On the combined influences of Young's modulus and stress ratio on the LEFM fatigue crack growth process: A new mechanistic approach

Abstract

A new mechanistic approach (NMA) was used recently to examine the physical aspects of LEFM (long) fatigue crack growth (FCG) process in crack-ductile materials in stages I and II. In this paper, NMA is extended to examine both the physical and analytical aspects of the combined effects of Young's modulus, E and stress ratio, R, in the same stages of the same materials. It is shown that, (i) with submicroscopic cleavage or reversed shear mechanism operating in the pure form, E is the most influential intrinsic "material" property controlling FCG, (ii) E-dependence of da/dN is a natural consequence of near-crack-tip displacement control proposed previously, and (iii) the demonstrated similarity of FCG curves and the existence of characteristic "pivot points" on these curves for a "class of materials" results from E-influence which continues even at a higher R. A simple analytical model based on "strain intensity factor," Kε, which contains E-influence implicitly and controls da/dN in all materials irrespective of class, is proposed. Model-predicted Kε-based theoretical values of threshold, "Idealised Master Growth Curves (IMGCs)" and mechanism transition point, all agreed excellently with experimental data for at least three classes of materials, i.e. steels, Al-alloys and Ti-alloys at extreme R-values of 0 and ≥ 0.6. The Kε-parameter concept is used here to raise the status of the analysis of the E-effect from a simple "normalisation" to that of direct data "representation". Using NMA existing empirical relations are given some sound theoretical base. In addition to aiding in a clearer physical understanding of the FCG process, the unique IMGCs developed for different R-values are considered useful in quick, accurate and conservative life estimations, and performing failure analyses usually required in selection and design of materials. Copyright © 1996 Elsevier Science Ltd.