The Critical Volume Method in Fatigue Analysis

Abstract

In recent years we have developed several new methods for predicting the endurance limits of components which fail by fatigue from stress concentrations. These methods were arrived at by improving two existing theoretical models: the critical volume (process zone) concept and the equivalent stress-intensity concept. Our approach renders these methods suitable for application to components of any three-dimensional geometry and allows the necessary material constants to be calculated from well-known existing material properties: the plain-specimen endurance limit and the crackpropagation threshold. This paper summarises the results of an extensive validation programme involving both notched specimens and industrial components. A high degree of reliability was obtained, with predictions of fatigue limit loads or stresses falling almost always within 20% of the experimental values. In the latter half of the paper we consider the mechanistic basis of these theories, and present a number of ideas in which these essentially empirical models are linked to the underlying processes of fatigue crack initiation, short crack growth and long crack growth. These mechanistic models are able to give some useful insights, but as yet we are not able to completely explain the excellent predictive success of these methods.