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Blast, Impact and Survivability Research Unit (BISRU), Department of Mechanical Engineering, University of Cape Town. Latin American Journal of Solids and Structures 1 (2004), p. 319-339. This paper reports on a particularly accurate and precise physical and experimental data capture and data processing system for high strain rate testing using the split Hopkinson pressure bar. The data processing incorporated the first four Pochammer-Chree modes and proved to be suitable for the tests that were carried out. To vindicate the design of the facility, two materials were tested, namely mild steel and copper, in both the as-received and annealed conditions. The actual test data, once appropriately processed was compared with both the Johnson-Cook (JC) and Cowper-Symonds (CS) high strain rate materials models, as well as with other published data. The correlation with previously published constants for the JC and CS models was not particularly good, due to perceived differences in the stress levels and prior work hardening, heat treatment condition and microstructure of these earlier tests, as these variables affect the constitutive equation constants. Using other published data, however, the correlation was improved, suggesting there is scope for modification and improvement of these constitutive equation parameters in the classic approaches.
E-Book Content
www.lajss.org Latin American Journal of Solids and Structures
1 (2004)319–339
Material testing at high strain rate using the split Hopkinson pressure bar S.T. Marais, R.B. Tait, T.J. Cloete and G.N. Nurick∗ Blast, Impact and Survivability Research Unit (BISRU) Department of Mechanical Engineering, University of Cape Town Private Bag Rondebosch, 7701, Cape Town Abstract This paper reports on a particularly accurate and precise physical and experimental data capture and data processing system for high strain rate testing using the split Hopkinson pressure bar. The data processing incorporated the first four Pochammer-Chree modes and proved to be suitable for the tests that were carried out. To vindicate the design of the facility, two materials were tested, namely mild steel and copper, in both the as-received and annealed conditions. The actual test data, once appropriately processed was compared with both the Johnson-Cook (JC) and Cowper-Symonds (CS) high strain rate materials models, as well as with other published data. The correlation with previously published constants for the JC and CS models was not particularly good, due to perceived differences in the stress levels and prior work hardening, heat treatment condition and microstructure of these earlier tests, as these variables affect the constitutive equation constants. Using other published data, however, the correlation was improved, suggesting there is scope for modification and improvement of these constitutive equation parameters in the classic approaches. Keywords: split Hopkinson bar, dynamic material response, material characterisation
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Introduction
As a result of increasing demand to improve analysis of fast manufacturing techniques as well as safety in structures, it is necessary to determine accurate and reliable material properties at high strain rates. Conventional screw-driven or servo-hydraulic methods of testing materials at high strain rates are not entirely adequate as the strain rates are limited and also there are oscillations and stress waves set-up within the testing apparatus at high speeds. These oscillations and stress waves impair the transducer load cell reading, thus making the data obtained more complex to interpret reliably. These limitations were overcome with the development of the Hopkinson pressure bar (SHPB) [9], subsequently extended by Davies [5] and Kolsky [13]. Even with such a SHPB facility, data instrumentation, data capture and proce