A Practical Inverse Identification of Johnson–Cook Parameters at Intermediate Strain Rates Using Split Hopkinson Pressure Bar Test

Abstract

A practical inverse method based on the hybrid experiment-finite element (FE) simulation is proposed for identifying strain rate sensitivity of a metal covering intermediate to dynamic loading conditions. The methodology uses the dynamic split Hopkinson pressure bar (SHPB) test for measuring mechanical responses at medium strain rates by optimizing temperature increase, non-uniform strain rate distributed in the non-standard notched SHPB specimens. From the standard dynamic SHPB test, the thermal softening index of the Johnson–Cook (JC) model is first determined by fitting the FE simulation to temperature changes in the specimen. The discrepancy between the measured and predicted flow stresses with the conventional JC model can be attributed to the assumption of constant strain rate sensitivity. Therefore, the new approach using the notched SHPB specimens under dynamic loadings is introduced to identify mechanical responses covering a broader range of strain rate. Finally, the strain rate sensitivity parameter in the JC model as a function of strain rate is evaluated through the inverse FE scheme, in which the sigmoidal function is determined to be optimum by predicting the flow stresses under wider range of strain rate, especially in the intermediate range of strain rate. The present study provides a new methodology based on hybrid experiment and numerical simulation to fill the gap in predicting mechanical responses between quasi-static and dynamic tests using commonly available tensile test and SHPB test. Graphical Abstract: (Figure presented.)