The Effect of Environment on Corrosion Induced Fatigue Crack Formation and Early Propagation in Al 7075-T651

Research provides insight into fatigue crack formation and progression in the size regime intermediate to safe-life and damage tolerance approaches; of particular interest are the influences of corrosion-induced degradation and time-cycle dependent loading environment effects.  Quantitative analysis of crack formation life (N_i), microstructurally small crack (<500 µm) propagation kinetics (da/dN), and the effect of cold loading environment, enable validation of mechanism-based modeling.  Pristine and corroded (L-S surface) 7075-T651 specimens were fatigued at 23°C, -50°C and -90°C under various-applied stresses.  Microscopy of programmed loading induced crack surface marks produced an unparalleled N_i and small crack da/dN data base.  Results show that fatigue formation involves a complex interaction of elastic stress concentration due to a 3-dimensional macro-pit coupled with local micro-feature (and constituent) induced plastic strain concentration.  Such interactions cause high N_i variability, but from an engineering perspective, a broadly corroded surface should contain an extreme group of features driving N_i to ~0.  At low-applied stresses, N_i consumes a significant portion of total life, which is well predicted by coupling elastic-plastic FEA with empirical low-cycle fatigue life models.  All pristine and corroded da/dN were uniquely correlated using complex continuum stress intensity (K) and crack opening solutions which account for the stress concentrating formation feature.  Multiple crack growth regimes, typical of environment enhanced fatigue in Al alloys, were observed but not captured by prominent mechanics-based small crack models.  Furthermore, neither local closure nor slip-based models captured the +150% variability in da/dN attributed to microstructure.  Low temperature loading produces a 10-fold increase in N_i and even larger reductions in da/dN due to elimination of H-enhanced cracking by reduced external water vapor pressure, lower H producing crack tip reaction rate, and slower H diffusion.  Engineering level models are validated using these high fidelity experimental results, informing next generation prognosis methods for realistic airframe environments.