A detailed physical analysis of the R-effect on lefm fatigue crack growth - I. On the combined roles of critical zones, LEFM parameters and stress ratio

Abstract

A recently proposed comprehensive new approach that accurately explained and quantitatively predicted LEFM (long) Fatigue Crack Growth (FCG) behavior of metals and alloys in three stages, involved on a "general" or "phenomenological" level the combined roles of four interrelated variables, e.g. a critically stressed crack-tip zone, Vc; LEFM parameters, Kmax and ΔK; two growth mechanisms, "submicroscopic cleavage" and "reversed shear"; and stress ratio, R. For obtaining a deeper and clearer understanding of the FCG process, the new approach is extended here to examine the physical roles of these in greater detail. This paper focuses mainly on the roles of Vc, R and LEFM parameters in stage I growth, including the threshold condition. Vc is divided into three types and the conditions of the importance of each are specified. Emphasizing the role of Kmean against R it is shown that this parameter, which does not appear to "control" FCG directly, plays a unique role of inducing a mechanism-transition that changes a Kmax-controlled stage I growth into a ΔK-controlled stage II growth. The transition-ΔK values at different R, ΔKcl(R) which were determined empirically earlier, are estimated theoretically here. These estimates showed fairly good agreement with experimental data for steels and Al-alloys. In addition, the physical basis for the observed positions and slopes of R = 0 and R ≥ 0.6 stage I growth curves is given using the concepts of the new approach, which is fully independent of the crack-closure arguments. Copyright © 1996 Elsevier Science Ltd.