New load-line-displacement based η factor equation to evaluate J-integral for SE(B) specimen considering material strain hardening and no-crack displacement effect

The η factor plays an important role in the experimental evaluation of J-integral and J-R curve. While for a deeply cracked SE(B) specimen, established η factor value is available; for shallow cracked specimen, there is a variation in the η values proposed by different researchers. Although, it has been found that η factor depends on the material strain hardening behavior, most of the available equations are derived based on elastic-perfectly plastic material behavior. Additionally, almost all the existing η factor solutions do not consider the subtraction of no-crack displacement while calculating the strain energy although basic definition demands it. Hence effort has been made in this work to cover these gap areas. Plastic η factors (ηpl) have been calculated for a wide range of relative crack depth (a/W) and Ramberg-Osgood material strain hardening exponent 'n' with the help of elastic-plastic FE analysis. Calculated results are validated using literature values. The variation of ηpl with a/W and 'n' has been studied considering the effect of no-crack displacement. Based on these analyses, a closed-form equation has been proposed which is applicable for a wide range of a/W and material strain hardening exponents. The ηpl was shown to be independent of applied load level and various R-O parameters.