Combined Effects Testing for Aluminum Alloy Environment Assisted Cracking

Abstract Aircraft structures and components are subjected to dynamic environmental and mechanical conditions during both ground and flight operations. Aerospace structures are fabricated from mixed materials that also include protective surface finishes. Atmospheric corrosion, especially at fasteners, may initiate stress corrosion cracking (SCC) and corrosion fatigue. Performing relevant laboratory and outdoor atmospheric testing of these material systems and accurately accounting for combined effects of corrosion and mechanical loads is challenging. Recent research has focused on developing systems for assessing environment assisted cracking (EAC) under both static and dynamic mechanical loading conditions. The test systems have been used to quantify the effects of select coatings and galvanic crevice corrosion processes on the development and propagation of environment assisted cracks. Increasing ∆K fatigue tests have shown that drying and relative humidity thresholds are significant for SCC initiation and propagation for susceptible alloys. Two regimes of cracking have been observed: SCC at low stress intensity ranges and corrosion fatigue dominated propagation at higher stress intensity ranges. The operative mechanisms and stress dependent behavior have been shown to depend on environmental conditions, alloy SCC susceptibility, crack orientation, and loading frequency among other factors. It is expected that the developed test systems and methods will be broadly applicable to improving our understanding of the environment, loads, and material interactions that are significant to aircraft structural integrity. Further study in this area will ultimately lead to improved assessment of material-environment compatibility and reduction of risk from corrosion degradation.