEM Input Wizard .......................................................................................... 20

4.1.3 Retain Model Panel ......................................................................................... 21 4.1.4 Select Items to Retain Panel ........................................................................... 22

4.1.5 Materials to Retain Panel ................................................................................ 23

4.1.6 Coordinate Systems to Retain Panel ............................................................... 24

4.1.7 Boundary Conditions to Retain Panel ............................................................. 24 4.1.8 Components/Nsets to Retain as Mesh Facets Panel ....................................... 25 4.1.9 Surfaces to Retain for Contact/Constraint Panel ............................................ 26

4.1.10 Components/Nsets for Residual Stress Panel ............................................... 26 4.2 Close ...................................................................................................................... 27

4.3 Save As… .............................................................................................................. 27 4.4 Read Results........................................................................................................... 29 4.5 Playback…. ............................................................................................................ 30

4.6 Quit - Ctrl-Q ........................................................................................................... 31 5. Edit Menu Wizards and Dialog Boxes......................................................................... 31

5.1 Undo - Ctrl-Z ......................................................................................................... 31 5.2 Redo ....................................................................................................................... 32

5.3 Preferences… ......................................................................................................... 32 5.3.1 Folders Tab ..................................................................................................... 32 5.3.2 Window Tab.................................................................................................... 33 5.3.3 3D View Tab ................................................................................................... 34 5.3.4 Display Tab ..................................................................................................... 34

5.3.5 ANSYS Tab .................................................................................................... 35 5.3.6 ABAQUS Tab ................................................................................................. 36

5.3.7 NASTRAN Tab .............................................................................................. 37

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5.3.8 FEAWD Tab ................................................................................................... 37

5.3.9 Meshing Tab ................................................................................................... 38 6. Cracks Menu Wizards and Dialog Boxes .................................................................... 39

6.1 New Flaw Wizard .................................................................................................. 39

6.1.1 Flaw type panel ............................................................................................... 40 6.1.2 Crack type panel ............................................................................................. 41 6.1.3 Elliptical crack panel ....................................................................................... 41 6.1.4 Single-front Through-the-thickness crack panel ............................................. 42 6.1.5 Two-front Through-the-thickness crack panel ................................................ 43

6.1.6 Long shallow surface crack panel ................................................................... 43 6.1.7 Two-front Elliptical crack panel ..................................................................... 44 6.1.8 User-defined crack panel ................................................................................ 45 6.1.9 Advanced Geometry button ............................................................................ 46

6.1.9.1 Refine ....................................................................................................... 47 6.1.9.2 Reset ......................................................................................................... 47

6.1.9.3 Edit ........................................................................................................... 48 6.1.10 Ellipsoidal void panel ................................................................................... 49

6.1.11 Flaw translation and rotation panel ............................................................... 50 6.1.11.1 Orient with Vectors ................................................................................ 50

6.1.12 Crack Front Mesh Template Panel ............................................................... 51

6.1.12.1 Meshing Parameters Dialog ................................................................... 52 6.1.12.2 Advanced Template Options Dialog ...................................................... 53

6.1.13 Flaw Insertion ................................................................................................ 55 6.2 Flaw from Files ..................................................................................................... 55 6.3 Multiple Flaw Insert ............................................................................................... 56

6.4 Stress-Intensity Factor ........................................................................................... 58

6.4.1 M-integral or Displacement Correlation Panel ............................................... 58 6.4.2 SIF Plot Panel ................................................................................................. 60

6.5 Grow Crack ............................................................................................................ 61

6.5.1 Crack Growth Parameters Wizard ................................................................... 62 6.5.1.1 First Growth Parameters Panel ................................................................. 62

6.5.1.2 Fatigue Growth Rate Model...................................................................... 66 6.5.1.3 Quasi-Static Growth Model ...................................................................... 74

6.5.1.4 Constant Amplitude Panel ........................................................................ 74 6.5.1.5 Variable Amplitude Panel ......................................................................... 77

6.5.2 Advanced Crack Growth Parameters Wizard .................................................. 82 6.5.2.1 First Advanced Growth Parameters Panel ................................................ 82 6.5.2.2 Second Advanced Growth Parameters Panel ............................................ 87

6.5.2.3 Third Advanced Growth Parameters Panel............................................... 89 6.5.2.4 Fourth Advanced Growth Parameters Panel ............................................. 91

6.5.3 Crack Growth Wizard ...................................................................................... 94 6.5.3.1 First Crack Growth Panel.......................................................................... 94 6.5.3.2 Second Crack Growth Panel ................................................................... 100

6.6 Read Crack Growth Wizard ................................................................................. 101 6.7 Grow/Merge Cracks Wizard ................................................................................ 102

7. Materials Menu Wizards and Dialog Boxes .............................................................. 104

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7.1 Material Properties ............................................................................................... 104

7.1.1 View Material ............................................................................................... 104 7.1.2 New Material ................................................................................................ 105

7.2 Apply Material ..................................................................................................... 106

7.3 Apply Coordinate System .................................................................................... 106 8. Wizards and Dialog Boxes for Loads Menu .............................................................. 107

8.1 Map Current State ................................................................................................ 107 8.2 Crack Face Pressure/Traction .............................................................................. 107

8.2.1 Constant Crack Face Pressure Panel ............................................................. 108

8.2.2 1-D Radial Residual Stress Distribution Panel ............................................. 109 8.2.3 2-D Radial Residual Stress Distribution Panel ............................................. 111 8.2.4 Surface Treatment Residual Stress Distribution Panel ................................. 112 8.2.5 Mesh-Based Stress Distribution Panel .......................................................... 114

9. Wizards and Dialog Boxes for Analysis Menu .......................................................... 115 9.1 Static Crack Analysis ........................................................................................... 115

9.1.1 Fdb File Name Panel ..................................................................................... 115 9.1.2 Analysis Code Panel ..................................................................................... 116

9.1.3 ANSYS Options Panel .................................................................................. 116 9.1.4 ANSYS local/global model connection Panel .............................................. 118 9.1.5 ANSYS Script Panel ..................................................................................... 120

9.1.6 ANSYS Command Line Panel...................................................................... 120 9.1.7 ABAQUS Options Panel............................................................................... 121

9.1.8 ABAQUS local/global model connection Panel ........................................... 122 9.1.9 ABAQUS Script Panel .................................................................................. 123 9.1.10 ABAQUS Command Line Panel ................................................................ 124

9.1.11 NASTRAN Options Panel .......................................................................... 124

9.1.12 NASTRAN local/global model connection Panel ...................................... 126 9.1.13 NASTRAN Script Panel ............................................................................. 128 9.1.14 NASTRAN Command Line Panel .............................................................. 128

9.2 Crack Growth Analysis ........................................................................................ 128 9.2.1 Crack Front Smoothing Panel ....................................................................... 128

9.2.2 Crack Growth Increments Panel ................................................................... 129 9.2.3 Analysis Code Panel ..................................................................................... 130

9.2.4 ANSYS Analysis .......................................................................................... 131 9.2.5 ABAQUS Analysis ....................................................................................... 131 9.2.6 NASTRAN Analysis ..................................................................................... 131

10. Fatigue Menu Wizards and Dialog Boxes ............................................................... 131 10.1 SIF History ......................................................................................................... 131

10.2 Discrete Crack Life ............................................................................................ 138 11. Fretting Menu Wizards and Dialog Boxes............................................................... 145

11.1 Read Model and Results ................................................................................. 145 11.2 Import Nucleation Data................................................................................... 148 11.3 Fretting Crack Nucleation ............................................................................... 152 11.4 Extract Subvolume .......................................................................................... 159 11.5 Turn Off Region Color.................................................................................... 161

12. Advanced Menu Wizards and Dialog Boxes ........................................................... 162

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12.1 Edges Wizard ..................................................................................................... 162

12.2 No Crack Regions .............................................................................................. 163 12.3 Write Crack Front Data ...................................................................................... 165 12.4 Write COD Data ................................................................................................ 166

12.5 Write Template Data .......................................................................................... 167 12.6 View Response................................................................................................... 168 12.7 Create Growth History ....................................................................................... 170 12.8 Crack Growth Sequence .................................................................................... 172

13. FRANC3D Command Language and Session Files ................................................ 173

14. FRANC3D Python Extensions ................................................................................ 173

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1. Introduction

FRANC3D Version 6 is a program that inserts and extends cracks and/or voids in pre-

existing finite element meshes. This manual describes the components of the graphical

user interface of the program. It also includes underlying theory and concepts where

appropriate.

There are three separate tutorial documents that describe the program usage when

combined with the three supported commercial FE codes: ANSYS, ABAQUS and

NASTRAN. In addition, there is a Benchmark document that describes a number of

crack configurations for which there are analytical or handbook solutions for stress

intensity factors (SIFs) where the FRANC3D SIFs are compared to the existing SIFs.

To get started using FRANC3D, one can choose one of the tutorials and follow the steps.

One can consult this reference document for more details at any of the FRANC3D

modeling steps in the tutorials. One should also try to reproduce some of the Benchmark

examples or choose your own model for validation.

In this document, menu buttons, dialog or wizard panel titles, and buttons on these panels

are indicated by bold text. Fields, labels and selectable options inside the dialog or

wizard panels are indicated by underlined text. Tab labels and pane labels inside the

dialog and wizard panels are indicated by italicized-underlined text. This formatting is

intended to be consistent with the tutorials.

2. General and 3D View Manipulation

An image of the main FRANC3D window is shown in Figure 2.0.1. Most of the screen is

used by a 3D graphics window that displays the current model.

When a model is first displayed, the 3D view is set such that the viewer is looking toward

the center of the model from a position in the positive z direction. The distance from the

viewer to the model is set so that the full model is visible.

For most operations, one will likely want to change the view to see a different side of the

model and/or to view details more closely. As with many other programs, the view is

changed by moving the mouse in the 3D graphics window with the appropriate

combination of mouse buttons and keyboard keys depressed. There are eight functions

for view manipulation and selection. A unique combination of mouse buttons and

keyboard keys is defined for each of the functions. The button and key assignments can

be changed using the 3D View tab in the Preferences dialog box (described in Section

4.4).

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Figure 2.0.1 FRANC3D main window.

The view functions are (using the default mouse and keyboard key assignments):

Rotate (left mouse button, no keyboard keys): Mouse motion rotates the model on the

screen. The mouse can be moved in any direction. The axis of rotation is defined to be

perpendicular to the mouse motion and passes through the current center of rotation. The

center of rotation is defined as the horizontal and vertical center of the graphics window

at a location midway between the front and back clipping planes (described below).

Pan (center mouse button or wheel, no keyboard keys): Mouse motion pans or drags the

model around the screen. The mouse can be moved in any direction, and the model

moves in such a way as if it were following the mouse.

Zoom/Spin (right mouse button, no keyboard keys): These are actually two different

types of updates activated by the same mouse key. If the mouse is moved up or down it

will appear as if the viewer is getting closer or farther, respectively, from the model. If

the mouse is moved left or right, the model rotates about an axis that is perpendicular to

the plane of the screen and passes through the horizontal and vertical centers of the

window.

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Front Clip (center mouse button or wheel plus shift key): A cutting or clipping plane can

be moved parallel to the screen from the viewer toward the model. This can be used, for

example, to clip away the front of a model to see internal features such as cracks.

Moving the mouse upward "pushes" the clipping plane away from the viewer toward the

model, cutting away the portion of the model closest to the viewer. Moving the mouse

downward "pulls" the plane away from the model and toward the viewer.

Back Clip (right mouse button or wheel plus shift key): A cutting or clipping plane can

be moved parallel the screen from behind the model toward the viewer. This can be used,

for example, to clip away parts of the model that may be making the view confusing (this

is especially useful when the model is displayed in "wireframe" mode without polygons).

Moving the mouse upward "pushes" the clipping plane away from the viewer. Moving

the mouse downward "pulls" the clipping plane toward the viewer, which will cut away

the back portion of the model.

Recenter (left mouse button plus shift, ctrl, and alt keys): Sometimes it is convenient to

change the center of rotation. The horizontal and vertical position of the center of

rotation is always the center of the screen and can effectively be changed by panning the

model. The location of the center in the direction perpendicular to the screen is set by the

current location of the front and back clipping planes. However, it is difficult and

somewhat non-intuitive to set its location using the front and back clip functions. The

Recenter function presents a graphical tool for setting the center of location. This is a

multi-step process. In the first step, while depressing the shift, ctrl and alt keys, one

should depress and release the left mouse button while the cursor is at the horizontal and

vertical location that one wants as the new center of rotation. A dialog box then will pop

up, Figure 2.0.2 – left panel, which contains a 3D view of the model shown as though one

had walked 90 degrees around the model to the left. It also shows a green line that is a

projection of the point one picked in the previous step along the depth direction in the

model. In this step, one should select a point horizontally (using left mouse button plus

the shift key only) along the projection to set the depth location of the new center of

rotation. A small box will be displayed at the new center location, Figure 2.0.2 – right

panel. Select Accept to complete the re-centering process; the model view will be

updated in the FRANC3D model view window to reflect the new center.

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Figure 2.0.2 Recenter View dialog.

2.1 View Controls

The View Controls box along the right hand side of the FRANC3D main window

provides additional options for manipulating the view of the model, Figure 2.1.1.

Figure 2.1.1 View control panel.

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Graphical Element Toggles: Graphics in 3D display windows consist of a combination

of markers, vectors, polygons, and text. The toggles in this box will turn these items on

or off independently.

Update Gain Controls: These dials adjust the gain between mouse motion and model

motion for rotations and translations. Rotating to the right increases the gain, rotating to

the left decreases it.

Axes Toggle: This turns the axes display on or off.

F/B Colors Toggle: This turns the front/back coloring on or off. By default, the front and

back side of a surface are shaded differently. This toggle is useful when looking at the

crack surface where there are two coincident surfaces.

Named camera positions: The list of named camera positions is displayed in the list box.

There should always be a reset view provided as the default (d) view. The icons below

allow one to save additional camera positions to the list as well as saving them to a file.

Camera positions can be read from a file, renamed, or deleted.

Preset Controls: These icons provide quick access to preset views that place the viewer

along one of the Cartesian axes. The two icons on the right switch the view from

perspective (the default) to orthotropic.

Mesh Toggle: This toggles the surface mesh on and off.

3. Menus

A brief summary of the menus from the menu bar shown in Figure 2.0.1, is provided

here. In general, selecting a menu item leads to a wizard or dialog and these are

described in more detail in Sections 4-12.

3.1 Help

The Help menu, Figure 3.1.1, is on the far-right side of the menu bar and has options for

accessing the FRANC3D documentation either locally, if installed, or on-line from

FAC’s www server (www.fracanalysis.com). If the user selects the Reference, Tutorial

for…, or Benchmark menu options, the appropriate documentation will be displayed in

a web browser window if the files have been installed locally and the file paths set. If the

file paths have not been set, the user is presented with the dialog box shown in Figure

3.1.2, and can select the appropriate index.html file. Alternatively, if the Online

Documentation menu option is selected, the list of FRANC3D documents that are

available from FAC’s web server are displayed in a web browser.

www.fracanalysis.com

10

Figure 3.1.1 Help Menu.

Figure 3.1.2 FRANC3D documentation file location dialog.

3.1.1 About Dialog

The last menu item displays the About dialog, which contains the version number, the

build number and date, acknowledgements, and a list of supporting agencies, Figure

3.1.3. We also wish to acknowledge the help and support of all of our users for their

contributions.

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Figure 3.1.3 About dialog box.

3.2 File

The File menu is shown in Figure 3.2.1. Each of the menu items is listed below.

Figure 3.2.1 File Menu.

Open ... - Displays an Open File dialog box that allows one to read a model, which is

described in Section 4.1. A number of different types of files can be read, including:

.fdb, .cdb, .inp, .bdf, and .model. The .fdb files are FRANC3D database (restart) files.

The .cdb files are ANSYS database files in ASCII format. The .inp files are ABAQUS

input files. The .bdf are NASTRAN database files. The .model files are mesh files in

FAC's internal format.

If an ANSYS .cdb file is selected, the ANSYS version of the FEM Input wizard is

invoked; this wizard is described in Section 4.1 and is similar for all the other file types

(except for the .fdb restart files).

12

Close - Closes the current model so that a new model can be read.

SaveAs ... - Displays a Save File As dialog box, which is described in Section 4.3. This

option allows one to save the current model to a file. Models can be saved in .fdb, .cdb,

.inp, or .model formats. (See Open for file extension descriptions.)

Hint: By using Open and SaveAs, FRANC3D can be used as a file format translator.

However, all features of a model might not be translated.

Read Results ... - Invokes the Read Results File dialog, which is described in Section

4.4.

Playback ... - Invokes the Playback Session File dialog, which is described in Section

4.5, allowing the user to read in a session log file and reproduce the saved commands.

Quit - Closes the model and exits the program.

3.3 Edit

The Edit menu is shown in Figure 3.3.1. Each of the menu items is listed below.

Figure 3.3.1 Edit Menu.

Undo - Undo the last executed command. This feature is not completed yet.

Redo - Redo the last command that was undone. This feature is not completed yet.

Implementation note: the mechanism is in place to support Undo and Redo, but the

details have not yet been implemented.

Preferences... - Invokes the Preferences dialog box, which is described in Section 5.3.

3.4 Cracks

The Cracks menu is shown in Figure 3.4.1. Each of the menu items is listed below.

13

Figure 3.4.1 Cracks Menu.

New Flaw Wizard ... - Invokes the New Flaw wizard, which is described in Section 6.1.

Flaw From Files ... - Invokes the Locate Flaw File dialog, described in Section 6.2,

followed by a subset of the New Flaw wizard, described in Section 6.1.10 - 11. This

option allows one to select a flaw description from a file, or from multiple files, and

insert it into the current model.

Multiple Flaw Insert ... Invokes the Multiple Flaw wizard, which is described in Section

6.3.

Compute SIF's ... - Invokes the Compute SIF's wizard, which is described in Section

6.4. Note that displacements from a completed analysis must be available for this option

to be enabled.

Grow Crack ... - Invokes the Crack Growth wizard, which is described in Section 6.5.

Note that displacements from a completed analysis must be available for this option to be

enabled.

Read Crack Growth ... - Invokes the Read Crack Growth wizard, which is described in

Section 6.6.

Grow/Merge Cracks ... - Invokes the Crack Growth/Merge wizard, which is described

in Section 6.7. This feature is not completed yet.

3.5 Materials

The Materials menu is shown in Figure 3.5.1. Each of the menu items is listed below.

Figure 3.5.1 Materials Menu.

14

Material Properties ... - Invokes the Material Properties wizard, which is described in

Section 7.1.

Apply Material ... - Invokes the Apply Material to Elements dialog, which is described

in Section 7.2.

Apply Coordinates ... - Invokes the Apply Coordinate System to Elements dialog,

which is described in Section 7.3.

3.6 Loads

The Loads menu is shown in Figure 3.6.1. Each of the menu items is listed below.

Figure 3.6.1 Loads Menu.

Map Current State ... - Invokes the Map Current State dialog, which is described in

Section 8.1. Note that a previous model and previous results must be available for this

option to be enabled. This option is not completed yet and the menu option is inactive.

Crack Face Pressure/Traction ... - Invokes the Crack-Face Tractions dialog, which is

described in Section 8.2.

3.7 Analysis

The Analysis menu is shown in Figure 3.7.1. Each of the menu items is listed below.

Figure 3.7.1 Analysis Menu.

Static Crack Analysis ... - Invokes the Static Crack Analysis wizard, which is described

in Section 9.1.

Crack Growth Analysis ... – Invokes the Crack Growth Analysis wizard, which is


15

3.8 Fatigue

The Fatigue menu is shown in Figure 3.8.1. Each of the menu items is listed below.

Figure 3.8.1 Fatigue Menu.

SIF History ... - Invokes the SIF History wizard, which is described in Section 10.1.

Discrete Crack Life ... - Invokes the Discrete Crack Life wizard, which is described in

Section 10.2.

3.9 Fretting

The Fretting menu is shown in Figure 3.9.1. Each of the menu items is listed below.

Figure 3.9.1 Fretting Menu.

Read Model and Results ... - Invokes the Fretting Model Import wizard, which is


Import Nucleation Data ... - Invokes the Fretting Data Import/Analysis wizard, which

is described in Section 11.2.

Fretting Crack Nucleation ... - Invokes the Fretting Crack Nucleation wizard, which is


Extract Subvolume ... - Invokes the Extract Subvolume wizard, which is described in

Section 11.4.

Turn Off Region Color ... - Invokes the Region Color dialog, which is described in

Section 11.5.

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3.10 Advanced

The Advanced menu is shown in Figure 3.10.1. Each of the menu items is listed below.

Figure 3.10.1 Advanced Menu.

Edges Wizard ... - Invokes the Edge Extraction wizard, which is described in Section

12.1.

No Crack Regions ... - Invokes the No Crack Regions dialog, which is described in

Section 12.2.

Write Crack Front Data ... - Invokes the Write Crack Front Data dialog, which is

described Section 12.3.

Write COD Data ... - Invokes the Write COD Data wizard, which is described Section

12.4.

Write Template Data ... - Invokes the Write Template Data dialog, which is described

Section 12.5.

View Response ... - Invokes the View Response dialog, which is described Section 12.6.

Create Growth History ... - Invokes the Create Growth History dialog, which is


Crack Growth Sequence ... - Invokes the Crack Growth Sequence dialog, which is


17

4. File Menu Wizards and Dialog Boxes

The wizards and dialog boxes for the File menu options are described in this section.

4.1 Open…Ctrl-O

The File Open menu option allows the user to read either a FRANC3D restart file, with

.fdb extension, or an uncracked finite element model file. The supported file types

include: ANSYS .cdb, ABAQUS .inp, NASTRAN .bdf and FAC’s .model format.

These files are all ASCII format. The .model format will not be discussed here.

The Model File dialog is displayed, Figure 4.1.1, which allows the user to specify the

desired file type using the File Filter. This dialog allows the user to change directories.

The file name can be selected with the left-mouse button or typed into the field. The user

should select Accept once the file name has been entered.

This dialog contains one extra check box at the bottom that is only activated if the file

filter is set to Ansys Files. The Read extra ANSYS load step files check box allows the

user to read ANSYS specific .s## files that contain boundary conditions for load steps (or

cases). If this box is checked, once the ANSYS .cdb file is accepted, a second dialog is

presented, Figure 4.1.2, which allows the user to select one or more of the .s## files. The

boundary condition data is imported from each selected file and stored in separate load

cases.

Figure 4.1.1 Model File dialog box.

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Figure 4.1.2 ANSYS load step file dialog box.

4.1.1 FRANC3D Restart Files

The user can choose a .fdb file from the Model File dialog, Figure 4.1.1. FRANC3D

reads the contents of this file and any files that are referenced inside the .fdb file. The

.fdb file is organized in blocks with each block having a title and a version number. The

blocks and their content are summarized here.

F3DMETA (

Uncracked finite element file name, type, and retained data.

)

HISTORY_SUMMARY (

Summary of the crack step file names.

)

STATICMETA (

Analysis type, input and results file names.

)

FLAWSURF (

Description of crack geometry: surface patches, crack front vertices, and template

parameters.

)

CRACKFRONTIDS (

List of crack front identifiers.

)

TEMPLATENODES {

List of crack front mesh template node ids.

}

TEMPLATENODECOORDS {

List of crack front mesh template node coordinates.

}

SIF_COMP_PARAMS (

19

Stress intensity factor computation parameters.

)

GROWTH_PARAMS (

Crack growth computation parameters.

)

TEMPLATE_PARAMS (

Crack front mesh template parameters.

)

MATERIAL_DATA (

Material properties.

)

CRACKFACENODES {

List of crack face nodes with the local normal to the crack surface.

}

EDGE_EXTRACT_PARAMS (

Edge extraction parameters.

)

MESH_PARAMS (

Meshing parameters.

)

CRACK_GROWTH_DATA (

Crack growth history data.

)

SAVED_VIEWS {

List of saved camera positions.

}

If one selects a .fdb file, FRANC3D attempts to read all the relevant files. If a file

referenced by the .fdb file is missing, the user is prompted to locate that file, Figure

4.1.1.1. Note that you need the original uncracked finite element model file to perform

crack growth simulations. FRANC3D will also ask the user if the model path will be the

current working directory, Figure 4.1.1.2. Note that the working (default) directory can

be set in the Preferences dialog (see Section 5).

Figure 4.1.1.1 Locate File dialog box for finding missing files.

20

Figure 4.1.1.2 Model File Path dialog box to set working directory.

4.1.2 FEM Input Wizard

The user can switch the File Filter in the Model File dialog, Figure 4.1, from Franc3D

Database Files (.fdb) to Ansys Files (.cdb), Abaqus Files (.inp) or Nastran Files (.bdf).

The process for reading an ANSYS .cdb file is essentially the same as the process for

reading .inp or .bdf files. Reading ANSYS files has one extra option, however, as

described in the previous section. Thus, the description below refers to ANSYS, but can

be applied to the other file types as well.

The FE Mesh File Input wizard is invoked from the File: Open… menu item if the user

selects an ANSYS, ABAQUS or NASTRAN mesh file. The File Open dialog is

presented, Figure 4.1.2.1, and depending on the File Filter, the list of files that are

presented will vary. In Figure 4.1.2.1, the File Filter is set to ANSYS, and the Read extra

ANSYS files check box is activated. With this box checked, the next dialog prompts the

user to select one or more .s## files, Figure 4.1.2.2, which contain the load step boundary

condition data.

Figure 4.1.2.1 Model File Open dialog with File Filter set to ANSYS cdb files and the

Read additional files check box activated.

21

Figure 4.1.2.2 Locate .s## files dialog for reading ANSYS load steps.

The rest of the wizard panels that follow the File Open dialog are basically the same for

all the supported finite element input file types. The ANSYS specific panels are

described next.

4.1.3 Retain Model Panel

The FEM Input wizard allows the user to specify what is retained from the finite

element file (in this example an ANSYS cdb file). The first panel allows the user to

retain everything or to retain selected items, Figure 4.1.3.1. Note that the nodes and

elements will be retained by default even if nothing else is retained. The View .cdb

button invokes a text box with the ANSYS .cdb information displayed, Figure 4.1.3.2.

Note that similar View buttons will be available for ABAQUS and NASTRAN. If the

user chooses to retain everything, then there is nothing more to do except click Finish

and wait for the model to load.

In general, the only time one would retain everything is when using FRANC3D as a file

format translator.

22

Figure 4.1.3.1 Retain everything or selected items.

Figure 4.1.3.2 View ANSYS cdb file dialog.

4.1.4 Select Items to Retain Panel

If the user chose to select the items to be retained, the next panel, Figure 4.1.4.1, lists all

of the supported data that is present in the file. Materials, coordinate systems, boundary

conditions, contact and constraint information, and node components are supported. The

user can choose to select none, all, or some of the items presented. If an item, such as a

coordinate system, does not exist in the input file, then that item is not presented in the

dialog box.

23

Figure 4.1.4.1 Select items to retain panel.

4.1.5 Materials to Retain Panel

If the user chooses to select individual materials to retain, the next panel, Figure 4.1.5.1,

lists all of the materials that are present in the file. The basic material properties required

to compute stress intensity factors are retained regardless of this selection. If the model is

self-contained and will not be merged with a global model, then the materials should be

retained.

If there are multiple materials, the regions will be separated by default based on the

material ids. The boundaries between material regions can be ignored so that a single

volume is created and the selected or first material id will be assigned to all elements.

This is accomplished by checking the Ignore material id boundary option in Figure

4.1.5.1.

24

Figure 4.1.5.1 Materials to retain panel.

4.1.6 Coordinate Systems to Retain Panel

If the user chooses to select individual coordinate systems to retain, the next panel, Figure

4.1.6.1, lists all of the coordinate systems that are present in the file. If the model is self-

contained and will not be merged with a global model, then the coordinate systems

should be retained.

Figure 4.1.6.1 Coordinate systems to retain panel (portion of full panel shown).

4.1.7 Boundary Conditions to Retain Panel

If the user chooses to select individual boundary conditions to retain, the next panel,

Figure 4.1.7.1, lists all of the boundary condition types that are present in the file. The

boundary condition types can be retained either with or without retaining the mesh facets

and nodes where the boundary conditions are applied; this is true for all but the

temperature boundary conditions because only surface mesh facets can be retained and

temperatures are generally applied throughout the volume. If the boundary conditions are

retained without retaining the mesh facets, the boundary conditions will be mapped to the

remeshed surfaces.

Note that the mesh should not be retained on a surface where a crack will be inserted.

25

Figure 4.1.7.1 Boundary conditions to retain panel.

4.1.8 Components/Nsets to Retain as Mesh Facets Panel

If the user chooses to select individual mesh facet groups to retain, the next panel, Figure

4.1.8.1, lists all of the node components that are present in the file. If a component is

selected, then all the nodes and the mesh facets on the surface of the mesh will be

retained. Be careful not to retain facets where the crack will be inserted as this will cause

errors later. Single nodes will not be retained at this time, only complete facets are

retained.

The Inverse retain mesh option allows the user to retain all but the selected component.

Figure 4.1.8.1 Components/Nsets to retain as mesh facets panel.

26

4.1.9 Surfaces to Retain for Contact/Constraint Panel

If the user chooses to select individual surfaces for contact/constraint, the next panel

Figure 4.1.9.1, lists all of the surfaces (components, nsets, or other) that are present in the

file. The mesh facets can be retained with the selected surfaces. If the mesh facets are

not retained, the contact/constraint conditions will be mapped to the remeshed surfaces.

Note that the mesh should not be retained on a surface where a crack will be inserted.

Figure 4.1.9.1 Surfaces to retain for contact/constraint panel.

4.1.10 Components/Nsets for Residual Stress Panel

If the user chooses to select surfaces for residual stress, the next panel, Figure 4.1.10.1,

lists all of the surfaces (components, nsets, or other) that are present in the file. The mesh

facets are not retained with these surfaces.

These surfaces are used in conjunction with the crack face traction option where a

residual stress normal to the selected surface is defined. This feature is described in more

detail in Section 8.2.4.

27

Figure 4.1.10.1 Components/Nsets for residual stress panel.

4.2 Close

The File Close menu item closes the current model without quitting FRANC3D. If a

model is open and has not been saved, the Save Model Warning dialog, Figure 4.2.1, is

displayed. If the user wishes to save the model, the Cancel button can be selected.

Selecting the OK button will close the model without saving.

Figure 4.2.1 Save Model Warning dialog.

4.3 Save As…

The File Save As menu item can be used to save the initial uncracked input file in any of

the supported formats; FRANC3D will act as a translator. The File Save As option can

also be used to save a FRANC3D restart (fdb) file of a cracked model if the user does not

want to perform an analysis of the model. The dialog shown in Figure 4.3.1 allows one

to switch the File Filter to save in the desired format.

The default is to save a FRANC3D restart (fdb) file. If the user enters a .fdb file name

and selects OK, the File Output Options dialog is presented, Figure 4.3.2, and the user

can choose to save the finite element data in ANSYS, ABAQUS or NASTRAN format.

28

Figure 4.3.1 Save File As dialog.

Figure 4.3.2 Fdb Output Options dialog for choosing analysis code file format.

The next panel, Figure 4.3.3, allows the user to specify output options: elements can be

defined as linear or quadratic, nodes and elements can be renumbered, and boundary

condition (bcs) data and solution information can be output or suppressed. If the user

switches the File Filter in Figure 4.3.1, then the dialog shown in Figure 4.3.2 is skipped.

When the user selects Finish in Figure 4.3.3, the files are written in the appropriate

format.

29

Figure 4.3.3 Fdb Output Options dialog for defining output parameters.

4.4 Read Results

Results for one or multiple load cases can be read into the FRANC3D database. The load

case results can be used in conjunction with the crack growth wizard (see Section 6.5).

The File Read Results menu item invokes the dialog shown in Figure 4.4.1, which

allows the user to choose the analysis results file to be read. The user can switch the File

Filter if it is not correct. The user can either replace or add load case results, using the

radio buttons (Replace current set of results or Add these results) at the top of the dialog.

For ANSYS, the results file will either be a .dtp file that is created using the FRANC3D

generated macros or a simple nodal listing of displacements saved to a .dsp file. The .dtp

file will contain displacements, temperatures if not equal to the reference temperature,

and contact pressures on crack surfaces if such exist. For ABAQUS, all results will be

contained in the .fil file. FRANC3D includes commands in the .inp file that instructs

ABAQUS to write results in ASCII format to the .fil file, including displacements,

temperatures and crack surface contact pressures. For NASTRAN, all results will be

contained in a .pch file, which will include displacements, temperatures and crack surface

contact pressures. The .dtp, .fil and .pch file can include results for multiple load cases.

If the results file contains data for multiple load steps, all of the data is read and stored in

the FRANC3D database. Thus, one should be careful when entering the frequency for

output in the analysis codes; for instance, in ABAQUS, the output frequency is usually

set to 99999. Only the results for load cases (or steps) where SIFs are to be computed are

needed by FRANC3D. In fact, only the results for the crack front template mesh are

needed to compute SIFs.

30

Figure 4.4.1 Read results file dialog.

4.5 Playback….

The File Playback menu item allows playback of recorded session (log) files. Each time

the FRANC3D program is executed, a session##.log file is saved. This file records the

commands that the user executes through the menus and dialogs. A sample .log file

might look like this:

OpenMeshModel(

model_type=ANSYS,

file_name='C:\cube\cube.cdb',

retained_mats=[ALL],

retained_bcs=[ALL])

InsertFileFlaw(

file_name='C:\cube\cube_05.crk')

RunAnsysAnalysis(

file_name='C:\cube\cracked_cube',

flags=[QUADRATIC],

executable='C:\ANSYS Inc\v121\ansys\bin\WINX64\ANSYS121.exe',

command='"C:\ANSYS Inc\v121\ansys\bin\WINX64\ANSYS121.exe" -b -p

struct -i "C:\cube\cracked_cube.macro" -o "C:\cube\cracked_cube.out"',

license=struct)

ComputeSif(

load_case_factors=[[1,1,1]])

31

A complete description of the FRANC3D command language is provided in Section 12.

Selecting the Playback menu item invokes the Playback Session File selection dialog,

Figure 4.5.1. The user selects the desired session file and then selects OK. FRANC3D

will read and execute the commands from the session file.

Figure 4.5.1 Playback session file selection dialog.

4.6 Quit - Ctrl-Q

The File Quit menu item exits FRANC3D. As for the File Close menu item (see Section

4.2), the Save Model Warning dialog (see Figure 4.2.1) will be displayed if the model

has not been saved.

5. Edit Menu Wizards and Dialog Boxes

The wizards and dialog boxes for the Edit menu options are described in this section.

Note that the first two menu items, Undo and Redo, are inactive in Version 6.

5.1 Undo - Ctrl-Z

The Edit Undo menu item is intended to undo user actions. It is not active in Version 6.

32

5.2 Redo

The Edit Redo menu item is intended to redo actions that have been undone. It is not

active in Version 6.

5.3 Preferences…

Selecting the Edit Preferences menu item invokes the Preferences dialog, which allows

one to set program-wide configurations. These preferences are stored in a database that

is read when FRANC3D is started. Some settings might not take effect until the program

is restarted. The Preferences dialog has nine tabs, and each of these will be described

next.

5.3.1 Folders Tab

In the Folders tab, one can set the default working directory and the help file directory,

Figure 5.3.1.1. The working directory is the folder where the model files will be read

from and written to by default. The user can override the default directory when reading

the FRANC3D restart file as described in Section 4.1.1.

The Help File directory is the top level directory for the web-help files described in

Section 3.1. Help files are accessed from the Help menu. The web-help files can be

accessed over the internet, and they can be installed locally by the user. These will

consist of folders labeled as: FRANC3D_V6_Reference.web or similar for the various

FRANC3D documents.

Figure 5.3.1.1 Folders tab of Preferences dialog.

33

5.3.2 Window Tab

In the Window tab, one can set the font and colors used in the graphical user interface

windows, Figure 5.3.2.1. The GUI font Select button will pop up a dialog box that will

allow one to select from the available fonts, Figure 5.3.2.2. Clicking any of the color

swatches (colored rectangles) will pop up a dialog box, Figure 5.3.2.3, which allows one

to select a new color.

The Defaults button restores all values to default settings.

Figure 5.3.2.1 Window tab of Preferences dialog.

Figure 5.3.2.2 Font selection dialog.

34

Figure 5.3.2.3 Color selection dialog.

5.3.3 3D View Tab

The 3D View tab is used to set the combinations of mouse buttons and keyboard keys that

are used to invoke the view and select functions, Figure 5.3.3.1. One should ensure that

each function has a unique set of buttons and keys. The view manipulations were

described in Section 2.


Figure 5.3.1.1 3D View tab of Preferences dialog.

5.3.4 Display Tab

The Display tab allows the user to set the colors used for the 3D model view, Figure

5.3.4.1. Clicking any of the color swatches will pop up a dialog box (see Figure 5.3.2.3),

35

which allows one to select a new color. The line width can be set also; the default is 1,

but this value can be increased for thicker lines. The program can be restarted or the

model reread for this setting to take effect.


Figure 5.3.4.1 Display tab of Preferences dialog.

5.3.5 ANSYS Tab

The ANSYS tab allows the user to set default ANSYS analysis parameters, Figure 5.3.5.1.

Configuration details that do not change frequently, such as the path to the ANSYS

executable and the license type can be set here. These values will then appear as default

values for every ANSYS analysis.

Ansys Executable stores the path to the ANSYS executable program. The Browse button

will pop up a file browser that can be used to locate the program.

License Type stores the ANSYS license type.

Solver allows one to specify the ANSYS equation solver.

Total and Database Memory options allow one to set the amount of memory that will be

requested on the ANSYS command line (see ANSYS documentation for details).

Number of Processors is self-explanatory.

36

Figure 5.3.5.1 ANSYS tab of Preferences dialog.

5.3.6 ABAQUS Tab

The ABAQUS tab allows the user to set default ABAQUS analysis parameters, Figure

5.3.6.1. Currently, only the location of the ABAQUS executable (or batch) file location

can be set in the preferences. The Browse button will pop up a file browser that can be

used to locate the program. For MSWindows, one should choose the abaqus.bat file from

the ABAQUS Commands folder.

Figure 5.8 ABAQUS tab of Preferences dialog.

37

5.3.7 NASTRAN Tab

The NASTRAN tab allows the user to set default NASTRAN analysis parameters, Figure

5.3.7.1. The location of the NASTRAN executable file location can be set in the

preferences. The Browse button will pop up a file browser that can be used to locate the

program. The user can also choose the default setting for pyramid elements, depending

on their version of NASTRAN. MSC NASTRAN does not support pyramid elements, so

all pyramid elements in FRANC3D must be split into two tetrahedral elements. NX

NASTRAN supports pyramid elements, and these are preferred.

Figure 5.3.7.1 NASTRAN tab of Preferences dialog.

5.3.8 FEAWD Tab

The FEAWD tab allows the user to set default FEAWD analysis parameters, Figure

5.3.8.1. The location of the FEAWD executable file location can be set in the

preferences. The Browse button will pop up a file browser that can be used to locate the

program. Note that FEAWD is one of FAC’s in-house analysis codes.

38

Figure 5.3.8.1 FEAWD tab of Preferences dialog.

5.3.9 Meshing Tab

The Meshing tab allows the user to set default meshing parameters, Figure 5.3.9.1. The

options are listed here.

Max volume elements sets a limit on the number of volume elements FRANC3D can

generate.

Max backtrack restarts sets a limit on the number of FRANC3D volume meshing

attempts. If the volume meshing gets stuck, the program will backtrack and restarts.

Coarsen crack mouth turns on/off the mesh coarsening along the crack mouth.

Local surface refinement turns on/off the local surface mesh refinement.

Condense mesh turns on/off condensing of the mesh node and element numbering.

Start node id sets the starting node number if condensing is turned on.

Start element sets the starting element number if condensing is turned on.

Volume mesh using allows the user to choose between FRANC3D, ANSYS and

ABAQUS volume meshing.

For ANSYS/ABAQUS write files only allows the user to write the surface mesh and the

commands to generate the volume mesh from the surface mesh to files without actually

running ANSYS or ABAQUS. This gives the user the option of sending the files to a

39

different computer or modifying the commands. During the meshing phase of crack

insertion, FRANC3D will prompt the user to save the surface mesh files, and then will

wait for the volume mesh file to be selected.

Figure 5.3.9.1 Meshing tab of Preferences dialog.

6. Cracks Menu Wizards and Dialog Boxes

The wizards and dialog boxes for the Cracks menu options are described in this section.

6.1 New Flaw Wizard

The New Flaw wizard leads the user through the process of creating and orienting a

parametrically defined flaw. This wizard contains a number of different panels; however,

normally one will not see all panels. Choices in some of the panels determine which

panels are shown next. The overall flow is illustrated in Figure 6.1.1. Note that the

elliptical and through crack boxes house a couple of crack shapes.

The individual wizard panels are described below.

40

Figure 6.1.1 New Flaw wizard flow diagram.

6.1.1 Flaw type panel

The first panel determines the flaw type, Figure 6.1.1.1. This panel allows one to specify

that either a crack (zero volume flaw) or a void (finite volume flaw) should be inserted.

It also allows one to save the flaw description to a file. The default is to add the flaw to

the model without saving to a (.crk) file. The second radio button (Save to file and add

flaw) allows the user to save the flaw to a file and add it to the model. The third option

(Save to file only) saves the flaw to a file without adding it to the model.

Figure 6.1.1.1 New Flaw wizard first panel selects flaw type.

41

6.1.2 Crack type panel

The current crack types include: 1) an elliptical crack with one crack front – top left icon

in Figure 6.1.2.1, 2) a through-the-thickness crack with one front – bottom left icon, 3) a

through-the-thickness crack with two fronts – top middle icon, 4) a long-shallow surface

crack shape to be used instead of long thin ellipses – bottom middle icon, 5) an elliptical

crack shape with two fronts – top right icon, or 6) a user-defined crack – bottom right

icon). The crack fronts are indicated by the thicker lines. Selecting the crack shape icon

specifies the crack type that will be shown in the next panel.

Figure 6.1.2.1 New Flaw Wizard crack (zero volume flaw) types.

6.1.3 Elliptical crack panel

Single front elliptical cracks are defined by entering the semi-axes lengths (a and b).

Once values have been specified the length of the axes, the ellipse is displayed in the 3D

view window, Figure 6.1.3.1. The ellipse is displayed in its local orientation, which is in

the x-y plane and centered at the global Cartesian origin.

The Advanced Geometry button is common to all predefined crack shapes and will be

described in Section 6.1.9.

42

Figure 6.1.3.1 New Flaw Wizard elliptical crack parameters panel.

6.1.4 Single-front Through-the-thickness crack panel

Single-front through-the-thickness cracks are specified using three lengths (a – c) and the

width (d), Figure 6.1.4.1. If set appropriately, the three lengths can be used to define a

straight or a quadratic shape crack front. The crack is displayed in its local orientation,

which is in the x-y plane with one corner at the global Cartesian origin.



Figure 6.1.4.1 New Flaw Wizard single-front through-crack parameters panel.

43

6.1.5 Two-front Through-the-thickness crack panel

Two-front through-the-thickness center cracks are specified using six crack lengths (a – f)

and the crack width (g), Figure 6.1.5.1. If set appropriately, the six lengths can be used to

define either straight or a quadratic shape crack fronts. The crack is displayed in its local

orientation, which is in the x-y plane with one mid-side at the global Cartesian origin.



Figure 6.1.5.1 New Flaw Wizard two-front through-crack parameters panel.

6.1.6 Long shallow surface crack panel

Long-shallow-surface cracks are specified using the crack length (a), the crack width (b),

and a corner radius (c), Figure 6.1.6.1. This crack shape can be used in place of high-

aspect ratio elliptical surface cracks. The crack is displayed in its local orientation, which

is in the x-y plane with the global Cartesian origin as shown in Figure 6.1.6.1.



44

Figure 6.1.6.1 New Flaw Wizard long shallow surface crack parameters panel.

6.1.7 Two-front Elliptical crack panel

Two-front elliptical cracks are defined by entering the outer semi-axes lengths (a and b)

and the inner semi-axes lengths (c and d). The user also has the option of turning off the

crack front for either the outer or inner front; this allows one to define a circumferential

crack in the outer or inner wall of a pipe.

Once values have been specified for the length of the axes, the ellipse is displayed in the

3D view window, Figure 6.1.7.1. The ellipses are displayed in the local orientation,

which is in the x-y plane and centered at the Cartesian origin. Both outer axes must be

larger than the inner axes to create a valid crack.



45

Figure 6.1.7.1 New Flaw Wizard two-front elliptical crack parameters panel.

6.1.8 User-defined crack panel

The user-defined crack requires a set of points that completely defines the crack

boundary geometry, including the crack front. Click on Define Points, Figure 6.1.8.1, to

display a dialog that allows one to enter the geometry points, Figure 6.1.8.2. The front

pts field in Figure 6.1.8.2 allows one to define points as crack front points using a value

of 1; non-front points should have this field set to 0. Note that unless one is defining an

embedded crack, there typically should be at least one point on the crack boundary that is

not a crack front point. The front points are displayed in blue (Figure 6.1.8.1) and

numbered corresponding to the listing shown in Figure 6.1.8.2. Points must be

consecutive around the boundary.

One can read the points from a file using the File Open menu in the dialog shown in

Figure 6.1.8.2. The file should be a simple ASCII .txt file with the format:

x y z flag

x y z flag

x y z flag

…

The Smooth Front Points option in Figure 6.1.8.1 allows the user to smooth and re-

parameterize the crack front points. This allows for more uniform crack surface

geometry as the boundary points are used with a surface triangulation routine to produce

the interior crack surface geometry.

46

Figure 6.1.8.1 User-defined crack panel.

Figure 6.1.8.2 User-defined crack points dialog.

6.1.9 Advanced Geometry button

Selecting the Advanced Geometry button on any of the predefined crack shapes leads to

the dialog shown in Figure 6.1.9.1. The crack shape shown here corresponds to the

single front ellipse with major and minor axes equal. Note that the triangular facets

represent the geometry, not a finite element surface mesh.

There are three buttons on the top menu bar: Refine, Reset and Edit. The crack

geometry is shown in the window. The menu options are described below.

47

Figure 6.1.9.1 Crack Advanced Geometry dialog.

6.1.9.1 Refine

The Refine button alters the discretization of the crack surface geometry. Although

cracks notionally have an analytical geometry (e.g. elliptical or quadratic), they are

actually defined geometrically by triangular cubic-Bezier patches. These patches only

approximate the analytical form. As the number of geometry patches is increased, the

approximation improves, but at the cost of additional processing time required to insert

the crack. The Refine button performs a uniform refinement of the patches, where each

patch is divided into four smaller patches. This is illustrated in the images in Figure

6.1.9.2. The Refine process can be repeated any number of times.

Refinement might be required for higher-aspect ratio elliptical cracks, specifically for the

ends of the major axis.

6.1.9.2 Reset

The Reset button sets the crack geometry back to the original default configuration.

48

Figure 6.1.9.2 Example of a Refined crack geometry. The left panel shows one level of

refinement from Figure 6.1.9.1, and the right panel shows a second level of refinement.

6.1.9.3 Edit

The Edit button invokes the Flaw Editor, Figure 6.1.9.3. This dialog allows the user to

redefine the default shape of the flaw. The vertices (black circles in Figure 6.1.9.3) can

be selected by holding down the Shift key and selecting the black circle with the left-

mouse button. The vertices turn red when they are selected (see right panel of Figure

6.1.9.3). The red circle can be moved by holding down the Shift key and dragging the

item to the new location using the left mouse button. The x and y coordinates of the

vertex are shown in the fields near the top of the dialog.

Unselect the vertex by holding the Shift key down and picking a point that is not close to

any vertex. The undo and redo icons (to the left of the coordinate fields) allow the user to

undo (or redo) any movement of the vertices that is not desired.

Select Accept and the revised flaw shape will be shown in the initial Advanced Geometry

window (see Figure 6.1.9.1).

Select Accept in the Advanced Geometry window (see Figure 6.1.9.1) to return to the

regular flaw insertion wizard panel. The user can then proceed to locate and orient the

edited flaw in the model.

Note that the flaw editor must be used before rotating and translating the flaw.

49

Figure 6.1.9.3 Flaw Editor dialog. Left panel shows the initial geometry and right panel

shows edited geometry.

6.1.10 Ellipsoidal void panel

Ellipsoidal voids are the only void shapes currently available. They are defined by

entering the three semi-axes lengths (a, b and c), Figure 6.1.10.1. The void is displayed in

the local orientation and centered at the Cartesian origin.

Figure 6.1.10.1 New Flaw Wizard ellipsoidal void parameters panel.

50

6.1.11 Flaw translation and rotation panel

Once a flaw (crack or void) is defined, it must be inserted (translated and rotated) into the

proper location relative to the unflawed body. The flaw orientation panel displays the

flaw and the model in a 3D graphics window, along with controls to translate and rotate

the flaw, Figure 6.1.11.1.

The translations move the local origin of the flaw to a location in the Cartesian space of

the body. It is usually simpler to specify the translations first and then "zoom in" on the

display of the flaw before specifying the rotations.

Up to three rotations about the local (rotated) Cartesian axes can be specified. These are

essentially Euler angles, but the order of the rotation axes can be specified. The rotations

follow a "right-hand-rule" where the thumb points in the positive direction along the axis

and the fingers curl in the positive rotation direction.

There is an Orient with Vectors button at the bottom of the flaw rotations panel. This

button is described in the next sub-section.

Figure 6.1.11.1 New Flaw Wizard orientation panel.

6.1.11.1 Orient with Vectors

The Orient with Vectors button provides another way to specify the flaw orientation,

Figure 6.1.11.2. This button invokes a dialog box that allows one to specify the local x-

and y-axes of the flaw oriented relative to the Cartesian coordinates of the body.

51

Figure 6.1.11.1 New Flaw Wizard orientation vectors dialog.

6.1.12 Crack Front Mesh Template Panel

For accurate stress-intensity factor computations, a pattern or "template" of elements with

controlled sizes and shapes is placed about all crack fronts. The template takes the form

of generalized cylindrical tubes of elements with the crack fronts serving as the axes of

the cylinders. Wedge shaped elements are placed immediately adjacent to the crack

fronts. These are surrounded by rings of brick elements.

Figure 6.1.12.1 shows the Crack Front Mesh Template wizard panel with an elliptical

surface crack being inserted into a cube. The user can turn off the crack front template

elements with the use crack-front template toggle. In some cases, a crack front template

mesh cannot be added to the model; a crack front that touches or crosses a material

boundary currently is one such case.

The overall radius of the template can be adjusted with the Template Radius field. The

default value of the radius is based on the crack size and might be inappropriate. In

addition, the template radius can be decreased if a user wishes to do a mesh

refinement/convergence study.

The template elements are displayed in the 3D graphics window superimposed on the

flaw and model geometry. By default only one half of the template elements are shown.

This allows one to view the progression of element sizes as the elements transition from

the crack front to the outer boundary of the template. One can view the full template by

turning on the Display Full Template toggle.

The Simple Template Intersections Only toggle allows the user to terminate the template

inside the model surfaces. For cases where the template will intersect the model surface

at shallow angles or at corners, this option allows a crack to be inserted with template

elements along the bulk of the crack front.

The Meshing Parameters button at the bottom left is used to specify surface and volume

meshing parameters as well as choosing the volume mesher; this is described in Section

52

6.1.12.1. The Advanced Options button allows the user to adjust the template elements

and is described in Section 6.1.12.2.

Figure 6.1.12.1 Crack front mesh template panel.

Note that this panel is displayed even if the flaw does not have any crack fronts (e.g. an

elliptical void). In this case, simply ignore this panel and select finish.

6.1.12.1 Meshing Parameters Dialog

The Meshing Parameters dialog, Figure 6.1.12.2, is very similar to the Preferences

meshing tab described in Section 5.3.9. The user can control some aspects of the surface

and volume meshing.

The first parameter, Maximum Generated Elements, limits the total number of volume

elements that FRANC3D will create during volume meshing. The second parameter,

Maximum Volume Mesh Restarts, limits the number of volume meshing restarts. These

two options only work when FRANC3D is used for volume mesh. FRANC3D uses an

advancing front volume meshing algorithm; this algorithm can get stuck sometimes and

the algorithm is designed to backtrack (i.e., remove elements) and restart the meshing. If

the program fails to create a volume mesh after several restarts, then one can try volume

meshing with ANSYS or ABAQUS.

The next two options allow the user to control surface mesh refinement: on the crack

surface, using the Do Coarsen Crack Mouth Mesh option, and on the model surface in

areas where adjacent region boundaries are near, using Do Local Surface Refinement.

53

The user can force FRANC3D to condense and renumber the nodes and elements.

Turning on the Condense option activates the Start node id and Start element fields. The

default is to start the numbering at 1, but any integer value can be entered.

FRANC3D, by default, is used to mesh the volume, but ANSYS and ABAQUS can also

be used by selecting the appropriate radio button for Volume mesh using. Note that the

user must have a licensed working copy of these programs to use their meshing

algorithms. The user can select the ANSYS and ABAQUS executables, using the

Browse button, if not already set in the Preferences (see Section 5.3).

For ANSYS/ABAQUS write files only allows the user to write the surface mesh and the

commands to generate the volume mesh from the surface mesh to files without actually

running ANSYS or ABAQUS. This gives the user the option of sending the files to a

different computer or modifying the commands.

Figure 6.1.12.2 Meshing parameters dialog.

6.1.12.2 Advanced Template Options Dialog

The Advanced Template Options dialog allows the user to adjust some of the default

crack front mesh template parameters, Figure 6.1.12.3. The parameters are described

below and illustrated in Figure 6.1.12.4, which shows a typical cross-section of a crack-

front template.

Progression Ratio sets the relative width of the element (in the direction perpendicular to

the crack front) going from the crack front to the outer surface of the template. For

example, a ratio of 1 means that all the rings of elements will have the same width. For a

54

ratio of 1.5, the width of the elements in a ring will be 50% greater than the width of the

elements in the next ring as we approach the crack front.

Num Rings sets the number of rings of elements in the template.

Num Circumferential Elem sets the number of elements in the circumferential direction

around the crack front.

Max Aspect Ratio controls the aspect ratio of the quadrilateral faces on the outer surface

of the template that will trigger 1:2 or 1:3 transitions in the outer ring of elements.

Figure 6.1.12.3 Meshing parameters dialog.

first ring

second ring

third ring

first ring

second ring

third ring

template radiusnum rings = 3

num circumferential elems = 8

progression ration = /

Figure 6.1.12.4 Crack front template cross-section.

55

6.1.13 Flaw Insertion

After one selects Finish on the crack front mesh template panel (see Figure 6.1.12.1), the

flaw is inserted into the body and the model is remeshed. This can be a computationally

intense process and might take a significant amount time for large models. An

information box is displayed to give a sense of the current status of these operations,

Figure 6.1.13.1. The crack geometry is inserted into the model geometry first,

represented by the Doing geometric intersections... status. Once the crack geometry has

been inserted, trimmed, and tied to the model geometry, surface and then volume

meshing occurs. The final mesh is smoothed to improve the element quality.

Figure 6.1.13.1 Flaw Insertion Status dialog.

6.2 Flaw from Files

The user has the option of saving the flaw to a .crk file when defining a new flaw (see

Figure 6.1.1.1). The Flaw from Files menu option allows the user to read one or more

.crk files and insert the crack(s) into the model. The Locate .crk flaw file dialog, Figure

6.2.1, is displayed, which allows the user to select one or multiple .crk files.

The flaw is displayed with the model using the Flaw Insertion wizard panels, starting

from the Translations & Rotations panel (see Figure 6.1.11.1).

56

Figure 6.2.1 Locate .crk flaw file dialog.

Note that the rotations are only valid for cracks with an origin at (0,0,0); a message is

printed to the console window, Figure 6.2.2, to remind the user of this.

Figure 6.2.2 Console window message regarding flaw-from-file rotations.

6.3 Multiple Flaw Insert

The Multiple Flaw Insertion wizard allows the user to add multiple cracks to a model,

Figure 6.3.1. The user defines each crack using the Flaw Insertion wizard panels

described in Section 6.1. When all cracks have been defined, they can be added to the

model and/or saved to a file.

The user starts by selecting the Add button in the dialog shown in Figure 6.3.1. This

leads to the flaw insertion wizard panels described in Section 6.1. Either voids or zero-

volume cracks can be added.

Once a crack has been added to the list, it can be edited or deleted by selecting the flaw

name and then selecting the Edit or Delete button. All of the added flaws can be

displayed in the model, Figure 6.3.2, to ensure that cracks do not overlap or intersect.

The set of radio buttons at the bottom of the panel allow one to: 1) add the flaws to the

model without saving to a file, 2) add the flaws to the model after prompting for a file

name to save the flaws, or 3) save the flaws to a file without adding them to the model.

Select Accept to close the dialog and begin the process of crack insertion and re-

meshing. The flaw insertion status window described in Section 6.1.13 will be displayed.

57

Figure 6.24 Multiple flaw insertion - top level dialog.

Figure 6.25 Multiple flaw insertion – flaw display window.

58

6.4 Stress-Intensity Factor

The user can compute and plot the stress intensity factors (SIFs) after performing an

analysis of the cracked model; the analysis menu is described in Section 9. SIFs are

computed at mid-side nodes along the crack front.

6.4.1 M-integral or Displacement Correlation Panel

The Compute SIFs menu option brings up the SIF Computation dialog box, Figure

6.4.1. Either the M-integral or displacement correlation method can be chosen to

compute the SIFs. The displacement correlation option generally is used only for

checking the M-integral values and is valid for isotropic materials only. For models

where a crack front template cannot be added, displacement correlation currently is the

only option.

Figure 6.4.1 Compute SIFs panel allows one to choose either M-integral or displacement

correlation computations.

Include Thermal Terms: The thermal terms can be included in the M-integral

computation. The nodal temperatures must exist in the results database and the user

should supply the reference temperature if not already given with the initial input file.

Include Applied Crack Traction: Any applied crack face traction/pressure terms should

be included in the M-integral.

Include Contact Crack Pressure: Any induced crack face contact pressure should be

included in the M-integral.

59

Note that it is possible to have both applied pressure and contact pressure on a crack

surface and these terms will be summed. Note that the ABAQUS Version 6.10 contact

pressures have been found to be incorrect.

Plot Stress Intensity Factors: Turn on/off the display of the SIF plot dialog, which is


6.4.1.1 Compute SIFs for Multiple Load Cases

For models with multiple load cases, if the user chooses to read results for more than one

of the load cases, the Compute SIFs panel has an extra set of options, Figure 6.4.1.1.

The load cases in FRANC3D are numbered from 1 to n. The user can choose to include

one or more of the load cases when computing SIFs using the check boxes under the

Include column. Load factors can be applied to each of the included load cases as well.

There are three options for combining the SIFs from multiple load cases. The first option

is to keep the SIFs separate, the second is to sum all SIFs, and then third option is to

sequence the SIFs. This last option combines the SIFs such that each load case

contributes to the crack growth direction based on a vector sum, Figure 6.4.1.2.

If the user wishes to plot the SIFs for a particular load case, only that load case should be

selected before clicking the Finish button. Otherwise the first load case, the summed, or

the sequenced SIFs will be displayed depending on the selected (bottom) radio button.

Figure 6.4.1.1 Compute SIFs panel for a model with multiple load cases.

60

Figure 6.4.1.2 Sequence of SIFs from multiple load cases used to predict cumulative

crack growth direction (from Spievak et al., EFM, 68, 2001).

6.4.2 SIF Plot Panel

Stress-intensity factor distributions are displayed in this dialog, Figure 6.4.2.1. The left

side of the dialog is a 3D graphics window that displays the model. The right side of the

dialog is a tab box that shows graphs of the computed stress-intensity factor distributions

along the crack front. The mode I, II, and III stress-intensity factors (KI, KII and KIII

tabs) are computed using the M-integral by default.

Figure 6.4.2.1 SIF plot panel.

61

The J-integral tab displays a plot of the total J-integral distribution (total energy release

rate not segregated into modal components). The T-Str tab displays a plot the T-stress.

The Table tab shows a table of the three SIF modes along with the parametric location

along the crack front, the N Coord value in the left panel of Figure 6.4.2.1.

The Export tab allows one to export the SIF data in tabular form, Figure 6.4.2.1 – right

panel. The user can specify the type of delimiters to use, the grouping of the data, and

order of the data. The program initially assigns one end of the crack front as A and the

other end as B. The data is plotted along the crack front, normalized from 0 to 1.0 by the

crack front length, starting from point A.

Figure 6.4.2.2 SIF table tab (left panel) and export tab (right panel).

6.5 Grow Crack

When the Grow Crack menu option is selected, if stress intensity factors have not yet

been computed for the current model the Compute SIF's wizard is presented first. This

wizard was described in Section 6.4.

The Crack Growth Parameters wizard is presented next if crack growth has not yet

been performed for the model. This wizard allows the user to specify algorithms and

parameters to determine the local direction and relative extension of crack growth. This

base wizard presents simplified models appropriate for most crack growth scenarios. It

also gives one the option to invoke the Advanced Crack Growth Parameters wizard,

which gives additional options. The Crack Growth Parameters wizard is described in

section 6.5.1. The Advanced Crack Growth Parameters wizard is described in section

6.5.2.

After the Compute SIF's and Crack Growth Parameters wizards are presented, if

necessary. The Crack Growth wizard is presented. This wizard allows one to specify

62

the amount of crack extension, crack front fitting algorithm and parameters, and crack

front element template parameters. This wizard is described in section 6.5.3.

6.5.1 Crack Growth Parameters Wizard

The Crack Growth Parameters wizard allows one to specify algorithms and associated

parameters that are used to predict the local direction of crack propagation and the

relative local amount of crack extension. The direction and extension are computed for

all growth evaluation points, which in most cases are the mid-side nodes of all crack front

elements.

6.5.1.1 First Growth Parameters Panel

The first panel in the wizard presents options for units, type of crack growth, algorithms

for predicting kink angles, and beta factors for mixed-mode kink angle algorithms, Figure

6.5.1.1.1.

Figure 6.5.1.1.1 First growth parameters wizard panel.

63

6.5.1.1.1 Crack Growth Units

The first option in the panel is Units. This tells the program how to interpret the

dimensions and boundary conditions in the FEM model. It is not a units conversion

feature. The options include metric (stresses in MPa, lengths in mm) and two options for

US customary units (stresses in psi, lengths in inches, and stresses in ksi, lengths in

inches). The unit information is used primarily for labeling plots. They are also used to

select the appropriate parameter values if the NASGRO v3.0 crack growth rate model is

specified and the material is selected from the built-in material database.

6.5.1.1.2 Growth Type

Crack growth predictions are made assuming either fatigue driven or quasi-static crack

growth. The fatigue growth algorithm uses a fatigue crack growth rate model (

da dN vs.

K ) in conjunction with the computed stress intensity factors in order to predict the

relative amount of crack growth for all crack front points. Currently there is only one

quasi-static model available, which uses an exponential function to determine relative

amount of growth for font points (§6.5.1.3).

For fatigue crack growth, one must also specify whether the crack will be subjected to

constant or variable (spectrum) load cycles.

6.5.1.1.3 Kink Angle

The local direction of crack propagation is determined by the "kink" angle, which is

defined by the amount that the crack will deviate from the self-similar direction measured

in a plane perpendicular to the crack front. The kink angle is the angle illustrated in

Figure 6.5.1.3.1.

Figure 6.5.1.3.1 Schematic illustration of the definition of the kink angle.

FRANC3D provides five algorithms for determining the kink angle, Maximum Tensile

Stress (MTS), Maximum Shear Stress (MSS), maximum generalized stress, maximum

strain energy release rate, and planar growth.

64

Maximum Tensile Stress

The MTS theory predicts that a crack will propagate in the direction where the "hoop"

stress,

, is maximum. For materials with isotropic stiffness properties, the hoop stress

is related to the resolved mode I stress intensity factor,

K Ir , by the relation

KIr() 2r cos

2KI cos

2

23

2KII sin

(6.5.1.1.1)

The expression for materials with anisotropic stiffness properties is considerably more

complex (see for example Banks-Sills, Wawrzynek, Carter, Ingraffea, and Hershkovitz,

"Methods for computing stress intensity factors in anisotropic geometries: Part II –

arbitrary geometry," Engng. Fracture Mech., 74, 9, 1293-1307, 2007) but can be

expressed symbolically as

KIr() 2r f I KI ,KII ,KIII ,, (6.5.1.1.2)

where

characterizes the angle cosines between the material property axes and the local

crack-front coordinate system.

If an isotropic material toughness is specified (the default behavior), a numerical

algorithm is used to find the angle that maximizes

K Ir (and

) in equation 6.5.1.1.1 or

6.5.1.1.2. If anisotropic toughness properties are specified, the kink angle is defined as

the angle that maximizes the ratio

KIr() Kp(), where

Kp () is a measure of the

materials directionally dependent resistance to crack growth (§6.5.2.3).

Maximum Shear Stress

Some materials exhibit crack growth in the direction of high shear stress, especially for

conditions of high shear loading at the crack front. The MSS theory predicts that a crack

will propagate in the direction where the resolved shear stress,

s r2 z

2 , is

maximum. For materials with isotropic stiffness properties, the components of the shear

stress are related to the resolved mode II and III stress intensity factors by the relations

KIIr () r 2r

1

2cos

2KI sin KII (3cos 1)

KIIIr () z 2r KIII cos

2

(6.5.1.1.3)

Symbolic expression for materials with anisotropic stiffness properties are

65

KIIr () r 2r f II KI ,KII ,KIII ,,

KIIIr () z 2r f III KI ,KII ,KIII ,,

(6.5.1.1.4)

where

characterizes the angle cosines between the material property axes and the local

crack-front coordinate system.

If an isotropic material toughness is specified (the default behavior) a numerical

algorithm is use to find the angle that maximizes the expression

Ksr() IIKII

r () 2

IIIKIIIr ()

2

(6.5.1.1.5)

where

II and

III are user supplied parameters that can be used to tailor predicted shear

crack growth direction to match assumed or predicted behavior (§6.5.1.1.4). Note that

MSS driven crack growth is not currently fully understood and there is no general

agreement on how to select the beta factors.

If anisotropic toughness properties are specified, the kink angle is defined as the angle

that maximizes the ratio

Ksr() Kp().

Maximum Generalized Stress

The generalized stress criterion predicts that the crack will grow in the direction that

corresponds the greater resolved stress intensity factor (

K Ir or

Ksr ) from the MTS or MSS

criteria. The beta factors (§6.5.1.1.4) can be set to tune the point of transition from

tensile to shear fracture, which will vary for different materials.

As a practical matter, if one portion of a crack front is determined to be in tensile fracture

while another part is in shear fracture, there will likely be an abrupt discontinuity in the

predicted kink angles. Such a crack will probably not mesh successfully and if it does the

crack geometry probably will not be realistic. In such a case, the user should select either

the MTS or MSS criteria explicitly.

Maximum Strain Energy Release Rate

The maximum strain energy release rate criterion predicts that the crack will grow in the

direction that maximizes the rate of change of potential energy in the system due to crack

growth. For materials with isotropic toughness, this angle is determined by numerically

maximizing the expression

G() KIr()

2

IIKIIr ()

2

IIIKIIIr ()

2

(6.5.1.1.6)

66

where the resolved stress intensities are defined in equations 4.7.1.2.1 - 4.7.1.2.4. For

materials with anisotropic toughnesses (§6.5.2.3) the expression

G() KIr()

Kp ()

2

IIKII

r ()

Kp ()

2

IIIKIII

r ()

K p ()

2

(6.5.1.1.7)

is maximized. The beta factors (§6.5.1.1.4) can be used to tune the behavior for assumed

or observed material behavior.

Planar

The planar crack growth option forces the crack to grow with a zero kink angle (self

similar growth). It is recommended that this option be selected if it is known that the

crack is expected to grow in a plane. This will avoid small undulations that may develop

in the predicted crack surface due to numerical "noise" that can arise when computing the

kink angles using one of the other criteria.

6.5.1.1.4 Mixed Mode Beta Factors

The mixed mode beta factors,

II and

III are used to tune the maximum shear stress and

maximum strain energy density kink angle criteria for specific assumed or observed crack

growth behavior. The values are also used if the mixed-mode option is selected for

defining an equivalent stress intensity factors (§6.5.2.4.1).

6.5.1.2 Fatigue Growth Rate Model

If the Fatigue crack growth option is selected (§6.5.1.1.2), the second wizard panel

presented allows one to select a fatigue crack growth rate model and set the appropriate

parameters, Figure 6.5.1.2.1.

67

Figure 6.5.1.2.1 Fatigue crack growth rate model panel.

Paris

The Paris growth rate model defines a simple power-law relationship between the crack

growth rate and stress intensity factor range

da dN C(K)n (6.5.1.2.1)

where

da dN is the crack growth rate,

K is the stress intensity factor range, and C and

n are user supplied parameters. Note that in the Paris the crack growth rate is not an

explicit function of the R-ratio (

R Kmin Kmax ). In general, the value of the C parameter

will change for different R-ratios and it is up to the user to provide an appropriate value.

The Paris model is not sensitive to near-threshold or near-critical crack growth behavior.

Walker

The Walker growth model augments the Paris model to introduce a R-ratio sensitivity. It

is expressed as

da

dNC

K

(1 R)1m

n

(6.5.1.2.2)

68

where m is a material dependent parameter that usually has a value near 0.5. The R

dependence drops out at R = 0, so the C value provided should correspond to the C for R

= 0. The Walker model is not sensitive to near-threshold or near-critical crack growth

behavior.

NASGRO version 5

NASGRO is a fatigue life prediction program initially developed at the NASA Johnson

Space Flight Center and now supported and maintained by Southwest Research Institute.

Built into the NASGRO program is an analytical crack growth rate equation that

incorporates a number of features observed in real materials such as sensitivity to near-

threshold and near-critical growth, sensitivity to the R-ratio, and some small crack

sensitivity. The cost for the flexibility in the growth model is that there are sixteen fitting

parameters. Some are common material properties (e.g., yield stress), but most must be

determined by performing regression on empirical data.

The NASGRO package includes a database of pre-computed parameters for a wide range

of materials and product forms. However, as of NASGRO version 4, this database is

only accessible to members of the NASGRO consortium. As a practical matter, this

means that this model will be most useful to users who have access to the NASGRO

program and can access the parameters for a material of interest (see NASGRO version 3

below).

The NASGROv5 crack growth rate equation is:

da

dNC

1 f

1 R

n 1K thK

p

1Kmax

Kc

q (6.5.1.2.3)

where C, n, p and q are empirical constants. C and n are similar to the constants in the

Paris and Walker equations. However, while C in the Paris model should be selected to

correspond to the R-ratio of interest, and C in the Walker equation should be for R = 0, in

the NASGRO equation the C should correspond to a high R where closure effects are no

longer significant (usually an R of about 0.7)

f is a function that accounts for the crack front being open for only a portion of the load

cycle due to plasticity induced crack front closure. It is defined as

f Kop

Kmaxmax(R,Ao A1R A2R

2 A3R3) R 0

A0 A1R 2 R 0

(6.5.1.2.4)

where the coefficients are given by:

69

A0 (0.825 0.34 0.05 2) cos

2

Smax 0

1

A1 (0.415 0.071)Smax

0

A2 1 A0 A1 A3

A3 2A0 A1 1

(6.5.1.2.5)

In these equations,

is a plane stress/strain constraint factor and

Smax 0 is the ratio of

the maximum applied stress to the flow stress. Both are assumed to be fitting parameters

in this application.

Kth is the threshold stress intensity factor range, below which there is no crack growth.

It is approximated by the following empirical equations:

K th

K1* 1 R

1 f

(1RC thp)

(1 A0)(1R )C th

p

R 0

K1* 1 R

1 f

(1RC thm )

(1 A0)(C th

pRC th

m )R 0

(6.5.1.2.6)

where

K1* K1

a

a a0

1 2

(6.5.1.2.7)

and

Cth is an empirical constant,

K1 is the observed threshold for a high R-ratio, and

a0

is a small crack parameter (usually 0.0015in, 0.0381 mm).

Kc is the fracture toughness. In FRANC3D, one has the choice to use the plane strain

fracture toughness,

KIc , the part-through toughness,

KIe , or compute it from:

Kc KIc (1Bke(Ak t t0 )

2

) (6.5.1.2.8)

where

t0 2.5 KIc ys 2

(6.5.1.2.9)

Ak and

Bk are empirical constants, and t, the "thickness" is specified by the user (in many

3D applications the notion of thickness is not well defined and engineering judgment

must be used to set this value).

70

More details about this model can be found in the NASGRO theory manual.

The View/Edit Parameters button brings ups the NASGRO v5 Parameters dialog box

shown in Figure 6.5.1.2.2. The NASGRO model parameters can be entered in this dialog

box. There are also options to load parameters from a file and write the current

parameters to a file. The Label field is a user assigned text string.

Figure 6.5.1.2.2 The NASGRO v5 Parameters

NASGRO version 3

NASGRO version 3 is an earlier version of the NASGRO crack growth rate equation.

The current version of the equation (version 5) does a better job of modeling observed

crack growth rates for some conditions. This earlier version is included in FRANC3D

because version 3 of NASGRO, the last public domain version, included a database of

equation parameters for a wide variety of materials and product forms. That database is

accessible from within FRANC3D.

The main difference between the NASGROv3 equation and the NASGROv5 equation is

the expression for the threshold stress intensity factor range. In version 3, the expression

Kth K04

tan1(1R)

a

a a0

1 2

(6.5.1.2.10)

is used rather than eqns. 6.5.1.2.6 and 6.5.1.2.7.

The View/Edit Parameters button brings ups the NASGRO v3 Parameters dialog box

shown in Figure 6.5.1.2.3. The NASGRO model parameters can be entered in this dialog

71

box. There are also options to load parameters from a file and write the current

parameters to a file. The From Database button brings up a wizard, which allows one to

select a material and product form from a database. Figure 6.5.1.2.4 shows as series of

panels from this wizard.

Figure 6.5.1.2.3 The NASGRO v3 Parameters

Figure 6.5.1.2.4 Panels from the NASGRO v3 material parameters database wizard.

72

User Table

The User Table option allows one to read table of crack growth rates. The program will

interpolate within this equation to find a crack growth rate for a given K and R. The

table data must be read from a file. The Browse button allows one to select a file. The

required format for the file is described at the end of this subsection. The model label is a

user supplied text string. The View Table button brings up a dialog box showing the

prescribed growth rate data in a graphical and tabular form (Figure 6.5.1.2.5).

Figure 6.5.1.2.5 The User Growth Model display dialog box.

Given query values

Kquery and

Rquery, crack growth rates are extracted from the table

using the following algorithm:

1.

Rl and

Ru values are found that bracket the query value,

Rl Rquery Ru.

2.

Kll ,

Klu,

Kul , and

Kuu are found where

Kll Kquery Klu and

Kul Kquery Kuu, with

Kll and

Klu values associated with

Rl and

Kul and

Kuu values associated with

Ru .

3. Linear interpolation in log/log space is used to find intermediate growth rate data,

73

logda

dN l

log

da

dN(K ll ,Rl )

log

da

dN(K lu,Rl )

da

dN(K ll ,Rl )

log Kquery K ll log K lu /K ll

and

logda

dN u

log

da

dN(Kul,Ru)

log

da

dN(Kuu,Ru)

da

dN(Kul,Ru)

log Kquery Kul log Kuu /Kul

4. Finally, normal linear interpolation is used to find the final growth rate

da

dNda

dN l

da

dN u

da

dN l

(Rqueary Rl )

(Ru Rl )

If during the interpolation process a query value (

Rquery or

Kquery) is found to be larger

or smaller than the largest or smallest corresponding value in the table, the query value is

replaced with the largest or smallest, respectively, available value.

Not that, as with all interpolation procedures, the accuracy of the results is dependent on

the density of the data available for interpolation. Very sparse tables can lead to

inaccurate and in some cases unrealistic crack growth predictions.

User Table File Format

The user table file format consists of a table of da/dN versus dK1 for given R values.

The R-ratio is used to label the columns of dK1 data. The rows represent the da/dN vs

dK1 data. The file is an ASCII text file.

dadN/R -1 -0.6 0 0.6 1.0000e-12 2.0469 2.1150 2.3531 2.6193 4.5249e-11 2.6618 2.6828 2.8066 2.8113 3.0422e-10 3.3872 3.3906 3.4624 3.2371 2.0454e-09 4.6844 4.7119 4.8342 4.4684 9.2459e-08 10.0399 10.4677 11.6805 12.6507 6.2164e-07 13.9172 14.6618 16.6286 17.2132 7.8884e-06 18.9059 19.8545 21.9998 20.0972 5.3036e-05 21.7701 22.6584 24.4075 20.8054 1.0000e-04 22.5218 23.3648 24.9475 20.9193

74

6.5.1.3 Quasi-Static Growth Model

If the Quasi-Static crack growth option is selected (§6.5.1.1.2), the second wizard panel

presented is shown in figure 6.5.1.3.1. Currently, the only quasi-static growth model

available in FRANC3D is based in a power law relationship among the crack front

points. The crack extension for any crack front point i is computed as

ai amedianK i

Kmedian

n

(6.5.1.3.1)

where

amedian is the specified crack extension for the crack-front point with the median

K value,

Kmedian is the median K value, and

K i is the stress intensity factor at crack front

point i, and n is a user supplied parameter that can be set on the panel.

There is no theoretical basis for selecting an n value. In practice, values in the range of 2

to 3 seem to match observed crack shapes reasonably well.

Figure 6.5.1.3.1 Quasi-static crack growth rate model panel.

6.5.1.4 Constant Amplitude Panel

If the Fatigue and Constant Amplitude crack growth options are selected (§6.5.1.1.2), the

third wizard panel presented is shown in figure 6.5.1.4.1.

75

Figure 6.5.1.4.1 Constant amplitude fatigue model panel.

6.5.1.4.1 Crack Extension Type

There are two options for specifying the amount of crack extension at each crack growth

step: specified median extension or specified applied cycles. For specified median

extension, the crack extension at crack front point i is computed from the expression

ai amedian

da

dN i

(K i,Ri,...)

da

dN median

(Kmedian,Rmedian,...)

(6.5.1.4.1)

where

amedian is the crack extension specified for the point with the median stress

intensity factor range,

da dNi is the crack growth rate computed at point i using the

specified growth rate model (§6.5.1.2), and

da dNmedian

is the crack growth rate

computed at the crack front point with the median stress intensity value. The crack

growth rates at crack front points will be functions of the corresponding

K , R (with the

exception of the Paris model), and other model dependant parameters.

Note that selection of the reference

K value to use in equation 6.5.1.4.1 is somewhat

arbitrary. The maximum, minimum, and average

K are reasonable values to consider.

The median value is used in FRANC3D because experience shows that the maximum and

minimum values frequently occur where a crack front meets a free surface where the

accuracy of the computed stress intensity factors are known to be least accurate. In some

cases, the inaccuracy of these maximum and minimum values can lead to a very

unintuitive relationship between the reference specified amount of crack extension and

76

the actual predicted crack front. In extreme cases, inaccurate maximum and minimum

values can have a significant impact on the average value leading to an unintuitive

behavior for this measure also.

The expression for crack extension at crack front point i using the specified cycles option

is

ai N da

dN i

(Ki,Ri,...) (6.5.1.4.2)

where N is the specified number of cycles and

da dNi is the crack growth rate computed

at point i using the specified growth rate model (§6.5.1.2).

The Specified number of applied cycles approach seems more intuitive than the Specified

median crack front extension approach for many analysts. However, the fatigue crack

growth model are highly nonlinear functions so trial-and-error might be required when

using the applied cycles approach to find a number of cycles that will not give

exceedingly large or exceedingly small predicted crack extensions. Exceedingly large

values might give inaccurate results, while exceedingly small crack extensions might

require many crack growth steps or, in some cases, might cause meshing problems.

WARNING, if the Specified number of applied cycles is used to grow the crack, it is

strongly suggested that one does not use the sum of the specified cycles for a series of

crack growth steps as an accurate prediction of the fatigue life. Crack extensions

computed using this approach are made assuming that the stress intensity factor range

will remain constant over the step. In the vast majority of crack growth analyses the

stress intensity factors increase over each step. This means that the sum of the cycles

specified for all the steps will be a lower bound (unconservative) estimate of the fatigue

life. It is suggested that the FRANC3D Fatigue Life wizard (Section 10.2) or an external

fatigue life prediction program be used to calculate life (e.g., NASGRO or AFGROW).

These programs allow for a variation of the stress intensity factor over a crack growth

step.

6.5.1.4.2 Constant Amplitude R

For constant amplitude loading, the user must specify an R value that relates the stress

intensity factors computed in the analysis,

Kcomp , to the stress intensity factor range used

to compute crack growth rates. The corresponding expressions are

Kmax Kcomp,

Kmin RKmax , and

K Kmax Kmin (6.5.1.4.3)

If multiple load cases are being used, there are additional options for setting these values.

These are described in section 6.5.2.1.

77

6.5.1.4.3 Advanced Options

The Advanced Options button brings up the Advanced Options wizard described in

section 6.5.2.

6.5.1.5 Variable Amplitude Panel

If the Fatigue and Variable Amplitude crack growth options are selected (§6.5.1.1.2), the

third wizard panel presented is shown in figure 6.5.1.5.1.

Figure 6.5.1.5.1 Variable amplitude fatigue model panel.

6.5.1.5.1 Crack Extension Type

The Crack extension type options are described in Section 6.5.1.4.1.

6.5.1.5.2 Load Spectrum

The Load spectrum section allows one to specify a load spectrum (variable amplitude

load schedule) that is used when computing crack growth rates. A spectrum is defined as

a series of load ranges. Associated with each load range is an optional repeat count (the

default value is one). One can also specify a multiplier and offset that will be applied to

all values in the spectrum (e.g., to support the use of a normalized spectrum).

78

If only one load case is used in the analysis, and load range i has specified values

Smax,iraw

and

Smin,iraw the expressions used for computing the stress intensity factor ranges and R ratio

crack for range i are

Smax,i sSmax,iraw t ,

Smin,i sSmin,iraw t (6.5.1.5.1)

Kmax,i Smax,i Kcomp ,

Kmin,i Smin,i Kcomp (6.5.1.5.2)

Ki Kmax,i Kmin,i and

Ri Kmin,i Kmax,i (6.5.1.5.3)

where s is the spectrum multiplier and t is the spectrum offset. The

Ki and

Ri are used

to compute the crack growth rate for stress range i. If multiple load cases are being used

there are additional options for setting these values. These are described in section

6.5.2.1.

For variable amplitude loading, the crack growth rates used for computing the relative

amounts of crack growth for crack front points (eqns. 6.5.1.4.1 and 6.5.1.4.2) are the

average crack growth rates computed for one pass through the spectrum. That is

da

dN

da

dN i

(K i,Ri,...)i1

n

n (6.5.1.5.4)

where

da dN is the crack growth rate computed for any given crack front point n is the

number of load ranges in the spectrum.

The corresponding predicted kink angle is weighted average kink angle computed as

kink

da

dN i

(K i,Ri,...) kink,i(KI ,i,KII ,i,KIII ,i,...)i1

n

da

dN i

(K i,Ri,...)i1

n

(6.5.1.5.5)

where

kink,i is the kink angle determined (implicitly) for range i using one of the criteria

described in section 6.5.1.1.3.

In the Variable amplitude panel, the Spectrum File field gives the path and file name for

the currently specified spectrum file (it is not editable). The Spectrum Label is a user

specified text string that can be used to identify the spectrum data. The Spectrum

multiplier and Spectrum offset are the s and t values, respectively, used in equation

6.5.1.5.1.

79

The Groups in spectrum and Group units values allow the user to set how plots and

reports are labeled. Analysts often find it convenient to group spectrum ranges into

larger units that represent meaningful engineering concepts (e.g. hours, flights, blocks,

etc.). The Groups in spectrum value specifies how many such groups are in the spectrum.

The Group units value is a label that identifies the group type. These values are primarily

useful in the Fatigue Life wizard (Section 10.2) where, for example a fatigue life plot

might show the crack length as a function of the number of flights, hours, blocks, or in

user specified units. The default value for the Groups in spectrum is one, and the default

value of Group units is "passes". The spectrum groups information is not used during

crack growth predictions, but the information can be specified in this wizard because, if

set, the load spectrum data will be transferred to the Fatigue Life wizard automatically.

The Read Load Spectrum button brings up the load spectrum input wizard. Load

spectrum data is assumed to be stored as columns of data in a text file. The columns can

be separated either by white space (spaces and tabs) or commas. The delimiters need to

be used consistently throughout the whole file.

The first panel in the wizard is a standard file selection dialog. The second panel is

shown in Figure 6.5.1.5.2.

Figure 6.5.1.5.2 The second load spectrum wizard panel.

The first option specifies if the data in the file represents sets of maximum and minimum

values from an already "counted" spectrum or if the data represents load points that will

be cleaned and counted by the program using a range-pair algorithm. If the file contains

only one column of data, only the latter option will be available.

The second option allows column delimiter to be specified.

80

The Data preview section displays how the file data will be segregated into columns

based on the specified delimiter type. It also gives the option to ignore the first line in the

file so that the first line can be used for identifying information or column labels.

If the Read a paired spectrum option is selected, the third panel is that shown in Figure

6.5.1.5.3.

Figure 6.5.1.5.3 The third load spectrum wizard panel for a paired file.

The pull-down boxes over each column are initially all set to "ignore". These can set to

identify the columns that specify the maximum, minimum, and, optionally, the repeat

count associated with each load range.

If the clean and pair option is selected and there is more than one column in the file then

the third panel is that shown in Figure 6.5.1.5.4. A pull-down box can be used to identify

the column that contains the load point data.

81

Figure 6.5.1.5.4 The third load spectrum wizard panel for a unpaired file.

The Plot Load Spectrum button brings up the Load Spectrum Display dialog. This is

shown in Figure 6.5.1.5.5. The Scaled Cycles tab shows the spectrum values after they

have been scaled and offset (eqn. 6.5.1.5.1). The tab shows the values as they appear in

the spectrum file. The magnifying glass icons allow one to view a smaller or greater

segment of the spectrum and the arrow icons allow one move forward and backward

through the spectrum.

Figure 6.5.1.5.5 The load spectrum display dialog box.

82

6.5.1.5.3 Advanced Options

The Advanced Options button brings up the Advanced Options wizard described in

section 6.5.2.

6.5.2 Advanced Crack Growth Parameters Wizard

The Advanced Crack Growth Parameters wizard allows one to specify advanced

algorithms and associated parameters to be used to predict the local direction of crack

propagation and the relative amount of crack extension. Because these algorithms are

only used in a minority of crack growth simulations, their selection and configuration has

been moved to the advanced wizard. This simplifies the basic crack growth parameters

wizard used for most crack growth scenarios.

6.5.2.1 First Advanced Growth Parameters Panel

One Load Case

The first panel presented by the advanced growth parameters wizard will depend on if the

program detects that there are results available for a single load case or for multiple load

cases. If results for only one load case are available, the first panel presented is shown in

Figure 6.5.2.1.1.

Figure 6.5.2.1.1 The first panel presented when only one case is available.

83

The Scale option allows the user to specify a linear scaling for the computed stress

intensity factors. This can be used, for example, if stress analysis was performed using

normalized or otherwise scaled loads and one now wishes to scale the loads (and

computed stress intensity factors) to be scaled to the actual in-service values.

Multiple Load Cases, Constant Amplitude

If there are results available for more than one load case and constant amplitude loading

is selected (§6.5.1.1.2), the first wizard panel that will be presented is shown in Figure

6.5.2.1.2.

Figure 6.5.2.1.2 The first panel presented for constant amplitude and multiple load cases.

The purpose of this panel is to allow the analyst to specify how the stress intensity factors

that correspond to the various load cases will be grouped together to define

Kmax and

Kmin, which are used to determine the

Kand R values used in the fatigue crack growth

rate model (§6.5.1.2).

The first option is to set

Kmax equal to the sum of the stress intensity factors for all load

cases and

Kmin equal to the

Kmax times the constant amplitude R-ratio (§6.5.1.4.2):

Kmax K i

i1

n

and

Kmin RKmax . (6.5.2.1.1)

84

This is the default behavior.

The second option sets

Kmax equal to the sum of the stress intensity factors from one or

more load cases and

Kmin equal to sum from one or more different load cases:

Kmax K i

i1

n

and

Kmin K j

j1

m

(6.5.2.1.2)

This option is useful for cases where there is there is a nonlinear relationship between the

structural response in the maximum and minimum loading configuration. This might

happen, for example, if contact or other nonlinearities are present.

Actually selection of load cases for the max and min groups is done on the following

wizard panel.

The third option sets

Kfixed equal to the sum of the stress intensity factors from one or

more load cases and

Kvariable equal to sum from one or more different load cases:

Kfixed K i

i1

n

and

Kvariable K j

j1

m

. (6.5.2.1.2)

These are combined to define

Kmax and

Kmin as

Kmax Kfixed Kvariable and

Kmax Kfixed RrelKvariable (6.5.2.1.3)

where

Rrel can be specified on the following wizard panel.

Two common applications for this option are to model combined high and low cycle

fatigue (HCL/LCF) and a fixed "dead" load with an alternating "live" load. These are

illustrated in Figure 6.5.2.1.3.

(a) (b)

Figure 6.5.2.1.3 Common applications for the fixed/variable load option: (a) combined

high and low cycle fatigue, and (b) fixed dead load and variable live load.

85

The fourth option is for Sequentially applied load cases. Typical examples of this are

rolling contact or meshing of gear teeth, where a continually varying load location and

magnitude is approximated by a series of load steps with each step in its own load case.

The sequential loading algorithm assumes that

Kmin 0 and

Kmax is equal to the

maximum

KI of all the load cases and

Kmax MAX(KI , i ) . (6.5.2.1.4)

In many such applications, the mode mixity changes thought the load sequence, which

affects the predicted kink angle. An algorithm has been implemented that predicts the

total effective kink angle for the full loading sequence from the vector sum of the

predicted crack growth angles and extension from each of the load steps in the sequence

(see Figure 6.4.1.2). That is:

kink tan1 y

x

(6.5.2.1.5)

with

x ai cosii1

n

and

y ai sinii1

n

. (6.5.2.1.6)

where i is the current step in the sequence and n is the total number of steps in the

sequence (number of load cases). The kink angle for step i

i f (KI , i,KII , i ,KIII , i ) (6.5.2.1.7)

is computed using the selected kink angle algorithm (§6.5.1.1.3). The crack extension for

step i is computed as

ai atotal

KI , i KI , i1

MAX(KI , i )KI , i Kopening

0 KI , i Kopening

(6.5.2.1.8)

with

atotal computed from the selected crack growth rate model (§6.5.1.2) using

K MAX(KI , i ),

R 0,

N 1. In eqn. 6.5.2.1.7

KI , 0 0 and

Kopening f MAX(KI , i ) NASGRO v3& v5

0 Paris,Walker, Table

(6.5.2.1.9)

where f is defined in eqn. 6.5.1.2.4.

86

Multiple Load Cases, Variable Amplitude

If there are results available for more than one load case and constant variable loading is

selected (§6.5.1.1.2), the first wizard panel that will be presented is shown in Figure

6.5.2.1.4.

Figure 6.5.2.1.4 The first panel presented for variable amplitude and multiple load cases.

The first option is to set

Kmax equal to the sum of the stress intensity factors from one or

more load cases times the load spectrum maximum value for the current load range,

Smax , and

Kmin equal to the sum from one or more different load cases time the

minimum load range value:

Kmax Smax K i

i1

n

and

Kmin Smin K j

j1

m

. (6.5.2.1.8)

The second option sets

Kfixed equal to the sum of the stress intensity factors from one or

more load cases and

Kvariable equal to the sum from one or more different load cases

(eqn. 6.5.2.1.2) and computes the maximum and minimum values as

Kmax Kfixed SmaxKvariable and

Kmin Kfixed SminKvariable . (6.5.2.1.9)

This allows one to model a static load with a superimposed variable amplitude load.

87

6.5.2.2 Second Advanced Growth Parameters Panel

The second Advanced Growth Parameters wizard panel presented will appear slightly

different depending on which load case strategy (§6.5.2.1) was selected. The various

panels are shown in Figures 6.5.2.2.1 through 6.5.2.2.3.

Figure 6.5.2.2.1 The second wizard panel if a simple sum load strategy was selected.

Figure 6.5.2.2.2 The second wizard panel if a Max/Min sum load strategy was selected.

88

Figure 6.5.2.2.2 The second wizard panel if a Fixed/Variable sum load strategy was

selected.

6.5.2.2.1 Load Case Multipliers and Offsets

The Load Case Multipliers and Offsets allows the user to set parameters for each of the

load cases. By default, all load cases are selected to be Active. If the load case is set to

be "inactive" it is not considered when computing stress intensity factor ranges.

The Scale is a load factor or multiplier that will be applied to the load cases. The load

and the corresponding stress intensity factors will be scaled by this amount.

If the Max/Min sum or Fixed/Variable load case strategies are selected then the Grouping

options allows one to assign load cases to the appropriate group. In this case, one cannot

proceed to the next panel until at least one load case is assigned to each of the groups.

6.5.2.2.2 Relative R for Fixed/Variable Loads

If constant amplitude loading (§6.5.1.1.2) and a fixed/variable load case strategy have

been selected, the Relative R for Fixed/Variable Loads item will be enabled. This value

will be used to determine the minimum stress intensity factor as described in section

6.5.2.1 and illustrated in Figure 6.5.2.1.3.

89

6.5.2.3 Third Advanced Growth Parameters Panel

The third Advanced Growth Parameters panel allows one to specify the nature of a

material’s resistance to crack growth. This panel is shown in Figure 6.5.2.3.1. The

default setting is that a material is isotropic in toughness. In this case, the direction of

crack growth will be predicted based on the near crack-tip stress fields only. As

mentioned in section 6.5.1.1.3, if an anisotropic resistance to crack growth is specified

then the kink angle will be the angle that maximizes the expression

Kr () Kp() , where

K r is the appropriate resolved stress intensity factor (see §6.5.1.1.3) and

K p is the

resistance to crack growth.

Figure 6.5.2.3.1 Third advance crack growth parameters wizard panel.

Currently, FRANC3D supports one anisotropic crack growth resistance model. It is an

orthotropic model based on six principal material toughnesses. These toughnesses are

illustrated in Figure 6.5.2.3.2. In the figure, the e's are the material property axes, which,

in general, will not be aligned with the global Cartesian or the crack front coordinate

systems. The first toughness subscript identifies the material axis perpendicular to the

crack plane and second subscript gives the direction of propagation.

90

Figure 6.5.2.3.2 Orientations for the six principal material toughnesses.

Figure 6.5.2.3.3 shows the crack-front orientation and defines the a and n vectors. x, y,

and z are local crack-front coordinate axes that, in general, are not aligned with the global

Cartesian coordinate axes.

is the local kink angle.

Figure 6.5.2.3.3 Definition of the crack-front a and n vectors.

The crack orientation and kink angle sensitive local resistance to crack growth

Kp() is

given by

K p(n,a) n12

K12

l

n22

K22

l

n32

K32

l

1 2l

(6.5.2.2.1)

91

Ki (a) 1 ai2 a j

2

Kij2

n

ak2

Kik2

n

1 2n

(6.5.2.2.2)

where

Kij and

Kik are the principal toughnesses (no summation on repeated indices),

ai

and

ni are the Cartesian components of the a and n vectors, and l and n (no subscript)

are shape parameters. The n shape parameters control the shape of the toughness

envelope in the principal planes. For example, Figure 6.5.2.3.3 shows the variation in

K3

as function of

assuming that crack front coordinates are aligned with the Cartesian

coordinates. The l shape parameter has a similar effect on the variation of the toughness

between the principal planes.

Figure 6.5.2.3.3 The variation in

K3 as function of

assuming that crack front

coordinates are aligned with the Cartesian coordinates.

6.5.2.4 Fourth Advanced Growth Parameters Panel

The fourth Advanced Growth Parameters panel is shown in Figure 6.5.2.4. This panel

allows one to set how mixed model loading and negative minimum stress intensity

factors are handled when computing crack growth rates. Within FRANC3D, crack

growth rates are computed as:

da

dN f Keffective

equivalent,Reffectiveequivalent

, ... , (6.5.2.4.1)

92

where the equivalent values account for mixed mode behavior and the effective values

account for the possibility that the crack might be closed for a portion of the cycle.

Figure 6.5.2.4.1 Forth advance crack growth parameters wizard panel.

6.5.2.4.1 Mixed-mode equivalent K

FRANC3D computes separate stress intensity factors for all three modes of fracture (

KI ,

KII , and

KIII ). However, conventional crack growth rate models (§6.5.1.2) consider

only one stress intensity factor range. By default, FRANC3D uses the mode one stress

intensity factors when computing the maximum and minimum effective stress intensity

factors

Kmaxequivalent KI ,max

Kminequivalent

KI ,min

. (6.5.2.4.2)

FRANC3D also provides the option to use an equivalent stress intensity factors that is a

function of all three modes of fracture

Kmaxequivalent KI ,max

2 IIKII ,max 2 (IIIKIII ,max )

2

Kminequivalent

KI ,min2 IIKII ,min

2 (IIIKIII ,min)

2

(6.5.2.4.3)

93

where

II and

III are user defined modal weighting parameters. Note that these are the

same values described in section 6.5.1.1.4. If the values are modified on this panel it will

affect the values used to compute kink angles.

In both cases, the equivalent stress intensity factor range and R-ratio are computed as

Kequivalent Kmaxequivalent Kmin

equivalent

Requivalent Kminequivalent

Kmaxequivalent

. (6.5.2.4.4)

6.5.2.4.2 Effective Delta K

Effective values of the stress intensity factor range and the R-ratio account for the

situation where the crack is actually closed during a portion or all of a load cycle. The

default behavior in FRANC3D is to check for a closed crack for the complete cycle and

to set the stress intensity factor range and R-ratio to zero:

Keffectiveequivalent

0 Kmax

equivalent 0

Kequivalent Kmaxequivalent 0

(6.5.2.4.5)

and

Reffectiveequivalent

0 Kmax

equivalent 0

Requivalent Kmaxequivalent 0

. (6.5.2.4.6)

This default behavior allows for negative R-values.

FRANC3D allows the option to truncate the stress intensity factor range to eliminate the

negative (closed) portion of the cycle. In this case, the expressions are

Keffectiveequivalent

0 Kmaxequivalent 0

Kequivalent Kminequivalent

0

Kmaxequivalent Kmin

equivalent 0

(6.5.2.4.7)

and

Reffectiveequivalent

0 Kmaxequivalent 0

Requivalent Kminequivalent

0

0 Kminequivalent

0

(6.5.2.4.8)

94

Note that there are a number of mechanisms that have been identified as being able to

keep the crack closed even when the current "remote" loading is positive. The most

significant being plasticity induced crack closure. This is one proposed explanation for

the observed dependency of crack growth rates on the R-ratio. The effective values

computed here do not explicitly take this into account (plasticity induced closure is

handled implicitly by some of the available crack growth rate models), they only consider

"global" closure where the currently applied "remote" load is negative.

6.5.3 Crack Growth Wizard

The Crack Growth wizard allows one to specify the amount of extension and the fitting

and crack front template mesh parameters. There are two panels, both of which display

the model and allow the user to view the new crack front as the parameters are adjusted.

This wizard will be invoked once SIFs have been computed and growth parameters have

been set.

6.5.3.1 First Crack Growth Panel

The first crack growth wizard panel, Figure 6.5.3.1.1 allows one to adjust the median

extension or number of cycles and to set the fitting and extrapolation parameters to

provide the best fit function through the computed new crack front points. The model is

displayed in the upper 3D model view window and the user can select a camera position

that shows the computed new crack front points.

The lower third of the window usually contains two panes: the crack extension and the

front fitting options pane. For multiple crack fronts, there is an extra Grow crack fronts

pane, Figure 6.5.3.1.2.

6.5.3.1.1 Crack Extension Pane

The Crack extension pane allows one to switch between median extension and number of

cycles. The user can also set the values for these two parameters here. The crack front

points will be redrawn in the model view window as these values are adjusted.

The Edit Growth Params button allows the user to modify the crack growth parameters

that were previously defined, see Section 6.5.1.

The Show SIFs button allows the user to display the SIFs, see Section 6.4.

95

6.5.3.1.2 Grow Crack Fronts Pane

When there are multiple crack fronts in a model, an extra Grow crack fronts pane (see

Figure 6.5.3.2) allows one to grow specific crack fronts using the grow check-boxes. In

addition, the extension for a selected crack front can be scaled up or down by adjusting

the associated factor.

Figure 6.5.3.1.1 First crack growth wizard panel – extension and fitting options.

96

Figure 6.5.3.1.2 Crack growth wizard extension and fitting options for multiple crack

fronts.

6.5.3.1.3 Front Fitting Options Pane

The Front fitting options pane includes seven options for curve fitting, fields for

extrapolating the ends of the fitted curves, and the option to discard crack front points at

either end. The curve fits options are: 1) kink angle/extension poly, 2) fixed order poly,

3) Hermitian, 4) cubic spline, 5) moving poly, 6) partial smoothing, and 7) no smoothing.

There are described in more detail below.

The polynomial order for any of the poly-fitting options can be set in the fixed poly order

field. The default order is 3.

The ends of the fitted curves can be extrapolated to ensure that the curves intersect the

model surface. It is required that the crack surface geometry intersect the model surface.

New crack surface geometry is created between the current and new crack fronts.

FRANC3D tries to ensure that the curves are extrapolated far enough, but the user also

can visually determine whether the extrapolation is sufficient.

A set of points at either end of the crack front can be ignored. If the end points cause

problems in fitting or extrapolating, the user can ignore some of these end points.

The create .crk file option allows the user to save the new crack geometry to a file before

inserting it into the model. The write .frt file button allows the user to save the front

points on the current and new front to an ASCII file. This file can be read using the Read

Crack Growth wizard, which is described in Section 6.6.

The Kink Angles and Extension buttons bring up plots of the computed kink angle,

Figure 6.5.3.1.3 – left panel, and crack front extension, Figure 6.5.3.1.3 – right panel,

along the crack front, respectively.

97

Figure 6.5.3.1.3 Plots of kink angle and crack front extension along the crack front.

Kink Angle/Extension Poly Fit

Using the kink angle and extension data, a best fit polynomial is fit through the data,

using polynomial orders up the value entered in the fixed poly order field. The curve-fits

can be extrapolated and then new extrapolated crack front points are defined based on

these and also on the existing crack front geometry.

As an example, consider a penny-surface crack with the kink angle and extension shown

in Figure 6.5.3.1.3. Figure 6.5.3.1.4 shows the initial crack boundary edges, the actual

crack surface, which is shaded grey, the new front points as green dots, and the curve fit

shown as a blue line. The black dots on the original crack front correspond to the fitted

front points, where the extension and kink angle are applied to produce the new fitted

points. The kink angle and extension curves are extrapolated to compute these new end

points that fall outside the model.

98

Figure 6.5.3.1.4 New front fit based on kink angle and extension curve-fits.

Fixed order poly through points

The fixed order polynomial option is the simplest and most commonly used fitting

option. A polynomial is fit through the new front points in Cartesian space based on a

least-squares fit. The order of the polynomial is provided in the fixed poly order field.

This polynomial can be extrapolated a small distance to make sure the new front

intersects the model surface. Best results are obtained with low order polynomials and

limited extrapolation; if the crack front does not suit a simple polynomial, one of the

other fitting options should be chosen.

Hermitian closed poly

The Hermitian polynomial is used for fitting closed crack fronts such as interior cracks.

It is not active if all of the cracks are open-ended surface cracks. The Hermitian fit is

performed using four equal segments, thus it works best for circular or elliptical shapes.

Cubic spline

A cubic spline fit can be used for crack fronts that do not allow for a simple low order

polynomial fit. A cubic spline will follow arbitrary curves. The ends of the crack front

are extrapolated using a linear segment through the end points for open-ended surface

cracks.

99

Moving polynomial

A moving polynomial fit uses the polynomial order given in the fixed poly order field. A

subset of the crack front points is used for successive curve fits, moving along the full

crack front in increments from one end to the other. Like the cubic spline fit, this option

is good for arbitrary curves. The ends of the crack front are extrapolated using a linear

segment through the end points for open-ended surface cracks.

As an example of crack fronts that do not work well with a simple polynomial fit,

consider the crack fronts in Figure 6.5.3.1.5. Figure 6.5.3.1.6 shows these same crack

fronts using a moving polynomial fit; similar results can obtained for cubic spline fits. In

the first example in Figure 6.5.3.1.5, am 8th

order polynomial fits the data, but

extrapolating a high order polynomial does not work well; in this case the extrapolated

curve does not intersect the model surface. A low order polynomial does not provide a

good fit through the data. In the second example, a 12th

order polynomial cannot capture

the curve. As shown in Figure 6.5.3.1.6, a moving polynomial (or spline fit) follows the

data and provides good extrapolated end points.

Figure 6.5.3.1.5 New front fit based on a simple polynomial fit.

Figure 6.5.3.1.6 New front fit based on a moving polynomial fit.

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Partial smooting/extrapolation

Partial smoothing is designed for crack fronts where part of the crack front does not

advance. An example of this is a through crack in a plate subject to bending where half

of the crack is open and half is closed.

Figure 6.5.3.1.7 New front fit based on a partial smoothing fit.

No smoothing or extrapolation

The final option removes all smoothing and extrapolation. This option will generally

only be used when reading crack growth from a file where the front points have been

defined by the user and no fitting or extrapolation is desired.

6.5.3.2 Second Crack Growth Panel

The second crack growth wizard panel, Figure 6.5.3.2.1 allows one to set the crack front

mesh template parameters. The model is displayed in the upper 3D model view window

and the user can select a camera position that shows the template. The lower third of the

window contains one pane for the template parameters. The template parameters and

buttons are identical to those for inserting a new crack, see Section 6.1.12.

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Figure 6.5.3.2.1 Second crack growth wizard panel – crack front mesh template options.

6.6 Read Crack Growth Wizard

The Read Crack Growth wizard consists of two panels. The first, Figure 6.6.1, allows the

user to specify the file name containing the new crack front points.

Figure 6.6.1 Read new crack front - file import dialog.

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The format for this file is simply: "x y z" ; one set of coordinates per line.

The second panel, Figure 6.6.2, displays the new front points within the model and allows

the user to specify the fitting, extrapolation and template parameters. The front fitting

options pane is identical to that described in Section 6.5.3.1.3. The flaw template pane is

identical to that described in Section 6.5.3.2.

Figure 6.6.2 Read new crack front - display panel.

6.7 Grow/Merge Cracks Wizard

The Grow/Merge Cracks wizard is not activated yet. The current implementation

allows co-planar cracks whose advancing crack fronts intersect, Figure 6.7.1, to be

merged to create a single crack, Figure 6.7.2. The implementation requires manual

selection of the front points that will be merged. These front points are written to a text

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file that can be read using the new flaw user-defined crack algorithm that was described

in Section 6.1.8. This capability will be enhanced before it is finally activated.

Figure 6.7.1 Grow/Merge Cracks – two co-planar cracks whose advancing fronts

intersect.

Figure 6.7.2 Grow/Merge Cracks – two co-planar cracks merged.

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7. Materials Menu Wizards and Dialog Boxes

The wizards and dialog boxes for the Materials menu options are described in this

section.

7.1 Material Properties

This wizard allows one to define a new material or to view the existing materials, Figure

7.1. If model that is read into FRANC3D contains material data, the user can view the

material data but cannot define a new material. If the model does not contain material

data, the user can add a material. The left panel of Figure 7.1.1 corresponds to the case

where material data exists, and the right panel corresponds to the case where there is no

material data.

Figure 7.1.1 Material properties dialog – view material or define a new material.

7.1.1 View Material

For the first case, where a material is defined, the user can view the material data. A list

of materials identified by name or number is presented, Figure 7.1.2 – left panel. The

user selects the material name and the properties and their values are then displayed,

Figure 7.1.2 – right panel. Temperature dependent material data is displayed based on

ascending temperatures.

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Figure 7.1.2 View material properties dialogs; left panel lists the materials identified by

name or number and right panel displays the properties.

7.1.2 New Material

For the case of no defined material, the user is presented with a different set of dialog

boxes that allow a material to be defined. The user first must choose between isotropic

and orthotropic material types, Figure 7.1.3. The user can set the material id and the

temperature value also. Depending on the selection of the material type, the dialog

shown in the left or the right panel of Figure 7.1.4 will be displayed. The user can set the

appropriate values and then Finish.

Figure 7.1.3 New material – isotopic or orthotropic.

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Figure 7.1.4 New material properties for isotropic (left panel) or orthotropic (right

panel).

7.2 Apply Material

The Apply Material dialog, Figure 7.2.1, allows one to apply a material id to all the

elements in the current model. Select the material from the list and its id will be attached

to all of the elements in the model.

In practice, this option is rarely used. The elements created during crack insertion and

remeshing will obtain their material data from the original uncracked finite element

model.

Figure 7.2.1 Apply Material id panel.

7.3 Apply Coordinate System

This Apply Coordinate System dialog, Figure 7.3.1, allows one to apply a coordinate

system id to all the elements in the current model. Select the coordinate system from the

list and its id will be attached to all of the elements in the model.

In practice, this option is rarely used. The elements created during crack insertion and

remeshing will obtain their coordinate system from the original uncracked finite element

model.

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Figure 7.3.1 Apply Coordinate System id panel.

8. Wizards and Dialog Boxes for Loads Menu

The wizards and dialog boxes for the Loads menu options are described in this section.

8.1 Map Current State

The Map Current State option is intended to allow one to map initial conditions or current

state information, such as stress or strain, from the current model to the model after crack

insertion or growth. Mapping is done by interpolating data from the current mesh based

on element shape functions to points, either nodes or gauss points, of the new mesh.

This menu item is currently disabled.

Temperatures and other boundary condition data that must be transferred from one model

to the next occurs automatically.

8.2 Crack Face Pressure/Traction

The Crack Face Pressure/Traction option allows one to define crack face pressures or

tractions. This capability is often used to simulate residual stresses. It can also be used

in a local/global modeling approach where the local model containing the crack is

analyzed using crack face tractions based on the stresses from the uncracked global

model.

The first panel of the wizard is shown in Figure 8.2.1. The user selects Add to create a

new entry in the list. Once an entry has been added, it can be edited or deleted by

selecting the name and then the Edit or Delete button.

The Crack Face Traction/Residual Stress Type panel is displayed next, Figure 8.2.2; it

allows one to choose the type of crack face loading. The choices are: 1) Constant crack

face pressure, 2) 1-D Radial residual stress distribution, 3) 2-D Radial residual stress

distribution, 4) Surface treatment residual stress distribution, and 5) Residual stress

defined on a mesh. These types will be described in more detail below.

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In addition to choosing the type of crack face traction, the user can specify the load case

(or load step) for this boundary condition. The default is load case 1. The user must

ensure that sufficient constraints exist for this load case before analyzing.

Figure 8.2.1 Crack Face Tractions – top level dialog.

Figure 8.2.2 Crack Face Tractions – type dialog.

8.2.1 Constant Crack Face Pressure Panel

The constant crack face pressure panel allows one to specify a uniform pressure on the

crack face, Figure 8.2.3. A positive pressure value will tend to open the crack.

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Figure 8.2.3 Constant Crack Face Pressure panel.

8.2.2 1-D Radial Residual Stress Distribution Panel

The 1-D radial residual stress distribution panel allows one to specify a radial distribution

of stress that can be applied to the crack face, Figure 8.2.4. The stress varies along a

radius from some origin. The user can specify the distribution axis as well as the axis

offset. The user can select the Define Distribution button to interactively define the

distribution, Figure 8.2.5.

Figure 8.2.4 1-D radial residual stress distribution panel.

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Figure 8.2.5 User defined 1-D radial residual stress distribution plot.

As an example, consider a corner crack in a brick, Figure 8.2.6 – left panel. The crack

has a radius of 2.0 and the corner is located at x=0, y=5, and z=5. A 1-D radial

distribution is defined, Figure 8.2.7 – right panel, with the distribution axis set to y and

the axis offset defined as x=0 and z=5. The crack face traction distribution is shown in

Figure 8.7 – left panel. To visualize the pressure that is applied to the crack, the ANSYS

color contours for the pressure values are shown in Figure 8.2.7 – right panel. This

image shows how the radial pattern of the pressures with the origin at the crack corner.

Figure 8.2.6 A corner crack in a brick (left panel) with a 1-D radial residual stress (right

panel).

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Figure 8.2.7 A 1-D radial residual stress distribution (left panel) applied to the corner

crack and shown as color contours in ANSYS (right panel).

8.2.3 2-D Radial Residual Stress Distribution Panel

The 2-D radial residual stress distribution panel allows one to specify a stress distribution

that varies in 2 directions (axial and radial) that can be applied to the crack face, Figure

8.2.8. The stress varies with radius from some origin. The user can select the Define

Distribution button to interactively define the distribution, Figure 8.2.9.

Figure 8.2.8 2-D radial residual stress distribution panel.

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Figure 8.2.9 User defined 2-D radial residual stress distribution plot.

8.2.4 Surface Treatment Residual Stress Distribution Panel

The surface treatment residual stress distribution panel, Figure 8.2.10, allows one to

specify a stress distribution that varies in a direction that is normal to a surface that has

been retained as a “residual stress surface” when reading the finite element file. The

stress varies with distance from the surface. The user can select the Define Distribution

button to interactively define the distribution, Figure 8.2.11.

Figure 8.2.10 User defined surface treatment residual stress panel.

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Figure 8.2.11 Surface treatment residual stress distribution plot.

As an example, a brick model is defined with the top surface (positive y-axis) defined as

a “treated_surface” node component, Figure 8.2.12 – left panel. When reading the model

into FRANC3D, this component is retained as a residual surface (see Section 4.1.10).

The stress distribution shown in Figure 8.9 is applied as the surface treatment crack face

traction. The distribution acts normal to the “treated_surface”, and distance will be

measured in the -y direction. The resulting crack face traction distribution on the crack

shown in Figure 8.2.12 (left panel) can be visualized using ANSYS, Figure 8.2.12 – right

panel.

Figure 8.2.12 Example of a surface treatment residual stress crack face traction.

There are a number of things that will be noted here. Consider the crack shown in the left

panel of Figure 8.2.13 and the residual stress distributions shown in the right panel of

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Figure 8.2.13. First, if the nodes on the crack surface are at a depth that is greater than

the residual stress, no crack face tractions are computed. Using distribution ‘a’, the lower

third of the crack will not have crack face tractions. Second, if the mesh on the crack

surface is coarse compared to the residual stress distribution, the computed crack face

traction will not adequately capture the residual stress distribution. Based on the coarse

mesh shown in the left panel of Figure 8.2.14, the computed traction value at the node

indicated by the red circle, using distribution ‘a’ from Figure 8.2.13, would be close to

zero. If a pseudo-refined mesh is defined, as in the middle and right panels of Figure

8.2.14, then the ‘a’ distribution can be captured and appropriately distributed onto the

nodes of the actual coarse mesh. Ideally, one would use a suitably refined mesh in the

model to avoid this problem.

Figure 8.2.13 Surface treatment residual stress applied as crack face traction up to the

defined depth of the distribution.

Figure 8.2.14 Surface treatment residual stress applied as crack face traction with

automatic pseudo-mesh refinement to capture the distribution.

8.2.5 Mesh-Based Stress Distribution Panel

The mesh-based stress distribution panel allows one to specify a stress distribution based

on a finite element mesh and stress file, Figure 8.2.15. The user chooses the mesh and

associated stress file, normally an uncracked model. The program queries this data to

compute tractions on the crack face. The uncracked model element shape functions are

used to interpolate the results onto nodes of the remeshed cracked model.

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Figure 8.2.15 Mesh based stress distribution panel.

9. Wizards and Dialog Boxes for Analysis Menu

The wizards and dialog boxes for the Analysis menu options are described in this section.

9.1 Static Crack Analysis

This Static Crack Analysis menu item allows one to set up and run a static (single step,

no crack growth) deformation analysis. One would use this, for example, if one had

inserted a crack into a body and wanted to compute stress-intensity factors for this crack

without performing any crack growth. The ANSYS, ABAQUS and NASTRAN wizard

panels are discussed below; most of the wizard panels are the same for all of the analysis

codes, but there are some differences as will be noted below.

9.1.1 Fdb File Name Panel

The first panel, Figure 9.1, allows one to specify the name of the .fdb file, which stores

the mesh and flaw information for this model. This file name will be used for all the files

created for this static analysis, with different file extensions applied. The .fdb extension

will be added if the user does not type it into the File Name field.

Note that the uncracked file name must not be re-used here; the uncracked model is

needed for crack growth and should not be overwritten.

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Figure 9.1 Fdb file name dialog.

9.1.2 Analysis Code Panel

The user next must choose the analysis code, Figure 9.2. ANSYS, ABAQUS or

NASTRAN can be chosen, and subsequent panels will differ slightly.

Figure 9.2 Analysis code panel.

9.1.3 ANSYS Options Panel

If the user chooses ANSYS in Figure 9.2, the next wizard panel, Figure 9.3, allows the

user to set options and analysis parameters for ANSYS.

The first box in Figure 9.3 contains a number of options. Quadratic elements are required

to compute SIFs, thus linear elements are disabled. The element series can be selected,

either the older Solid-92 or the newer Solid-186 elements. The user can choose between

quarter-point and midside nodes for the wedge elements at the crack front. The material

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data and coordinate systems can be written to the cdb file; for a local/global model, this

data is typically contained in the global .cdb file. The user can choose to have ANSYS

write nodal temperatures when the solution is done; the nodal temperatures will be

written to the results file (.dtp extension) if any nodal temperature is not equal to the

reference temperature.

The second box contains the ANSYS executable file name and license string. These

values should be set in the Preferences (see Section 5.3.5) but can be modified or set

here.

The third box allows the user to select the global model to connect to the local cracked

model. If there is no global model, this option should be unchecked as in Figure 9.3.

The fourth box allows the user to define boundary conditions. Retained boundary

conditions can be transferred. Crack face tractions, if defined (see Section 8.2) can be

applied. Crack face contact conditions can be applied also.

The fifth box allows the user to specify the solution information. The default is to use

solution information from the input .cdb files. This information can be suppressed or

defined in an external file.

The final box contains two buttons, View/Edit Command and View/Edit Script, which

will be described in Sections 9.1.5-6. The user also can choose to write the files only,

without running ANSYS. This can be used to verify the files before running, or if the

files will be transferred to a different computer before running.

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Figure 9.3 ANSYS options and analysis parameters.

9.1.4 ANSYS local/global model connection Panel

If the user checked Connect to global model in Figure 9.3, the global model file must be

selected, Figure 9.4. The Browse button can be used to find the file.

Figure 9.4 ANSYS connect to global model box.

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If the user connects to a global model file, the Finish button in Figure 9.3, switches to a

Next button. The Local/global model connection panel, Figure 9.5, defines how the local

and global models will be combined. Note that the user cannot proceed to this panel if

the Connect to global box is checked and the Global model filename has not been

entered.

Figure 9.5 ANSYS local/global model connection panel.

The simplest method to connect a local crack model to a global model is through node-

merging. If the user retained surface mesh facets on the cut-surfaces for the local model,

then these surfaces can easily be glued together by merging nodes. Alternatively,

constraint equations or contact conditions can be defined between the cut-surfaces of the

local and global models. The ANSYS macro that FRANC3D generates will contain

necessary commands to join the two .cdb files together.

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Extra local/global contact/constraint can be defined, lower box of Figure 9.5, for surfaces

other than the crack faces and cut-surfaces. The user chooses the master and slave

surfaces from the list of components. The material id, real properties id, and element

type id can be set.

9.1.5 ANSYS Script Panel

An ANSYS script is generated that will instruct ANSYS to merge the local model with

the global model, if necessary, and perform a deformation/stress analysis. If the user

selects the View/Edit Script button in Figure 9.5, the script is displayed in a separate

panel, Figure 9.6, and can be edited if desired.

Figure 9.6 ANSYS Script panel.

9.1.6 ANSYS Command Line Panel

If the user selects the View/Edit Command button in Figure 9.5, the ANSYS command

is displayed in a separate panel, Figure 9.7. The command line can be edited if desired.

Figure 9.7 ANSYS Command Line panel.

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9.1.7 ABAQUS Options Panel

If the user chooses ABAQUS in Figure 9.2, the next wizard panel, Figure 9.8, allows the

user to set options and analysis parameters for ABAQUS.

The first box in Figure 9.8 contains a number of options. Quadratic elements are required

to compute SIFs, thus linear elements are disabled. The ABAQUS solid elements can be

modified using an extra letter in the designation, either M for modified, H for hybrid, or

R for reduced. The user can choose between quarter-point and midside nodes for the

wedge elements at the crack front. The user can also choose between wedge, constrained

collapsed bricks and collapsed bricks that allow for blunted crack fronts. Note that if one

wants to have ABAQUS compute the J-integral, collapsed brick elements must be used.

The number of integration rings will correspond to the number of mesh template element

rings at the crack front.

The second box contains the ABAQUS executable file name. This should be set in the

Preferences (see Section 5.3.6) but can be modified or set here. A Python script can be

executed on the .inp file prior to running ABAQUS.







solution information from the input .inp files. This information can be suppressed or




without running ABAQUS. This can be used to verify the files before running, or if the


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Figure 9.8 ABAQUS options panel.

9.1.8 ABAQUS local/global model connection Panel



Figure 9.9 ABAQUS connect to global model box.


Next button. The Local/global model connection panel, Figure 9.10, defines how the

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local and global models will be combined. Note that the user cannot proceed to this

panel if the Connect to global box is checked and the Global model filename has not been

entered.

Figure 9.10 ABAQUS local/global model connection panel.





local and global models.

9.1.9 ABAQUS Script Panel

An ABAQUS script can be created that will be added to the .inp file that FRANC3D

generates. If the user selects the View/Edit Script button in Figure 9.10, the script is

displayed in a separate panel, Figure 9.11, and can be edited if desired. The additional

instructions to ABAQUS should be added to the appropriate section; commands that will

include other files can be added here.

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Figure 9.11 ABAQUS Script panel.

9.1.10 ABAQUS Command Line Panel

If the user selects the View/Edit Command button in Figure 9.10, the ANSYS command

is displayed in a separate panel, Figure 9.12. The command line can be edited if desired.

Figure 9.12 ABAQUS Command Line panel.

9.1.11 NASTRAN Options Panel

If the user chooses NASTRAN in Figure 9.2, the next wizard panel, Figure 9.13, allows

the user to set options and analysis parameters for ABAQUS.

The first box in Figure 9.13 contains a number of options. Quadratic elements are

required to compute SIFs, thus linear elements are disabled. The user can choose

between wedge and constrained collapsed bricks, as well as choosing between quarter-

point and mid-side nodes for the crack front elements. Note that MSC NASTRAN does

not support pyramid elements whereas NX NASTRAN has implemented a CPYRAM

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element. The pyramid elements are preferred, but if the user has MSC NASTRAN, the

pyramid elements must be split into two tetrahedral elements.

The second box contains the NASTRAN executable file name. This should be set in the

Preferences (see Section 5.3.7) but can be modified or set here.







solution information from the input .bdf file. This information can be suppressed or




without running NASTRAN. This can be used to verify the files before running, or if the


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Figure 9.13 NASTRAN options panel.

9.1.12 NASTRAN local/global model connection Panel



Figure 9.14 NASTRAN connect to global model box.


Next button. The Local/global model connection panel, Figures 9.15-16, defines how the

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local and global models will be combined. Note that the user cannot proceed to this

panel if the Connect to global box is checked and the Global model filename has not been

entered. For NASTRAN, the global model file can be either a .bdf or a .dat file. The .dat

file will include the .bdf files. In Figure 9.14, the global model file is a .dat file, which

leads to the panel shown in Figure 9.15. If the user selects a .bdf file, the connection

panel will list the node sets and allow the user to choose the sets for merging, Figure

9.16.

Figure 9.15 NASTRAN local/global model connection panel – with a .dat file.

Figure 9.16 NASTRAN local/global model connection panel – with a .bdf file.

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local and global models.

9.1.13 NASTRAN Script Panel

A NASTRAN script is not yet supported.

9.1.14 NASTRAN Command Line Panel

If the user selects the View/Edit Command button in Figure 9.13, the NASTRAN

command is displayed in a separate panel, Figure 9.17. The command line can be edited

if desired.

Figure 9.17 NASTRAN Command Line panel.

9.2 Crack Growth Analysis

This wizard allows one to set up and run a series of crack growth analyses. The first set

of wizard panels are the same as for the Grow Crack wizard described in Section 6.5.

The SIF computation and interpretation along with the crack growth direction and

extension criteria must be defined. The subsequent panels are described next.

9.2.1 Crack Front Smoothing Panel

The crack front smoothing panel, Figure 9.2.1, provides crack front fitting and template

options.

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Figure 9.2.1 Crack growth analysis wizard panel for fitting and template parameters.

9.2.2 Crack Growth Increments Panel

The next panel, Figure 9.2.2, allows the user to specify the number of crack growth steps

and the median crack growth increment per step or the number of cycles per step

depending on the choice for extension type made in the previous panel. There are three

possible choices for entering the data per step: a constant value for all steps, a linear

variation, and a user-defined set of values.

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Figure 9.2.2 Crack growth analysis wizard panel for median crack growth increments.

9.2.3 Analysis Code Panel

The next panel, Figure 9.2.3, allows one to specify the analysis code - either ANSYS,

ABAQUS or NASTRAN. The user should also enter the base file name. All crack

growth steps will use this filename as the base name; a crack step number will be

appended. Finally, the current crack growth step number can be set; this number is used

to create the file name (e.g. base_name_STEP_002).

Figure 9.2.3 Crack growth analysis wizard panel for analysis code and base file name.

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9.2.4 ANSYS Analysis

If the user chose ANSYS in the previous panel, then the ANSYS analysis panels are

displayed; these are identical to those described for static crack analysis (see Sections

9.1.3-9.1.6).

9.2.5 ABAQUS Analysis

If the user chose ABAQUS in the previous panel, then the ABAQUS analysis panels are

displayed; these are identical to those described for static crack analysis (see Sections

9.1.7-9.1.10).

9.2.6 NASTRAN Analysis

If the user chose NASTRAN in the previous panel, then the NASTRAN analysis panels

are displayed; these are identical to those described for static crack analysis (see Sections

9.1.11-9.1.14).

10. Fatigue Menu Wizards and Dialog Boxes

The wizards and dialog boxes for the Fatigue menu options are described in this section.

10.1 SIF History

The SIF history dialog, Figure 10.1.1, allows the user to display SIF history data. By

default, the SIF history data is stored in the FRANC3D restart (.fdb) file. However, the

user can import data into the dialog through the Open Full History option of the File

menu in this dialog.

The crack front lines are displayed in the left-side model window and the path options are

displayed on the right-side of the dialog. The Front K’s and Path K’s tabs (above the

model window) allow one to display the SIFs along the crack front, Figure 10.1.2 – left

panel, or the SIFs along the defined path through the crack front, Figure 10.1.2 – right

panel. The Export tab allows one to save selected data to an ASCII text file and will be

described later.

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Figure 10.1.1 SIF history dialog.

Figure 10.1.2 SIF history dialog – Front K’s tab (left) and Path K’s tab (right).

10.1.1 Geometry Tab

Traditional fatigue lifing methodologies require a SIF history that consists of a single

valued stress intensity factor (K) and a single valued crack size (a) for each crack

increment. Developing such a "K-history" is straightforward for a 2-D analysis, but is

considerably more complicated for 3D analyses. In 3D, one has a distribution of K

values along a crack front, and there might not be an obvious crack dimension that

uniquely characterizes the crack “length”.

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Three different algorithms are available in the SIF History dialog (see Fig 10.1). The

results for the three algorithms are illustrated below using three different crack growth

simulations. It should be noted that while the crack growth might appear nearly planar in

these three examples, the algorithms work for non-planar growth.

10.1.1.1 Constant Normalized Distance

The first algorithm uses a constant, user-specified, normalized distance along the crack

fronts. Figure 10.1.3 shows the computed paths for 50% normalized distance. The figure

shows that this approach works very well for the first crack and reasonably well for the

third crack. For the middle crack, however, the path is somewhat sinuous; this can be

even more pronounced for crack fronts that transition from corner- to through-cracks.

Figure 10.1.3 Computed K paths for a constant normalized distance of 50%.

10.1.1.2 Nearest Point on Next Front

The second algorithm is a modification of the first approach. The analyst selects a point

on the initial crack front. The algorithm then searches for the nearest point on the next

crack front and makes this the next point in the history. The process is repeated for all

remaining crack fronts. The K's in the history are the interpolated values at the "nearest

points", and the crack length is the distance from the crack "origin" to the initial crack

front point plus the (straight line) distances between successive "nearest points".

This algorithm is illustrated in Figure 10.1.4. For all three cracks, the path was started at

locations of 10%, 20%, ... 90% along the initial crack front. The algorithm performs very

well for the first crack. For the middle crack, some of the paths converge into one path at

10% of the distance along the crack front. This is because of the path constraints, which

are described in Section 10.1.1.4. For the third crack, most of the paths merge together

due to these path constraints.

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Figure 10.1.4 Computed K paths using nearest next point. Paths are at 10% increments

with a 10% and 90% minimum and maximum threshold.

10.1.1.3 Intersection of Fronts with a Plane

The third algorithm defines a plane nominally perpendicular to the crack surface and

gives the path as the points where the crack fronts intersect the plane. This algorithm is

illustrated for the first of the three cracks above in Figure 10.1.5.

Figure 10.1.5 Computed plane intersection points for the slot crack.

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10.1.1.4 Path Constraints

There is a user-settable threshold that prevents the computed paths from getting too close

to the free surfaces where the accuracy of the computed stress intensity factors is

questionable. The user can set both the minimum and maximum thresholds in the dialog.

10.1.1.5 Path Curve Fitting

A best fit, in a least-squares sense, through the path computed by one of the three

algorithms. Figure 10.1.6 shows the least-squares fit to the paths shown in Figure 10.1.3.

A quadratic curve was used for the first crack, and as expected, there is little

improvement in the path for this case. A cubic curve was used for middle crack, which

yields a path that is aesthetically more pleasing. A fifth order curve was used for the

third crack and gives some improvement over the initial computed path by smoothing the

kinks.

Figure 10.1.6 Computed least-squares fits to the paths of Figure 10.1.3.

10.1.1.6 Input Curve Options

An issue related to computing a smooth K-history is computing a smooth K distribution

along a crack front for a single crack step. Because the K's are numerical results, they

tend to be "noisy" and frequently show spurious behavior near free surfaces where the

assumptions and numerical techniques used to compute the K can break down. In some

cases, it is desirable to fit a smooth curve through the numerical results. A least-squares

polynomial fit is applied, Figure 10.1.7. In the figure the computed K's for four crack

fronts have been fit with cubic polynomials. Note that this capability should be used with

some caution as the K distribution might not fit a simple polynomial very well.

136

computed and fit SIF's

20000

25000

30000

35000

40000

45000

50000

55000

60000

65000

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

normalized distance

K

Figure 10.1.7 Computed K distributions (blue) along with polynomial fits (red).

10.1.1.7 History Starting Condition

10.1.2 Export Tab

137

Figure 10.1.8 SIF history dialog – Export tab.

10.1.3 Settings Menu

138

10.2 Discrete Crack Life

The Discrete Life wizard allows the user to compute fatigue cycles.

The first wizard panel that is presented after starting the program is shown in Figure 10.3.

This panel allows the analyst to first set the units, either SI or US. Note that if the SIF

history data is not in one of these two units, the history data will need to be converted.

The next option on this panel is a choice between constant amplitude and variable

amplitude loading. This choice will affect the appearance of the subsequent panel. The

third option provides a choice between one or multiple load cases. Typical uniaxial

fatigue testing utilizes a single load case, but it is possible to have multiple load cases, as

for combined low cycle and high cycle fatigue.

The analyst has the option of reading previously stored session files and/or saving the

current session file for later use, by selecting the appropriate button on this panel.

Figure 10.3 Fatigue life wizard - first panel.

If the analyst chooses Constant Amplitude for the Loading Type, the next panel is as

shown in Figure 10.4 – left panel. Alternatively, if the analyst chooses Variable

Amplitude, the next panel is as shown in Figure 10.4 – right panel. The difference is the

removal of the R ratio field and the addition of the Load Spectrum options.

139

Figure 10.4 Fatigue life panel for Constant Amplitude loading (left) and for Variable

Amplitude loading (right).

The first section of either panel allows the analyst to read the SIF history data. The

default is to import FRANC3D's .spt file format, but other txt file formats are supported.

Once the data has been imported, the analyst can View (see Figure 10.4) the data in the

form of an XY plot, Figure 10.5. Note that this plot shows all three SIF modes for the

path, unlike the SIF History module described previously.

The Load Spectrum section allows the analyst to read variable amplitude (spectrum)

loads from a file. Several file formats are supported, but the simplest consists of a

column formatted .txt file where the first column is the minimum (or maximum) load, the

second column is the maximum (or minimum), and an optional third column contains the

cycle count. The third column defaults to 1 if not present. Once the spectrum data has

been imported, it can be Viewed also, Figure 10.6.

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Figure 10.5 XY plot of Mode I, II and III SIFs versus crack length

Figure 10.6 XY plot of spectrum load cycles

The next panel is shown in Figure 10.7. It allows the analyst to choose one of five

options for the crack growth rate model. The simplest is the Paris model, requiring only

two parameters. The required fields are highlighted in yellow until the analyst enters

values. The other options include Walker, NASGRO v3 and v4, and user-defined crack

growth rate data.

141

Figure 10.7 Fatigue life panel for crack growth rate models. Paris model chosen in left

panel and NASGRO v3 model chosen in the right panel.

The NASGRO v3 option has more parameters than Paris, but it accounts for the non-

linear crack growth rate, in particular at the high and low ends of the crack growth rate

curve. The NASGRO v3 material database contains data for a wide variety of materials,

and this data is included in FRANC3D and accessible by selecting the From Database

button. The dialog boxes shown in Figure 10.8, for a titanium alloy as an example, allow

the analyst to automatically fill in the required NASGRO v3 parameter fields from this

database.

142

Figure 10.8 NASGRO version 3 material database dialog boxes.

143

The next wizard panel, Figure 10.9, allows the analyst to choose the integration

technique, the equivalent KI value, and the retardation, and then provides a number of

stopping checks (i.e., conditions that would terminate the integration). Selecting Next on

this panel leads to the final panel, preceded by the session summary sheet, Figure 10.10.

Figure 10.9 Fatigue life panel for integration and threshold options.

Figure 10.10 Fatigue life session summary sheet.

144

The session summary sheet describes all of the options leading up to the final panel

where the cycle computation occurs, Figure 10.11. The analyst selects the Compute Life

button to fill the spread sheet. By selecting the Plot Results button, this data can also be

plotted in an XY plot, Figure 10.12, which shows cycles versus crack length. The data

can be saved to a file using the Save To File button. The initial crack length can be

modified from the default of 0.0 to get a cycle count starting from a prescribed initial

crack length. The entire session can be saved to a .fsn file for later reuse by selecting the

Save Session File button.

Figure 10.11 Fatigue life panel shows cycles versus crack length.

Figure 10.12 XY plot of cycles versus crack length

145

11. Fretting Menu Wizards and Dialog Boxes

The Fretting menu is accessed from the FRANC3D main menu bar at the top of the

FRANC3D window, Figure 11.1.

Figure 11.1: FRANC3D main menu bar contains the Fretting menu.

11.1 Read Model and Results

FRANC3D is able to read ANSYS, ABAQUS and NASTRAN files for fretting analysis.

The finite element (FE) information is contained in .cdb, .inp and .bdf files, respectively,

Figure 11.1.1. The FE information consists of nodes, elements and usually a definition of

the contact surfaces. Once the FE model is imported, analysis results must be imported.

A typical fretting test is conducted by applying a normal load (P) and then applying a

maximum and minimum tangential (shear) load (Qmax and Qmin). The dialog box shown

in Figure 11.1.2. allows the user to select one or more of these load cases. At minimum,

the P+Qmax load is required. If the P+Qmin load is not selected, FRANC3D assumes that

the minimum stress state is at zero. The P only loading is not required for most fretting

nucleation models.

146

Figure 11.1.1 Fretting dialog for importing FE model data.

Figure 11.1.2 Fretting dialog for importing FE results data.

Selecting the Browse button from the dialog in Figure 11.1.2. leads to the dialog

shown in Figure 11.1.3. The user can select the results files from ANSYS, ABAQUS

or NASTRAN. The ANSYS .str files are simply the nodal stress listings saved to a

file. Corresponding strain (.stn), displacement (.dsp) and contact status (.con) listings

should also exist with the same name prefix. ABAQUS results are read from .fil files.

NASTRAN results are read from .pch files. Note that strains are not required for

some of the fretting nucleation models.

147

Figure 11.1.3: Fretting dialog for locating results files.

Selecting the FINISH button from the dialog in Figure 2.3 leads to the Data Filter dialog

shown on the left side of Figure 11.1.4. The user can let FRANC3D select all material

regions and let FRANC3D automatically determine the contact surfaces from the FE

model and results data. Selecting Next on this dialog leads to the dialog on the right,

which allows the user to either go Back to change prior selections, Cancel the entire

operation, or Finish. Selecting Finish concludes the user interaction and FRANC3D

then processes the user selections for the FE model and results.

Figure 11.1.4 Fretting dialog for choosing all or selected material regions and auto

or selected contact surfaces.

If the user chose user select for both Materials and Contact surfaces on the dialog shown

in left panel of Figure 11.1.4, a sequence of dialog boxes is displayed to allow the user to

select the required data, Figure 11.1.5. The first dialog shown in the upper left panel of

148

Figure 11.1.5 displays the material identifiers and has a check box for coloring the

regions. By default, if there are multiple material regions, the regions will be given

different colors when the model is displayed in the FRANC3D window. If there is only

one material region or if the user unchecks the Color different materials box, then the

model is displayed in the default model color (grey).

The next dialog shown in the upper right of Figure 11.1.5 displays the contact surfaces

and named sets or surfaces that are part of the FE model data. The user must first select

the surface(s) that is the contact (fretting pad) surface. The next dialog shown in the

lower left of Figure 11.1.5 allows the user to select the target (fretting specimen) surface

and the final dialog shown in the lower right of Figure 2.6 allows the user to specify the

edge of contact nodes if desired. Selecting Next on this dialog leads to the dialog on the

right side of Figure 11.1.4, which will conclude the user interaction for the Fretting Read

Model and Results menu item. FRANC3D might automatically switch the definition of

the contact (or master) and target (or slave) surfaces based on the analysis results, in

particular the contact status and pressure.

Figure 11.1.5 Fretting dialog for selecting material regions and contact surfaces.

11.2 Import Nucleation Data

FRANC3D is able to import experimental fretting fatigue test data and perform curve fit

operations on this data. The first dialog box that is displayed after selecting the Import

149

Nucleation Data menu item is shown in Figure 11.2.1. The user should select the file

containing the raw experimental data. The format for this text file should be:

sigma_axial cycles

---------------------------

675.3 2.23e4

655.7 2.63e4

533.8 4.45e4

Figure 11.2.1 Fretting dialog for importing experimental fretting nucleation data.

Selecting the Browse button in the dialog shown in Figure 11.2.1 leads to the file

selection dialog shown in Figure 11.2.2. The user can select the appropriate .txt file.

Once the data file is selected, the user selects Next to arrive at the dialog shown in

Figure 11.2.3. The fretting data is shown in tabular form. Selecting the Plot/Edit

button leads to the dialog shown in Figure 11.2.4.

150

Figure 11.2.2 Fretting dialog for selecting the fretting nucleation data file.

Figure 11.2.3 Fretting dialog shows fretting nucleation data in tabular form.

The dialog shown in Figure 11.2.4 displays the fretting nucleation data in tabular and

graph form. The user can edit the data in the table by selecting the Edit menu item

and switching from the default Read Only mode. The user can also perform curve

fitting operations on the data using the Fit menu. The usual fretting nucleation model

uses a power law relationship with four coefficients to relate the fretting nucleation

cycles on the horizontal axis with the fretting parameter on the vertical axis. This

power law is highlighted in Figure 11.2.5, which also shows the fit through the data.

The goodness-of-fit is displayed on the top of the plot with the equation. The

151

coefficients can be output along with the fitted data by selecting the Save to File

button on the dialog in Figure 11.2.3. This text file is formatted as:

y=a(x^b)+c(x^d)

parameters: a=364.771 b=-0.0532003 c=442836 d=-0.681027

fitted data:

22300 698.23168

63777 439.16081

105254 365.42131

146731 327.89836

Figure 11.2.4 Fretting dialog for plotting fretting nucleation data.

152

Figure 11.2.5 Fretting dialog for plotting fretting nucleation data with power law

curve fit through the data.

11.3 Fretting Crack Nucleation

FRANC3D computes fretting fatigue crack nucleation cycles and initial crack location

(and possibly orientation) based on one or more of the fretting nucleation models that

have been implemented. These include: the equivalent stres, the critical plane shear

stress amplitude, the critical plane Smith-Watson-Topper, and a crack-analog model. The

model equations and parameters are briefly summarized here. For more details, refer to:

“Three-dimensional Simulation of Fretting Crack Nucleation and Growth”, B.J. Carter,

E.C. Schenck, P.A. Wawrzynek, A.R. Ingraffea, and K.W. Barlow, to be submitted to

Engineering Fracture Mechanics.

The equivalent stress model is defined by:

eq = 0.5 (psu)w (max)

1-w (1)

where psu and max are functions of the nodal stress components and w is a material

dependent fitting parameter. Based on curve fitting of experimental data, the equation, in

the form of the power law shown in Figure 2.11:

eq = aNib + cNi

d (2)

relates eq to Ni, where coefficients a – d are material dependent fitting parameters. For

example, for Ti-6Al-4V, a = 52476, b = -0.6471, c = 450.85 and d = -0.03582.

153

The critical plane shear stress amplitude model is defined by:

crit =max/G (1-R)m (3)

wheremax is the maximum shear stress on the critical plane, Ris the shear stress ratio, G

is the shear modulus and m is a material dependent fitting parameter. Substituting crit

for eq in equation (3), one can compute Ni given the appropriate values for the

coefficients a – d. For Ti-6Al-4V, a = 6.46, b = -0.686, c = 4.65x10-3 and d = 1.22x10-2.

The critical plane Smith-Watson-Topper model is defined by:

crit = max (f2 / E) (2Ni)

2b + f2Ni)

b+c (4)

where max is the maximum principal stress and represents the normal strain amplitude

on the critical plane; this equation is already in the form of equation (3). E is the elastic

modulus, f and f are material strength parameters, and b and c are fitting parameters.

For Ti-6Al-4V, f = 376.6 MPa, b = 1.78x10-4, c = -0.767 and f = 32.4.

The crack analog model is quite different from the above three models. First, the fretting

parameter (Q) is only computed at the nodes along the EOC. Second, the FE analysis

must be conducted with bonded contact conditions rather than the standard contact (that

allows for slip and requires a coefficient of friction). Q can be related to JII from linear

elastic fracture mechanics:

Q = (JII E/(1-))

1/2 (6)

where is Poisson’s ratio. Ni is computed from:

log Ni = ( – Q ) / (7)

where and are material dependent fitting parameters. Q = Qmax - Qmin, is determined

from the maximum and minimum load cycles.

The first dialog box that is displayed after selecting the Fretting Crack Nucleation

menu item is shown in Figure 11.3.1. The user can select one of the four fretting

nucleation models. Selecting Next leads to the dialog shown in Figure 2.13, which lets

the user define the Equivalent Stress fretting model parameters. Note that the user can

select the fitting coefficients from the previous section based on a curve fit through the

experimental data. If these coefficients are not available, this option for curve fit values

is disabled. The other three models have similar dialog boxes as shown in Figures 11.3.2

- 11.3.5.

154

Figure 11.3.1 FRANC3D Fretting dialog to select fretting nucleation model.

155

Figure 11.3.2 Fretting dialog to set equivalent stress fretting model parameters.

Figure 11.3.3 Fretting dialog to set critical shear stress model parameters.

156

Figure 11.3.4 Fretting dialog to set Smith-Watson-Topper model parameters.

Figure 11.3.5 Fretting dialog to set RAI Q model parameters.

157

The dialog boxes shown in Figures 11.3.2 - 11.3.4 have a check box for Volume

Averaging. Rather than computing the fretting parameters based on node-point stress

(and strain) values, the user can choose to do volume averaging. If this box is checked,

the next dialog box that will be displayed is shown in Figure 11.3.6. The user can adjust

the size of the displayed sphere by changing the Average over length value. Note that

stress values are not averaged across material region boundaries. Typical length values

used in the literature are based on the material grain size.

Figure 11.3.6 Fretting dialog for volume averaging of stress (and strain).

Selecting Next on the dialog shown in Figure 11.3.6 leads to the dialog shown in Figure

11.3.7. This dialog shows the model and allows the user to view the contact surfaces, the

edge of contact, the fretting nucleation cycles and the fretting parameter values. The

latter two items can be displayed as text or using color contours.

158

Figure 11.3.7 Fretting dialog for displaying fretting nucleation cycles and parameter

values and contours.

Selecting the Save to File button on the dialog shown in Figure 11.3.7 leads to the file

save dialog shown in Figure 11.3.8. The fretting nucleation data is saved to a text file

with the following format:

master surface

node x y z parameter cycles

19222 0.49648972 0.91291108 0.15 25 19283

20917 0.52414545 0.94056681 0.12 29 18453

20881 0.51283174 0.92925310 0.12 24 19898

159

Figure 11.3.8 Fretting dialog for selecting file to save fretting nucleation predictions.

The user can go Back and choose a different fretting nucleation model to compare

predictions. Selecting Finish on the dialog shown in Figure 2.18 closes the dialog.

11.4 Extract Subvolume

FRANC3D is able to extract a subvolume (or portion) of the full model that is positioned

such that it contains the predicted fretting crack nucleation site. The dialog shown in

Figure 11.4.1 is displayed when the user selects the Extract Subvolume menu item. The

user can choose the size, shape, location and orientation of the cutting-region (the

magenta region in Figure 11.4.1) as well as specifying the material region from which to

extract the subvolume. Selecting Next on this dialog leads to the dialog shown in Figure

11.4.2. The global and local portions of the model should be saved. The initial crack can

also be saved to a file (if it is oriented correctly). Note that some fretting models do not

predict the initial crack orientation; in this case, the predicted orientation is based on the

maximum principal stress direction. Selecting Finish on the dialog shown in Figure

11.4.2 closes the dialog. The full fretting model remains active if the user wishes to re-

evaluate the fretting nucleation using a different model or with different parameters.

Discrete crack growth simulations can be conducted using the saved files.

160

Figure 11.4.1 Fretting dialog for extracting a subvolume.

161

Figure 11.4.2 Fretting dialog for saving a subvolume and its corresponding ‘global’

model counterpart.

11.5 Turn Off Region Color

FRANC3D will display material regions with different colors. The Turn Off Region

Color menu item allows the user to select the material regions to be colored using the

dialog shown in Figure 11.5.1. The regions that are checked are drawn in solid colors

while the unchecked regions are drawn as wireframes. Selecting Accept will draw

colored regions while selecting Cancel will convert all regions to the default (grey)

model color. This option is purely for display purposes so that interior regions can be

viewed in complex multi-region models.

162

Figure 11.5.1 Fretting dialog for turning off/on colored regions.

12. Advanced Menu Wizards and Dialog Boxes

The wizards and dialog boxes for the Advanced menu options are described in this

section.

12.1 Edges Wizard

The Edge Extraction dialog, Figure 12.1.1, allows the user to control the geometry that

is extracted from the finite element facets. The angle threshold can be adjusted to

increase or decrease the number of surfaces by blending facets together.

163

Figure 12.1.1 Edge extraction dialog.

For cylindrical surfaces such as a hole in a plate, the user should add edges to break the

surface into at least two surfaces. By selecting the Add button for Edge Lines in Figure

12.1.1, the dialog in Figure 12.1.2 is displayed to allow the user to define edges.

Figure 12.1.2 Add edge dialog.

12.2 No Crack Regions

The no crack regions dialog box, Figure 12.2.1, allows the user to insert a crack into a

single region in a multi-region model. Each of the regions can be displayed using the

Displayed Region selection; the selected region is colored while the other regions are

shown as wireframes. The user selects the region that will not be cracked by checking

the appropriate region id in the No Crack Regions pane.

164

When a crack is inserted that crosses the material boundary, Figure 12.2.2, the crack

geometry is trimmed so that the final meshed crack model does not have any crack

surface in the selected region, Figure 12.2.3.

Figure 12.2.1 No crack regions dialog.

Figure 12.2.2 Inserting a crack into a two-region model where the second region is

designated as a no-crack region.

165

Figure 12.2.3 Crack inserted into one region of a two-region model.

12.3 Write Crack Front Data

The Save File As dialog, Figure 12.3.1, is presented so that crack front data can be saved

to a file. The saved file is an ASCII text file with extension is .fcg. The content of the

file consists of the SIF data for each crack front for each step of crack growth.

Figure 12.3.1 Write crack front data - Save File As dialog.

The .fcg file format is briefly described here. This file can be imported into the SIF

history dialog.

StepData: 4 # number of crack growth steps LoadFactors: 1 # load factors for each load case 1 1 1 FrtData: 1 # number of crack fronts in current step Start: -0.5 0.0 10 # start of crack front Stop: 0.5 0 10 # end of crack front

166

FitFrt: -0.5407 -0.00021 10.045 ….. 0.556 -0.00022 10.045 # list of fit new points FitOld: -0.498 0.0 9.967 …… 0.490 0.0 9.906 # list of current front points FitExt: 0.089 …… 0.15387 # list of extensions at each point FitParams: FIXED_ORDER_POLY 3 0 0 3 3 # fitting parameters FrtPts: 24 # number of front points NPos: 0.02090 # parametric location of first point Ks: 0.9082 0.00074 -0.00083 # SIFs at point Js: 7.4872e-005 # J-integral at point Ts: -0.6669 # T-stress at point Coord: -0.49788262 1.0932882e-016 9.9672525 # coordinate of point Axes: -0.9979 0.0 -0.0645 0.0 1.0 0. 0 0.0645 0.0 -0.9979 # local axes KAng: -0.00163 # kink angle at point Ext: 0.0481 # extension at point DKEE: 0.9082 # delta K1 effective REff: 0 # effective R ration NodeId: 13371 # node id ENDFP # end of data for first point ……. # repeat from NPos to ENDFP for each point on crack front NPos: 0.97917452 # end parametric position on crack front Ks: 0.9084 0.000642 0.0008515 # same data as for each front point Js: 7.4896e-005 Ts: -0.6654 Coord: 0.4979 1.249e-017 9.967 Axes: 0.9979 0.0 -0.064 0.0 1.0 0.0 0.06425 0.0 0.9979 KAng: -0.001414 Ext: 0.060896 DKEE: 0.90844 REff: 0 NodeId: 7272 ENDCF # end of current crack front ENDCS # end of current crack growth step ……. # repeat from FrtData to ENDCS for each crack growth step ENDFCGD # end of file

12.4 Write COD Data

The Write COD Data menu item allows the user to save crack displacement data to a

file. The first dialog that is displayed, Figure 12.4.1, prompts the user to enter a distance

back from the crack front where crack displacements will be computed. The default is to

compute the values at the entered distance from crack front nodes. If the user turns this

option off, values are computed at geometric points along the crack front.

167

Figure 12.4.1 Write crack front data - Save File As dialog.

Select Accept on the dialog in Figure 12.4.1, and the dialog shown in Figure 12.4.2 is

displayed. This dialog allows the user to choose which data will be written to the file.

The file is saved by selecting the Create File button, which invokes the usual Save As

File dialog. The file is an ASCII text file.

Figure 12.4.2 Write COD data – create file dialog.

12.5 Write Template Data

The Save File As dialog, Figure 12.5.1, is presented so that template data can be saved to

a file. The data is saved to an ASCII text file with .std extension.

168

Figure 12.5.1 Write template data - Save File As dialog.

The file format for this file is briefly described here.

STD_TEMPLATE_DATA { VERSION: 1 NUM_FRONTS: 1 CRACK_FRONT: 0 49 25 OPEN CORNER_SET: 0 # corner node ids of template element ring 11978 12907 11421 12440 11888 6756 7686 7222 8147 5762 6227 10350 10816 973 1436 4673 5142 8373 8835 …. SIDE_SET: 0 # side node ids 6938 3386 6470 14975 6003 2013 616 1081 3384 2479 2916 6383 2450 7784 6385 6854 9277 8246 8815 4877 8353 3061 1198 1662 7643 3527 7180 13551 6713 14015 14362 6338 13898 4288 ….. NODE_COORDS: 1781 # list of all node ids and coordinates 7321 -0.52397588 -0.01178516 9.9308077 11252 -0.47625961 1.0479856e-014 9.7249057 …. NODE_DISP: 1781 # list of all node ids and displacements 7321 2.34113830186791e-005 0.000939063262674639 -0.000290174323814175 11252 2.24602369565477e-005 0.000940419013229567 -0.0002799458448084 …. }

12.6 View Response

The View Response menu item allows one to view the deformed shape of the current

model. Analysis results are required to activate this menu item. The dialog shown in

169

Figure 12.6.1 is displayed. The Display Mesh Overlay can be turned off to remove the

surface mesh facets from the model view. The deformed shape can be displayed by

turning the Display on Deformed Geometry option and entering a non-zero magnification

factor, Figure 12.6.2.

Figure 12.6.1 View Response dialog showing undeformed mesh.

Figure 12.6.2 View Response dialog showing the deformed shaded solid and the original

wireframe outline.

170

12.7 Create Growth History

The Create Growth History dialog, Figure 12.7.1, allows a user to create and edit the

SIF history data. A list of the crack growth steps is provided, and then for each step the

crack front data for each crack front is given. The data was briefly described in Section

12.3. The dialog has three menu items: File, Edit and Plot. The File menu has four

options, Figure 12.7.2. The Open History menu item invokes the usual File Open

dialog with the .fcg file filter. The Add Step menu item invokes the usual File Open

dialog with the .sif file filter. The Add List menu item invokes the usual File Open

dialog with the .txt file filter; the .txt file should contain a list of .sif files. The last menu

item, Save History invokes the usual File Save As dialog for saving the data to a new

.fcg file.

Figure 12.7.1 Create Growth History dialog.

Figure 12.7.2 File menu for Create Growth History dialog.

171

The Edit menu option has one item: Set Start Condition, which invokes the dialog

shown in Figure 12.7.3. This dialog allows the user to set the start condition for the SIF

history be either setting a location or providing an initial length. This initial length

should be the initial crack length, but this length is somewhat ill-defined for a general 3D

crack.

Figure 12.7.3 Crack Start Condition dialog from Growth History dialog.

The Plot menu option has one item: Plot Crack Fronts, which invokes the dialog shown

in Figure 12.7.4. This dialog allows the user to view the crack front boundaries for all of

the crack fronts for all of the crack growth steps.

Figure 12.7.4 Plot crack fronts dialog from Growth History dialog.

172

12.8 Crack Growth Sequence

The Crack Growth Sequence menu item invokes the dialog boxes shown in Figure 12.8.1

and 12.8.2. The first dialog shows the model using the camera position for the current

model in FRANC3D. The second dialog allows the user to cycle through the crack steps

using the Next, Prev, First, and Last buttons. The user can snap images from this

window to create a crack growth sequence or animation or movie clip of the crack

advance. Select Close when done.

Figure 12.8.1 Crack Growth Sequence dialog showing the ‘first’ crack step.

Figure 12.8.2 Crack growth sequence dialog lets the user cycle through the crack steps

shown in Figure 12.8.1.

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13. FRANC3D Command Language and Session Files

The FRANC3D command language is described here. These commands are written to

session##.log file by FRANC3D as GUI operations are completed. The session files can

be played back as described in Section 4.5.

A complete description of the command language is provided in a separate document:

FRANC3D Command Language Version 6.

14. FRANC3D Python Extensions

The FRANC3D command language is replicated in the Python module. Each command

has a Python counterpart. The Python module is named PyF3D.dll or .pyd. A separate

executable called Fcl2Py.exe is available that converts the session .log files containing

FRANC3D Commands into equivalent Python commands. A user can then modify this

script or write their own.

A complete description of the command language is provided in the FRANC3D

Command Language Version 6 document.

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