Improving Prediction of Limit Bending Load for Wall-Thinned Pipes by Reconsidering the Effect of Biaxiality

In this paper, a failure criterion applicable to large-strain finite element analysis (FEA) results was studied to predict the limit bending load M_c of the groove shaped wall-thinned pipes, under combined internal pressure and bending load, that experienced cracking. In our previous studies, Meshii and Ito [1] considered cracking of pipes with groove shaped flaw (small axial length δ_z in Fig. 1) was due to the plastic instability at the wall-thinned section and proposed the Domain Collapse Criterion (DCC). The DCC predicted M_c of cracking for small δ_z by comparing the von Mises stress σ_Mises with the true tensile strength σ_B. However, it was indicated that the predictability of M_c was not necessarily sufficient. Thus, in this work, attempts were made to improve the accuracy of M_c prediction with a perspective that multi-axial stress state might affect this plastic instability. As a result of examination of the various failure criteria based on multi-axial stress, it was confirmed that the limit bending load of the groove flawed pipe that experienced cracking could be predicted within 5 % accuracy by applying Hill’s plastic instability onset criterion [2] to the outer surface of the crack penetration section. The accuracy of the predicted limit bending load was improved from DCC’s error of 15% to 5%.