Abstract In this paper, the dynamic deformation behaviors of 2 aluminum alloys (AA6016, AA5182) and 2 steels (v 1158, FeP04

Geng Changgang

Li Xiangping

Lu Xin

CiteSeerX

49 
Materials Characterization at High Speed by 
Dynamic Tensile Tests* 
Lu Xin1, Li Xiangping2, Geng Changgang1 
1 Beijing Research Institute of Mechanical and Electric Technology, China 
2 Fiat Research Center, Italy 
Abstract 
In this paper, the dynamic deformation behaviors of 2 aluminum alloys (AA6016, AA5182) 
and 2 steels (v 1158, FeP04) are investigated. Firstly, the test method, an improved 
Hopkinson pressure bar test, is introduced. Then, the test results are presented and 
analyzed. The strain rate range used in this study is between 290 s-1 and 1750 s-1.The test 
results show that the yield stresses of all four tested materials increase along with the 
increase of strain rate, but the ductility does not. An interesting finding is that the tested Al 
alloys demonstrated larger elongation than those of steels. In conventional tensile tests, 
as it is well known, the tensile elongation of low carbon steels is usually much higher than 
the one of Al alloys. So, it can be predicted that the aluminum alloys could have better 
formability than the steels in high speed forming processes, though the underlying 
mechanism is not fully understood. 
Key Words: 
High speed, Material test, Aluminum alloy, Steel  
1 Introduction 
In recent years, the automotive industry has been facing three major challenges: 1) the 
increasingly strict legislations on environment protection; 2) higher safety standards; 3) 
more sophisticated customer demands and, therefore, more product segmentations. To 
meet these challenges, the automotive makers have been in continuous pursuit of new 
                                            
 
* The authors would like to thank Ministry of Science and Technology of the People´s 
Republic of China and National Natural Science Foundation of China (NSFC) for their  the 
financial support (2002DFG00036 and 50375016). 
 2nd International Conference on High Speed Forming – 2006 
 
 
50  
materials and new manufacturing technologies in order to be able to provide lighter and 
safer cars with lower costs by using lightweight materials like aluminum alloys and more 
flexible manufacturing processes. In this context, innovative processes such as 
electromagnetic forming (EMF) and electrohydraulic forming (EHF) received more 
attention and have become the subject of many research projects. EMF and EHF 
technologies are based on high-voltage discharge of capacitors: through the conductive 
coil or the electrodes in water the large pulsing energy discharged from the capacitor 
banks can generate high pressure and deform the workpiece in very high velocity. The 
advantages of these technologies include low investment in one sided dies [1], increased 
formability [2], reduced wrinkling and springback [3]. Strain rates in these forming 
processes are usually in the range of 102-103 s-1, much higher than the strain rates in 
which the conventional tensile tests are carried out (10-4 to 10-2 s-1). For the design and 
analysis of high speed forming processes the characteristics and deformation behaviors of 
workpiece materials must be known. However, today there exist only limited dynamic 
material test data. On the other hand, the need for vehicle crashworthiness design and 
optimization poses further demands for systematic materials testing under dynamic 
conditions. The combined needs make the investigation of material dynamic 
characteristics more imperative. 
 The actual high speed material testing techniques include dropweight machines [4, 
5], split hopkinson pressure bars (SHPBs) [6, 7], Taylor impact [8], and shockloading by 
plate impact [9]. These testing methods have a common problem related to the disturbed 
wave by mechanical oscillation. In this paper, a new method based on “rotating disk 
indirect bar-bar tensile impact apparatus (RSHTB)” is presented. The novelty of the 
RSHTB method consists in the fact that a destructive short metal bar is used to solve the 
problem of the mechanical oscillation, using its fracture to filter the oscillation wave. As a 
result, the reliability of the acquired data is significantly improved.  
 In this paper, the dynamic deformation behaviors of 2 aluminum alloys (AA6016, 
AA5182) and 2 steel (V1158, FeP04) materials are investigated by means of the RSHTB  
dynamic tensile test method.  
2 Experimental procedure 
The schematic drawing of a RSHTB device is shown in Figure 1. The novelty with the 
RSHTB device is the introduction of a mechanical filter. When the mass 1 is impacted by 
hammer 3 on the rotating disk the produced rectangular impulse tensile stress wave is 
transmitted to input metal stick 5 from the destructive short metal stick 2. The short metal 
stick 2 is deformed until fracture occurs. So, an incident wave is generated by impact 
between 2 and 5, other than between 3 and 1. In this way, the incident rectangular wave 
reached at the input metal stick 5 is mechanically filtered, hence it is clean and stable with 
a very little disturbed wave. This is very important for obtaining reliable test results. In 
addition, the height and width of the incident rectangular wave can be adjusted easily with 
this kind of device to obtain the strain rate range 101-3.5x103s-1. The test sample’s shape, 
dimension, and the clamping method are illustrated in Figure 2.  
 2nd International Conference on High Speed Forming – 2006 
 
 
51 
 
1-mass; 2-the destructive short metal stick; 3-hammer; 4-strain cage; 5-input stick; 6-sample; 7-
output stick; 8-supper dynamic strain meter; 9-TCL wave shape recorder; 10-connect switch; 11-
computer; 12-printer. 
Figure 1: Apparatuses t for RSHTB 
 
 
Figure 2: Dimension of test sample and its linking type 
From the theory of one-way stress wave the force and the displacement in the two end s 
of test sample are expressed as follows: 
P t E A t ti r1 1 1( ) [ ( ) ( )]= +ε ε                     (1) 
 
 2nd International Conference on High Speed Forming – 2006 
 
 
52  
P t E A tt2 2 2( ) ( )= ε                             (2) 
 
u t C di r
t
1 01 0
( ) [ ( ) ( )]= −∫ ε τ ε τ τ                (3) 
 
u t C dt
t
2 02 0
( ) ( )= ∫ ε τ τ                          (4) 
 
The E1, A1, C01 and E2, A2, C02 are the elasticity modulus, cross section area, and 
longitudinal wave speed of elasticity respectively for the end surfaces of both the input 
stick and output stick. The εi(t), εr(t), εt(t) are the strain signals of the incident wave, the 
reflective wave, and the transmission wave respectively.  
It is supposed that the stress and strain are uniform in the sample without cracks, 
thus in the uniaxial tension sample, the stress, strain, and strain rate are the following: 
 
σ ε εS
S S
i tt A
p t P t
A
E A t E A t( ) [ ( ) ( )] [ ( ) ( )]= + = +1
2
1
21 2 1 1 2 2
           (5) 
 
{ }ε ε τ ε τ τ ε τ τs
s s
i r t
tt
t
l
u t u t
l
C d C d( ) [ ( ) ( )] [ ( ) ( )] ( )= − = − − ∫∫
1 1
1 2 01 02 00
   (6) 
 
ε ε ε εs
s
i r tt l
C t t C t( ) [ ( ( ) ( )) ( )]= − −1 01 02         (7) 
 
where As and ls are the cross-area and length of the gauge. When the diameters of input 
stick and output stick are same we have:  
 
E E E A A A C C C1 2 1 2 01 02 0= = = = = =, ,  
,thus 
ε ε εi r tt t t( ) ( ) ( )+ =                (8) 
 
 
σ ε ε ε εs
s
i r t
s
tt
EA
A
t t t EA
A
t( ) [ ( ) ( ) ( )] ( )= + + =
2
         (9) 
 
∫ ∫ −=−−=
t t
ti
s
tri
s
s dl
C
d
l
C
t
0 0
00 )()([
2
)()()([)( ττετεττετετεε    (10) 
 
ε ε ε ε ε εs
s
i r t
s
i tt
C
l
t t t C
l
t t( ) [ ( ) ( ) ( )] [ ( ) ( )]= − − = −0 02          (11) 
 
 2nd International Conference on High Speed Forming – 2006 
 
 
53 
From equation (8) to (11), the stress, stain, and strain rate of the test sample can be 
obtained at anytime so long as strain signals of input stick and output stick are measured 
precisely. With the dynamic tensile test device and the test specimen described above the 
largest reachable strain rate is about 2.2x103s-1. 
3 Results and discussions 
The engineering stress – engineering strain curves of AA6016 and AA5182 are shown in 
Figure 3 and Figure 4 respectively. And in Figure 5 and Figure 6 the curves of engineering 
stress vs. engineering strain for two steels V1158 and Fep04 are shown. It can be clearly 
seen that for all four materials the yield stress increases with strain rate, but the ductility 
showed a complex picture. The increase of yield stress of steels is much more evident 
than that of Al alloys, the ductility of Al-alloy is even higher than that of the steel. A 
comparison between all 4 materials under similar strain rate conditions is shown in Figure 
7.  
 
Figure 3: Engineering stress vs. engineering strain of AA6016 alloy 
 2nd International Conference on High Speed Forming – 2006 
 
 
54  
 
Figure 4: Engineering stress vs. engineering strain of AA5182 alloy 
 
 
Figure 5: Engineering stress vs. engineering strain plot of V1158 steel 
 
 2nd International Conference on High Speed Forming – 2006 
 
 
55 
 
Figure 6: Engineering stress vs. engineering strain plot for FeP04 steel 
In conventional quasi static tests, as it is well known, the ductility of low carbon steels is 
much better than that of aluminum alloys. For example, the total elongation of FeP04 steel 
sheet can be as high as 45% in conventional quasi static tensile tests (see Figure 8).  
 
Figure 7: Engineering stress vs. engineering strain plot for all 4 materials under different 
deformation strain rates: 6016(380 S-1),5182(370 S-1),1158(290 S-1)andFep0.4(390 S-1) 
 2nd International Conference on High Speed Forming – 2006 
 
 
56  
For 5000 and 6000 series aluminum alloys the total tensile elongations are usually less 
than 25%. However, it is interesting to note that the dynamic test results showed the 
contrary trend. The aluminum alloys demonstrated a higher total elongation than that in 
quasi static test, although the gain is not large. On the other hand, the steel materials 
reached a much lower total elongation than that obtained in conventional tests, 
unexpectedly lower than that of aluminum alloys tested in this investigation. This 
observation is contrary to the results of some previous studies, which stated that the 
fracture strain of bcc-steels often was enhanced by 20 to 50% at strain rates about 103s-
1[10] and attributed the possible reasons to twinning and adiabatic heating [11]. The 
increase in ductility of aluminum alloys is in agreement with the results obtained in other 
studies [12]. 
 
TENSILE CURVE
0
50
100
150
200
250
300
350
0 10 20 30 40 50
Elongation (%)
St
re
ss
 (M
Pa
)
Tensile test
 
Figure 8: Engineering stress vs. engineering strain curve of FeP04 obtained with 
conventional quasi static tensile tests 
In order to understand the experimental phenomena observed in this study, an analysis 
on the constitutional stress – strain relationship will be helpful. The strain hardening 
exponent and strain rate sensitivity are important parameters for the explanation of 
material flow behavior. Since the data presented above are in engineering stress and 
engineering strain, true stress and true strain data are needed to get the strain hardening 
exponent n and the strain rate sensitivity value m. True stress and true strain are 
calculated according to the following equations:     
 
 2nd International Conference on High Speed Forming – 2006 
 
 
57 
)1( es +⋅=σ          (12) 
 
)1(log ee +=ε          (13) 
 
where 
 e is the engineering strain 
 ε is the true strain 
 S is the engineering stress 
 σ is the true stress 
The n and m values are then calculated according to the slope method. The n values are 
obtained from log-log plot of true stress and true strain, the selected strain range is 
between 0.05 and 0.10. For each material the strain hardening exponent at each strain 
rate is calculated and the results are included in Table 1. Similarly, for each material the m 
value is obtained from log-log plot of true stress and strain rate, the true stress values 
corresponding to true strain o.15 were taken. The results are given in Table 2.  
 
Materials n-values (at true strain 0.05 – 0.10) in different strain rates 
AA6016 0.118 (380s-1) 0.203 (870s-1) 0.195 (1750s-1) 
AA5182 0.225 (370s-1) 0.197 (800s-1) 0.235 (1520s-1) 
V1158 0.0014 (290s-1) 0.0015 (800s-1) 0.0026 (1630s-1) 
FeP04 0.0021 (360s-1) 0.0032 (810s-1) 0.0048 (1530s-1) 
Table 1: Strain hardening exponent (corresponding to strain range 0.05 – 0.10) 
Material AA5182 AA6016 V1158 FeP04 
m-value 0.0350 0.0377 0.0624 0.0717 
Table 2: Strain rate sensitivity value m (calculated at true strain 0.15) 
In metal forming, particularly in stretching and bending deformations, the strain hardening 
exponent is an indicator of the formability of the material. A higher level of this exponent is 
desirable since it results in a better distribution of the strain. In conventional tensile tests 
at room temperature the n values for FeP04 range between 0.23 and 0.25. From table 1 it 
can be seen that while for the Al alloys the strain hardening exponents are still close to 
0.20, those for steels, however, approach to zero. Without strain hardening the steels 
continuously lose the ability to sustain a stable deformation and, as a result, the total 
elongations are quite low. As to strain rate sensitivity, for engineering metals at room 
temperature the m values are in the range of -0.005 to 0.015. The m value is the measure 
of a material to resist necking and is particularly important for superplastic deformation at 
high temperatures, but considered less important in quasi static deformation at room 
temperature. From Table 2 it can be noted that the two steel materials possess much 
 2nd International Conference on High Speed Forming – 2006 
 
 
58  
higher m values than the two aluminum alloys. That is why in the steels a significant 
increase in yielding strength is observed.  
 The above mentioned analysis can explain why the steels investigated in the 
present study demonstrated lower ductility at high speed deformation, but the reason why 
the aluminum alloys showed better ductility is not understood. For the purpose of EMF 
and EHF, the results described above reveal that aluminum alloys are ideal materials for 
high speed forming processes because of their lower strain rate sensitivity and high 
ductility properties under dynamic conditions, in addition to the advantage of good 
conductivity which is useful for EMF processes. On the other hand, the high strain rate 
sensitivity of steels in high speed deformation is a great advantage for automotive 
applications since this can contribute to the improvement of crashworthiness.   
4 Conclusions 
The rotating disk indirect bar-bar tensile impact apparatus is proved to be one of the best 
methods for high speed dynamic material tensile tests. According to the test results, under 
dynamic conditions the aluminum alloys showed better ductility than in conventional quasi 
static tests and at the same time there is a noticeable increase in yield strength. The two 
steel materials investigated in this paper demonstrated quite high strain rate sensitivity in 
high speed tensile deformation with a significant increase in yield strength in comparison 
to quasi static conditions. The ductility of the two steels is found to be lower than that 
obtained in conventional quasi static tensile tests due to close-to-zero strain hardening 
exponents. These mechanical properties mean that aluminum alloys are very suitable for 
high speed forming applications, while the high strain rate sensitivity of steels can bring 
about big advantages in automotive applications in terms of crashworthiness.   
References 
[1] Wilson, F. W.: High Velocity Forming of Metals. ed. American Society of Tool and 
Manufacturing Engineers. Prentice-Hall,Inc,Englewood Cliffs, N.J. 1964 
[2] Golovashchenko, S.; Mamutov, V.; Dmitriev, V.; Sherman, A.: Formability of sheet 
metal with pulsed electromagnetic and electrohydraulic technologies. Prooceedings 
of TMS symposium “Aluminum-2003,”San-Diego, 2003, p.99-110. 
[3] Golovashchenko S.: Numerical and experimental results on pulsed tubes calibration, 
Proceedings of a 1999 TMS Symposium “Sheet Metal Forming Technology”, San-
Diego,1999, p.117-127. 
[4] Radford, D. D.; Walley, S.M.; Church, P.; Field, J.E.: Dynamic upsetting and failure 
of metal cylinders: Experiments and analysis. J. Phys. IV France 110(2003)263-268. 
[5] Swallowe, G. M.; Lee, S.F.: A study of the mechanical properties of PMMA and PS 
at strain rates of 10-4 to 103 s-1 over the temperature range 293-363K. J. Phys. IV 
France 110(2003) 33-38. 
[6] Albertini, C.; Cadoni, E.; Labibes, K.: Study of the mechanical properties of plain 
concrete under dynamic loading. Exper. Mech. 39 (1999) 137-141. 
[7] Gorham, D.A.: Measurement of stress-strain properties of strong metals at very high 
strain rates. Inst. Phys. Conf. Ser. 47 (1980) 16-24. 
 2nd International Conference on High Speed Forming – 2006 
 
 
59 
[8] Couque, H.; Walley, S.; Lichtenberger, A.; Chartagnac, P.; Dormeval, R.; Petit, J.: 
Test recommendation: Dynamic compression testing using the Talor test, publ. 
DYMAT, Arcueil, France (2001) 
[9] See the website www.dymat.org. 
[10] Hartmann, K.-H.; Kunze, H.-D.; Meyer, L.W.: Metallurgical Effects of lmpact 
Materials. In: Shock Waves and High Strain Rate Phenomena in metals. Hrsg.M.A. 
Meyers and L.E.Murr, Plenum Press, New York 1981, S. 325-337. 
[11] Reinders, B.-O.; Kunze, H.-D.: Influence of Mechanical Twinning on the Deformation 
Behavior of Armco lron. In: Shock Waves and High-Strain-Rate Phenomena in 
Materials Eds. M.A. Meyers, L.E.Murr, K.P. Staudhammer, M. Dekker, 1990, S. 127-
136. 
[12] Xin, L.; Zhongren, W.; Jintao, H.: A Comparison of Mesomechanism of Dynamic 
Deformation between Copper and Aluminum. Journal of Materials Processing 
Technology 70 ( 1997)228-230. 
Acknowledgement:  
The research and experimental activities were partially carried out in the context of the 
Fifth Framework Project of the European Unione “Electromagnetic Forming of Tube and 
Sheet Metal for Automotive Parts (Acronym: EMF; Project Number: GRD2-2001-50057; 
Contract Number: G3RD-CT-2002-00798). The authors want to thank European 
Community for the financial support through the European Commision “GROWTH” 
Programme. 
 
 


Materials Characterization at High Speed by Dynamic Tensile Tests

http://citeseerx.ist.psu.edu/viewdoc/download;jsessionid=59334F9F9F7BBBB4CC5E5B274FDF3012?doi=10.1.1.1079.9072&rep=rep1&type=pdf

Materials Characterization at High Speed by Dynamic Tensile Tests

Abstract

Similar works

Full text

Available Versions

CiteSeerX