An experimental program has been conducted on a Type 316L stainless steel to analyze the behavior of surface and embedded metallurgical defects in components.
Crack closure measurements have been performed under plane stress and plane strain conditions, under constant amplitude loading, and after overloads.
The crack opening load was measured by four different methods including surface, center, and bulk measurements.
Fatigue crack growth rates were determined, at different R ratios, under constant amplitude and under random loading conditions, using a stationary Gaussian sequence (I = 99%). Numerical analysis of the crack closure phenomenon has been carried out.
The principal results obtained are: 1. Immediately after overload, crack blunting occurs, and there is no crack closure: this fact is observed using both the different experimental measurement techniques and the calculations. 2. Under plane stress conditions, calculations have shown that crack closure decreases after overload over a distance of ∼1 mm, then increases; good agreement was found between experiments and calculations. 3. Under plane strain conditions, calculations have shown that crack closure rapidly increases after overload, the initial value is reached after ∼1 mm propagation; this was not detected experimentally because a pure plane strain state did not exist in the tested specimens. 4. The fatigue crack growth rates, da/dN, under random loading were expressed in terms of ΔKrms and ΔKeq. A better correlation of fatigue crack growth rates under constant amplitude loading and random loading was found using ΔKeq compared to ΔKrms.
The equivalent load approach is appropriate for describing the crack growth under this narrow band sequence that does not present a strong interaction between cycles. It may not be applicable to other load spectra.