Study on the Parameters Applicable to the Stress Estimation of Fatigue Fracture Surface without Striations

Abstract

In the failure analysis of fatigue fracture, the estimation of applied stress by examining the fatigue fracture surface is important in order to investigate the cause of accident. A fractographic method using striations has been established. Striations, however, can be observed only when a crack propagated at relatively high rate. In the case of fracture accident after a long period of operation under low stress condition, striations are rarely observed on the fracture surface. The estimation of applied stress is almost impossible in such a case. Therefore, an estimation method which can be applied in the low stress intensity region without striations is desirable. In this study, the high frequency current impedance method and the hardness measurement method were investigated as candidates aiming the application to failure analysis. The former uses the electrical impedance of fatigue fracture surface. A positive relationship was found between the electrical impedance and stress intensity factor range. The latter uses micro hardness which was measured directly on the fatigue fracture surface. The hardness of fatigue fracture surface was correlated with the stress intensity factor range. These parameters might be used as parameters in the estimation of applied stress by examining the fatigue fracture surface without striations formed in low stress intensity region.