Development of High Strain Rate Mechanical Testing for Metallic Materials

摘要

Recent developments of high strain rate servo-hydraulic systems and high speed videoudimaging equipment have made the simple tensile test covering the quasi-static touddynamic range possible. However, obtaining reliable material data from the raw dataudproduced from such tests requires a good understanding of the unique set of problemsudthis testing technique can present.udTensile tests covering the quasi-static to 760 s-1 strain rate range were performed onudOxygen Free Electrolytic (OFE) copper, Al 6061-T6 and a Ta-2.5%W alloy. Modifiedudstandard sized tensile specimens were used in all tests and evaluations of the higherudstrain rate tests were carried out to understand the specimen dynamics at these highudstrain rates. Digital Image Correlation was used to measure strain at the higher strainudrates and was ideal as a non-contact extensometer and could provide an indication ifuddynamic equilibrium is maintained throughout the test. The work strongly suggestsudthat each material and specimen geometry will have its own strain rate threshold atudwhich stress equilibrium is maintained. Appropriate methods were also necessary inudprocessing the raw dynamic output to extract meaningful material data from the tests.udData obtained from the tests were successful in evaluating the materials behaviourudover the quasi-static to dynamic strain rate range. The materials responded in audtypical manner to that expected of their crystal structure and stacking fault energy,udagreeing with results available from open literature. The tests performed in tensionudwere compared with tests carried out in compression and showed the strain rateudsensitivity in tension did not differ substantially to that in compression.udThree constitutive material models were assessed, the Johnson-Cook (J-C) model wasudfound 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 MechanicaludThreshold Stress (MTS) model.