Surface Crack Life Prediction: An Overview

Successful application of fracture mechanics methodology to component fatigue life prediction requires knowledge of flight cycle loading, local stress and material conditions, da/dN characterization for the material, and the stress intensity factor(s) for the appropriate crack geometry(ies). It is generally assumed that all of the aforementioned conditions are well within current state-of-the-art design procedures for through-thickness cracks. It is further assumed that the part-through, surface crack stress intensity factor distribution is now sufficiently easily characterized that accurate fatigue life predictions can be made for this class of crack geometries. Major unresolved issues for surface cracks exist for small surface cracks, which are initiated within surface layers of machining stress and flight cycle-induced plastic zones near structural notches. The issues include definition of the material properties within the surface layer, unknown surface “values” of the stress intensity factor distribution, crack growth through highly varying stress/material property fields at the surface, and the applicability of macrocrack growth models to small cracks. Further experimental and analytical effort is required in order to achieve accurate fatigue life prediction design methodologies for small surface crack problems in real structures.