ASM Metals HandBook Vol. 8 - Mechanical Testing and Evaluation
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boundaries occurs. According to information previously mentioned, segregation of impurities at interfaces can
lead to several different phenomena that involve a reduction in the value of the following materials parameters:
surface energy, grain-boundary diffusivity, and boundary cohesive strength (Ref 56, 59, 60, 61, and 67). The
occurrence of one or more of these phenomena would adversely influence ductility, either by resulting in
nucleating cavities (the growth and interlinkage of which during deformation may cause premature failure) or
by weakening boundaries.
Fig. 21 The average elongation to failure as a function of iron content for an initial grain size of 2.5 μm,
testing temperature of 473 K, and an initial strain rate of 1.33 × 10
-4
s
-1
. Source: Ref 56
References cited in this section
13. P.K. Chaudhury and F.A. Mohamed, Effect of Impurity Content on Superplastic Flow in the Zn-22% Al
Alloy, Acta. Metall., Vol 36, 1988, p 1099
14. P.K. Chaudhury, V. Sivaramakrishnan, and F.A. Mohamed, Superplastic Deformation Behavior in
Commercial and High Purity Zn-22 pct Al, Metall. Trans. A, Vol 19, 1988, p 2741
23. F.A. Mohamed and T.G. Langdon, Creep at Low Stress Levels in the Superplastic Zn-22% Al
Eutectoid, Acta Metall., Vol 23, 1975, p 117
24. P. Shariat, R.B. Vastava, and T.G. Langdon, An Evaluation of the Roles of Intercrystalline and
Interphase Boundary Sliding in Two-Phase Supersonic Alloys, Acta Metall., Vol 30, 1982, p 285
28. K. Duong and F.A. Mohamed, Effect of Impurity Content on Boundary Sliding Behavior in the
Superplastic Zn-22% Al Alloy, Acta Mater., Vol 46, 1998, p 4571
29. R.B. Vastava and T.G. Langdon, An Investigation of Intercrystalline and Interphase Boundary Sliding
in the Superplastic Pb-62% Sn Eutectic, Acta Metall., Vol 27, 1979, p 251
30. F.A. Mohamed, S. Shen-Ann, and T.G. Langdon, The Activation Energies Associated with Superplastic
Flow, Acta Metall., Vol 23, 1975, p 1443
33. P.K. Chaudhury, K.T. Park, and F.A. Mohamed, Effect of Fe on the Superplastic Deformation of Zn-22
pct Al, Metall. Mater. Trans. A, Vol 25, 1994, p 2391
38. A. Ball and M.M Hutchinson, Super-plasticity in the Aluminum-Zinc Eutectoid, Met. Sci., Vol 3, 1969,
p 1
39. A.K. Mukherjee, The Rate Controlling Mechanism in Superplasticity, Mater. Sci. Eng., Vol 8, 1971, p
83
40. R.C. Gifkins, Grain-Boundary Sliding and Its Accommodation during Creep and Superplasticity,
Metall. Trans. A, Vol 7, 1976, p 1225
41. J.H. Gittus, Theory of Superplastic Flow in Two-Phase Materials: Role of Interphase-Boundary
Dislocations, Ledges, and Diffusion, J. Eng. Mater. Technol. (Trans. ASME), Vol 99, 1977, p 244
42. A. Arieli and A.K. Mukherjee, A Model for the Rate-Controlling Mechanism in Superplasticity, Mater.
Sci. Eng., Vol 45, 1980, p 61
43. V. Paidar and S. Takeuchi, Superplastic Deformation Carried by Grain Boundaries, Acta Metall. Mater.,
Vol 40, 1992, p 1773
49. R.C. Gifkins, “Mechanisms for Low-Stress Region I of Superplasticity,” in Proceedings of a
Symposium on Strength of Metals and Alloys, R.C. Gifkins, Ed., Pergamon, Oxford, 1982, p 701
50. F.A. Mohamed, Interpretation of Superplastic Flow in Terms of Threshold Stress, J. Mater. Sci., Vol 18,
1983, p 582
52. R.C. Gifkins, Grain Rearrangements during Superplastic Deformation, J. Mater. Sci., Vol 13, 1978, p
1926
56. X.G. Jiang, S.T. Yang, J.C. Earthman, and F.A. Mohamed, Effect of Fe on Ductility and Cavitation in
the Superplastic Zn-22 pct Al Eutectoid, Metall. Mater. Trans. A, Vol 27, 1996, p 863
59. R.Z. Valiev and T.G. Langdon, An Investigation of the Role of Intragranular Dislocation Strain in the
Superplastic Pb-62% Sn Eutectic Alloy, Acta Metall. Mater., Vol 41, 1991, p 949
60. Z.R. Lin, A.H. Chokshi, and T.G. Langdon, An Investigation of Grain Boundary Sliding in
Superplasticity at High Elongations, J. Mater. Sci., Vol 23, 1988, p 2712
61. R.C. Gifkins, Ductility and Strain-Rate Control Mechanisms in Superplasticity, Scr. Metall. Mater., Vol
25, 1991, p 1397
62. M.J. Mayo and W.D. Nix, A Micro-Indentation Study of Superplasticity in Pb, Sn, and Sn-38 wt.% Pb,
Acta. Metall., Vol 36, 1988, p 2183
63. M.J. Mayo and W.D. Nix, Direct Observation of Superplastic Flow Mechanisms in Torsion, Acta.
Metall., Vol 37, 1989, p 1121
64. K.T. Park and F.A. Mohamed, Effect of Impurity Content on Cavitation in the Superplastic Zn-22 pct
Al Alloy, Metall. Trans. A, Vol 21, 1990, p 2605
65. K.T. Park, S.T. Yang, J.C. Earthman, and F.A. Mohamed, The Effect of Impurities on Ductility and
Cavitation in the Superplastic Zn-22% Al Alloy, Mater. Sci. Eng. A, Vol 188, 1994, p 59
66. A. Yousefiani, J.C. Earthman, and F.A. Mohamed, Formation of Cavity Stringers during Superplastic
Deformation, Acta. Mater., Vol 46, 1998, p 3557
67. H. Riedel in Fracture at High Temperatures, B. Ilschner and N. Grant, Ed., MRE Springer-Verlag,
1986, p 116
68. F.A. Mohamed, Modification of the Burke-Nix Ductility Expression, Scripta Metall., Vol 13, 1979, p 87
69. F.A. Nichols, Plastic Instabilities and Uniaxial Tensile Ductilities, Acta Metall., Vol 27, 1980, p 663
70. L.H. Lin, J.P. Hirth, and E.W. Hart, Plastic Instability in Uniaxial Tension Tests, Acta Metall., Vol 29,
1981, p 819
71. F.A. Mohamed and T.G. Langdon, Flow Localization and Neck Formation in a Superplastic Metal, Acta
Metall., Vol 29, 1981, p 911
72. F.A. Mohamed, M.I. Ahmed, and T.G. Langdon, Factors Influencing Ductility in the Superplastic Zn-22
Pct Al Eutectoid, Metall. Trans. A, Vol 8, 1977, p 933
Superplastic Deformation at Elevated Temperatures
Farghalli A. Mohamed, University of California, Irvine
Grain Growth during Testing
During superplastic flow, concurrent grain growth (deformation-enhanced grain growth or strain-induced grain
growth) takes place. In some cases, the amount of concurrent grain growth is more significant than that arising
from stress-free annealing or in the shoulder of a tensile specimen (static grain growth) (Ref 73). The
occurrence of significant concurrent grain growth during superplastic flow can lead to the following effects: (a)
the observation of a false region I (Ref 46, 47), (b) a loss of ductility in region II (Ref 56, 74), (c) erroneous
values for creep rates measured in region II when the applied stress acting on a single specimen was changed
during stress-increase experiments from a very low value (Ref 74), (d) the predominance of strain hardening in
the stress-strain curves at high temperatures (Ref 75), and (e) an increase in the level of cavitation (Ref 76).
(Excessive concurrent grain growth results in less accommodation of boundary sliding by boundary migration
or boundary diffusion.)
Three comments are in order regarding concurrent grain growth. First, concurrent grain growth is very
significant, and its associated effects, such as strain hardening, become noticeable when experiments are
performed without an annealing treatment before testing (Ref 46, 75). Second, the results of a recent
investigation on the superplastic flow of Zn-22A1 doped with iron have shown that the effect of concurrent
grain growth on cavitation is not as significant as that of the level of iron (Ref 56). Third, region I behavior is
genuine and is not related to the occurrence of concurrent grain growth; a well-defined region I, with high
values of the apparent stress exponent and the apparent activation energy, was observed in creep experiments
involving negligible grain growth.
References cited in this section
46. G. Rai and N.J. Grant, On the Measurements of Superplasticity in an Al-Cu Alloy, Metall. Trans. A, Vol
6, 1975, p 385
47. A. Arieli and A.K. Mukherjee, On Concurrent Grain Growth and Deformation Mechanism in Region I
for Superplastic Zn-22% Eutectoid Alloy, Scr. Metall., Vol 13, 1979, p 331
56. X.G. Jiang, S.T. Yang, J.C. Earthman, and F.A. Mohamed, Effect of Fe on Ductility and Cavitation in
the Superplastic Zn-22 pct Al Eutectoid, Metall. Mater. Trans. A, Vol 27, 1996, p 863
73. C.H. Cacers and D.S. Wilkinson, Superplastic Behavior of a Zn-22 pct Al-0.5 Pct Cu Alloy, Metall.
Trans. A, Vol 17, 1985, p 1873
74. A. Yousefiani and F.A. Mohamed, Superplastic Flow and Cavitation in Zn-22% pct Al Doped with Cu,
Metall. Mater. Trans. A, Vol 29, 1998, p 1653
75. B.P. Kashyap and T. Tangi, On the Contribution of Concurrent Grain Growth to Strain Sensitive Flow
of a Superplastic Al-Cu Eutectic Alloy, Metall. Trans. A, Vol 18, 1987, p 417
76. D.W. Livesey and N. Ridley, Effect of Grain Size on Cavitation in Superplastic Zn-22% Al Eutectoid, J.
Mater. Sci., Vol 17, 1982, p 2257
Superplastic Deformation at Elevated Temperatures
Farghalli A. Mohamed, University of California, Irvine
Acknowledgments
This work was supported by the National Science Foundation under grant No. DMR-810422. Thanks are also
due to Professor James Earthman for useful discussion, and to my graduate students, Ahmadali Yousefiani,
Kim Duong, and Zhigang Lin, for their assistance.
Superplastic Deformation at Elevated Temperatures
Farghalli A. Mohamed, University of California, Irvine
References
1. M.M.I. Ahmed, F.A. Mohamed, and T.G. Langdon, Neck Formation and Cavitation in the Superplastic
Zn-22 pct Al Eutectoid, J. Mater. Sci., Vol 14, 1979, p 2913
2. A.K. Mukherjee, Deformation Mechanisms in Superplasticity, Annu. Rev. Mater. Sci., Vol 9, 1979, p
191
3. R.C. Gifkins, Mechanisms of Super-plasticity, Proc. of a Symp. on Superplastic Forming of Structural
Alloys, N.E. Paton and C.H. Hamilton, Ed. (San Diego), Metallurgical Society of AIME, 1980, p 2
4. O.D. Sherby and J. Wadsworth, Superplasticity—Recent Advanced and Future Directions, Prog. Mater.
Sci., Vol 33, 1989, p 169
5. A.H. Chokshi, A.K. Mukherjee, and T.G. Langdon, Superplasticity in Advanced Materials, Mater. Sci.
Rep., Vol 10, 1993, p 237
6. R. Sawle, Commercial Application of Superplastic Sheet Forming, Proc. of a Symp. on Superplasticity
Forming of Structural Alloys, N.E. Paton and C.H. Hamilton, Ed. (Warrendale, PA), TMS-AIME, 1982,
p 307
7. W.A. Backofen, I.R. Turner, and D.H. Avery, Superplasticity in an Al-Zn Alloy, Trans. ASM, Vol 57,
1964, p 980
8. W.B. Morrison, Superplasticity of Low-Alloy Steels, Trans. ASM, Vol 61, 1968, p 423
9. J. Hedworth and M.J. Stowell, The Measurement of Strain-Rate Sensitivity in Superplastic Alloys, J.
Mater. Sci., Vol 6, 1971, p 1061
10. G.B. Gibbs, Creep and Stress Relaxation Studies with Polycrystalline Magnesium, Philos. Mag., Vol
13, 1966, p 317
11. A. Arieli and A. Rosen, Measurement of Strain-Rate Sensitivity Coefficient in Superplastic Ti-6A1-4V
Alloy, Scr. Metall., Vol 10, 1976, p 471
12. E.W. Hart, A Phenomenological Theory for Plastic Deformation of Polycrystalline Metals, Acta.
Metall., Vol 18, 1970, p 599
13. P.K. Chaudhury and F.A. Mohamed, Effect of Impurity Content on Superplastic Flow in the Zn-22% Al
Alloy, Acta. Metall., Vol 36, 1988, p 1099
14. P.K. Chaudhury, V. Sivaramakrishnan, and F.A. Mohamed, Superplastic Deformation Behavior in
Commercial and High Purity Zn-22 pct Al, Metall. Trans. A, Vol 19, 1988, p 2741
15. S. Yan, J.C. Earthman, and F.A. Mohamed, Effect of Cd on Superplastic Flow in the Pb-62 wt% Sn
Eutectic, Philos. Mag. A, Vol 69, 1994, p 1017
16. B.Y. Chirouze, D.M. Schwartz, and J.E. Dorn, The Mechanism of Basal Creep in Mg-12 at.% Li from
500 to 885 K, Trans. ASM, Vol 51, 1967, p 51
17. K.L. Murty, F.A. Mohamed, and J.E. Dorn, Viscous Glide, Dislocation Climb and Newtonian Viscous
Deformation Mechanisms of High Temperature Creep in Al-3Mg, Acta Metall., Vol 20, 1972, p 1009
18. M.S. Soliman, T.J. Ginter, and F.A. Mohamed, Philos. Mag. A, Vol 48, 1993, p 63
19. F.A. Mohamed and T.G. Langdon, A Comparison of Constant Strain Rate and Creep Testing
Procedures in Superplasticity, J. Mater. Sci., Vol 17, 1982, p 1925
20. H.I. Huang, O.D. Sherby, and J.E. Dorn, Activation Energy for High Temperature Creep of High Purity
Aluminum, Trans. MS/AIME, Vol 23, 1956, p 117
21. F.A. Mohamed and T.G. Langdon, The Determination of the Activation Energy for Superplastic Flow,
Phys. Status Solidi (a), Vol 33, 1976, p 375
22. J.E. Bird, A.K. Mukerjee, and J.E. Dorn, Correlations between High-Temperature Creep Behavior and
Structure, Proc. of a Symp. on Quantitative Relation Between Properties and Microstructure, D.G.
Brandon and A. Rosen, Ed., Israel University Press, 1969, p 22
23. F.A. Mohamed and T.G. Langdon, Creep at Low Stress Levels in the Superplastic Zn-22% Al
Eutectoid, Acta Metall., Vol 23, 1975, p 117
24. P. Shariat, R.B. Vastava, and T.G. Langdon, An Evaluation of the Roles of Intercrystalline and
Interphase Boundary Sliding in Two-Phase Supersonic Alloys, Acta Metall., Vol 30, 1982, p 285
25. T.G. Langdon, The Effect of Surface Configuration on Grain Boundary Sliding, Metall. Trans., Vol 3,
1972, p 797
26. R.C. Gifkins, Factors Influencing Deformation of Superplastic Alloys, Mater. Forum, Vol 15, 1991, p
82
27. K.T. Park, S. Yan, and F.A. Mohamed, Boundary Sliding Behavior in High-Purity Pb-62% Sn, Philos.
Mag. A, Vol 72, 1995, p 891
28. K. Duong and F.A. Mohamed, Effect of Impurity Content on Boundary Sliding Behavior in the
Superplastic Zn-22% Al Alloy, Acta Mater., Vol 46, 1998, p 4571
29. R.B. Vastava and T.G. Langdon, An Investigation of Intercrystalline and Interphase Boundary Sliding
in the Superplastic Pb-62% Sn Eutectic, Acta Metall., Vol 27, 1979, p 251
30. F.A. Mohamed, S. Shen-Ann, and T.G. Langdon, The Activation Energies Associated with Superplastic
Flow, Acta Metall., Vol 23, 1975, p 1443
31. F.A. Mohamed and T.G. Langdon, Creep Behavior in the Superplastic Pb-62% Sn Eutectic, Philos.
Mag. A, Vol 32, 1975, p 697
32. S.H. Vale, D.J. Eastgate, and P.M. Hazzledine, The Low Strain Rate Behavior of Superplastic Zn-Al
Eutectoid Alloy, Scr. Metall., Vol 13, 1979, p 1157
33. P.K. Chaudhury, K.T. Park, and F.A. Mohamed, Effect of Fe on the Superplastic Deformation of Zn-22
pct Al, Metall. Mater. Trans. A, Vol 25, 1994, p 2391
34. F.A. Mohamed and T.J. Ginter, The Stress Dependence of Subgrain Size in Al, J. Mater. Sci., Vol 17,
1982, p 2007
35. F.A. Mohamed and T.G. Langdon, Deformation Mechanism Maps for Superplastic Materials, Scr.
Metall., Vol 10, 1976, p 759
36. S.A. Shei and T.G. Langdon, The Mechanical Properties of a Superplastic Quasi-Single Phase Copper
Alloy, Acta Metall., Vol 26, 1978, p 638
37. M.F. Ashby and R.A. Verrall, Diffusion-Accommodated Flow and Superplasticity, Acta Metall Vol 21,
1973, p 149
38. A. Ball and M.M Hutchinson, Super-plasticity in the Aluminum-Zinc Eutectoid, Met. Sci., Vol 3, 1969,
p 1
39. A.K. Mukherjee, The Rate Controlling Mechanism in Superplasticity, Mater. Sci. Eng., Vol 8, 1971, p
83
40. R.C. Gifkins, Grain-Boundary Sliding and Its Accommodation during Creep and Superplasticity,
Metall. Trans. A, Vol 7, 1976, p 1225
41. J.H. Gittus, Theory of Superplastic Flow in Two-Phase Materials: Role of Interphase-Boundary
Dislocations, Ledges, and Diffusion, J. Eng. Mater. Technol. (Trans. ASME), Vol 99, 1977, p 244
42. A. Arieli and A.K. Mukherjee, A Model for the Rate-Controlling Mechanism in Superplasticity, Mater.
Sci. Eng., Vol 45, 1980, p 61
43. V. Paidar and S. Takeuchi, Superplastic Deformation Carried by Grain Boundaries, Acta Metall. Mater.,
Vol 40, 1992, p 1773
44. H.W. Hayden, S. Floreen, and P.D. Goodell, The Deformation Mechanisms of Superplasticity, Metall.
Trans., Vol 3, 1972, p 833
45. R.C. Gifkins and T.G Langdon, Comments on Theories of Structural Superplasticity, Mater. Sci. Eng.,
Vol 36, 1978, p 27
46. G. Rai and N.J. Grant, On the Measurements of Superplasticity in an Al-Cu Alloy, Metall. Trans. A, Vol
6, 1975, p 385
47. A. Arieli and A.K. Mukherjee, On Concurrent Grain Growth and Deformation Mechanism in Region I
for Superplastic Zn-22% Eutectoid Alloy, Scr. Metall., Vol 13, 1979, p 331
48. J. Weertman, Steady-State Creep of Crystals, J. Appl. Phys., Vol 28, 1957, p 1185
49. R.C. Gifkins, “Mechanisms for Low-Stress Region I of Superplasticity,” in Proceedings of a
Symposium on Strength of Metals and Alloys, R.C. Gifkins, Ed., Pergamon, Oxford, 1982, p 701
50. F.A. Mohamed, Interpretation of Superplastic Flow in Terms of Threshold Stress, J. Mater. Sci., Vol 18,
1983, p 582
51. F.A. Mohamed and T.G. Langdon, The Transition from Viscous Glide to Dislocation Climb, Acta
Metall., Vol 22, 1974, p 779
52. R.C. Gifkins, Grain Rearrangements during Superplastic Deformation, J. Mater. Sci., Vol 13, 1978, p
1926
53. I.I. Novikov, V.K. Portnoy, and V.S. Levchenko, Investigation of Structural Changes during
Superplastic Deformation of Zn-22% Al Alloy by Replica Locating Technique, Acta. Metall., Vol 29,
1981, p 1077
54. S.T. Yang and F.A. Mohamed, On the Characteristics of the Threshold Stress for Superplastic Flow in
Zn-22 pct Al, Metall. Mater. Trans. A, Vol 26, 1995, p 493
55. H. Gleiter and B. Chalmers, High Angle Grain Boundaries, Prog. Mater. Sci., Vol 16, 1973
56. X.G. Jiang, S.T. Yang, J.C. Earthman, and F.A. Mohamed, Effect of Fe on Ductility and Cavitation in
the Superplastic Zn-22 pct Al Eutectoid, Metall. Mater. Trans. A, Vol 27, 1996, p 863
57. M.P. Seah, Grain Boundary Segregation and the T-t Dependence of Temper Brittleness, Acta Metall.,
Vol 25, 1977, p 34
58. P. Gas, M. Guttmann, and J. Bernardini, The Interactive Co-Segregation of Sb and Ni at the Grain
Boundaries of Ultra-High Purity Fe-Base Alloys, Acta Metall., Vol 30, 1982, p 1309
59. R.Z. Valiev and T.G. Langdon, An Investigation of the Role of Intragranular Dislocation Strain in the
Superplastic Pb-62% Sn Eutectic Alloy, Acta Metall. Mater., Vol 41, 1991, p 949
60. Z.R. Lin, A.H. Chokshi, and T.G. Langdon, An Investigation of Grain Boundary Sliding in
Superplasticity at High Elongations, J. Mater. Sci., Vol 23, 1988, p 2712
61. R.C. Gifkins, Ductility and Strain-Rate Control Mechanisms in Superplasticity, Scr. Metall. Mater., Vol
25, 1991, p 1397
62. M.J. Mayo and W.D. Nix, A Micro-Indentation Study of Superplasticity in Pb, Sn, and Sn-38 wt.% Pb,
Acta. Metall., Vol 36, 1988, p 2183
63. M.J. Mayo and W.D. Nix, Direct Observation of Superplastic Flow Mechanisms in Torsion, Acta.
Metall., Vol 37, 1989, p 1121
64. K.T. Park and F.A. Mohamed, Effect of Impurity Content on Cavitation in the Superplastic Zn-22 pct
Al Alloy, Metall. Trans. A, Vol 21, 1990, p 2605
65. K.T. Park, S.T. Yang, J.C. Earthman, and F.A. Mohamed, The Effect of Impurities on Ductility and
Cavitation in the Superplastic Zn-22% Al Alloy, Mater. Sci. Eng. A, Vol 188, 1994, p 59
66. A. Yousefiani, J.C. Earthman, and F.A. Mohamed, Formation of Cavity Stringers during Superplastic
Deformation, Acta. Mater., Vol 46, 1998, p 3557
67. H. Riedel in Fracture at High Temperatures, B. Ilschner and N. Grant, Ed., MRE Springer-Verlag,
1986, p 116
68. F.A. Mohamed, Modification of the Burke-Nix Ductility Expression, Scripta Metall., Vol 13, 1979, p 87
69. F.A. Nichols, Plastic Instabilities and Uniaxial Tensile Ductilities, Acta Metall., Vol 27, 1980, p 663
70. L.H. Lin, J.P. Hirth, and E.W. Hart, Plastic Instability in Uniaxial Tension Tests, Acta Metall., Vol 29,
1981, p 819
71. F.A. Mohamed and T.G. Langdon, Flow Localization and Neck Formation in a Superplastic Metal, Acta
Metall., Vol 29, 1981, p 911
72. F.A. Mohamed, M.I. Ahmed, and T.G. Langdon, Factors Influencing Ductility in the Superplastic Zn-22
Pct Al Eutectoid, Metall. Trans. A, Vol 8, 1977, p 933
73. C.H. Cacers and D.S. Wilkinson, Superplastic Behavior of a Zn-22 pct Al-0.5 Pct Cu Alloy, Metall.
Trans. A, Vol 17, 1985, p 1873
74. A. Yousefiani and F.A. Mohamed, Superplastic Flow and Cavitation in Zn-22% pct Al Doped with Cu,
Metall. Mater. Trans. A, Vol 29, 1998, p 1653
75. B.P. Kashyap and T. Tangi, On the Contribution of Concurrent Grain Growth to Strain Sensitive Flow
of a Superplastic Al-Cu Eutectic Alloy, Metall. Trans. A, Vol 18, 1987, p 417
76. D.W. Livesey and N. Ridley, Effect of Grain Size on Cavitation in Superplastic Zn-22% Al Eutectoid, J.
Mater. Sci., Vol 17, 1982, p 2257
Introduction to High Strain Rate Testing
Sia Nemat-Nasser, University of California, San Diego
Introduction
HIGH STRAIN RATE TESTING is important for many engineering structural applications and metalworking
operations. In structural applications, various components must be designed to function over a broad range of
strain rates and temperatures. In metalworking operations, materials undergo large amounts of strains at various
temperatures and strain rates. The constituent materials must, therefore, be characterized at the strain rates and
temperatures of the intended application. Conventional servohydraulic machines are generally used for testing
at quasi-static strain rates of 1 s
-1
or less (Fig. 1). With special design, it is possible to attain greater strain rates,
up to about 100 s
-1
, with conventional load frames. For higher strain rates, other test methods are required.
Table 1 summarizes various methods in terms of the ranges of the strain rates that they can achieve.
Table 1 Experimental methods for high strain rate testing
Applicable strain rate, s
-1
Testing technique
Compression tests
<0.1 Conventional load frames
0.1–100 Special servohydraulic frames
0.1–500 Cam plastometer and drop test
200–10
4
Hopkinson (Kolsky) bar in compression
10
3
–10
5
Taylor impact test
Tension tests
<0.1 Conventional load frames
0.1–100 Special servohydraulic frames
100–10
3
Hopkinson (Kolsky) bar in tension
10
4
Expanding ring
>10
5
Flyer plate
Shear and multiaxial tests
<0.1 Conventional shear tests
0.1–100 Special servohydraulic frames
10–10
3
Torsional impact
100–10
4
Hopkinson (Kolsky) bar in torsion
10
3
–10
4
Double-notch shear and punch
10
4
–10
7
Pressure-shear plate impact
Fig. 1 Strain rate regimes and associated instruments and experimental conditions
The articles in this Section describe various methods for high strain rate testing. Several methods have been
developed, starting with the pioneering work of John Hopkinson (Ref 1) and his son, Bertram Hopkinson (Ref
2, 3). Based on these contributions and also on an important paper by Davies (Ref 4), Kolsky (Ref 5) invented
the split-Hopkinson pressure bar, which allows the deformation of a sample of a ductile material at a high strain
rate, while maintaining a uniform uniaxial state of stress within the sample. The basic concept of the Kolsky
apparatus involves a test sample sandwiched between an input and output bar, as described in detail in the
article “Classic Split-Hopkinson Pressure Bar Testing” in this section. This technique provides a capability to
measure the stress-strain response of ductile materials at a high strain rate, usually between approximately 50 s
-
1
and 10
4
s
-1
, depending on the sample size, over the entire stress-strain curve. Strains exceeding 100% can be
achieved with the Hopkinson bar method. The maximum strain rate that can be attained in a Hopkinson bar
varies inversely with the length of the test specimen. The maximum strain rate is also limited by the elastic limit
of the Hopkinson bars that are used to transmit the stress pulse to the test sample. These basic factors of the
Hopkinson bar method and some specialized compression and tension tests are discussed in the article “High
Strain Rate Tension and Compression Tests” in this section. An overview of shear test methods (other than the
torsional Kolsky bar method) is also provided in the article “High Strain Rate Shear Testing.”
The most important characteristic of Kolsky's split-Hopkinson compression apparatus is that it allows high
strain rate deformation while the sample is, in fact, in dynamic equilibrium, that is, the stress gradient is
essentially zero along the sample. It is thus possible to develop the uniaxial stress-strain response of many
materials at a variety of strain rates. Because the response of most materials depends on both the strain rate and
the temperature, the technique allows developing constitutive relations that express the uniaxial stress to the
corresponding strain rate and temperature. From such results, one is able to produce experimentally based,
three-dimensional constitutive models for numerous materials.
The series of articles on split-Hopkinson compression testing begins with the article “Classic Split-Hopkinson
Pressure Bar Testing,” which is followed by the article “Recovery Hopkinson Bar Techniques.” One limitation
of the classic split-Hopkinson bar technique had been the repeated loading of the sample by the stress pulses
that travel back and forth along the bars. During the 1990s, techniques were developed to trap the reflected
pulses at the free ends of the Hopkinson bars, making it possible to subject a sample to a single stress pulse and
then recover the sample without it being subjected to any additional loading (Ref 6). Using this recovery