Determination of Johnson-Cook constitutive model coefficients considering initial gap between contact faces in SHPB test

When a material is deformed at high speed, it is well known that the flow stress is highly dependent on the strain rate and temperature. So, many studies of the stress-strain relationship at high strain rate condition have been conducted using the Split Hopkinson Pressure Bar (SHPB) test. Studies about the effects of friction, eccentricity, declination, and joints were also conducted. However, no study has been conducted about the effects of gaps formed by surface roughness of the contact faces. In this study, effect of initial gap between specimen and bars was investigated by SHPB test. The process of compressing initial gaps formed by rough places on contact surfaces was described as the initial gap compression effect. The proposed effect on measured stress-strain relation was confirmed experimentally and numerically by adjusting the surface roughness. Considering the initial gap compression effect, an equivalent gap compensation method was proposed for efficient finite element analysis. A methodology for determining Johnson-Cook (JC) coefficients using the optimization-based inverse method and the proposed equivalent gap model was presented. Also, the friction coefficients were measured to increase the accuracy of the determined coefficient. As a result, it was shown that accurate JC coefficients could be determined using the proposed methodology; the determined JC coefficients were considered to be reliable for various strain rates. Also, stress-strain relations obtained with and without initial gap compensation were compared to show the necessity of gap compensation. Consequently, it was revealed that consideration of the initial gap compression effect is essential for accurately determining plastic constitutive model parameters.