Development of High Strain Rate Mechanical Testing for Metallic Materials

Abstract

Recent developments of high strain rate servo-hydraulic systems and high speed video imaging equipment have made the simple tensile test covering the quasi-static to dynamic range possible. However, obtaining reliable material data from the raw data produced from such tests requires a good understanding of the unique set of problems this testing technique can present. Tensile tests covering the quasi-static to 760 s-1 strain rate range were performed on Oxygen Free Electrolytic (OFE) copper, Al 6061-T6 and a Ta-2.5%W alloy. Modified standard sized tensile specimens were used in all tests and evaluations of the higher strain rate tests were carried out to understand the specimen dynamics at these high strain rates. Digital Image Correlation was used to measure strain at the higher strain rates and was ideal as a non-contact extensometer and could provide an indication if dynamic equilibrium is maintained throughout the test. The work strongly suggests that each material and specimen geometry will have its own strain rate threshold at which stress equilibrium is maintained. Appropriate methods were also necessary in processing the raw dynamic output to extract meaningful material data from the tests. Data obtained from the tests were successful in evaluating the materials behaviour over the quasi-static to dynamic strain rate range. The materials responded in a typical manner to that expected of their crystal structure and stacking fault energy, agreeing with results available from open literature. The tests performed in tension were compared with tests carried out in compression and showed the strain rate sensitivity in tension did not differ substantially to that in compression. Three constitutive material models were assessed, the Johnson-Cook (J-C) model was found to represent the experimental results of the OFE Cu and Al 6061-T6 materials well, but did not give such a good fit to the Ta-2.5%W material. The Zerilli-Armstrong (Z-A) model provided a good fit to Ta-2.5%W but not the OFE Cu and Al 6061-T6 materials. No satisfactory fit was achieved using the Mechanical Threshold Stress (MTS) model.