Advances in computer-aided crack length measurement during fatigue crack growth testing

Abstract

The accurate measurement of crack length is one of the most important aspect of fatigue crack growth rate (FCGR) testing. Of the various methods available for crack length measurement, compliance technique is very popular due to the facilities it provides for easy automation. In the compliance technique, compliance crack length (CCL) rela-tions are used for correlating the compliance, computed from measurements of displacements & loads during fatigue cycling, to the crack length contained in the specimen. CCL relations are specific not only to the specimen geometry, but also to the location on the specimen body at which displacements are measured. This specificness is not very conducive to the experimentalist as it introduces errors in the measured crack length if the location of displacement measurement is not accurately maintained. With variations in specimen geometry and size, the accu-rate positioning of displacement measurement transducers is not an easy task. In order to provide greater flexi-bility in the use of the compliance technique, a new scheme has been proposed in this paper. Modelling the defo-rmation of a fracture mechanics specimen during fatigue cycling as rotation of two rigid hinge about a hringe point, the relationship between the location of the hinge-point with crack length has been established using finite element analysis for the single-edge notched three point bend specimen. Further -an iterative method has been developed which can be implemented in the background software for on-line crack length measurement. It has been shown that the iterative method converges rapidly to give the crack length with high accuracy.