Proposal of an Estimation Method for Temperature Dependence of Flow Stress in a Wide Range of Strain Rates Based on a Difference between Indentation Sizes

Abstract

In general metallic materials, flow stress during plastic deformation depends on plastic strain, plastic strain rate and temperature. At an extremely high strain rate more than 10⁴ s^-1, the flow stress tends to drastically increase with strain rate. Moreover, some researchers reported that the flow stress also increases with temperature at such high strain rate. This indicates that the temperature dependence of the flow stress must be evaluated under a corresponding strain rate condition. However, a conventional split-Hopkinson pressure bar method is difficult to evaluate the temperature dependence due to the measurement principle. In this study, a simple estimation method for the temperature dependence of the flow stress based on the difference between the indentation sizes was proposed. In this method, the temperature dependence is estimated from the difference between the indentation sizes formed at room and high temperatures via a high-velocity impact test, a drop-impact test, and an indentation test with a spherical impactor. The fundamental equation in the proposed method was derived based on the energy conservation during impact process and the expanding cavity model combined with the Johnson-Cook (JC) flow stress model. The equation of thermal conduction was also introduced into the cavity model. The verification with the finite element analysis revealed that the temperature dependence of the flow stress can be accurately estimated under various materials, temperatures, strain rates, and radii of impactor by the proposed method.