Application of Strip Model to Crack Tip Resistance and Crack Closure Phenomena

This paper systematically investigates plasticity effects in fatigue crack propagation in structural materials. These effects lead to the crack closure phenomena and the crack tip resistance to propagation due to residual compressive stresses. A simple application of the strip-model, elastic-plastic analysis leads to the exact distribution of the residual stresses at the crack tip. The compressive stresses are explicitly demonstrated and computed with their significance discussed. Experimental techniques based on ultrasonic surface waves are used to directly measure the effects of the residual stresses. Structural materials of aluminum, steel, and titanium are used for these studies. The experimental data clearly reveal the extents of crack closure and crack tip resistance to applied loadings. It is found that the amount of crack closure increases with the actual crack length before gross scale plastic flow occurs. From this investigation, it is particularly significant that the concepts of crack closures and crack tip resistive forces to propagation are not only demonstrated by direct experimental means, but also their extents can be quantitatively measured. Such measurements will facilitate modifications in analytic models for predicting crack growth rates.