Fatigue extrusions, slip band cracking and a novel hybrid concept for fatigue crack closure close to the crack tip

Small fatigue cracks in the aluminium alloys 2024-T351 and 8090-T8771 have been observed to grow by a heterogeneous process of three-dimensional slip band decohesion. Local, finely-divided crack path excursions at slip bands, the pull-out of small sections of decohered slip band and the formation of fatigue extrusions at slip bands produced micro-roughness on the crack surfaces. The hybrid form of plastic deformation induced surface roughness provides a potential source of crack closure. This may be effective in reducing the nominal crack growth driving force at small cracks and near threshold for long cracks when the crack tip opening displacement is small. A higher magnitude of closure at small mode I crack sizes and a spread in the levels of closure for a given crack size are consistent with the dependence of the slip band related micro-roughness on the individual crack path. Fatigue extrusions imply a dynamic form of crack closure that could develop immediately at the crack tip even for fatigue at high mean load. Complex influences of crack size, grain orientation and crack path on crack closure and crack growth rate indicate that a stochastic approach to determining fatigue life is appropriate for the critical small crack regime in these and similarly behaving materials.