Investigation on the method to estimate fracture properties of semi-elliptical surface cracked plate under three-point bending

Abstract

This study develops the model of load–displacement relationship for a semi-elliptical surface cracked plate under three-point bending, based on the principle of equivalent energy density in elastoplastic conditions. Semi-analytical expressions of the equivalent stress intensity factor (SIF) K_Ⅰ and the equivalent J-integral are subsequently derived. The parameters of the load–displacement model are calibrated by elastoplastic finite element analysis, and the accuracy and applicability are validated. This work investigates the distributions of K_Ⅰ and J-integral along the front of surface cracks. The distribution functions of K_Ⅰ and J-integral are established, accounting for the effects of ellipse ratio a/c, relative depth a/t, and parametric angle ϕ. Additionally, this work proposes a compliance method for measuring instantaneous crack propagation. Based on these findings, the methods for evaluating the fatigue crack growth rate and quasi-static ductile fracture of semi-elliptical surface cracked plate under three-point bending are developed. By using A508-Ⅲ pressure vessel steel, fatigue crack growth and quasi-static fracture tests are conducted on the plates with varying surface crack geometries and dimensions. The results reveal that the crack growth rate is lowest at the deepest point of the surface crack and highest at the surface point. Compared to through cracks, surface crack exhibits slower fatigue growth rates, with significant differences in the Paris model parameters. The results of quasi-static fracture tests demonstrate that the J-resistance curve is highly sensitive to the surface crack morphology and is lower for surface cracked specimens compared to through cracked specimens.