On alternative strain rate analysis for direct impact method

Abstract

The direct impact method provides a higher sample deformation rate in comparison with the classical Split Hopkinson Pressure Bar method. The principal assumption of strain rate calculation for the direct impact method is constancy of velocity of the impacted face of the specimen during the impact. This assumption delivers reliable results for materials with relatively low yield strength and hardening rate. The present study proposes an alternative procedure for calculation of the specimen strain rate in order to improve accuracy of the direct impact method for wide range of metals and alloys. The proposed procedure is based on the assumption that the transmitted pulse can be splitted into parts, one of which corresponds to stress and the other to strain rate. The procedure has been validated by finite element analysis and the semi-analytical modelling of the direct impact tests of pure copper C101 and additively manufactured Inconel 718 alloy. The proposed method qualitatively changes the shape of the stress-strain curve by adding an unloading area. The numerically estimated accuracy of the method is limited by 4.5% for Inconel 718 and 0.6% for copper C101. The proposed alternative analysis of the strain rate for a direct impact has a number of advantages over the classical methods and can be used in the study of various materials in a wide range of strain rate.