MEGN 537 – Probabilistic Biomechanics Applying the AMV Method with a Finite Element Model Anthony J Petrella, PhD

MEGN 537 – Probabilistic Biomechanics

Applying the AMV Method with aFinite Element Model

Anthony J Petrella, PhD

The Model

• Can be analytical or computational

• MATLAB for analytical• Finite elements for computational

• ABAQUS• ANSYS• SolidWorks Simulation (COSMOS)

Parameterization

• The most challenging step for an FE model• Some variables are easy

• Material properties• Loading• Already parametric

• Some variables are more difficult• Dimensional parameters• Affect the mesh

Typical Steps for Prob FE

1. Setup model in usual way → model database

2. Add or confirm parameters in FE model

3. Script(s) to update model parameters and regenerate FE mesh

4. Run FE solution

5. Script to extract scalar result(s) of interest

6. Encapsulate above in NESSUS to automate

ExampleSyringe Sealing Pressure

1. Setup model in usual way → model database


1. Setup model in usual way → model database• Axisymmetric• Rigid barrel, plunger rod• Rubber stopper• Note initial interference!

• First, resolve interference• Second, apply fluid pressure

Rigid Barrel

Rigid PlungerRod


2. Add or confirm parameters in FE modelrib1_od

rib2_od

rib2_width

rib1_width

rib1_rad

rib2_rad

plunger_od


2. Add or confirm parameters in FE model

a. rib1_od, rib2_od = 0.2915 ± 0.0015

b. rib1_rad, rib2_rad = 0.008 ± 0.001

c. rib1_width = 0.048 ± 0.002

d. rib2_width = 0.058 ± 0.002

e. plunger_od = 0.2275 ± 0.0005


3. Script(s) to update model parameters and regenerate FE mesh• All good FE codes have a scripting language…• For ABAQUS it is Python• ANSYS Parametric Design Language (ADPL)• SolidWorks Simulation uses VB scripting

• The script simply automates mouse clicks• There is typically a “replay” file from which you can

extract the needed commands

Try This…

• Download ABAQUS files• Double-click CAE file• Change rib1_od to 0.285• Close CAE file and save changes

• Now open abaqus.rpy file in a text editor• These are the Python commands that perform the task you

executed above with mouse clicks


4. Run FE solution• Use command-line

5. Script to extract scalar result(s) of interest• Created in the same way as before using the replay file• One may extract multiple results of interest, but each

must be a scalar – this is how prob analysis works


6. Encapsulate in NESSUS to automate…» copy ..\..\axi_10ml_syringe.cae» copy ..\..\script_extract.py» abaqus cae nogui=script_update» abaqus j=temp inp=axi_10ml interactive» abaqus cae nogui=script_extract» del *.rpy.*

Note: script_update is your input file, so it’s automatically copied to each trial folder by NESSUS

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6. Encapsulate in NESSUS to automate» copy ..\..\axi_10ml_syringe.cae» copy ..\..\script_extract.py» abaqus cae nogui=script_update» abaqus j=temp inp=axi_10ml interactive» abaqus cae nogui=script_extract» del *.rpy.*

Model Database – allows for re-meshing;must be copied to the working directoryto be operated on by script_update.py

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Extraction script – the default ABAQUS outputis a binary results database, so this script isneeded to extract scalar results to a text file;this will be a similar situation for any FE code

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Update script – changing dimensions changes the mesh,so you need a script to operate on the model database andregenerate the mesh after all dims are updated; this filecontains all the parameters that are prob variables modified byNESSUS, so it is copied to the working directory automatically

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FE solution – this is the ABAQUS command to run thesolution for a single trial

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Extract results – run the extraction script to read the binaryresults file from ABAQUS and write scalar output to a text file

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Output

• There are two output files of interest…• cpress.out – this is the peak contact

pressure on the 1st rib• cfn1_initial.out – radial component of

contact force after assembly• Line 1: force on 1st rib• Line 2: force on 2nd rib• Line 3: total force on both ribs



Clean up any unnecessary files

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Files Summary

axi_10ml_syringe.cae (model database, need to copy)script_update.py (input for NESSUS, copied automatically)script_extract.py (extraction script, need to copy)

axi_10ml.inp (ABAQUS input created when update script is run)temp.odb (ABAQUS binary output created when FE solution is run)

cpress.out (text output file created by extraction script)cfn1_initial.out (text output file created by extraction script)

These first three files are the only ones you need to begin the analysis…the others below are created automatically.

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Documents

MEGN 537 – Probabilistic Biomechanics Applying the AMV Method with a Finite Element Model Anthony J Petrella, PhD