A NEW UNIFIED PHENOMENOLOGICAL MODEL FOR FATIGUE CRACK INITIATION, SHORT CRACK PROPAGATION, LONG CRACK PROPAGATION AND CLOSURE EFFECTS

A unified, physical-phenomenological model has been developed for predicting fatigue crack initiation and growth under uniaxial external loadings. In the model, Stage I fatigue crack advance (initiation and growth) is estimated using the range in crack tip non-elastic shearing displacement, whereas Stage II fatigue crack advance (initiation and growth) is calculated using the range in non-elastic crack tip opening displacement and the local peak tensile stress. The effects of "weak" surface grains, grain boundaries, other relevant microstructual features and redistribution of the local stresses during crack closure and re-opening have been incorporated into the model in order to represent the actual local mechanical conditions at the crack tip under various loading conditions.; The model has been used to predict the differences in the growth behavior of short and long cracks, using 304 stainless steel as an example. The effects of tensile mean stresses and variable amplitude loadings on fatigue life and crack growth behavior were also predicted.; The role played by the weakness of the surface grains in the behavior of short cracks has been investigated experimentally using the dc electrical potential technique. For this study, a titanium alloy, Ti - 6Al - 4V was used. The experiment involves comparing growth rates using an unconstrained surface flawed sample (in which the material is free to shear in and out along the plane of the crack) and another in which sliding in and out along the crack is constrained mechanically. Application of surface constraint reduces the growth rate of short surface cracks at low applied stress levels. This effect is believed to occur because the constraint reduces the plastic strain concentration which the free surface effect would otherwise produce. Hence, the experiment provides direct evidence of the free surface effect. At high stresses, surface constraint is believed to be due to the fact that at high strains, the whole grain becomes increasingly filled by dislocations leading to a smaller interaction by individual slip bands with grain boundary. This effect is believed responsible for the decrease in the depth of the "well" (minimum in crack growth rate as the crack extends) with increase in applied stress range.