Direct Compression Loading Using the Pre-stretched Bar Technique: Application to High Strains Under Moderate Strain Rates

Abstract

The mechanical properties of ductile materials are of high interest in the domain of energy absorption, high speed forming processes, machining processes and ballistics. The range of the intermediate strain rates has always been required for the main testing devices, particularly for classical Split Hopkinson Pressure Bars. But due the transient aspect of the loading conditions, high ductility levels and moderate rates of strain are not easily to attain. As the length of the striker is mostly limited to 1 m due to practical reasons, the duration time of metallic strikers is theoretically close to 0.4 ms and then limited to 0.04 in total strain at 100/s. However, it is admitted that a long duration time is required to determine mechanical responses when high plastic strains are observed under real-life conditions. The technique of the pre-stretched bar is a good candidate to determine high strain responses of metallic materials up to fracture for the range of moderate rates of strain. The main challenge consists in the initiation of the elastic waves’ system to load samples under direct compression. The authors propose here a new configuration of the pre-stretched technique where the pre-stretched part is preserved but the compression loading generated through the sample is radically modified with comparison to the tensile testing version developed by Albertini in the 70′. The strategy is based on FEM approach to generate a representative model of the classical pre-stretched bars combined with analytical calculations. The experimental tests are performed on samples extracted from Aluminum and brass alloys so as to validate the modified pre-stretched bars designed for compression loadings at moderate strain rates.