Long Fatigue Cracks — Microstructural Effects and Crack Closure

Fracture mechanics technology is an effective tool for characterizing the rates of fatigue crack propagation. Generally, fatigue crack growth rate (da/dN) in each loading cycle can be presented as a function of stress intensity range (ΔK), where ΔK = Kmax — Kmin, Kmax and Kmin are the maximum and the minimum stress intensities, respectively. A typical fatigue crack growth rate curve of da/dN versus ΔK can be divided into three regimes, i.e., Stage I (near-threshold), Stage II (Paris), and Stage III (fast) crack growth regions, as shown in Figure 1.Depending on the region of crack growth, fatigue crack growth behavior can be sensitive to microstructure, environment, and loading conditions [e.g., R (load) ratio = Kmin / Kmax]. In the nearthreshold region, fatigue crack growth rates are very slow, ranging from approximately 10−10 to 10−8 m/cycle. In this region, the fatigue crack growth rate curve eventually reaches a threshold stress intensity range, ΔKth, below which the crack would not grow or grow at an extremely slow rate. Typically, the value of ΔKth is operationally defined as the stress intensity range which gives a corresponding crack growth rate of 10−10 m/cycle. In the nearthreshold region, the influence of microstructure, environment, and load ratio on the rates of crack propagation is very significant.