A detailed physical analysis of the r-effect on lefm fatigue crack growth-II. On the combined roles of growth mechanisms and stress ratio

Abstract

A new-approach-based previous detailed physical analysis procedure that examined the combined roles of critically stressed crack-tip zone, Vc, LEFM parameters and stress ratio, R, in explaining the R-influence on the LEFM (long) fatigue crack growth (FCG) process is extended here to examine in detail mainly the combined roles of two intrinsic crack growth mechanisms (ICGM), i.e. "Submicroscopic Cleavage" (SMC) and "Reversed Shear" (RS) and stress ratio. This paper is limited to analysing FCG in stages I and II of "crack-ductile" materials. Alternatively, new definitions and interpretations for FCG threshold and ICGM are given in terms of atomic level displacements in Vc. On this basis, two mechanism-based "physical" models of the growth process applicable to stages I and II, respectively, are proposed. It is suggested that Kmax-dependence of the static-elastic-fracture and ΔK-dependence of slip-plane "shear-decohesion" produced by "slip reversal" in Vc are the real causes for the observed R-sensitivity and R-insensitivity of FCG in stages I and II, respectively. It is concluded that, except for increasing the magnitude of displacements and hence da/dN with AK the basic FCG process with RS mechanism remains the same, irrespective of ΔK and R values. Macroscopic and some microscopic experimental evidences are cited in support of these models. The study reported is intended to provide a more detailed, systematic and comprehensive view of these FCG mechanisms, and to further strengthen and widen the scope and applicability of the highly capable and successful new mechanistic approach which is fully independent of crack-closure arguments. Copyright © 1996 Elsevier Science Ltd.