Effect of Water Vapor Pressure on the Fatigue Crack Propagation Rates in Aerospace Aluminum Alloys 7075-T651 and 2199-T86

The objective of this research is to examine the effect of high purity water vapor on fatigue crack growth rates (da/dN) in aluminum alloys (AA7075-T651 and AA2199-T86). For decreasing ΔK testing (either at a constant stress ratio of R = 0.5 or constant a maximum stress intensity of Kmax = 16.5 MPa√m protocols), growth rates declined with decreasing water vapor pressure over a wide range of ΔK-values. This behavior demonstrated consistency with current environmental theories where crack growth is limited by molecular flow or H-diffusion in the crack tip process zone. A local minimum in da/dN, referred to as the threshold transition regime (TTR), was observed for low ΔK and low to intermediate ranges of water vapor pressures [1]. An initial decline in Stage I growth proceeded towards the UHV threshold before transitioning to increasing da/dN with further decreases in ΔK before merging with growth rates typical of high water vapor pressures at very low ΔK. This behavior was observed for decreasing ΔK at both constant Kmax and constant R testing conditions. The dip in growth rates corresponded to a transition from flat-transgranular (Stage II) cracking to crystallographic slip band cracking (SBC; Stage I) for decreasing da/dN [1].
It is proposed that the da/dN minimum is attributed to roughness-induced changes in the mass transport behavior that alters the PH2O at the crack tip. As the ∆K decreases in low PH2O environments, the cross-slip becomes limited which leads to a higher degree of slip band cracking. This increased crack wake roughness impedes the flow of water vapor molecules from the crack mouth to the crack tip, resulting in a decreased crack tip PH20 when compared to the PH2O of the bulk environment at the crack mouth. The subsequent rise in da/dN with decreasing ΔK in the TTR is explained based on an increased supply of water vapor to the crack tip due to turbulent-convective mixing. Specifically, after a certain critical level of roughness is reached in the crack wake, the PH2O transport mechanism changes to convective turbulent mixing as a result of crack wake asperity contact. The convective mixing increases the supply of PH20 to the crack tip that helps to reignite the hydrogen environment embrittlement process and results in the subsequent rise in da/dN rates.
This research addresses four primary objectives to validate and extend the threshold transition regime hypothesis presented in the Burns et al. paper [1]. First, quantitatively investigate the crack front evolution in the threshold transition regime to evaluate the proposed molecular transport based explanation. Second, further investigate the interaction between molecular flow and the degree of roughness using targeted experimental evaluation. Third, quantitatively evaluate the degree of asperity contact in the crack wake, as pertinent to the proposed turbulent mixing hypothesis and the post-minima regime behavior. Fourth, extend to characterize the environmental fatigue crack growth behavior of a 3rd generation Al-Cu-Li alloy (2199-T86) with a different slip character than 7075-T651. In toto, this work focuses on understanding the interacting effects of PH20, crack wake roughness, and ΔK on crack closure and water vapor transport to the crack tip.
Analysis detailed above indicated that the threshold transition behavior observed in 7075-T651 and 2199-T86 is governed by molecular transport that can be affected by the loading protocol, specimen geometry, and/or testing configuration. While the false threshold behavior resulting in the dip in da/dN is real and repeatable, such behavior is geometry and molecular flow path dependent and should not be incorporated into fracture mechanics based predictions. Following appropriate testing procedures would help to ensure data that accurately represent the increased fatigue resistance of 7075-T651 and 2199-T86 at high altitudes. The understanding developed in this investigation will better inform protocols in selecting environment appropriate crack growth rates for linear elastic fracture mechanics (LEFM) modeling.