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Testing for Crash & Safety Simulation

Hubert Lobo

DatapointLabs New York, USA

CAHRS Workshop

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DatapointLabs

Research quality material testing

ISO 17025 production environment

Results in 5 days (48 hour RUSH service)

Web-based quotation & data delivery

Domain expertise in CAE material calibration

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expert material testing

TestPaks® = Materials testing +

CAE material parameter conversion

metal, plastic, foam, rubber, composites…

over 20 CAE software codes

testing

Your CAE

data conversion materials

+

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Topics

A test philosophy for representing rate dependency of materials

Experimental technique including sampling and specimen geometries

Assessment of crash material data quality, expected trends & validation

Specific comments for unfilled and fiber-filled polymers, foams, rubber and metals.

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Getting pertinent properties

Importance of measuring the right property

Artifact free data

Properly designed experiments

eg. not using crosshead displacement

to calculate strain

Traceable data (ISO 17025)

NIST traceable instruments

Certified trained technicians

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Getting the right samples

Spatial variation

Properties vary with location

Forming, stretching, molding…

Environmental variation

Ageing and conditioning

Process variation

Degradation from processing

Recycled materials

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Metals

Relatively well behaved

Models designed to match behavior

Challenges lie with post yield non-

Mises failure envelopes

Scaling of yield surface with strain rate

Work of Nakajima, Dubois, Hooputra

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Plastics

Not well behaved

Models not designed for plastics crash

simulation

Complex models are expensive

Can we develop best practices for

adapting common models to plastics

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Plastics Behavior - Basics

Non-linear elasticity

Elastic limit well below

classical yield point

Significant plastic

strains prior to yield

Post-yield with necking

behavior 0

5

10

15

20

25

30

35

0.0 5.0 10.0 15.0 20.0

Engineering Strain (%)

En

gin

ee

rin

g S

tre

ss (

MP

a)

data

Plastic Point

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Plastics Rate Effects

Modulus may depend on rate

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Plastics Rate Effects

Fail strain may be rate dependent

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Material Testing

Instron servo-hydraulic UTM

Dynamic load cell

Test at 0.01, 0.1, 1. 10, 100/s

strain rates

Temperatures: -40 to 150C

tens_slow.mpg

Tens.mpg

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Test Specimens

ASTM D638Type V

Preparation

CNC from plaque

CNC from part

Molded

Variability

processing

orientation

thickness

gate region

E1

E2

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Modeling simple ductile plastics

Modulus is not rate dependent

Large strains to failure

Post-yield necking

Plasticity curves vary with strain rate

Failure strain independent of strain rate

LS-DYNA, ANSYS, ABAQUS,

PAMCRASH

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Choosing EMOD

0

5

10

15

20

25

30

35

0.0 5.0 10.0 15.0 20.0

Engineering Strain (%)

En

gin

ee

rin

g S

tre

ss (

MP

a)

data

Plastic Point

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Post-yield with necking (Deihl)

0

10

20

30

40

50

60

70

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

Strain mm/mm

Str

ess

MP

a

UTM1-39.62-1

UTM1-39.62-2

yield point

necking starts

neck propagation

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Fail Limitations

When FAIL f(strain rate)

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0

5

10

15

20

25

30

35

40

1.E-02 1.E-01 1.E+00 1.E+01 1.E+02

Strain Rate(/s)

Te

nsile

Str

eng

th (

MP

a)

data

Cowper Symonds

Eyring

Modeling Rate Dependency

Cowper Symonds

Does not correlate

well with plastics

rate dependency

LCSR

Capture model

independent

behavior

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Eyring Model

Eyring Model

Yield stress v. log strain rate is

linear

Best form for plastics

Fit yield stress v. log strain rate

data to Eyring equation

Can submit to LSDYNA MAT24 as

table using LCSR

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MAT24 validation

0

10

20

30

40

50

60

0 0.05 0.1 0.15 0.2 0.25 0.3

Strain (mm/mm)

Str

ess (

MP

a)

LS-DYNA Simulation

Tensile Experiment

MAT 24 Model

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Brittle plastics

Modulus is rate dependent

Small strains to failure

Brittle failure

Failure strain decreases with

increasing strain rate

LSDYNA MAT19

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Methodology for MAT 19

Determine elastic limit at

quasi-static strain rate

Use elastic limit for von-

Mises yield

Define failure

failure stress v. strain

rate table

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Ductile-brittle transitions

Non-linear behavior

Failure depends on

strain rate

Models

LS-DYNA MAT89

PAMCRASH 103

Abaqus *ELASTIC

*PLASTIC,Rate

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Fiber Filled Plastics

Digimat MX

Material model reverse engineered

from standard experiment

Perform injection-molding simulation

Apply Digimat material model to

transfer data to crash simulation

Crash model has spatially oriented

properties

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Basic Digimat TestPak Protocol

Mold 100X300X3.16mm plaques

Edge gated on 100 mm end

Long flow length

Fully developed flow

Highly fiber orientation

Cut test specimens by CNC

5 specimens each (0°, 90° )

Obtain true stress-strain data

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Advanced Models

MATSAMP (LS-DYNA)

Standard rate dependent model

Add non-mises failure envelope

Compression

Shear

Add triaxiality

Post yield transverse strain

Add unloading

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Pros and Cons

Better failure envelope modeling

Greater cost

More complex model

Greater simulation accuracy in

difficult cases

Cost-benefit not certain for general

use

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Foams

Different deformation modes

Crushable

Elastic with or without damage

Visco-elastic

Large volumetric strain component

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Effect of Poisson’s Ratio = 0

Material compacts by eliminating air

No lateral deformation

Poisson’s Ratio -> 0

Axial strain volumetric strain

True for

open cell foams

crushable foams

May not be true for

closed cell foams

elastomeric foams

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Typical Stress-Strain Data

0.000

0.005

0.010

0.015

0.020

0.025

0.030

0.035

0.040

0.045

0.050

0 20 40 60 80 100

Engineering Strain (%)

En

gin

ee

rin

g S

tre

ss (

MP

a)

Zone 1 Zone 2 Zone 3

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Test Strategy

Compressive stress-strain

5 decades of strain rate

.01, .1, 1, 10, 100 /s

Temperatures

-100 to 150C

Optional tests

Tensile (for cut-off stress)

Shear (as required)

Hisp_comp.mpg

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Test Instruments

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PU Foam-stress strain

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1Strain (mm/mm)

Stre

ss (M

Pa)

0.01/s fit

0.1/s fit

1/s fit

10/s fit

100/s fit

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PU Foam- rate effects

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PU Foam recovery

0.000

0.005

0.010

0.015

0.020

0.025

0.030

0 20 40 60 80 100

Engineering Strain (%)

En

gin

ee

rin

g S

tre

ss (

MP

a)

Load

Unload

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Conclusions

Choice of material model depends on

material

test data

situation complexity

Proper selection = reasonable model

Simple improvements can add power

Validated models represent baseline

Models can be tuned for multi-axial loadings

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