New η-factor equation for evaluation of J-integral of shallow cracked CT specimen considering R-O material strain hardening

Conventional η factor equation in ASTM 1820 for CT specimen is based on elastic-perfectly plastic material behavior. Also, it is applicable for deeply cracked specimen (a/W > 0.45). However, actual material always shows strain hardening behavior. Additionally, it may be sometimes needed to test CT specimens with relatively lower a/W (i.e. <0.45) to match the low constraint condition of some actual crack geometry. For both these conditions, η factor equation is not presently available in the open literature. Based on elastic-plastic FEA, the present work proposes a new plastic η factor (ηpl) equation for CT specimen considering Ramberg-Osgood material hardening index. The proposed equation is applicable for both shallow and deeply cracked CT specimen. The independency of ηpl with respect to applied load level and other R-O parameters have also been checked. The accuracy of predictions of proposed ηpl equation has also been indirectly checked.