Effects of strain hardening and stress state on fatigue crack closure

A theoretical analysis is performed for the effect of strain hardening on fatigue crack closure in plane stress and plane strain conditions based on a modified strip yield model. It is shown that the effect of the strain hardening exponent n on the opening stress ratio is small, less than 1.3% in plane stress and about 10% in plane strain, respectively, for n>3. Comparing with available finite element results and experimental data in both stress states shows good agreement. When the opening stresses in plane stress and plane strain are applied to deal with fatigue crack growth data in very thin and thick plates respectively, the data for different stress ratios collapse into a single curve.