abaqus tutorial midsurface 072315

Mid-surfacing Tutorial UMass Lowell - FEA of Composites (22.514) 1

Modeling a Composite Rib-Section by Mid-Surfacing using

Conventional Shell Elements in Abaqus/CAE

Authors: Andrew Wysocki, Dimitri Soteropoulos

Instructor: Professor James Sherwood

University of Massachusetts, Lowell Finite Element of Composites (22.514)

Overview:

In this tutorial Abaqus/CAE 6.14 is used to analyze a rib section of an airfoil. A three-dimensional model

is imported and mid-surfaced, a process used to create a simplified shell (2-D) representation of a solid

model (Abaqus/CAE Users Manual v6.14, Section 35). Models comprised of shell elements are less

computationally expensive in comparison to three-dimensional solid elements. Additionally, in thin

sections shell elements account for bending responses better compared to models of a single element

thickness. In completion of this tutorial the user will analyze the rib section of a graphite-epoxy

composite wing section using the following workflow:

1. Import Geometry

2. Create Mid-surface Model

3. Define Composite Material Properties

4. Meshing and Mesh Verification

5. Geometry Repair

6. Creating a Composite Layup

7. Define Steps and Analysis Attributes

8. Viewing Results

Problem Description:

A four layer [0/90]S graphite-epoxy composite wing section rib is subjected to a bending load will be

examined. Predefined geometry will be imported into Abaqus/CAE 6.14 where the global displacement

will be analyzed. The purpose of this tutorial is to become familiarized with importing a 3-Dimensional

geometry and creating a 2-Dimensional mid-surface to be used as a shell model. The model used in this

tutorial can be found in Figure 1 a. The geometry selected for this tutorial is the rib section of a wing, as

shown in Figure 1 b. The rib section has a uniform thickness of 1.8 mm.

Image: http://www.zenithair.com/stolch701/7-construc.html


Figure 1. a.) Rib section geometry used in analysis, and b.) a diagram of a wing section assembly.

The wing section rib in Figure 1 a will be modeled with conventional shell elements. The orthotropic

properties used in this tutorial for a graphite-epoxy composite are shown in Table 1.

Table 1. Graphite-Epoxy Material Properties

Material Property Value

E1 155.0 GPa

E2 12.10 GPa

Nu12 0.248

G12 4.40 GPa

G13 4.40 GPa

G23 3.20 GPa

Opening Abaqus CAE

Open Abaqus CAE 6.14 by clicking Start Abaqus 6.14 Abaqus CAE. If the Start Session window

appears, close it.

Importing the Model Geometry

The first step in importing the model geometry is to locate the file in the directory. A mid-surface shell

will be generated by determining the surface between two faces of a 3-Dimensional model. From the

File menu select Open.

In the Open Database window, locate Left_Wing.cae and click OK.


A dialog box may appear, click Dismiss.

Creating a Mid-surface Model

Creating a mid-surface of a model generates a shell representation of the imported geometry. Using

shell elements can reduce the computational time when analyzing a model. To create a mid-surface

model make sure that Part is selected in the Module: menu above the Viewport.

In the Main Menu bar at the top of the window select Tools Mid-surface Assign

The user is prompted to Select the cells for mid-surfacing. Click the geometry, the edges should turn

red. Click Done.


The imported geometry has been removed and replaced as a reference representation. The reference

representation of the geometry should appear translucent brown.

At this point we have just defined what 3-D geometry we want to create a mid-surface from. The 2-D

surface that will be created will be offset between the top and bottom faces of the reference

representation. To offset a face from the reference geometry go to Tools Geometry Edit in the

Main Menu.

The Geometry Edit dialog will appear. Select Category Face and Method Offset in the Geometry

Edit dialog.


Under the Viewport, use the dropdown menu to select the top face of the reference geometry. This can

be done by selecting each feature individually, by face angle or by face curvature. In this tutorial the

faces are selected by angle. In the Select faces to offset dropdown menu select by face angle the

default value should be 20. Change it to 40. All faces that are joined at an angle of less than 40 will be

selected, which in this model is all of the faces on the top side.

Click the top surface of the geometry, the model should turn red. Click Done.

The Offset Faces dialog will appear. Click Auto Select next to Use target faces to compute distance with

Half the average distance selected.


The top surface should be red and the bottom surface should be purple.

Click OK to in the Geometry Edit dialog. A mid-surface plane has now been created from the reference

geometry.

At this point the reference representation can be removed since it will not be used in the analysis. Open

the Part Display Options (ViewPart Display Options). The reference representation can be toggled

by clicking the Show reference representation box and then pressing Apply. Click OK.


Meshing the Shell

Expand the part in the model tree and double click Mesh (Empty). Click the Seed Part icon and assign an

Approximate Global Seed Size: 10. Click Apply, and then click OK.

Click Mesh Part and then click Yes.


Verify the mesh to check the quality of the mesh (MeshVerify). Select the regions to verify by part,

click the part. Click Done.

The Verify Mesh dialog box will appear. Click Highlight. Elements that do not pass the default mesh

quality tests are highlighted yellow. Notice on the elements that are selected on the nose and tail of the

rib section.

There will also be an indication of the number of warnings below the viewport window.

Press Dismiss to close the Verify Mesh dialog box. Open the Part Module using the dropdown menu

above the main viewport.

Zoom into where the elements were highlighted when verifying the mesh. Note: you will need to zoom

in a considerable amount until you see something like the picture below.


These small pieces were caused by the imported geometry. If the reference representation is turned

back the cause of the problem can be seen (ViewPart Display Options).

Toggle the reference representation off.

Repairing Geometry

In this portion of the tutorial the imperfections caused by the intersection of the shell planes are

repaired. While in the Part Module, locate the region where that needs to be repaired and zoom in to it.

The first region to be repaired is the lower corner (pictured above) of the trailing edge of the rib section.

Open the Geometry Edit dialog box (ToolsGeometry Edit). Select Edge for Category and Repair Small

for Method double click Repair small. While holding Shift individually select the two edges to be

repaired. Click Done.


A prompt will appear stating that the mesh will become invalidated. Click OK.

A second prompt will appear to update the validity of the geometry. Click Yes.

The small imperfection has been fixed. Repeat the procedure for the filet on the opposite side of the

trailing edge.

Repair the filets on the leading edge in a similar fashion selecting the edges as shown below. Close the

Geometry Edit dialog box and cancel the procedure by clicking the red x below the viewport window.

Re-mesh the part by double clicking Mesh (Empty) in the Model Tree and then click the Mesh Part

button. Click Yes.


Defining Composite Material Properties

When defining properties in Abaqus CAE it is important to consider the unit system that you will be

working in. Consult Table 2 to ensure that a consistent system of units is used. For this tutorial the SI

(mm) unit system will be used.

Table 2. Consistent unit systems

Quantity SI SI (mm) US Unit (ft) US Unit (inch)

Length Force lbf

Mass kg slug

Time

Stress ( ) (

)

(

)

Energy J

Density

Double click Materials in the model tree and create a material as shown below. Change the name of the

material to Graphite-Epoxy. To define the material as a lamina with the properties found in Table 1 go to

MechanicalElasticityElastic. In the Type: dropdown select Lamina.


Click OK.

Note that the values for the elastic modulus are inputted in units of MPa.

Creating a Composite Layup

A composite layup will be created for this model, representing a four-ply laminate. For a composite

layup, region, material, ply orientation and thickness will be assigned. This step is similar to creating and

assigning a section for an isotropic material analysis. Expand the Parts entry in the model tree by clicking

the + to the left of it. Further expand the model tree by clicking the + next to the part called Left Wing-

72. Double click Composite Layups and create a four-ply layup with conventional elements as shown

below.


Click Continue and the Edit Composite Layup dialog box will appear. Double click Region under the

Plies tab and the dialog box will disappear. Drag the mouse to draw a box around the whole model and

click Done. The Edit Composite Layup dialog box will reappear and the four plies will show (Picked)

under their respective Region.

Double click Material in the Edit Composite Layup dialog box and the Select Material dialog box will

appear. If the dialog box does not appear, right click the Material column label and go to Edit Material

Because only one material has been created for this analysis, Graphite-Epoxy is preselected. Click OK to

assign this material to each of the plies.


Next, a relative thickness will be assigned to the ply. Double click Element Relative Thickness in the Edit

Composite Layup dialog box and the Thickness dialog box will appear. If the Thickness dialog box does

not appear, right click the column label Edit Thickness. The total thickness of the rib section is 1.8 mm

thick. Since the layup schedule is four-layers, the thickness of each layer is 1.8/4. Enter 1.8/4 in the

Specify Value: option and click OK.

The final step in creating the composite layup is to assign fiber orientation to each of the plies. The

stacking sequence for this laminate is [0/90]S therefore rotation angles will be defined as such in the

laminate. The rotation angle is calculated with respect to the 1-Reference axis. Enter 0 in the Rotation

Angle boxes.

Check the material orientation by clicking the CSYS column.


Ensure that the material is defined as desired. Be sure to note which way the 1- and 2-directions of the

layup are oriented. The Layup Orientation should default to Definition Part global.

Note how the 1- and 2-directions follow the contours of the part. Discrete material orientations can also

be added to define a local material orientation for the part.

Creating an Instance

Expand the Assembly option in the model tree and double click Instances. In the Create Instance dialog

box that appears, select Left Wing-72 -1 and create an instance of the part called Left Wing-72-mesh-1

by clicking OK.


Creating a Step

Double click Steps (1) in the model tree and create a General Static, General Step. Accept all defaults

for this step. Click Continue, and then click OK.

Creating Sets for Analysis Attributes

In this portion of the analysis sets are created to simplify the application of boundary conditions and

loads. Under Parts in the model tree, double click Sets. The Create Set dialog box will appear, name the

set TrailingEdge under Type select Node. Click Continue.


Click in the Views Toolbar to change the orientation to the Top. The Views Toolbar can be activated

by View Toolbars Views.


At the bottom of the Viewport window, change Select the nodes for the set to by feature edge. Hover

the cursor over the nodes on the trailing edge of the rib section. When the nodes are highlighted, click

once to select them.

Click Done.

Repeat for the other side, name the Set LeadingEdge.

Applying a Fixed Boundary Condition

Double click BCs in the model tree to create a Mechanical, Symmetry/Antisymmetry/Encastre

boundary condition on the trailing edge of the rib section. In the Create Boundary Condition dialog

window, change Name: to Fixed select the Category to be Mechanical and the Type to be

Symmetry/Antisymmetry/Encastre. Click Continue.

On the bottom of the viewport click Sets

In the Region Selection dialog box select the set created for the trailing edge of the rib section.


Click Continue. Select ENCASTRE (U1=U2=U3=UR1=UR2=UR3=0) in the Edit Boundary Condition dialog

box. Click OK.

Applying a Displacement Boundary Condition

A load is to be applied to the model at the top left nodes so as to develop a bending load in the curved

plate. Double click BCs (1) in the model tree and create a Mechanical, Displacement/Rotation type

boundary condition. Ensure that Step-1 is selected in the Step: option in the Create Boundary

Condition dialog box. Change the Name: to DISP.


Click Continue

In the Region Selection dialog box, select the Set created for the LeadingEdge node set. Click

Continue.

Check the box next to U3: and enter a value of -1.

Click OK. Small orange arrows pointing in the negative z direction will appear at the nodes.

Create and Run a Job

Double click Jobs in the model tree and create a Job called Rib_Section. Click Continue


In the Edit Job dialog box, accept all defaults and click OK.

Expand the Jobs (1) option in the model tree and right click the Rib_Section job and click Submit.


When the analysis has completed, the Message Area at the bottom of the screen should look similar to

the figure shown below.

Viewing Results

Right click the job Rib_Section (Completed) in the model tree and click Results.

You will automatically be entered into the Visualization module where the model has turned green and

is oriented in an isometric view. Click the Plot Contours on Deformed Shape icon in the Visualization

module and the Von Mises stress contour will appear.


The displacement contour of UZ is to be viewed for the laminate. At the top of the screen, change the

dropdown selection from S to U and from Magnitude to U3.


The range of displacement values can be read from the legend in the viewport. To increase the font size

of the legend go to ViewportViewport Annotation Options. In the Viewport Annotation Options click

Set Font. The Select Font window will appear, change Size to 14.


Conclusion

This concludes the Modeling a Composite Rib-Section by Mid-Surfacing using Conventional Shell

Elements tutorial. Mid-surfacing was used to create a 2D shell representation from a 3D solid geometry

of a composite rib structure. While mid-surfacing is both a common and helpful tool when working with

thin walled parts, further geometric editing and fixing may be required prior to meshing. These

imperfections will create distorted elements and should be taken care of in the preprocessing stage. A

typical decrease in run time and overall element count will be obtained by using a mid-surfaced part

rather than a full 3D model.

abaqus tutorial midsurface 072315

Documents