球体衝突試験に基づく流動応力のひずみ速度依存性評価手法の提案

抄録

In metallic materials, the flow stress significantly depends on the strain rate. At high strain rates exceeding 10⁴ s^-1, the flow stress tends to increase drastically with increasing strain rate. However, the conventional split-Hopkinson pressure bar method is difficult to evaluate the strain rate dependency of the flow stress at higher than 10⁴ s^-1. In this study, a simple estimation method for the material constant C in the Johnson-Cook flow stress model, which is associated with the strain rate dependency, was proposed. In this method, the material constant C is estimated from the indentation sizes formed via a ball impact test and an instrumented ball indentation test. The fundamental equation was theoretically derived based on the energy conservation and the heat conduction equation during impact process. Then, the average strain rate during impact process was determined via a finite element analysis. The verification with the finite element analysis revealed that the strain rate dependency can be accurately estimated at higher than 10⁴ s^-1 by the proposed method.