First MIT Conference on Computational
Fluid and Solid Mechanics, Massachusetts Institute of Technology
Cambridge, Massachusetts, USA. June 12-15, 2001.
Nonlinear finite element modeling of surfacecracks in a nuclear pressure pipe
F. Labbea and J.R. Donosob
Universidad Tecnica Federico Santa Maria,
a
Mechanical Engineering Department and bMaterials Science Department.
Casilla 110-V,Valparaíso, CHILE
flabbe@usm.cl
Abstract
Surface cracks are often encountered in power plants component and have been recognized as a
major origin of potential catastrophic failure for such components.Failure assessment of cracks in
nuclear pressure pipes has been done currently by a one-parameter fracture mechanics approach.
The J-integral is the one-parameter used, and has been proved to beuseful to predict ductile crack
initiation. However, when shallow surface cracks and/or a fully plastic condition develops around
the crack, the J-integral alone does not describe completely the cracktip stress field and a second
parameter is necessary to consider. In addition, defects like surface cracks produces a very
complex stress field at the close vicinity of the crack tip due tothree-dimensional effects, highly
nonlinear large deformations, microestructural fracture process, etc. Hence, a detailed nonlinear
three-dimensional stress analysis needs to be done.
In this paper, anonlinear finite element modeling of the three-dimensional elastic-plastic stress
field around a crack tip in a pipe, is used to evaluate a second fracture mechanics parameter Q. The
pipe is subjectedto a pressure loading and the Q parameter measures the degree of triaxiality
around the crack tip.
The crack is modeled as a three-dimensional semi-elliptic inner surface flaw in the axialdirection. The depth-to-length ratio of the elliptic crack is a/2c = 0.1, and the depth-to-thickness
ratios analyzed are from shallow crack (a/t = 0.125) to a deep crack (a/t = 0.75). The finite element... [continua]

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