Constraint characterization and fracture initiation analysis for semi-elliptical surface cracks in reactor pressure vessel

The constraint and fracture initiation for semi-elliptical surface cracks in an actual large scale reactor pressure vessel (RPV) have been investigated by finite element analysis. The results show that the overall constraint level of the cracks increases when the crack depth a/t increases and crack aspect ratio a/c decreases. For most cracks with a/t = 0.25-0.8 and a/c = 0.2-2, the constraint level is higher than the standard plane strain specimen, and the conventional fracture assessment may possibly yield non-conservatism. The aspect ratio a/c has obvious effects on the distributions of the new unified constraint parameters Ad* and the elastic-plastic stress intensity factor KJ and the fracture initiation position along the crack fronts. For low aspect ratio cracks with a/c < 1.0, fracture initiation probably appears in the deepest region of the crack. For high aspect ratio cracks with a/c >= 1.0, fracture initiation is likely to occur in the near-surface region. To improve the accuracy of fracture assess-ment for cracks in the RPV, the constraint effect requires to be incorporated. The engineering estimation methods of the parameters Ad* and KJ at the crack initiation positions have been provided and verified.