The effects of strain rate and strain-rate history on the strength of metals at high strains have been investigated in two series of tests at room temperature using the split Hopkinson-bar method to obtain shear strain rates up to 3000 s-1. In the first series, various materials were subjected to nearly constant low and high strain rates in torsion, and apparent rate sensitivities were determined by comparison of the flow stresses at a given strain. The flow stresses were found to increase with rate even at large shear strains (~0.5), the effect of adiabatic heating being relatively small. In the second series, strain-rate changes of about six orders of magnitude were rapidly imposed at shear strains up to 0.6. The results indicate that the flow stress depends on the strain-rate history, and may be greater or less than that obtained at the same strain and strain rate in a constant-rate test. The differences in the observed behavior of copper, titanium, and mild steel are discussed, and it is shown that the data for copper are consistent with a proposed form of constitutive relation involving a functional of the strain rate.

Strength of Metals and Alloys at High Strains and Strain Rates

This study investigates the dynamic shear deformation and fracture behaviour of pure polycrystalline tungsten. Both wrought and recrystallized specimens are subjected to simple shear at room temperature and at strain rates ranging from 2×10 −3 to 4×10 3 s −1 by means of a torsional split Hopkinson bar. The effects of strain rate on flow response, strain rate sensitivities, thermal activation volume and deformation mechanisms are investigated. Fracture phenomena are analyzed by scanning electron microscopy to evaluate damage initiation and propagation. Compared with quasi-static rate tests performed at 2×10 −3 and 3×10 −3 s −1 , we find that strain rate obviously influences shear strength and ductility. Recrystallized tungsten exhibits better ductility and lower shear strength. Fracture feature observations show that, at low strain rate, recrystallized tungsten's equiaxial grain structure fractures by simple intergranular cleavage from grain boundary sources, whereas at high strain rate there is a mixture of intergranular and transgranular cleavage. Wrought tungsten's elongated fibrous structure, at low strain rate, fractures by an alternating mixture of intergranular and transgranular cleavage, but at high strain rate fractures by transgranular cleavage propagating across the whole specimen. We briefly discuss possible dynamic fracture mechanisms.

Dynamic shear deformation and failure behaviour of pure polycrystalline tungsten

Full understanding of the thermomechanical behaviors of materials at high strain rates and high temperatures are of great importance from not only scientific meaning but also practical value in engineering structure design and safety assessment. Great efforts have been made for abilities of operation the split Hopkinson bars, the most popular technique for experimental determination of mechanical behaviors of materials over the strain rates from 102 to 104 s−1 over the past 70 years, at high temperatures since 1960s. A review of experiment work is presented in this paper to give an overview of the development of experimental techniques at high temperatures based on Hopkinson bar systems. The principles of the split Hopkinson bar requires the loading bars avoiding temperature gradient or keeping relatively low temperature when performing high temperature testing. Techniques such as performing temperature gradient corrections, rapid heating or using special designed automatically assembled systems were proposed by researchers to enable the operation of the split Hopkinson bars at temperature as high as possible. Moreover, to the application of high speed photographic technique for capturing the dynamic deformation process of the specimen in high temperature Hopkinson bar testing, some key issues of eliminating the strong thermal radiation induced lights oversaturation and de-blurring of images due to insufficient exposure at high temperature and high strain rate condition, as well as fabrication of high contrast speckle pattern for high temperature digital image correlation measurement were also proposed. The technique can now enable the split Hopkinson bar testing to be performed at high temperature up to 1873 K under the loading conditions of compression or tension with the in situ observation and full field measurement of deformation as well. The paper concludes with summaries of the most important achievements and highlighting of the prospects, trends and remaining challenges for future research.

Experimental methods for determination of mechanical behaviors of materials at high temperatures via the split Hopkinson bars

To investigate the mechanical properties of a binary Al–2·8Li alloy at high strain rates in as received (solution treated) and aged conditions, quasistatic and dynamic torsion tests were carried out. With increasing strain rate, the strength and ductility of the as received alloy increased and a change of the fracture mode from intergranular to transgranular was also observed. However, in the aged alloy, the intergranular fracture mode was predominant at both quasistatic and dynamic rates. At the grain boundary offsets on the fracture surface of the aged alloy, a large number of very fine dimples were observed. This evidence suggests that a ductile precipitate free zone may be present there.MST/1043

Dynamic fracture of binary Al-Li alloy in torsion

A MACHINE FOR PRODUCING SQUARE TORSION PULSES OF MICROSECOND DURATION. Technical Report No. 1

A method has been developed to measure the related rotation of the flanges of a thin-walled tubular specimen during a torsion test. The method, which is based on the Moire-fringe technique, is capable of use at the high rates of strain encountered during a Hopkinson-bar test, as well as at low rates of strain. In the application described, the specimen gauge length is very short, but the method could be used for specimens of considerably longer gauge length.Direct calibration of the system is easily carried out at low angular velocities. The method can then be used to measure directly the specimen strain during a Hopkinson-bar test, and thus to check the value derived from measuremets of torsional waves in the elastic bars. Results of such comparisons are given, and it is found that the values given by the two method agree well, the differnce being attributable largely to inaccuracies in the torque measurement. The new method permits the determination of specimen deformation during the later stage...

J D Campbell

Papers

An optical method for direct measurements of strain in a torsional Hopkinson-bar apparatus