A comparison of microcrack closure load development for stage I and II cracking events for Al 7075-T651

Abstract

Closure stress measurements for both Stage I and II surface microcracks of lengths comparable to grain size are made using a compliance technique for Al 7075-T651 specimens fatigued in air at 30 pct relative humidity. As previously reported for Stage II cracking in Al 2219-T851, residual crack openings are observed for zero load which can be empirically related to the crack closure stress. This functional dependence is subsequently used to calculate closure stress values for both Stage I and II events for microcracks in Al 7075-T651 on the basis of residual crack opening values. Stage I crack interaction with grain boundaries is found to induce closure stresses close to the maximum applied stress, explaining the effective crack retardation of Stage I cracks by grain boundaries observed for Al 7075. In these cases the closure load is found to slowly fall as cracking continues by a Stage II mode into a second grain. For Stage n cracking it is possible to estimate the closure load from the cracking path as observed from the surface.