Environmental Cracking of Aerospace Aluminum Alloys in High Altitude Environments

Aluminum alloys have been used extensively in a number of different industries due to their advantageous properties. These properties include high-strength-to-weight ratio, as well as good workability and strong corrosion resistance. In many applications, environmental degradation mechanisms are responsible for a significant number of failures. Recent work has demonstrated that a significant portion of loading for airframe structural components can occur at high altitudes where the environment is typified by low temperatures and low water vapor pressures. At these temperatures and pressures, crack growth kinetics of aluminum alloys are drastically reduced. As such, incorporating low temperature, low water vapor pressure effects into the next generation of airframe structural integrity management has the potential to increase accuracy and reduce over-conservatism.
This dissertation will further explore the effect of a high-altitude environment with a particular focus given to the role low water vapor pressure plays in the fatigue behavior of aerospace aluminum alloys. To this end, four tasks will be explored: (1) Efficacy of water vapor pressure over frequency (PH2O/f) as an exposure parameter to describe the environment. (2) Effect of limiting sample thickness to eliminate irregular crack front behavior at certain stress intensities (∆Ks) and water vapor pressures. (3) Extension of high-altitude behavior in AA 7075 to 2xxx series aluminum alloys. (4) Incorporation of high-altitude fatigue crack growth rates (FCGR) data into linear elastic fracture mechanics (LEFM) based models to determine the magnitude of fatigue life extension.
While researchers had previously studied the role of PH2O/f in describing the environment, this is the first study to comprehensively study this parameter across a wide range of water vapor pressures in the near-threshold region. Several critical findings on the limitations of using the exposure parameter were found. K-shed testing showed that exposure parameter performed better than water vapor pressure at describing the environmental cracking of 7075. Constant water vapor curves diverged between ∆Ks of 4 to 6 MPa√m. Constant K-hold testing revealed two regions, one where water vapor pressure describes the environment, and another where PH2O/f is a better proxy for environmental severity. At low to intermediate water content, the environment appears to be well-described by the PH2O/f exposure parameter. At higher water vapor content, the effect of frequency seems to be diminished, such that water vapor pressure is an adequate descriptor. These results show that while PH2O/f is widely used in literature, and it is the best singular parameter to describe environment, it is important that its limitations are understood.
Literature has previously shown that in certain test conditions, a sharp decrease in growth rates could occur termed the threshold transition regime (TTR). Testing of the TTR this behavior had been suggested to be due to limitations in the transport of water vapor to the crack tip. This was investigated by performing testing on multiple thickness samples to give insights into the possible mechanism controlling this behavior. Varying sample thickness testing did not show any impact on the fatigue crack growth behavior at any water vapor pressures and the fractures surfaces were very similar as well. This invalidates the hypothesis that shrinking the sample thickness the TTR might be eliminated. Modeling efforts using Franc3D confirmed that at as an irregular crack front forms, the stress intensity spikes in the center of the sample, likely resulting in cleavage fracture. Additionally, modeling of constant semi-elliptical cracks on the edge of the sample for different thicknesses are not consistent with experimental results. This indicates that the irregular crack front is unique for each sample thickness. Further work is needed to give insight into how the irregular crack front develops in order to better explain why changing sample thickness had no effect on the TTR behavior.
Environmental fatigue testing of aluminum alloy 2199 in high altitude environments established over a wide range of ΔK was performed. Reduction of PH2O at 23 ◦C resulted in a systematic reduction of crack growth rates, resulting from a reduction in H needed for the hydrogen embrittlement process. Above 165 Pa, changes in PH2O do not result in increased crack growth rates, indicating the environmental contribution reaches a saturation point. At 0.5 Pa FCGR matched those at ultra-high vacuum (UHV), indicating that at these low pressures, there is no longer any environmental contribution to cracking. Low-temperature testing showed that at temperatures above -15 ◦C no change in FCGR was observed. Decreased crack rates were observed at -30 ◦C and -50 ◦C. When temperature was again lowered to -65 ◦C at 0.5 Pa, temperature had no effect on crack behavior indicating that temperature is affecting environmental cracking only. This behavior was suggested to be due to dislocation interactions with H changing as temperature decreased.
Utilizing an LEFM program, AFGROW, the environmental effect on fatigue life on AA 7075-T651 in aerospace applications was able to be quantitatively predicted. Through several different modeling conditions, the fatigue life in low water vapor environments consistently produced significant increases in fatigue life. At low stresses, the relative change in fatigue life was greater across all different modeling conditions. While environmental effects are most potent in the near-threshold regime of the da/dN vs. ∆K relationship, targeted modeling demonstrates that the majority of the crack extension occurs at high stresses (thus high ΔK). This leads to the counter-intuitive conclusion that, despite the lower relative impact of environmental on the da/dN at high ∆K, the environmental effect at high stress levels has a more significant impact on the total fatigue life. Through these results, it is clear that implementation of low water vapor pressure, low temperature, environmental FCGR into the next generation of airframe structural integrity management models can have a profound impact.