let’s get together: using fasteners in fea
TRANSCRIPT
© 2016 Autodesk© 2016 Autodesk Join the conversation #AU2016
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
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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
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Fasteners
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What is a fastener?
Connector
Idealization
Bolt Weld JointSpring Rigid Body Elements
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FastenersGlobal vs Local
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Bolts
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Pre-processing Idealization methods Pre-loading Applications
Bolts
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Bolt – Pre-ProcessingBolt Calculator
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Bolt – Pre-ProcessingBolt Calculator
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Three main approaches
Contact
Solids
Beams and Bars
Bolts – Idealization Methods
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Bolt – IdealizationContact
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Bolts – Idealization MethodsSolids Hexagonal Head
Threads
Bevels
Strength Markings
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Bolts – Idealization MethodsSolids
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Bolts – Idealization Methods1D Elements
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Bolt – Pre-load
Why is tensioning important?
Direct impact on stiffness
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Bolt – Pre-loadBeamNastran In-CAD Fusion Simulation Mechanical
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No ability to add an axial pre-load to solid elements
Or is there?
Yes! Thermal loading
Bolt – TensioningSolid
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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
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Bolt – Pre-loadingSolid
α =𝐹𝑜𝑟𝑐𝑒
∆𝑇∗E ∗Area
α =700𝑙𝑏𝑓
1𝐹∆∗2.9e7 ∗0.25𝑖𝑛2∗𝑝𝑖
α = 1.229e-4 / F
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Case Study: Eccentric Bolt Loading
P = 1200 lbf L = 8” W = 6” t = .375”
Bolt Diameter is variable
P
1
43
2
Z
Y
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Case Study: Eccentric Bolt Loading
By the instantaneous center theory
Fm = 707 lbf
P
1
43
2
Fm
Z
Y
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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
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Case Study: Eccentric Bolt Loading
Diameter Mesh Density Linear or Non-linear
0.25”
0.50”
0.75”
0.03”
0.07”
0.2”
0.4”
0.6”
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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
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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
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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
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Case Study: Eccentric Bolt LoadingShells
Key tips for using plates Check your normal
direction
Use a contact offset
Select edges for bolt connector
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Case Study: Eccentric Bolt LoadingShells
Variance in shear forces regardless of mesh density
Correlation of displacements to solid models
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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
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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
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Welds
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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
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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
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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
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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
© 2016 Autodesk© 2016 Autodesk
Extracting Weld Loads | Autodesk Nastran In-CAD
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
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Extracting Weld Loads | Autodesk Nastran In-CAD
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
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Other Connectors
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Rigid Connectors
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Rigid
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Rigid and rods
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Rigid Interpolation
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Results
0
1
2
3
4
5
6
7
Interpolation Interpolation
NL
Rod Rod NL Ridgid Ridgid NL
Displacement VM
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Results
300
320
340
360
380
400
420
440
Ridgid Rod Interpolation Ridgid NL Interpolation
NL
Rod NL
Stess VM
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Cables
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Are always nonlinear Use preload or pre-slack
Cables
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2D Model
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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
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Buckling
Easy to switch in Nastran In-CAD
Vibration
Best to use a dedicated model for each cable
Other considerations
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3D and cables
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3D and cables
Use RGB’s to connect to the solid Turn on forces
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3D and cables
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