Prediction of residual stress effects on fracture instability using the Local Approach

Effects of residual stresses induced in a three-point bend CTOD specimen on brittle fracture initiation are studied on the basis of the Local Approach. Compressive residual stresses are produced near the crack tip by preloading at room temperature. The material used is a 780 MPa class high-strength steel. Fracture tests are conducted at low temperatures (-75 and -40 degreesC) after preloading. The preloading apparently increases the fracture load and critical CTOD levels at the onset of brittle fracture from the reference levels at monotonic loading, which is due to the compressive residual stress field around the crack tip. The present study employs the Weibull stress fracture criterion to evaluate the brittle fracture resistance of the preloaded specimen. The Weibull stress is calculated by a 3D elastic-plastic FE analysis. Under the assumption that the critical Weibull stress at brittle fracture initiation is a material property independent of preloading, the critical CTOD of the preloaded specimens can be predicted from the fracture test results of monotonically loaded specimens.