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Combining Experiments, PolyUMod,
Kornucopia, and Abaqus to Create
Accurate FE Scratch Simulations
DuPontTed Diehl, Li L in, Ye Zhu, John J. Podhiny, Richard T. Chou, and Jeffrey A. Chambers
Veryst Engineering, LLCJorgen S. Bergstrom, Xiaohu Liu
2013 SIMULIA Community Conference
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2
Overview
Goal of Project
Complexity of Scratch
Material Characterization
Material Testing
Constitut ive Approach, and Calibration
FEA Modeling Approach To Simulate Scratch
Correlating 3D Physical Scratch Data with FEA Simulation Results
Open Issues More To Do!
Conclusions
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Project Goals
Develop Abaqus models to predict scratch performance of polymeric
materials
Predict residual depth and residual shapeof scratch.
Including time dependent partial self-healing behavior of many
polymers.
Material Characterization Include nonlinear viscoplastic nature of polymers via PolyUMod.
Utilize efficient Ziggurat testing protocols and MCalibration for material
calibration.
Develop improved methods to compare 3D physical scratch test data toAbaqus scratch simulation results.
Utilize Kornucopia analysis tools to process and transform large
amounts of messy, 3-D data for correlations.
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Polymer Materials Used in Study
Three polymeric materials were studied.
DuPont Surlyn grades (an ethylene copolymer ionomer).
Surlyn 9950
Surlyn 1706/1707 blend
Materials are differentiated from each other by such factors as their
percent neutralization, acid content and other formulation make-up.
PMMA
A polymethylmethacrylate.
PMMA is noticeably different from the Surlyn materials.
PMMA is stiffer and harder.
PMMA is based on the molecule methyl methacrylate and is not an
ethylene copolymer ionomer.
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Complexities of Polymer Scratch
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Photographs of 3 different polymers after
being scratched by the same indenter
Indenter load
progressively
increased
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Complexities of Simulating Polymer Scratch
The following are required for viable simulations and scratch validation.
Material experiments that exercise the sample to generate sufficient data to
characterize elastic, inelastic, and recovery behavior, including rate/time
dependence and large strain response.
A polymer constitutive model capable of representing such complex behavior.
Robust FEA methods that can incorporate the advanced material model,
support sliding contact, and can handle extremely large strains which will
cause severe element distortions.
Detailed polymer scratch experiments with 3-D surface imaging technology to
capture the scratched profile for comparison to the FEA simulations.
A method to manipulate and correlate large amounts of data from tests and
models.
Including a method to correlate 3-D scratch surfaces.
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What about Polymer fracture?
This is important, but was notconsidered in th is in itial study.
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Material Characterization
Physical Tests and Constitutive Models
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The ZigguratTest Protocol
An efficient all-in-one material testing protocol with these key benefits:
Exercises elastic, inelastic, loading and unloading behavior.
Adds stress relaxation behavior at several strain levels in same test
Can be applied to compression (shown), tension, shear, equibiaxial
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Time
Com
pressiveStrain
Compre
ssiveStress
Compressive Strain
Applied Boundary Conditions Materials Response
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Application of ZigguratTest Protocol to
Polymers with Significant Plasticity
The previous slide presentedan idealizedZiggurat protocol.
Elastomer testing often looks quite
similar to picture on right.
Tests of polymers that exhibit
noticeable inelastic (plasticity)behavior often need a modified
ziggurat protocol.
Key goal of Ziggurat protocol
is to get several relaxation
holds on both the loading and unloading curves. Depending on relaxation behavior of material, time holds might be very
long compared to loading ramps of various strain segments.
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Ziggurat Protocol
(Surlyn 1706/1707)
Key items
Asymmetric strain/time BC
Very long time holds
Difficulty with strain control
(cross-head) vs. laser gage
strain measurements.
Laser measures used for
material calibration.
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0 10 20 30 40 50 60 70
0
50
100
150
0 10 20 30 40 50 60 70
0
50
100
150
Comp. engineering strain (%)
Comp.engineeringstress(MPa)
Comp.e
ngineeringstress(MPa)
Comp engineering strain (%)
a) Strain computed from
cross-head displacementb) Strain computed from
laser gages measuring
specimen height
0 10 20 300
10
20
30
40
50
60
Compeng.strain(%
)
Comp.eng.stress
(MPa)
0 10 20 300
25
50
75
100
125
150
Time (minute)
c) Strain and stress data as a function of time
Strain co mputed from laser gages
measuring specimen height
Idealized Protocol
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Ziggurat Protocol
(Surlyn 1706/1707)
Key items (continued)
Raw data (not shown):
35,000 points, non-uniform t.
Strain data had a small amount of noise
due to optical laser gages.
Data had start-up distortions in the initial
small-strain regime due to small geometry
irregularities in the samples.
Kornucopia utilized to clean-up and re-sample
data prior to MCalibration analysis. This improves both accuracy and efficiency of material law calibration
process.
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Ziggurat Protocol
(Surlyn 1706/1707)
Raw data clean-up steps:
Semi-automatic algorithm separates data into
various Ziggurat segments.
Strain holds and all the other segments
Each segment smoothed to remove noise
Distortion at the beginning of the test was
trimmed and corrected with linear extrapolation.
Each segment resampled
Load/unload segments 100 equal time
points
Strain holds 100 logarithmically spaced pts.
Data clean-up significantly improved the data quality and reduced the dataset
from 35,000 points to less than 1,000 points.
Reusable worksheet allowed for efficient, repeatable, and traceable data
cleanup of all Ziggurat tests for all materials.
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PolyUMod Three Network Model (TNM)
Material data exhibits non-linear viscoplastic response with significant flow
and partial recovery after unloading. Utilize PolyUMod library of user-material models for Abaqus
The Three Network Model is a rheological approach with non-linear
springs and dashpots capable of representing materials.
13
See papersreferences for
details of the
governing
equations
PolyUMod and
MCalibration are
software products
from Veryst
Engineering, LLC.
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PolyUMod Three
Network Model (TNM)
Total stress is given by sum of theCauchy stress tensor in each
network.
Stress response of each of the 3
networks
Hyperelastic Arruda-Boyce
eight-chain model.
The stiffness of network B is taken to evolve with the plastic strain
accumulation in Network A.
The rate of viscoplastic flow of Networks A and B is given by a power law
expression of the driving deviatoric shear stress and temperature, and theflow resistance is taken to be pressure dependent.
This material model exhibits true plasticity despite the presence of the
Network C component. The cause of the plasticity is the energy barrier
for flow that is inherent in the flow equations.
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Calibrating TNMwith Test Data via MCalibration
Utilize Compression with Friction load case,
MCalibration automatically creates an Abaqus FE model of theexperimental test setup and boundary conditions (including friction).
Allows model to closely represents the configuration of actual tests.
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Calibrating TNMwith Test Data via MCalibration
MCalibration performs nonlinear search to find material law parameters that
minimize error between model and actual test data. Several controls and options to allow user influence over process.
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TNMRepresentations of Material Test Data17
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TNMRepresentations of Material Test Data18
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Abaqus Simulation of Scratch
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Abaqus/Explicit Model
to Simulate Scratch
Key features & approach
Explicit model selected to more
robustly handle potential severe
deformations.
100 micron radius scratch tip
rigid surface
Polymer specimen was divided
into two regions (tied together)
Coarse region (500 C3D8R),
Fine region (55,000 C3D8R).
Adaptive re-meshing (by solver) used for fine mesh region.
Mass scaling is used to speed-up the simulation.
More to be said about this shortly.
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Abaqus/Explicit Model to Simulate Scratch
The scratch simulations were performed in three steps:
(example for Surlyn 1706/1707 blend simulation)
1. Indenter pushed vertically down into
material 60 m in 0.05 sec.
2. Indenter moved horizontally 0.6 mm
in 0.15 sec.
3. Indenter unloaded by moving vertically
upward 66 m in 0.05 sec.
Allow material to creep recover
(>20 seconds in the FEA model)
Notes:
Different scratch depths used for other materials
Simulations were performed in displacement control
Physical scratch experiments were performed in load control
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Step 1
Step 2
Step 3
GIFMP4
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http://c/Users/Ted/Documents/Presentations%20and%20Papers/other/ABAQUS/2013_conf/_Presentation/ScratchSimulation_Diehl_etal/FEA_scratch_scratch_1706-1707mix.mp4http://c/Users/Ted/Documents/Presentations%20and%20Papers/other/ABAQUS/2013_conf/_Presentation/ScratchSimulation_Diehl_etal/FEA_scratch_scratch_1706-1707mix.mp48/12/2019 DuPont Diehl 5222013
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Abaqus/Explicit Model to Simulate Scratch
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Mass Scaling
Issues with
Explicit Model
Compared to Courant stability ofmodel, scratch time is long,
especially for creep recovery.
Use staged mass scaling to
speed-up simulation.
Caused largeoscillations
Minimized by mass-
proportional Rayleigh
damping.
Segmented filtering by
Kornucopia shows noticeable
discontinuity caused by staged
mass-scaling.
Recent analysis shows we
were just too aggressive!
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0 5 10 15 20 250.06
0.04
0.02
0
0 1 2 3 40.06
0.04
0.02
0
0 1 2 3 4
0.06
0.04
0.02
0
0 5 10 15 20 250.06
0.04
0.02
0
Time (sec)
Displacement(mm)
D
isplacement(mm)
Displacement(mm)
Displacement(mm)
Noticeable
Discontinuity
Stage 2, t=0.3msec
Stage 1, t=0.029msec
Stage 3
t=3.0msec
All stages, No smoothing
Stage 1, smoothed
Stage 2, smoothed
Stage 3, smoothed
Stage 1, smoothed
Stage 2, smoothed
Stage 3, smoothed
Creep recovery of single node
in scratch region
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Physical Scratch Tests and
Correlations to Abaqus Simulations
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How to Compare Experiments and FEA???
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Visual image
Zygo optical scan
Abaqus top
surface
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Typical Physical Scratch Tests
Zygo 3-D optical surface profile (Surlyn 9950, 1.5N Load, ~ 1 day post scratch)
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Many
drop-outs
in data
Limited
3-D
viewing
Limited
data
analysis
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Typical Physical Scratch Tests
Zygo 3-D optical surface profile (Surlyn 9950, 1.5N Load, ~ 1 day post scratch)
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Improvements
after Data
Healing via
Kornucopia
and Mathcad
0 100 200 300 400 50030
40
50
60
Healed
Raw
Distance (micron)
Z(micron)
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Further Scratch Test Data Analysis
(PMMA ~ 1 day post scratch)
Collapsing
many surface
cross-sectionsinto a single
average
cross-section
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Further Scratch Test Data Analysis
(Surlyn 1706/1707 blend ~ 1 day post scratch)
Collapsing
many surface
cross-sectionsinto a single
average
cross-section
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Similar Post-Process of FEA Scratch Results
(Surlyn 1706/1707 blend ~ 25 seconds post scratch)
Collapsing
many surface
cross-sectionsinto a single
average
cross-section
FEA post-processing
required addit ional
surface remapping
of results to enable
slicing
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Final Cross-Section Correlations
Average scratch cross-sections show that Abaqus/Explicit models using
PolyUMod TNM material model correlates well across several diverse
materials.
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Open Issues
Polymer fracture
Difficult to model as many fracture locations can occur.
Need to assess viable routes for this very complicated.
Creep recovery over long t ime periods
The amount of mass scaling we used was shown to be a bit aggressive.
Lesser mass scaling would be better, but makes even longer run
times.
Switching from Abaqus/Explict back to Abaqus/Standard not possible
because of adaptive meshing approach used.
We only computed for 25 seconds of creep recovery physical tests were
for 1 day!
A DSP concept Frequency Warping.
Nonlinearly warp time-domain implication of visco material constants
so as to compress, in time, the short and long scale meanings of
constants. Similar to filter warping or structural frequency warping.
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Conclusions
Demonstrated that accurate models of Polymeric Scratch are possible.
Combined:
Detailed physical material tests (Ziggurat Protocol)
PolyUMod Three Network (material) Model for Abaqus
MCalibration material calibration
Abaqus/Explicit using mass scaling and adaptive re-meshing Kornucopia data analysis
Methodology developed and demonstrated using three different polymeric
materials, two DuPont Surlyn grades and PMMA.
Demonstrated efficient method to compare complex 3-D scratch results
Open issues
Polymer fracture
Improved methods to simulate long creep recovery times
Something to explore Frequency Warping
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