Effect of Simulated Accelerated Atmospheric Environment Conditions on Fatigue Crack Growth Rates

Corrosion significantly impacts the usable structural life of an airframe. Such systems are primarily designed to withstand a well-defined load spectrum that is a function of flight profile and mission type. However, the airframe is also exposed to a spectrum of environmental conditions including variations in temperature, humidity, atmospheric gas composition, salt concentration, pollution, and ultraviolet light exposure which tend to degrade structural capabilities over time. The detrimental influence of chlorides on fatigue performance for metallic materials has been documented in mechanical tests typically performed in an aqueous solution. Yet aqueous solutions are poor predictors of atmospheric corrosion phenomena. The presence of water vapor has generally been shown to accelerate fatigue crack growth rates (FCGR) when comparing baseline results to those from experiments at low-temperature, low-pressure, dry air, or in inert gas. Recent research has shown promise in improving the correlation between laboratory results and field exposures by incorporating ozone and salt into atmospheric corrosion studies. The current test program aims to incorporate those effects into the study of FCGR in aerospace aluminum. Full fatigue crack growth rate curves for both baseline laboratory air and accelerated atmospheric environments were produced. The effect of ozone in combination with relative humidity was examined and found to slow crack growth rates at mid-range ΔK values. The effect of wetting and drying on atmospheric fatigue crack growth rates was examined using constant ∆K testing and the drying cycle was observed to accelerate the fatigue crack growth in line with other research.