Effect of corrosion severity on fatigue evolution in Al–Zn–Mg–Cu

The effect of existing-localized corrosion on fatigue cracking of 7075-T6511 was established using crack surface marker-band analysis and a fracture mechanics model. The substantial reduction of fatigue life due to EXCO solution L–S surface pre-corrosion is nearly independent of exposure time after initial-sharp degradation, scaling with the evolution of pit-cluster size and initial stress intensity range with exposure time. Independent of exposure time, formation of a resolvable fatigue crack (∼10 μm) accounts for a similar-low (∼5%) fraction of total fatigue life at low stress range (σmax = 150 MPa, R = 0.1). Crack formation occurs at microscopic protrusions into the corroded volume. A corrosion-modified-equivalent initial flaw size (CM-EIFS); predicted with the AFGROW tool using measured initial aspect ratio, initiation cycles, and total fatigue life inputs; accurately represents the corrosion damage effect on fatigue for a range of exposures. The similar deleterious effect of several corroding environments for various-exposed surfaces is described by a lower-bound CM-EIFS with a 300 μm depth and 1200 μm surface length suggesting fatigue is governed by a microscopic pit-based topography. Either an approximate lower-bound, or specific CM-EIFS calibrated by limited measurements of fatigue life for service-environment exposed specimens, can be used to assess the impact of corrosion in a damage tolerant framework. Complexities (e.g., local H embrittlement, 3D pit geometry, topography dependent initiation, and microstructure sensitive small-crack growth) do not compromise the CM-EIFS estimation, but must be better understood for refined modeling.