HSE
Health & Safety
Executive
Dynamic tensile properties of
thin sheet materials
Prepared by HSL for the
Health and Safety Executive 2005
RESEARCH REPORT 303
HSE
Health & Safety
Executive
Dynamic tensile properties of
thin sheet materials
John Dutton
HSL
Broad Lane
Sheffield
S3 7HQ.
The European Structural Integrity Society (ESIS) has organised a cross European test programme
involving ten laboratories to assess test techniques for dynamic tensile testing. The results of this work
will be used to develop a European standard for dynamic tensile testing and data analysis. The Health
and Safety Laboratory has been invited to contribute to this project.
The production of dynamic properties of materials is particular important in the modelling of railway
vehicles to improve their crashworthiness. Due to the implications to health and safety, the work was
sponsored by Railway Inspectorate Technical Division and funded by the Health and Safety Executive.
This report outlines the experimental work carried out to determine the dynamic tensile properties of six
materials. Five of the materials were supplied by ESIS and the other was sourced from an extruded
beam taken from a carriage involved in the Ladbroke Grove incident. Tests were carried out at pseudo
static, 40 and 400 strain rates giving a reasonable spread of data. To enable the capture of data at
these strain rates a high-speed data logger was used along with a linescan camera to measure
specimen elongation. Test specimens were manufactured to a design developed by HSL, along with
the test apparatus and technique used.
This report and the work it describes were funded by the Health and Safety Executive (HSE). Its
contents, including any opinions and/or conclusions expressed, are those of the authors alone and do
not necessarily reflect HSE policy.
HSE BOOKS
© Crown copyright 2005
First published 2005
ISBN 0 7176 2949 X
All rights reserved. No part of this publication may be
reproduced, stored in a retrieval system, or transmitted in
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written permission of the copyright owner.
Applications for reproduction should be made in writing to:
Licensing Division, Her Majesty's Stationery Office,
St Clements House, 2-16 Colegate, Norwich NR3 1BQ
or by e-mail to hmsolicensing@cabinet-office.x.gsi.gov.uk
ii
CONTENTS
1
2
Introduction ........................................................................................................................... 1
Experimental ......................................................................................................................... 2
2.1
Material ......................................................................................................................... 2
2.2
Specimen Design........................................................................................................... 2
2.3
Test Rig Design............................................................................................................. 3
2.4
Linescan Camera Development..................................................................................... 3
2.5
Data Logging................................................................................................................. 3
2.6
Strain Gauge Location................................................................................................... 4
2.7
Static Tensile Tests........................................................................................................ 4
2.8
Dynamic Tensile Tests .................................................................................................. 4
2.9
Data Analysis ................................................................................................................ 5
3
Results................................................................................................................................... 6
3.1
Static Tensile Tests........................................................................................................ 6
3.2
Dynamic Tensile Tests at 40 and 400 Strain Rates ....................................................... 6
4
Discussion ............................................................................................................................. 9
4.1
Tensile Tests.................................................................................................................. 9
4.2
Specimen Vibration....................................................................................................... 9
4.3
Linescan Camera Accuracy........................................................................................... 9
5
Conclusions......................................................................................................................... 11
6
References ........................................................................................................................... 12
Appendix 1 - Figures................................................................................................................... 13
Appendix 2 – Tensile Test Graphs.............................................................................................. 31
iii
iv
EXECUTIVE SUMMARY
The determination of tensile properties of a material is particularly important where finite
element analysis is to be used. The incorporation of actual stress/strain data from a material,
improves the accuracy of a model. Determining tensile properties of a material at pseudo static
strain rates is relatively easy to carry out. As the strain rate is increased, problems are
encountered with dynamic artefacts introduced to the data. As the requirement for more detailed
analysis of events e.g. to determine the crash worthiness of railway vehicles, which have
occurred at dynamic speed, increases, the need for material properties at these rates becomes
important. This leads to the requirement to produce a technique that overcomes the problems
encountered at high strain rates.
The European Structural Integrity Society (ESIS) has organised a cross European test
programme involving ten laboratories to assess test techniques for dynamic tensile testing. The
results of this work will be used to develop a European standard for dynamic tensile testing and
data analysis. The Health and Safety Laboratory has been invited to contribute to this project.
The production of dynamic properties of materials is particular important in the modelling of
railway vehicles to improve their crashworthiness. Due to the implications to health and safety,
the work was sponsored by Railway Inspectorate Technical Division and funded by the Health
and Safety Executive.
This report outlines the experimental work carried out to determine the dynamic tensile
properties of six materials. Five of the materials were supplied by ESIS and the other was
sourced from an extruded beam taken from a carriage involved in the Ladbroke Grove incident.
Tests were carried out at pseudo static, 40 and 400 strain rates giving a reasonable spread of
data. To enable the capture of data at these strain rates a high-speed data logger was used along
with a linescan camera to measure specimen elongation. Test specimens were manufactured to a
design developed by HSL, along with the test apparatus and technique used.
The main findings of the test programme are: 1. Tensile properties have been determined for six materials, at three strain rates.
2. As the strain rate was increased the proof stress, ultimate tensile strength, percentage
elongation and energy absorbed increased, in the majority of the materials tested.
3. Averaging the output from two strain gauges attached to the test specimens, produced
data that was significantly cleaner than that on a single gauged specimen.
4. More work is required on data analysis. The use of filtering was found to be
unsatisfactory, whilst the use of curve fitting equations requires further work to
determine its validity.
5. The use of a linescan camera to measure strain at high test rates, produces data that is
reasonably accurate and easy to determine. The use of a faster scanning camera with
improved resolution will further increase the accuracy of data produced.
6. Once the data produced by HSL has been incorporated with that produced by the other
ESIS laboratories, a European standard on dynamic tensile testing and data analysis can
be developed.
v
7. The accuracy of the test results is sensitive to the specimen geometry, alignment of
fixtures, strain gauge position. These areas need further investigation.
vi
1
INTRODUCTION
The tensile properties of a material at pseudo static strain rates are readily available. When
modelling of a component is carried out using for example a finite element package, these
properties are used. If the strain rate applied to the model is increased, static properties could
still be used. To improve the accuracy of modelling of high strain rate events material properties
data are required at a strain rate as close as possible to the event being modelled. With this in
mind the European Structural Integrity Society (ESIS) has started a European inter laboratory
test project, which aims to produce a European standard for dynamic tensile testing and data
analysis.
As this is a novel area of research and has significant health and safety implications, the work
was sponsored by Railway Inspectorate Technical Division and funded by the Health and Safety
Executive. The use of modelling to analyse the crash worthiness of railway vehicles is common.
These analysts may be using material data that is unsuitable to simulate conditions during a
crash. The energy absorption requirements are becoming increasingly stringent in order to
protect passengers in trains and especially in the leading vehicle when trains are travelling at
high speed. Accurate high strain rate material properties data are required to adequately model
deformation at high strain rates and calculate the energy absorbed.
This report outlines the experimental work carried out to determine the dynamic tensile
properties of six materials. Five of the materials were supplied by ESIS and the other was
sourced from an extruded beam taken from a carriage involved in the Ladbroke Grove incident.
Tests were carried out at pseudo static, 40 and 400 strain rates giving a reasonable spread of
data. To enable the capture of data at these strain rates a high-speed data logger was used along
with a linescan camera to measure specimen elongation. Test specimens were manufactured to a
design developed by HSL, along with the test apparatus and technique used.
1
2
2.1
EXPERIMENTAL
MATERIAL
For the test programme six materials were used, five supplied by ESIS and one at the request of
the customer. The ESIS material consisted of two magnesium alloys, two steels denoted as 300
MPa and 500 MPa and one aluminium alloy. The additional material was sourced from an
extruded section removed from a railway carriage involved in the Ladbroke Grove incident. The
material was a 6000 series aluminium alloy. The analysis of the materials is presented in Tables
1 - 3.
Table 1 - Analysis of Aluminium Alloys
Material
ESIS 5000 Series
Aluminium
6000 Series
Aluminium
Fe
Mn
Al
Ti
Mg
Zn
Cr
Si
Cu
0.16
0.43
Bal
0.02
3.11
<0.02
<0.02
0.09
<0.02
0.22
0.09
Bal
0.07
0.46
0.03
0.04
0.74
0.08
Table 2 - Analysis of Steels
Material
ESIS Steel
300 MPa
ESIS Steel
500MPa
C
Fe
P
S
Mo
Mn
Ni
Cr
Si
Cu
V
0.014
Bal
0.006
0.008
<0.02
0.10
0.02
0.02
1.24
0.02
<0.02
0.10
Bal
0.020
0.005
<0.02
0.95
0.03
0.03
0.67
0.02
<0.02
Table 3 - Analysis of Magnesium Alloys
Material
ESIS Magnesium
1.9 mm Thick
ESIS Magnesium
2.2mm Thick
Fe
Mn
Al
Ni
Mg
Zn
Si
<0.01
0.35
4.90
<0.01
Bal
0.05
0.05
<0.01
0.29
8.84
<0.01
Bal
0.72
0.04
The 5000 series ESIS aluminium alloy conforms to BS EN 573-3 chemical analysis for EN AW
5154A. The 6000 series aluminium alloy taken from the railway vehicle conforms to the
analysis for EN AW 6005B. In addition, the magnesium materials were found to be AZ91
(2.2mm Thick) and an AM50 (1.9mm Thick) whilst the steel materials were low and high
strength car body sheets.
2.2
SPECIMEN DESIGN
The design of the specimens was influenced by the size of material supplied by ESIS. The
smallest section of material supplied were the magnesium alloys, which were supplied in strips
~150 mm long by ~ 62mm wide. A typical ‘Dumb Bell’ specimen design as found in the
standard BS EN10002-1 (1), was the geometry chosen. The design used can be seen in Figure 1.
A gripping arrangement was developed which attempted to prevent any deformation occurring
in the grips during the impact event. The use of four holes at each end of the specimen
distributed the stresses present in these regions during testing and so prevented any plastic
2
deformation in these regions. The addition of a clamping force from the grips further reduced
any possibility of deformation occurring in these regions.
2.3
TEST RIG DESIGN
To load the specimens in tension during the impact, the Rosand instrumented test machine used
at HSL employs a fork hitting a plate attached to the bottom of the specimen, as seen in Figure
2. The rig was manufactured from heavy gauge steel to produce a rigid structure and to
minimise its deflection during the impact tests. The design allowed for both the front and back
of the specimen to be visible. This arrangement enabled a linescan camera and a high-speed
video camera to have an unobstructed view of the specimen so that elongation could be
measured during a test.
2.4
LINESCAN CAMERA DEVELOPMENT
A Dalsa linescan camera running at 36 kHz with a 1024 pixel scan resolution was used to
capture an image of the elongation of the specimen during the test. The elongation is displayed
as a black and white image of edges marked on the specimen. To produce these edges, two
gauge lengths were marked on the specimens using strips of self-adhesive aluminium tape,
Figure 3. By painting the regions of the specimen not covered by the aluminium tape black, a
contrast between background and the tape was created which would be visible to the line scan
camera, Figure 4. The image shows the outer and inner markers as white regions and by
measuring the movement at the transition from white to black; the elongation of the specimen
can be determined. A bright source of light accurately focused on the specimen was required to
produce images of acceptable quality. The light source was required to be powered by direct
current to avoid flicker on the image due to mains voltage frequency.
To ensure the line scan camera produced a linear image across its full scan range, a grid was
produced of known increments, which was scanned by the linescan camera. The captured image
was then analysed to calculate the increments between the lines of the grid. These
measurements could then be compared to the known increment values. The camera was found
to be linear within ±1%.
Measurement of elongation from the image produced by the linescan camera, was carried out
using a National Instruments (NI) software package, which had the facility to find the edges of
the gauge lengths, by detecting the transition points from white to black. These points were then
followed line-by-line producing a data set, which contained elongation at known time intervals.
The data created by the NI software was then analysed in a spreadsheet. The strain rate of the
test was determined from the slope of strain vs. time data. A formula to determine displacement
from time was then derived from the data. This formula was then incorporated in a further
spreadsheet, used to analyse the strain gauge data, to produce displacement from the time base
recorded during a test.
2.5
DATA LOGGING
To enable capture data from the strain gauge and the piezo electric load cell during a test, a fast
logging rate was required. An Odyssey data logger was used for the test programme. It was
fully configurable and could log up to 10 million samples per second. Each channel could be
individually configured depending upon the expected voltage input. An input was set up to
enable a trigger signal from the Rosand machine to start the data logging. To ensure no data was
missed, a set amount of data was captured before and after the trigger.
3
2.6
STRAIN GAUGE LOCATION
To determine the best location for the strain gauge to be used during the test programme trials
were carried out on several specimens with five strain gauges located as in Figure 5. Each strain
gauge was logged using the data logger. The data was then analysed to establish the position,
which produced an output sensitive enough to capture the strain with as little impact artefacts
and noise. The centre position was the most appropriate region to capture the loading in the
gauge length of the specimen but the strain gauge became detached from the specimen, due to
bonding of the adhesive, before failure occurred. The two top and bottom off centre line gauges
were found to be too insensitive to the loading of the specimen. The two top and bottom centre
line strain gauges produced outputs which were sensitive to the loading of the specimen. The
bottom centre line strain gauge output was found to be adversely affected by dynamic impact
artefacts to a greater extent than the top strain gauge. It was decided that whilst the top centre
line strain gauge was affected by dynamic artefacts, it was to a lesser extent than the bottom
strain gauge and it was thus the best position to measure the loading of the specimen.
2.7
STATIC TENSILE TESTS
To enable comparison of the dynamic data with slower strain rate tests, tensile tests were carried
out according to the standard BS EN 10002-1 (1). The specimens had a strain gauge attached in
an identical position to those used in the dynamic tests. The output from the strain gauge, test
machine load and displacement was logged during the test. This data was then compared to the
load and displacement recorded during the dynamic tests.
2.8
DYNAMIC TENSILE TESTS
Three tests per material were carried out at 40 and 400 strain rate to give a total of thirty-six
tests. To establish the required impact velocities, initial calculations were made to give a
starting value to use during trial tests on spare material. Any adjustment of the impact velocity
could be made to ensure that no test specimens were wasted. The test setup was not varied
except in the test velocity, which was adjusted to give the 40 and 400 strain rates required. The
two velocities used were 1 and 10 m/s-1.
The 400 strain rate tests were carried out using an accelerated carriage to enable velocities of
approximately 10 m/s-1 to be reached. A mass of 8.8 kg was used which provided over 400
joules of energy during the test.
At the lower strain rate a heavy carriage was used which allowed 146kg of mass to be used. The
additional mass was required to enable enough potential energy to be available to break the
specimens at the low drop height used.
2.8.1
Strain Gauge Calibration
Before each test, two checks were carried out to ensure correct operation of the strain gauge and
amplifier.
2.8.1.1
Amplifier Range Check
During trial tests on spare specimens, the range of the amplifiers was adjusted to allow capture
of the maximum load from each material tested. The initial range chosen was based upon the
full range of the amplifier being equivalent to 4000µe using a calibration resistor. This setting
was found to be too low and a value of 10000 µe full range was selected. All the materials were
tested using this setting.
4
Prior to each test, the range of the amplifier was checked using 5000 µe and 10000 µe
calibration resistors. These checks were logged using the Odyssey data logger.
2.8.1.2
Static Load Calibration
To enable the output from the strain gauge attached to the specimen to be converted to load, a
calibration procedure was carried out. This involved loading the test specimen in a servo
hydraulic test machine to a predetermined load in ninety seconds. The target load varied
depending on the material used. The value was determined from the static tensile test data and
was a value below 40% of the yield load of the material being calibrated. During the loading
ramp, the output from the test specimen strain gauge and test machine load cell was logged
using the data logger. From this data, a graph of strain gauge voltage versus load cell voltage
was produced. A linear fit was made to the graph and from this an equation was produced which
related strain volts to load cell volts. The load ramp was carried out three times to give an
average fit equation.
Each specimen was calibrated in this manner prior to testing. The data was checked before a test
to ensure it was suitable to produce a valid strain output to load conversion.
2.9
DATA ANALYSIS
2.9.1
Static Tensile Tests
The static tensile results were obtained using a programme incorporated into a servo-hydraulic
test machines control software. The programme controlled the load rates and logged the
measurement channels during the test. The software calculated the 0.2% and 0.5% proof stress,
ultimate tensile strength and elongation, once the final specimen dimensions had been input.
2.9.2
Dynamic Tensile Tests
All calculations of 0.2%, 0.5% proof stress and ultimate tensile strength were carried out using
the data captured during the test. Attempts were made to filter the data to remove any dynamic
artefacts. No filter would satisfactorily smooth the data without adversely affecting the data. It
was decided that it was sensible to use curve fits to the raw data to allow further analysis to be
carried out.
An Excel spreadsheet was thus used to convert the raw data to stress/strain data. This data was
then transferred to a graph plotting software, where a range of curve fits was applied to the
whole of the data. The selection of the best curve fit to the raw data was subjective and
depended upon which appeared to produced the best fit to the initial rise of the stress, Figure 6.
This data was then transferred back to the Excel spreadsheet, where a linear fit was constructed
to the initial elastic portion of the curve fit plot. The upper and lower limits of the curve fit were
determined from the initial fit data and yet again were a subjective assessment. Offset lines of
0.002 and 0.005 strain were then added using the linear fit equation produced from the elastic
portion of the graph, examples are shown in Figures 7 and 8. The 0.2% and 0.5% proof stress
values were measured at the point at which the offset lines intersected the curve fit plot.
Elongation was measured directly from both the inner and outer markers on the specimen.
These values of elongation were then compared to the linescan values, to determine the
accuracy of that method of measurement.
5
3
3.1
RESULTS
STATIC TENSILE TESTS
One test was carried out per material at ambient temperature. The tests were carried out at a
strain rate of 0.0011s-1 (shown as 1.1E-3) and the results presented in Table 4.
3.2
DYNAMIC TENSILE TESTS AT 40 AND 400 STRAIN RATES
The tensile properties determined during the test programme are presented in Table 4. The
presence of dynamic artefacts on the captured data required the use of curve fitting software to
enable the yield and UTS values to determine. Appendix A contains traces of all the tests
carried out.
Table 4 – Tensile Properties of Materials Tested
0.5%
Average
0.2%
Strain
Proof
0.2%
Proof
Material
Rate
Stress
Proof
Stress
(Number of
Stress
Specimens)
[MPa]
[MPa]
[MPa]
[s-1]
Aluminium 5000
1.1E-3
111
111
111
Series (1)
Aluminium 5000
40
80-127
104
110-122
Series (3)
Aluminium 5000
400
135-148
143
144-170
Series (3)
Aluminium 6000
1.1E-3
248
248
253
Series (1)
Aluminium 6000
40
192-272
228
237-272
Series (3)
Aluminium 6000
400
287-307
282
276-326
Series (3)
Steel 300 MPa (1) 1.1E-3
183
183
191
Steel 300 MPa (3)
40
303-363
334
362-391
Steel 300 MPa (2)
400
340-393
367
360-437
Steel 500 MPa (1) 1.1E-3
407
407
424
Steel 500 MPa (3)
40
471-492
493
499-508
Steel 500 MPa (4)
400
500-564
526
582-595
Magnesium
1.1E-3
132
132
146
1.9mm Thick (1)
Magnesium
40
89-124
104
110-146
1.9mm Thick (3)
Magnesium
400
159-240
184
170-264
1.9mm Thick (3)
Magnesium
1.1E-3
161
161
186
2.2mm Thick (1)
Magnesium
40
165-199
184
200-233
2.2mm Thick (3)
Magnesium
400
205-267
236
220-283
2.2mm Thick (2)
6
Average
0.5%
Proof
Stress
[MPa]
Ultimate
Tensile
Strength
[MPa]
Average
Ultimate
Tensile
Strength
[MPa]
111
237
237
116
180-188
184
160
168-347
248
253
276
276
253
284-303
292
304
316-354
338
191
377
399
424
503
587
312
284-460
440-480
524
572-612
664-715
312
370
460
524
588
690
146
234
234
122
276-347
315
267
274-364
321
186
229
229
220
265-354
313
252
279-318
299
The percentage strain and energy absorbed to failure were determined and are presented in
Table 5.
Table 5 – Energy Absorbed and percentage Strain Measured
Average
Strain
Strain
Strain
Rate
Material
(Number of Specimens Tested)
[s-1]
[%]
[%]
[J]
Average
Energy
Under
Curve
[J]
Aluminium 5000 Series (1)
1.1E-3
29.6-30.9
30.3
39.9
39.9
Aluminium 5000 Series (3)
40
43.3-48.5
46.5
36.5-43.4
40.3
Aluminium 5000 Series (3)
400
54.2-55.4
54.8
41.3-78.7
59.5
Aluminium 6000 Series (1)
1.1E-3
6.9-8.0
7.5
16.9
16.9
Aluminium 6000 Series (3)
40
12.1-14.0
12.8
20-22
20
Aluminium 6000 Series (3)
400
14.1-18.4
15.8
19-29.6
24.4
Steel 300MPa (1)
1.1E-3
7.2
46.6
25.5
25.5
Steel 300MPa (3)
40
47.4-54.2
49.7
27.9-42.2
35.9
Steel 300MPa (2)
400
48.1-49.4
48.8
30-41.3
35.7
Steel 500 MPa (1)
1.1E-3
31.9
31.9
36.5
36.5
Steel 500 MPa (3)
40
34.8-38.6
36.5
47.4-48.2
47.7
Steel 500 MPa (4)
400
38.0-42.3
40.0
51.5-57.5
54.2
Magnesium 1.9mm Thick (1)
1.1E-3
11.2
11.2
14.5
14.5
Magnesium 1.9mm Thick (3)
40
15.6-18.6
16.9
14.8-20.5
18.4
Magnesium 1.9mm Thick (3)
400
14-17.4
15.9
17.2-22
20
Magnesium 2.2mm Thick (1)
1.1E-3
7.2
7.2
6.9
6.9
Magnesium 2.2mm Thick (3)
40
4-8.4
5.7
5.3-15
9.7
Magnesium 2.2mm Thick (2)
400
4-16.8
9.3
6.2-11
8.6
7
Energy
Under
Curve
3.2.1
Investigation of Specimen Vibration
It became evident during the dynamic tensile tests carried out at 40-strain rate, that some of the
materials exhibited severe ringing on the recorded signal. Checks were carried out to ensure that
the strain gauge amplifier and loading rig were not introducing this interference to the signal.
By examining one of the test traces beyond the failure of the specimen, the interference could
still be seen on the strain gauge output, Figure 9. A simple test was carried out which involved
manually flicking a broken specimen still held in the test grips, to induce it to vibrate. Whilst the
specimen vibrated its strain gauge output was logged, Figure 10. The resultant trace clearly
shows the similar frequency of vibration to that observed on the test traces.
To further investigate this anomaly an additional strain gauge was attached to two specimens, in
the same position as that used in previous tests but on the opposite face. The tests were carried
out as the previous tests but with an additional calibration required for the extra strain gauge. By
averaging out the data captured from the two strain gauges, it was hoped that a more readily
interpretable trace could be produced, Figures 11 - 12.
3.2.2
Accuracy of Linescan Camera
The majority of the tests had an error of between ~-8% to ~+10%. The theoretical minimum
error of the camera, in the test configuration used, was 0.08mm. This equates to approximately a
6% error on the specimen with the least amount of deformation. The largest errors measured
were mainly found on the 2.2mm thick magnesium specimens. The error of the camera ranged
from ~-8% to ~+58%. The majority of the larger errors were associated with materials, which
had the least amount of deformation prior to failure.
8
4
4.1
DISCUSSION
TENSILE TESTS
All materials tested at 40 and 400 strain rates exhibited UTS values higher than the static tensile
test values. The 0.2% proof stress of the majority of the specimens was higher at 40 and 400
strain rates than the static tests. The variation in proof stress’s measured highlights the problems
encountered in determining this property. The ESIS aluminium and magnesium materials were
the most difficult as they did not exhibit clear linear extension. This meant that any line
constructed to the data could effect the measurement of the proof value significantly. The UTS
values were not affected by this and so could be measured with greater accuracy. There is a
need for some further work to identify the most appropriate method of analysing this data.
The increase in proof and UTS was mirrored in the strain and energy to failure measurements
made. This increase can be seen in the stress/strain graphs for the three strain rates, Figure 13.
The graphs of static tests carried out, exhibit a slight dip in load, Appendix 2, Figures A, D, G,
J, M and P. This is due to the transition from extensometer control, used to measure the proof
stress, and position control, which is used to load the specimen to failure.
There is a measured drop in 0.2% proof stress at 40-strain rate, for the aluminium 5000 and
6000 series and the 1.9 mm thick magnesium, Figure 14. The real cause of this is not known but
it could be a material property or an artefact of the proof stress measuring technique. A similar
effect has been found in 5000 series aluminium with respect to flow stress (2). Whilst not the
same property as measured in this report, it does point to the possibility of a strain rate effect on
the mechanical properties of these materials. This anomaly needs further investigation.
The use of curve fitting software to allow determination of tensile properties produced traces
that appeared to give good correlation to the raw data. By ensuring that the fit data followed the
initial rise of the raw data accurately, it was hoped that the resultant data would give the
correlation required. Once HSL’s data has been incorporated with the other ESIS laboratories
data, this should lead to a consistent method of analysing the data from a dynamic tensile test
being developed. The HSL’s test methods and data analysis can then be compared with the
European standard.
4.2
SPECIMEN VIBRATION
The tests carried out at 40 and 400 strain were found to be affected by specimen vibration to
varying degrees. The tests carried out on the ESIS Aluminium and 500 MPa steel, with a strain
gauge attached on either side of the specimen, clearly show this vibration, Figures 11 and 12.
The averaging of the two outputs can be seen to smooth the data significantly. The use of two
strain gauges for future dynamic tensile tests is strongly recommended.
4.3
LINESCAN CAMERA ACCURACY
The error of the linescan camera seems to be poor when the maximum error was approximately
+58%. The theoretical error is approximately 6% but other factors increase this value. The
marking of the inner gauge lengths with adhesive tape was simple to do, but during the tensile
tests, the surface of the specimen deformed leading to a breaking of adhesion between it and the
tape used. As the tape lost adhesion, it would not move with the specimen. This led to a
reduction in measured deflection from the inner gauge length on the linescan image. The outer
gauge lengths did not have this problem as they were out of the deformation region. All the
9
Linescan measurements were made using the outer markers because of the above problem with
the inner markers.
The large inaccuracies observed were mainly on the 2.2mm thick magnesium specimens. Upon
examination of the broken specimens, it was evident that they had bent, Figure 15. There are
two possible explanations. Firstly, the set of 2.2mm thick magnesium specimens were machined
with minor alignment errors, which led to rotation during the impact event, Figure 16. The
linescan camera would capture this rotation, which would increase the deflection measured,
Figure 17. Because the specimens only exhibited small amounts of deformation, this rotation
error led to a large increase in the linescan inaccuracy. A second possible explanation is based
on the material itself. Magnesium has the hexagonal close packed (hcp) crystal structure. During
the rolling of the 2.2mm thick sheet a strong crystalline texture could have been developed. Due
to the restricted slip planes in hcp crystals the texture could affect the tensile failure mode so
that the specimens bent during the high strain rate deformation and fracture. Unfortunately it has
not been possible to explore this hypothesis further within the present contract.
The linescan camera has proved to be a good way of determining specimen deflection. If the
error when testing the 2.2mm thick magnesium specimens was due to its crystal structure and
not directly due to the linescan camera, the camera performance will be judged to have been
even better. In addition, as the performance of cameras improves further the accuracy of the
data will increase. Already the scan frequency of cameras has increased to >70kHz with a pixel
count of 2048.
10
5
CONCLUSIONS
1. Tensile properties have been determined for six materials, at three strain rates.
2. As the strain rate was increased the proof stress, ultimate tensile strength, percentage
elongation and energy absorbed increased, in the majority of the materials tested.
3. The use of averaging the output from two strain gauges attached to test specimens,
produced data that was significantly cleaner than a single gauged specimen was.
4. More work is required on data analysis. The use of filtering was found to be
unsatisfactory, whilst the use of curve fitting equations requires further work to
determine its validity.
5. The use of a linescan camera to measure strain at high-test rates, produces data that is
reasonably accurate and easy to determine. The use of improved cameras will further
increase the accuracy of data produced.
6. Once the data produced by HSL’s has been incorporated with that produced by the other
ESIS laboratories, a European standard on dynamic tensile testing and data analysis can
be developed.
7. The accuracy of the test results is sensitive to the specimen geometry, alignment of
fixtures, strain gauge position. These areas need further investigation.
11
6
REFERENCES
1. Metallic Materials – Tensile Testing – Part 1: Method of test at ambient temperature.
BS EN 10002-1: 2001.
2. Apps P, Dynamic properties of aluminium alloys literature review. Health and Safety
Laboratory report, HSL MM/04/23, 2004.
12
APPENDIX 1 - FIGURES
13
50mm
10mm
50mm
150mm
50mm
30mm
Figure 1- Specimen Design
14
Impact Forks
Piezo Electric Load Cell
Strain Gauge
Specimen
Loading Points
Bottom of
Specimen
Figure 2 – Front View of Test Rig
15
Top of Specimen
~50mm
Outer Gauge
Length
Aluminium
Tape
Marking
Gauge
Lengths
~25mm
Inner Gauge
Length
Figure 3 – Marking of Gauge Lengths for Linescan Camera
16
Outer Gauge Length (~50mm)
Inner Gauge Length (~25mm)
Approximate
Start of Impact
Top of
Specimen
Transition from Black to White
detected by NI Software. These
Edges are then tracked
Direction of Elongation
Figure 4 – Image Captured by Linescan Camera During Test
17
Time
[ms]
Top
Approximate
Position of
Strain Gauge
Bottom
Figure 5 – Location of Strain Gauges
18
400
Engineering Stress [MPa]
300
Raw Data
Fit Data
200
Fit to this region was
criteria for acceptance of
curve fit
100
0
0.00
0.05
Engineering Strain [e]
Figure 6 – Curve Fit to Data showing Fit to Initial Rise of Stress
19
400
350
Fit Data
Raw Data
Engineering Stress [MPa]
300
250
0.5% Proof Stress
200
0.2% Proof Stress
150
100
50
0
0.00
0.05
0.10
Engineering Strain [e]
Figure 7 – Curve Fit to Data Allowing Proof Stress to Be Measured (Aluminium 6000 Series 40 Strain Rate)
20
0.15
500
400
Engineering Stress [MPa]
Raw Data
Fit Data
300
0.5% Proof Stress
0.2% Proof Stress
200
100
0
0.0
0.1
0.2
Engineering Strain [e]
Figure 8 – Curve Fit to Data Allowing Proof Stress and UTS to be measured (Aluminium 6000 Series 400 Strain Rate)
21
0.3
4000
3500
3000
Strain Gauge [N]
2500
2000
Approximate Point of Failure
1500
1000
Vibration After Failure
500
0
6
8
10
12
14
16
18
20
22
24
26
28
-500
-1000
Time [ms]
Figure 9 – ESIS Steel 300 MPa Tested at 40 Strain Rate with ‘Ringing’ on Signal during and After Failure
22
30
32
0.3
0.2
Strain Gauge [V]
0.1
0
150
155
160
165
170
175
180
-0.1
-0.2
-0.3
Time [ms]
Figure 10 – Vibration Produce by Flicking Specimen
23
185
190
195
200
7000
Average
Back Gauge
6000
5000
Front Gauge
Load [N]
4000
3000
2000
1000
0
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
Time [ms]
Figure 11 - ESIS Steel 500 MPa Specimen Tested at 40 Strain Rate with Two Strain Gauges Attached
24
10.0
4500
Average
Back Gauge
4000
3500
Load [N]
3000
2500
Front Gauge
2000
1500
1000
500
0
0.0
2.0
4.0
6.0
8.0
10.0
12.0
Time [ms]
Figure 12 - ESIS Aluminium Specimen Tested at 40 Strain Rate with Two Strain Gauges Attached
25
14.0
400
350
Dynamic Test (400 Strain)
300
Stress [MPa]
250
Dynamic Test (40 Strain)
Static Test
200
150
100
50
0
0
0.05
0.1
0.15
0.2
Strain [e]
Figure 13 – Comparison of Tensile Tests at Different Strain Rates (Aluminium 6000 Series)
26
0.25
400
Aluminium 6000 Series
Aluminium 5000 Series
Magnesium 1.9mm Thick
Stress [MPa]
300
200
100
0
1.0E-04
1.0E-03
1.0E-02
1.0E-01
1.0E+00
1.0E+01
-1
Strain Rate [s ]
Figure14 – Variation in proof stress with increase in strain rate
27
1.0E+02
1.0E+03
Figure15 – Specimen Deformation (2.2mm Thick Magnesium)
28
Specimen Misalignment
leads to impact at this side
first
Figure16 – Drawing of Specimen Rotation during Dynamic Tensile Test
29
Rotation at this point will lead to an
increase in deflection observed by
the linescan camera
Inner Gauge Length
Edges Tracked in
Linescan camera Image
Figure17 – Schematic of Specimen Rotation during Impact
30
APPENDIX 2 – TENSILE TEST GRAPHS
31
400
Engineering Stress [MPa]
300
200
100
0
0.00
0.10
0.20
0.30
0.40
0.50
Engineering Strain [e]
Figure A – Static Tensile Test on Aluminium 5000 Series
32
0.60
0.70
400
Engineering Stress [MPa]
300
Test 3
Test 2
200
100
Test 1
0
0.00
0.10
0.20
0.30
0.40
0.50
Engineering Strain [e]
Figure B - Tensile Test at 40 Strain rate on 5000 Aluminium
33
0.60
0.70
400
Test 3
350
Test 1
Engineering Stress [MPa]
300
250
200
150
100
Test 2
50
0
0.00
0.10
0.20
0.30
0.40
0.50
Engineering Strain [e]
Figure C - Tensile Test at 400 Strain rate on 5000 Aluminium
34
0.60
0.70
450
400
350
Engimeering Stress [MPa]
300
250
200
150
100
50
0
0
0.05
0.1
Engineering Strain[e]
Figure D – Static Tensile Test on Aluminium 6000 Series
35
0.15
0.2
450
400
Engineering Stress [MPa]
350
Test 2
300
Test 3
250
Test 1
200
150
100
50
0
0.00
0.05
0.10
Engineering Strain [e]
Figure E - Tensile Test at 40 Strain rate on 6000 Aluminium
36
0.15
0.20
450
Test 1
400
Test 2
350
Test 3
Engineering Stress [MPa]
300
250
200
150
100
50
0
0.00
0.05
0.10
Engineering Strain [e]
Figure F - Tensile Test at 400 Strain rate on 6000 Aluminium
37
0.15
0.20
600
Engineering Stress [MPa]
500
400
300
200
100
0
0.00
0.10
0.20
0.30
0.40
Engineering Strain [e]
Figure G – Static Tensile Test on 300MPa Steel
38
0.50
0.60
600
Test 3
Engineering Stress [MPa]
500
400
300
Test 1
200
Test 2
100
0
0.00
0.10
0.20
0.30
0.40
Engineering Strain [e]
Figure H - Tensile Test at 40 Strain rate on 300 MPa Steel
39
0.50
0.60
600
Test 2
Test 4
500
Engineering Stress [MPa]
400
300
200
100
0
0.00
0.10
0.20
0.30
0.40
Engineering Strain [e]
Figure I - Tensile Test at 400 Strain rate on 300 MPa Steel
40
0.50
0.60
700
Engineering Stress [MPa]
600
500
400
300
200
100
0
0.00
0.10
0.20
0.30
Engineering Strain [e]
Figure J – Static Tensile Test on 500MPa Steel
41
0.40
0.50
700
Test 1
600
Test 2
Engineering Stress [MPa]
500
Test 3
400
300
200
100
0
0.00
0.10
0.20
0.30
Engineering Strain [e]
Figure K - Tensile Test at 40 Strain rate on 500 MPa Steel
42
0.40
0.50
1000
Test 3
900
800
Engineering Stress [MPa]
700
600
Test 2
500
Test 4
400
300
200
Test 1
100
0
0.00
0.10
0.20
0.30
Engineering Strain [e]
Figure L - Tensile Test at 400 Strain rate on 500 MPa Steel
43
0.40
0.50
400
350
Engineering Stress [MPa]
300
250
200
150
100
50
0
0.00
0.05
0.10
0.15
Engineering Strain [e]
Figure M – Static Tensile Test on 1.9mm Thick Magnesium
44
0.20
0.25
400
Test 3
350
Test 2
Engineering Stress [MPa]
300
250
Test 1
200
150
100
50
0
0.00
0.05
0.10
0.15
Engineering Strain [e]
zx
Figure N - Tensile Test at 40 Strain rate on 1.9mm Thick Magnesium
45
0.20
0.25
400
Test 4
Test 1
350
Engineering Stress [MPa]
300
250
Test 3
200
150
100
50
0
0.00
0.05
0.10
0.15
Engineering Strain [e]
Figure O - Tensile Test at 400 Strain rate on 1.9mm Thick Magnesium
46
0.20
0.25
400
350
Engineering Stress [MPa]
300
250
200
150
100
50
0
0.00
0.05
0.10
Engineering Strain [e]
Figure P – Static Tensile Test on 2.2mm Thick Magnesium
47
0.15
400
Test 3
350
Test 2
Engineering Stress [MPa]
300
250
Test 1
200
150
100
50
0
0.00
0.05
0.10
Engineering Strain [e]
Figure Q - Tensile Test at 40 Strain rate on 2.2mm Thick Magnesium
48
0.15
400
Test 3
350
Engineering Stress [MPa]
300
250
Test 1
200
150
100
50
0
0.00
0.05
0.10
Engineering Strain [e]
Figure R - Tensile Test at 400 Strain rate on 2.2mm Thick Magnesium
49
0.15
Printed and published by the Health and Safety Executive
C30 1/98
Printed and published by the Health and Safety Executive
C1.10
01/05
ISBN 0-7176-2949-X
RR 303
£20.00
9 78071 7 629497
Dynamic tensile properties of thin sheet materials
HSE BOOKS