Effect of superimposed monotonic fracture modes on the Δ K and Kmax parameters of fatigue crack propagation

Abstract

Fatigue crack growth is represented using the Unified Approach in terms of two crack-tip driving forces, Δ K and K max. In this approach, crack growth is related solely to fracture mechanics parameters. Using this approach, crack-growth trajectory path can be defined in terms of ΔK ∗ –K max ∗ , which are two limiting values (they are the same as the thresholds when d a/d N=0), for a given crack-growth rate. Pure fatigue is represented by a line ΔK ∗ =K max ∗ (a 45° line in the Δ K∗– K max plot). Normally, fatigue crack growth occurs along with superimposed environmental effects at low crack-growth rates (or low Δ K values) and/or with monotonic fracture modes at high crack-growth rates (or high Δ K values). Both of these superimposed effects are functions of the applied K max. They cause shifts in the ΔK ∗ –K max ∗ crack-growth trajectory maps. In the Unified Approach, the ΔK ∗ =K max ∗ line provides a reference that can be used to quantify the superimposed effects of environment and overload fracture. This paper presents a detailed analysis of these effects, taking as an example an idealized superimposition model. The model predictions are compared with the actual materials behavior of several aluminum alloys and their composites. In SiC particulate-reinforced Al composites, the coarse particulates ahead of the crack-tip fracture readily, causing rapid growth of fatigue crack. The fracture process is K max-dependent and hence this superimposed monotonic fracture process get reflected in the crack-growth trajectory map represented in terms of ΔK ∗ –K max ∗ . The analysis also shows that the relative ratio of monotonic to fatigue modes varies with load ratio, R. At low load ratios, due to low K max value, the crack-growth process is predominantly fatigue-dominated, while at high R-ratios due to high K max value crack growth is fracture-dominated. These regimes are reflected differently in the trajectory map. It is shown that crack-growth trajectory maps provide a powerful tool to investigate the changing fatigue mechanisms as a function of load ratio, crack-growth rate and environment. Without proper understanding and quantification of these mechanisms as function of crack-tip driving forces, any life-prediction methodology can only be empirical at best.