OS4-2-4 Estimation of applied stress in the near threshold of fatigue crack propagation utilizing high frequency current impedance and hardness measurement

Abstract

In the failure analysis of fatigue fracture accident of industrial machinery, the estimation of applied stress on the fatigue crack is an important issue in order to determine the cause of failure and to prevent another accident. The estimation of applied stress using fractographic technique has been already established. However, the estimation can be applicable in relatively high stress intensity factor region. Low stress intensity factor region, however, is much more important in the real engineering. The objective of this study is the development of estimation method of applied stress, which can be used in the near threshold region of crack propagation. High frequency current impedance method and fracture surface hardness method were examined. In the impedance method, there was a correlation between the high frequency current impedance of fatigue fracture surface and stress intensity factor range. In the hardness method, micro Vickers hardness was measured on the fatigue fracture surface. The hardness parameter of fatigue fracture surface was defined as the area of indentation mark in order to reduce the effect of surface roughness of the fatigue fracture surface. The hardness parameter had a correlation with the stress intensity factor range in the near threshold region. Each method can be used in the stress estimation in low stress intensity factor region.