mae 3272 - lecture notes - ansys

ANSYS Guest Lecture by R Bhaskaran 24-March-2014

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MAE 3272: Computational Modeling of Bicycle Crank using ANSYSfi

Rajesh BhaskaranMechanical & Aerospace

EngineeringCornell University

My Co-ordinates

Dr. Rajesh BhaskaranSwanson Director of Engineering SimulationMechanical & Aerospace Engineering

E-mail: [email protected] Office: 102 Rhodes Hall Office hours (held in Swanson Lab, 163

Rhodes): T 3-4 pm, W 3:30-4:30 pm Come for help with ANSYS


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Computer Labs with ANSYS

B7 Upson 318 Phillips ACCEL lab (Carpenter Hall) Swanson Lab (163 Rhodes)

Student Version of ANSYS (including Mechanical & FLUENT)

$25 for one-year license Provides access to all FEA, CFD and rigid-body

dynamics capabilities No CAD translators Limits on mesh size

Mesh size should be sufficient for MAE3272

Purchase instructions will be posted on Blackboard

Student version is 15.0. Labs are at 14.5. You can read 14.5 files into 15.0 but not vice versa


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ANSYS Node Limits

Node limit forANSYS Mechanical

Student version 32k

Version in computer labs

256k

Research version in Swanson lab

No limit

Get vague solution error when you exceed the node limit

Crank Arm Experiment

Strain Gage Rosette


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Testing Procedure

Static Test Dynamic Test

Static Test

Experimental Set-up

ANSYS Model

Strain Gage Rosette


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Close-Up View of Sample Rosette Locations

Physical Model ANSYS Model

ANSYS Modeling Procedure for Static Test Case

1. Hand calculations using beam theory

2. Import CAD geometry3. Mesh, Setup Physics and Solve4. Check results5. Add gauges to CAD geometry6. Calculate strain values for gauges7. Compare with measured values

Covered in MAE 3250


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Tutorials on ANSYS Modeling Procedure

Google cornellansys

URL : https://confluence.cornell.edu/x/uhPpB

Bike Crank Part 2:Covers ANSYS procedure for adding strain gages to the crank model





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Hand Calculations

Use Euler-Bernoulli beam theory See Pre-Analysis step in Bike Crank Part 2 tutorial




Will focus on these steps


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Static Test: Comparing Strain Measurements to ANSYS Values

Need to extract average normal strain in the area covered by strain gage

Static Test Case: Post-Processing

User inputsColor pictures& other results

ANSYS (Blackbox)

Displacements at selected points

Mathematical model

Numerical

Solution

Post-processing step

Postprocessing: Nodal displacements > , , w >

etc. > , etc. Additional post-processing: etc. > > average

strain over area covered by strain gauge) Transform and average


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Post-Processing in ANSYS

In ANSYS, use the post-processing capabilities of the shell181 element type to calculate Model each gauge as one shell181

element Post-processing sequence:

Crank nodal displacements > , , w > shell181 nodal

displacements shell181 post-processing: nodal

displacements > , , w > etc. > >

shell181: 4 nodes per element

Post-processing in ANSYS

Key point: shell181 nodal displacements are not added to the vector of nodal displacements Stiffness of gauge is ignored

Thickness of shell181 element is ignored Original stiffness matrix is not changed


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Strain Gage Modeling: Procedure

Create surface on crank face Plane > Sketch > Surface from sketch

Bond surface to crank face Should happen automatically

Mesh surface with one shell181 element Insert Commands Tell ANSYS to use shell181 purely for post-processing

Solve and view results Use solution coordinate system to get strain

component in direction ANSYS will do the transformation and averaging to find

Strain Gage Modeling: Commands

et: set element type et, matid, 181: set element type to 181 (i.e. shell181)

keyopt: set keyoption to control element behavior

keyopt, matid, 1, 2: set keyoption#1 to 2 (use shell181 elements for post-processing only)

See help for shell181 for more info


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Results: Solution Coordinate System

Local coordinate system for element

Verification of ANSYS Results

Check that: Boundary conditions on displacement and

traction are satisfied Equilibrium is satisfied: Reactions balance

applied load ANSYS results are reasonably independent of

the mesh ANSYS results compare well with Euler-

Benoulli beam theory


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Verification: Some Sample Results

ANSYS (0.05 edge sizing)



Beam Bending Theory

(micro-strains)

-1240 -1239 -1239 -1228

ANSYS vs. Experiment vs. Theory: Sample Results

Strain gauge Beam theory ANSYS Experiment % difference between ANSYS

& Expt.Rosette 1 Right 12 -6 -10 4 40Rosette 1 Center 317 311 351 6 11Rosette 1 Left 172 206 274.5 4. 25Rosette 2 Right -29 -42 -47.75 4 12Rosette 2 Center 6 6 7.75 2 22Rosette 2 Left 15 30 35.5 2 15


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ANSYS vs. Digital Image Correlation (DIC)

DIC: New measurement technique enabled by computers and digital cameras

Procedure: Paint random speckle pattern on

surface of part Track the movement of the

pattern as part is loaded Deduce displacements and strain

Digital Image Correlation:

ANSYS:

Figures provided by J. Borshoff


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Sources of Mismatch Between Measurements and ANSYS

Gauges are placed in regions of low strain Place gauges away from the neutral axis

Uncertainty in the locations and orientation angles of the gauges Results are particularly sensitive to the orientation

of the gauges

Gauges not glued on correctly Wobble in support used in static test

Not modeled in ANSYS

Dynamic Test

Model as quasi-static since loading rates are low Freeze-frame loads at any instant and analyze as static Ignores acceleration:

Creates bonded contacts at interfaces between parts


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Conclusion

Static test: Can get good comparison between ANSYS, strain gauge measurements & beam theory in regions of high strain

Dynamic test: Might not get good comparison Think about possible reasons

mae 3272 - lecture notes - ansys

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