Crack closure and microstructurally short fatigue crack growth rates

Previous research efforts modeling fatigue crack closure have assumed that the material surrounding the crack tip behaves isotropically. The current model implements a double slip geometry to simulate fatigue cracks traversing through the microstructure using finite elements. By varying the crystallographic orientation of the slip systems in the model, with respect to the crack growth direction, fatigue crack closure levels S-open/S-max, were found to vary in the range 0.02 to 0.35. Corresponding to this variation, a nonlinear crack growth law was used to estimate expected variations in crack growth rates for microstructurally short fatigue cracks when they grow through grains with different orientations. The resulting predictions for da/dN variations span the rand 1.5 less than or equal to da/dN(max)/da/dN(min) less than or equal to 60, corresponding to a change in the crack growth exponent, m, form 1 to 10.