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THE EFFECT OF STRAIN RATE ON

TENSILE PROPERTIES AND FRACTURE STRAIN

Gang Huang, Benda Yan and Hong Zhu

ArcelorMittal USA LLC

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Outline

• Background

• Purpose of the work

• Materials tested

• Results

– Strain rate effect on tensile properties

– Strain rate effect on fracture strain

• Conclusions

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Background

• Effect of strain rate on tensile properties and fracture behavior

is of critical importance to crash performance.

• There are two major issues for tensile testing at high strain

rates

– Severe load oscillation deteriorates quality of stress-strain

curve

– Existing strain measurement devices (strain gage, LVDT, or

Doppler extensometer) are not accurate and reliable at high

strain rates

• Data on the effect of strain rate on tensile properties/fracture

strain is scarce due to testing difficulties

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Purpose of Work

• Improve data quality of high strain rate tests by

using Digital Image Correlation (DIC) technique

• Understand strain rate effect on fracture strain

using DIC

– Tensile fracture

– Cut edge fracture

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Experiment

• Tensile test at high strain rate

– MTS servo-hydraulic testing machine

•20kip load capacity

•Maximum speed 5.5 m/s

• 3-point bending of hat section beam

– Quasi-static: 100kip MTS testing machine

– Dynamic: drop tower mass 380 kg, speed 8m/s

• Digital Image Correlation (DIC)

– Full field non-contact

deformation measurement

– Accurate output of

displacement and strain

Time t Time t + Δt

Time t + 2Δt

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High Strain Rate Test

High strain rate test with DIC system

Data acquisition

Image acquisition

Light source

High speed camera

Load washer

Slack adapter

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High Strain Rate Test

Top and bottom

fixtures Specimen

Load cell

Piezoelectric

load washer

Slack adapter

Actuator

Strain gauge for

load measurement

Random speckles for

strain measurement

Schematic of testing setup High strain rate specimen

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High Strain Rate Test – Load Signal Improvement

0

1000

2000

3000

4000

5000

0 1 2 3 4 5 6 7

Displacement (mm)

Lo

ad

(N

)

Load cell

Load washer

Improvement with strain gage

Strain rate: 226/s

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

0 1 2 3 4 5 6

Displacement (mm)

Lo

ad

(N

)

Load cell

Load washer

Improvement with strain gauge

Strain rate: 540/s

Load-displacement curves

of DP590 at 220/s

Load-displacement curves

of DP780 at 550/s

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Materials

Material Thickness

(mm) Coating

Yield

Strength

(MPa)

Tensile

Strength

(MPa)

Uniform

Elongation

(%)

Total

Elongation

(%)

Local

Fracture strain

(%)

EDDS 1.54 Uncoated 123 284 27.0 55.0 110

DP590 CR 1.4 Uncoated 347 612 17.0 28.0 73

590R 1.4 GA 485 618 14.3 22.1 66

DP780 1.4 EG 527 822 11.1 18.0 58

TRIP780 1.4 GA 503 892 15.0 21.3 46

DP980 1.4 Uncoated 636 1043 8.5 13.4 46

DP980 1.4 GA 645 1002 8.4 13.8 45

Tensile Properties

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Stress-strain Curves

0

100

200

300

400

500

600

700

800

0 0.1 0.2 0.3 0.4 0.5

Engineering strain

En

gin

eeri

ng

str

ess (

MP

a)

DP590-0.02/s

DP590-1/s

DP590-10/s

DP590-34/s

DP590-226/s

DP590-560/s

0

200

400

600

800

1000

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4

Engineering strain

En

gin

eeri

ng

str

ess (

MP

a)

DP780-0.008/s

DP780-1/s

DP780-10/s

DP780-34/s

DP780-231/s

DP780-540/s

Engineering stress-strain

curves of DP590

Engineering stress-strain

curves of DP780

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Stress-strain Curves

0

100

200

300

400

500

600

0 0.05 0.1 0.15 0.2 0.25 0.3

Engineering strain

En

gin

ee

rin

g s

tre

ss

(M

Pa

)

EDDS-0.02/s

EDDS-1/s

EDDS-10/s

EDDS-34/s

EDDS-138/s

EDDS-229/s

EDDS-570/s

Engineering stress-strain curves of EDDS

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Strain Rate Effect on Tensile Properties

Tensile properties 0.02 /s 1 /s 10 /s 34 /s 138 /s 229 /s 570 /s

UTS (MPa) 286 307 338 354 371 400 450

UE 0.271 0.251 0.2 0.214 0.233 0.217 0.195

TEL (5mm GL) 0.958 0.917 0.983 NA 0.942 0.9833 NA

Tensile properties 0.02 /s 1 /s 10 /s 34 /s 226 /s 560 /s

UTS (MPa) 630 660 672 690 696 723

UE 0.17 0.168 0.175 0.169 0.175 0.174

TEL (5mm GL) 0.547 0.467 0.517 0.55 0.517 0.533

Tensile properties 0.02 /s 1 /s 10 /s 34 /s 231 /s 540 /s

UTS (MPa) 820 833 857 862 867 880

UE 0.114 0.124 0.112 0.125 0.126 0.112

TEL (5mm GL) 0.408 0.4 0.392 0.4 0.4 0.417

Tensile properties of EDDS at different strain rates

Tensile properties of DP590 at different strain rates

Tensile properties of DP780 at different strain rates

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Strain Rate Effect on Tensile Strength

Relation between tensile strength and strain rate

200

300

400

500

600

700

800

900

0.01 0.1 1 10 100 1000

Strain rate (1/s)

Te

ns

ile

str

en

gth

(M

Pa

)

EDDS

DP590 CR

DP780 EG

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Strain Rate Effect on Elongations

Uniform elongation and total elongation

with respect to strain rate

0

0.2

0.4

0.6

0.8

1

1.2

0.0001 0.001 0.01 0.1 1 10 100 1000

Strain rate (1/s)

Elo

ng

ati

on

EDDS-TEL

DP590 CR-TEL

DP780 EG-TEL

EDDS-UE

DP590 CR-UE

DP780 EG-UE

Total

Elongation

Uniform

Elongation

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Strain Rate Effect on tensile fracture strain

Material Quasi –

static 1/s 10/s 120/s 220/s 500/s

Standard

deviation

EDDS 1.13 1.12 1.1 1.09 1.13 NA 0.018

DP590 CR 0.738 0.681 0.711 0.741 0.734 0.691 0.025

GA 590R 0.668 0.6295 0.661 0.6715 0.6446 0.5718 0.037

DP780 0.602 0.615 0.619 0.611 0.598 0.564 0.02

DP980 LC 0.4819 0.4762 0.4908 0.5202 0.5193 0.5174 0.02

Fracture strain of 5 materials at different strain rates

An animation showing the

speckles change of GA

590R at 200/s

(camera speed 4000

frame/s)

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Strain Rate Effect on fracture strain

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0.01 0.1 1 10 100 1000

Strain rate (1/s)

Fra

ctu

re s

tra

in

EDDS

DP590 CR

590R GA

DP780 CR

DP980 EG

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3-point Bending Beam Tests – Quasi-static

800

20

80 60

Test setup of quasi-static beam test and specimen

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3-point Bending Beam Tests – Dynamic

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Strain Rate Effect on Edge fracture strain

Material

Edge Fracture Strain

Notch from water-jet cutting Notch from punching

Beam-dynamic*

(~ 65/s)

Beam- quasi-static

(~ 0.46/s)

Beam- dynamic*

(~ 65/s)

Beam- quasi-static

(~ 0.46/s)

DP980 CR 0.35 0.39 0.36 0.34

DP980 GA 0.28 0.32 0.3 0.29

TRIP 780 GA 0.38 0.37 0.27 0.2

GA 590R 0.45 0.57 0.45 0.5

*Note: The fracture strain of

dynamic beam was determined

using FEA combined with experiment

(fracture strain of quasi-static beam

was directly measured using DIC)

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Conclusions

• Data quality of high strain rate testing is improved by

applying strain gauge in grip section and DIC technique.

• Strain rate has little effect on uniform elongation and total

elongation for Advanced High Strength Steels (AHSS) such

as DP590 and DP780.

• In agreement with existing data, the tensile strength

increases with strain rate.

• Strain rate exhibited insignificant effect on fracture strain

for the tested materials.

• DIC can be used as an effective tool in strain

measurement for plasticity and fracture studies for high

strength steels.