Classically determined effective ΔK fails to quantify crack growth rates

Decomposition of the resistance to fatigue crack growth into the intrinsic and extrinsic component is very important for understanding of fatigue failure mechanisms, relation to microstructure and modelling of residual fatigue life. Crack closure for four grades of steel were estimated by the difference between Kmax values and the effective ΔKeff values (measured at the load ratio R = 0.8) corresponding to the same crack growth rate. The results showed that crack closure values obtained by the difference Kmax – ΔKeff were not in agreement with the available crack closure models, both the Newman’s model of plasticity-induced closure and the results from finite element analysis. The discrepancies could not be explained by the effect of mean stress, specimen thickness, loading amplitude or T-stress. Therefore, the application of fracture mechanics to fatigue cracks should be revisited. It was pointed out that ΔKeff may not be a good parameter for quantification of the crack driving force, since the relationship between Kmax – Kcl and the cyclic plastic deformation at the crack tip might not be linear.