| The current research provides insight into fatigue crack formation and progression in the poorly understood size regime that bridges safe-life and damage tolerance approaches; particular attention is given to the influences of corrosion-induced degradation and time-cycle dependent loading environment effects. Quantitative analysis of crack formation life (Ni), microstructurally small crack (<500 microm) propagation kinetics (da/dN), and the effect of cold loading environment provide the means to validate mechanism-based modeling. Both 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 Ni and small crack da/dN database. Results show that fatigue crack 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 Ni variability, but, from an engineering perspective, a broadly corroded surface should contain an extreme group of features driving Ni to ∼0. At low-applied stresses, Ni 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 were observed, typical of environment enhanced fatigue in Al alloys. Such behavior is not captured by prominent mechanics-based small crack models. Furthermore, neither local closure nor slip-based models captured the order of magnitude variability in da/dN attributed to microstructure. Low temperature loading produces an order of magnitude increase in Ni, and even larger reduction in da/dN, due to elimination of H-enhanced cracking by reduced external water vapor pressure, lower crack tip reaction rate (to produce atomic-H), and slower H diffusion. Engineering level modeling approaches are validated using these high fidelity experimental results, informing next generation prognosis methods for realistic airframe environments. |
