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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.