Study on Validity of Stress Intensity Factor in CT Specimen for Materials With Low Yield Stress

Abstract

This paper discusses the validity of a stress intensity factor K for compact tension (CT) specimens of stainless and low-alloy steels by computing J-integral values. In fracture mechanics testing, specimen configurations and loading conditions are strictly regulated to meet the small-scale yielding (SSY) condition and obtain valid data. For some materials, particularly those without an obvious yield point, it is hard to meet the SSY condition. The regulations specified in fracture mechanics test standards are based on experimental results. The theoretical reason for the SSY condition is not necessarily clear and the condition seems conservative. This study focuses on a correlation with the stress intensity factors K and the J-integral values under the SSY condition. J-integral values of CT specimens were analyzed by elasto-plastic finite element analysis. Subsequently, the validity of the stress intensity factor K was checked by comparing K and J–integral values to determine whether CT specimens were under the SSY condition or not. To simplify the comparison, J-integral values were converted to K values, equivalent stress intensity factor KJ values. When K values were low enough to meet the SYY condition of the specimen, they equaled KJ values. Meanwhile, KJ values exceed K values in a high K region, which means the specimen is no longer under a SSY condition. It is possible to determine whether the specimen is under the SSY condition or not by comparing the values of KJ and K. This paper evaluated the validity of K values using the correlative relationship and showed that they were valid to some extent even if loading conditions and specimen configurations were outside the scope of the specification by the test standards. The validity of K values with 0.2% offset strength or flow strength instead of yield stress is also confirmed by this method.