The Effect of Geometry on Crack Formation

Publisher Summary This chapter discusses the effect of geometry on crack formation. To properly assess the fatigue integrity of components, the local material behavior must be understood, both from the point of view of the number of cycles required to initiate a crack (the crack formation life) and the subsequent crack growth, while the crack is still within the influence of the geometry of a stress concentrator. A fundamental approach necessitates the ability to determine precisely the division between nucleation and Stages I and II crack propagation. To estimate the crack formation life of a component, it is necessary to relate the conditions at the critical region to known material behavior and allow for influencing factors. If it is assumed that the conditions in a component can be represented by tests on plain specimens of the type used in obtaining fatigue data, this may provide the basis for assessing crack formation life. Two possible approaches are at present available for evaluating the strain at a concentration feature, namely, the Neuber and the Hardrath-Ohman or modified Stowell methods. The accuracy of the Neuber and modified Stowell methods is found to depend upon the shape of the cyclic stress strain curve and the degree of plastic strain.