let’s get together: using fasteners in fea

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Let’s get together: Using fasteners in FEA

Andrew Sartorelli David TruyensTechnical Support Specialist Simulation Business Development Specialist

© 2016 Autodesk© 2016 Autodesk

This class covers the use bolted connections, welds, and other fastener types to join parts within multi-body assemblies inside Autodesk Nastran In-CAD, Fusion Ultimate, and Simulation Mechanical. Models will be demoed to show off the different workflows and to show how various approaches can affect results.

Class summary


At the end of this class, you will be able to:

Explain the differences between various fastener approaches

Understand the limitations of using fasteners over solid model components

Use automated connectors like Bolted Connections

Develop skills to construct connectors manually

Key learning objectives


Fasteners


What is a fastener?

Connector

Idealization

Bolt Weld JointSpring Rigid Body Elements


FastenersGlobal vs Local


Bolts


Pre-processing Idealization methods Pre-loading Applications

Bolts


Bolt – Pre-ProcessingBolt Calculator


Three main approaches

Contact

Solids

Beams and Bars

Bolts – Idealization Methods


Bolt – IdealizationContact


Bolts – Idealization MethodsSolids Hexagonal Head

Threads

Bevels

Strength Markings


Bolts – Idealization MethodsSolids


Bolts – Idealization Methods1D Elements


Bolt – Pre-load

Why is tensioning important?

Direct impact on stiffness


Bolt – Pre-loadBeamNastran In-CAD Fusion Simulation Mechanical


No ability to add an axial pre-load to solid elements

Or is there?

Yes! Thermal loading

Bolt – TensioningSolid


Bolt – Pre-loadingSolid

Use thermal loading to induce axial compression

𝜎 =𝐹𝑜𝑟𝑐𝑒

𝐴𝑟𝑒𝑎→ 𝐹𝑜𝑟𝑐𝑒 = 𝜎 ∗

𝐴𝑟𝑒𝑎

𝜎

𝐸= 𝜀 → 𝜎 = 𝜀 ∗ E

𝜀 = α ∗ ∆𝑇 𝐹𝑜𝑟𝑐𝑒 = α ∗ ∆𝑇 ∗ E ∗ Area

𝐹𝑜𝑟𝑐𝑒 = α ∗ ∆𝑇 ∗ E ∗Area

α =𝐹𝑜𝑟𝑐𝑒

∆𝑇∗E ∗Area Assume ∆𝑇 = 1℃ Orthotropic material

α in axial direction from above

α in other directions should be ~0


Bolt – Pre-loadingSolid

α =𝐹𝑜𝑟𝑐𝑒

∆𝑇∗E ∗Area

α =700𝑙𝑏𝑓

1𝐹∆∗2.9e7 ∗0.25𝑖𝑛2∗𝑝𝑖

α = 1.229e-4 / F


Case Study: Eccentric Bolt Loading

P = 1200 lbf L = 8” W = 6” t = .375”

Bolt Diameter is variable

P

1

43

2

Z

Y



By the instantaneous center theory

Fm = 707 lbf

P

1

43

2

Fm

Z

Y


Case Study: Eccentric Bolt LoadingFree Body Diagram

Bolt 1

Fy = P/4 + Fm cos(θ)

Fy = 200lbf

Fz = Fm sin(θ)

Fz = 500lbf

1

43

2

Z

Y



Diameter Mesh Density Linear or Non-linear

0.25”

0.50”

0.75”

0.03”

0.07”

0.2”

0.4”

0.6”


Case Study: Eccentric Bolt LoadingComputed Force in Y direction for Bolt 1

150

160

170

180

190

200

210

0.03" 0.07" 0.2" 0.4" 0.6" .8" 1"

Fo

rce

, Lb

f

Mesh Sizing

Shear Force - Y, Linear

0.75" Diameter

0.5 Diamater

0.25 Diameter

Analytical Solution

155

160

165

170

175

180

185

190

195

200

205

0.03" 0.07" 0.2" 0.4" 0.6" .8" 1"

Fo

rce

Lb

f

Mesh Sizing

Shear Force - Y, Non-Linear

0.75" Diameter

0.5" Diameter

0.25" Diameter

Analytical Solution


Case Study: Eccentric Bolt LoadingComputed Force in Z direction for Bolt 1

0

100

200

300

400

500

600

0.03" 0.07" 0.2" 0.4" 0.6" .8" 1"

Fo

rce

, Lb

f

Mesh Sizing

Shear Force - Z, Linear

0.75" Diameter

0.5 Diamater

0.25 Diameter

Analytical Solution

0

100

200

300

400

500

600

0.03" 0.07" 0.2" 0.4" 0.6" .8" 1"

Fo

rce

Lb

f

Mesh Sizing

Shear Force - Z, Non-Linear

0.75" Diameter

0.5" Diameter

0.25" Diameter

Analytical Solution


Case Study: Eccentric Bolt LoadingMesh Sensitivity Study

0.00E+00

1.00E+05

2.00E+05

3.00E+05

4.00E+05

5.00E+05

6.00E+05

7.00E+05

0.030.070.20.4

Str

ess (

psi)

Mesh Size (in)

Stress - Linear Stress - Non-linear


Case Study: Eccentric Bolt LoadingShells

Key tips for using plates Check your normal

direction

Use a contact offset

Select edges for bolt connector


Case Study: Eccentric Bolt LoadingShells

Variance in shear forces regardless of mesh density

Correlation of displacements to solid models


Appropriate bolt size and preload should be done before FEA

Refining the mesh around the bolt holts doesn’t improve results!

Case Study: Eccentric Bolt LoadingTakeaways


Case Study: Eccentric Bolt LoadingTakeaways

0

50

100

150

200

250

0.03" 0.07" 0.2" 0.4" 0.6" .8" 1"

Fo

rce

Lb

f

Mesh Sizing

Shear Force - Y, Non-Linear

0.75" Diameter

0.5" Diameter

0.25" Diameter

Analytical Solution

0

100

200

300

400

500

600

0.03" 0.07" 0.2" 0.4" 0.6" .8" 1"

Fo

rce

Lb

f

Mesh Sizing

Shear Force - Z, Non-Linear

0.75" Diameter

0.5" Diameter

0.25" Diameter

Analytical Solution


Welds


Are we concerned with load transfer or weld failure?

Load transfer

Global modeling approach using contact

Weld failure

Local modeling approach to extract forces

Welds - Idealization Methods


1. All Solids

2. Plates with solid fillet welds

3. Plates with in-plane weld elements (Different properties)

4. Plates with shells mapping to theoretical weld face placement

5. Node-to-Node Rigid Links

6. Continuous Plate Intersections

7. Beam Stiffeners at Plate Intersections

Modeling of Seam Welds

1 3 4 5 62


All methods are used regularly in a variety of industries

Varying amounts of effort required based both on method and automated tools in FE pre-processor

Consider meshing error Shell-Solid interfaces Contact

None of them improve the local validity of in-weld stress but…

All transfer load with varying degrees of correctness

#1 – Solids may allow you to validate a weld size choice (Hard)

#6 – Allows calculation of the right weld size (Easy)

Modeling of Seam Welds


Extracting Weld Loads | Autodesk Nastran In-CAD

1,000# separate load case in each direction

2.0 in.

5.0 in. Weld Forces:Normal = 1,000 lb/5 in = 200 lb/inShear = 1,000 lb/5 in = 200 lb/inMoment = (1,000 lb * 2 in)/5 in

= 400 in-lb/in



Tip 1 – Align Element Direction to Surface Direction

Tip 2 – Plot Unaveraged Elemental Results

Shell Membrane FYShell Membrane FXYShell Moment MY

Tip 4 - Use Element Groups!

Tip 3 – Add Element Force to Output Set



Max = 500 lb/inAvg = 200 lb/in

Max = 271 lb/inAvg = 200 lb/in

Max = 390 in- lb/inAvg = 375 in- lb/in

Why so far off?OK to max Nodal force


Other Connectors


Rigid Connectors


Rigid


Rigid and rods


Rigid Interpolation


Results

0

1

2

3

4

5

6

7

Interpolation Interpolation

NL

Rod Rod NL Ridgid Ridgid NL

Displacement VM


Results

300

320

340

360

380

400

420

440

Ridgid Rod Interpolation Ridgid NL Interpolation

NL

Rod NL

Stess VM


Cables


Are always nonlinear Use preload or pre-slack

Cables


2D Model


When working with frame generator make sure to add work points at assembly level

A mast is Non-Linear due to cables and P-Delta effect

Setup


Buckling

Easy to switch in Nastran In-CAD

Vibration

Best to use a dedicated model for each cable

Other considerations


3D and cables


3D and cables

Use RGB’s to connect to the solid Turn on forces


3D and cables


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How did we do?

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© 2016 Autodesk. All rights reserved.

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let’s get together: using fasteners in fea

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