Failure modeling of a nuclear pressure pipe with a shallow surface flaw

摘要

Shallow surface cracks in nuclear pressure pipes have been recognized as a major origin of potential catastrophic failure for such components. Failure assessment has been done currently by a one-parameter fracture mechanics approach. The J-integral is the one-parameter used, and has been proved to be useful to predict ductile crack initiation. However, when shallow surface and/or a fully plastic condition develops around the crack, the J-integral alone does'not describe completely the crack tip stress field and a second parameter is necessary. In this paper, a modeling of the three-dimensional elastic-plastic stress field around a crack tip is used to evaluate a second fracture parameter Q. This parameter measures the degree of triaxiality around the crack tip and is very important in a two-parameter fracture mechanics approach. The defect considered in the pipe is a three-dimensional shallow surface crack. The Finite Element Method is used to evaluate the full crack-tip stress field. The material, an ASTM A 516 Gr. 70 steel used in the nuclear industry, is modeled with a Ramberg-Osgood power law and flow theory of plasticity. A finite deformation theory was used to account for the highly nonlinear behavior around the crack tip. Numerical results are presented for the J integral parameter, normal stress in the very near crack-tip and for the second parameter Q, for a wide applied pressure loading in the flawed pipe.