Determination of Paris law constants with a reverse engineering technique

Abstract

Paris law constants are commonly obtained with a well established procedure based on standard specimens, notched and pre-cracked. Pre-cracking produces through cracks with stable shapes, nearly straight, similar during all propagation. However, in several situations specimens with corner and surface cracks are recommended. In these cases cracks having significant propagation will continuously modify their shape, beginning with corner or surface geometries and subsequently transforming into through cracks, resulting into a transition region with significant crack shape modification. The aim of this paper is to determine Paris law constants from the analysis of crack shapes on the surface of fracture, in regions of intense shape modification. A double-U specimen of a new generation nickel base superalloy was used to obtain experimental crack shapes within transitory region as well as the number of load cycles between them. An automatic crack growth technique based on the finite element method (FEM) was employed to obtain fatigue constants from crack propagation data.