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The effect of low PH2O environments (pertinent to high altitude flight conditions) on the fatigue crack growth behavior of 7075-T651 were quantified via linear elastic fracture mechanics (LEFM) testing. Real-time compliance based crack monitoring enabled K-shed and constant ΔK testing at various PH2O/f levels to elucidate the mechanical and chemical driving forces that govern the observed behavior. A breakdown in fracture mechanics similitude for constant ΔK, PH20/f, and crack length experiments is observed due to varying levels of crack wake roughness. This behavior is understood based on the rate limiting molecular transport step of the H-environment embrittlement process. Specifically, crack wake roughness will lead to impeded transport in the crack growth direction, this leads to the development of an irregular crack front due to environment enhanced crack extension occurring on the crack flanks where the transport distance is low and slow crack progression at the specimen center. The resulting through thickness gradients in the environmental and mechanical driving forces for crack extension provide the means to understand the breakdown of similitude and variation of the crack growth rates. It is necessary to incorporate this understanding into the data generation protocol for these environment specific da/dN vs. ΔK relationships in order to accurately incorporate environment into LEFM based damage tolerant approaches.

The objective of the study is to evaluate the fatigue crack growth of a third-generation Al-Li-Cu alloy (2199-T86) in environments relevant to high-altitude flight. Reduction of PH2O at 23 °C resulted in a general reduction of crack growth rates. A local minimum in growth rates were observed at intermediate ΔK and PH2O and was attributed to roughness induced impedance of water transport through the crack wake. Decreasing temperature had no effect on growth kinetics down to −15 °C, but decreased crack rates were observed at −30 °C and −50 °C.