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Mechanical Testing and Characterization of a

Thermoplastic Copolyester based Elastomer

(DSM Engineering Plastics Arnitel® EM400)

J. Bergstrom1, J. Harding2, S. Brown1, G. Freeburn1

1Veryst Engineering, LLC, 47A Kearney Rd, Needham MA, 02494 2DSM Engineering Plastics, Inc, 2267 West Mill Rd, Evansville IN, 47720

5/5/2015

Jorgen Bergstrom, Ph.D.

Ph.D. in polymer modeling from M.I.T.

Principal Engineer at Veryst Engineering

Former Lecturer in the department of Mechanical Engineering at M.I.T.

Creator of PolymerFEM.com, PolyUMod®, and MCalibration®

Areas of expertise: Polymer mechanics

Polymer failure analysis

Polymer testing and characterization

Finite element simulations of polymers

User-material model development

5/7/2015 Confidential 2

Veryst Engineering

Founded 2006

www.veryst.com

Located in Needham, MA (20 minutes west of Boston)

Ten Full-Time Engineers

7 PhD’s (three former MIT faculty)

15,000 ft2 office with 5000 ft2 lab

Veryst mission is to use fundamental science to solve

complex design, manufacturing, and failure analysis

problems

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Our Agenda

We aim to be the best in the world for testing, modeling

and simulation of nonlinear, coupled problems

Modern engineering materials are not simple:

Yield, cycle, creep, relax, wear, anisotropic, degrade

Manufacturing changes properties

Operate at different temperatures and rates

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Our Products and Services

Modeling and Simulation

Mechanical Testing

Material testing (tension, compression, creep etc., low and high

strain rate and temperature)

Custom testing

Root Cause Failure Analysis

Educational: Online and in-person classes

Software products/libraries: PolyUMod® and

MCalibration®

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Outline

Thermoplastic elastomers are widely used in the

automotive industry for their combination of strength,

processing characteristics, and performance

Many modeling challenges exist as this class of

materials display properties of both rubber and plastic

This presentation will showcase testing and modeling

techniques for these complex materials

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Goal of study: Experimentally examine a TPE-E and

calibrate a suitable material model

Arnitel® Copolyester Elastomers

Arnitel® is a family of copolyester based elastomers manufactured by DSM. These copolyesters combine the performance characteristics of elastomers with the processing features of thermoplastics, providing benefits in processing and productivity. Arnitel copolyesters consist of alternating hard and soft segments. The hard segments are crystalline polybuthylene-terephthalate (PBT); the soft segments are amorphous polyesters or polyethers. The ratio of soft to hard segments and the composition of the soft segment can be varied, thus creating a wide range of properties.

Main characteristics of Arnitel®

Excellent flexural fatigue endurance

High impact strength, even at sub-zero temperatures

High tear and abrasion resistance

Good resistance to chemicals and weathering

Extremely high load-bearing capability relative to other elastomers

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Arnitel® Copolyester Elastomers

Arnitel® copolyester elastomers are available with a hardness range from 25 to 74 Shore D. In addition, grades suited for injection molding, extrusion and blow molding are also standard.

The range of available Arnitel grades covers a broad variety of applications where flexibility, durability, high and low temperature performance, and/or mechanical strength are required.

Arnitel® in Automotive Typical Applications:

Air Ducts

Air Bag Covers

CVJ Boots and Bellows

Convoluted Tubing

Hydraulic Hoses

Body Plugs

Wire and Cable Housings

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Test Specimens

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Left: ASTM D638 Type 4 dogbone specimen, ASTM D638 Type 5 dogbone specimen,

compression specimen (12.7mm diameter), ball drop specimen (14.2mm diameter)

Right: Specimens painted with “speckle” pattern for Digital Image Correlation (DIC)

Uniaxial Tension and Compression

Slow and

intermediate

strain rates

(0.001/s to

0.1/s)

Strain is

measured using

Digital Image

Correlation

(DIC)

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3.0

2.8

2.6

2.4

2.2

2.0

1.8

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0

Eng. Strain

High Rate Tension and Compression

Custom drop tower system

For determining the tensile

and compressive response of

all polymers, including films

The strain is measured using

a high speed camera and

Digital Image Correlation

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Tension vs Compression

Important to test

both tension and

compression

since the

response can be

pressure

dependent

Arnitel is almost

symmetrical

between tension

and compression

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Tension

Compression

Monotonic Uniaxial Tension

Tension at different strain

rates

The yield stress

increases with strain rate

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110/s

0.001/s

Uniaxial Tension – Stress Relaxation

Uniaxial tensile

loading at

different strain

rates

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Uniaxial Tension – Stress Relaxation

Stress relaxation

response

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Uniaxial Tension – Stress Relaxation

Stress relaxation

response

The amount of

relaxation

increases with

strain

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Uniaxial Tension – Stress Relaxation

The material

response is not

linear

viscoelastic

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No

rmal

ize

d S

tre

ss

Increasing strain

High Rate Uniaxial Compression

The compressive load is applied in 1 ms

Max applied true strain: -0.8

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Uniaxial Compression

The compressive

response at -700/s

is 2X higher than at

0.1/s

The higher stiffness

and yield strength

is important for

crash simulations

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Poisson’s Ratio

The Poisson’ s

ratio was

determined

from the DIC

data

Average value:

0.47

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Material Models

Arnitel is a highly non-linear viscoplastic material:

The stress-strain response is non-linear

The stress increases with strain rate

The stress relaxes if the strain is held constant

The material recovers during unloading

The material is temperature dependent

Predicting these behaviors in a Finite Element model

requires an advanced material model:

Bergstrom-Boyce (BB) model (native in Abaqus, ANSYS, LS-

DYNA)

Parallel Network (PN) model (from the PolyUMod library)

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Material Models

Material models were

calibrated using the

MCalibration®

software from Veryst

Engineering

The MCalibration

software can

calibrate all Abaqus

and ANSYS material

The MCalibration

software is available

from Veryst

Engineering

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Calibration: BB Model

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Average error in

predictions: 6% Average error in

predictions: 18%

Parallel Network (PN) Model

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The Parallel Network (PN) model is a micromechanism inspired modeling

framework that is commercially available from Veryst Engineering

Arruda-Boyce

Eight-Chain

Model

Network A:

• Neo-Hookean hyperelastic

• Non-linear viscoplastic (power-

law flow with damage evolution)

Network B:

• Neo-Hookean hyperelastic

• Non-linear viscoplastic (power-

law flow with damage evolution)

Parallel Network (PN) Model

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Average error in

predictions: 5% Average error in

predictions: 4%

Validation Testing: Ball Impact

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Custom ball impact test system

High strain rate compressive response of soft polymers

Can test very thin films and bulk materials

Validation Testing: Ball Impact

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Validation Testing: Ball Impact

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FE Simulation of the 30 in ball

impact using the PN model

The max strain rate is -2500/s

Validation: BB Model

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Average error of

FE prediction: 22%

Validation: PN Model

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Average error of

FE prediction: 8%

Conclusions

Arnitel® EM400 is a highly non-linear thermoplastic

copolyester based elastomer with excellent mechanical

properties

The material undergoes stress relaxation and creep

under static load

The yield stress and hardening increases with strain rate

These behaviors can be accurately modeled in a FE

simulation if a suitable material model is calibrated

The Parallel Network model accurately captures all

experimentally observed behaviors

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