An Evaluation of Aging Aircraft Material Properties

This paper evaluates changes in the behavior of aging aircraft materials which result from aging and/or corrosion that occurs during service usage. Specifically, tensile stress-strain curves, cyclic stress life (SN) tests, and fatigue crack growth tests are used to characterize the “aged aircraft” material. These properties are compared with handbook properties for virgin material of the same pedigree. The materials tested are aluminum alloys obtained from fuselage and wing panels of retired KC-135 aircraft. Computer controlled tests were conducted using specimens machined from the retired aircraft components. Different surface configurations for testing were used to observe the effects of aging and the effects of corrosion on material behavior. In the crack growth specimens, various levels of corrosion were observed, thus the crack growth rates could possibly be quantified as a function of the level of corrosion present in the material. The resulting SN and da/dN-ΔK curves for the “aged” only materials were compared with the fatigue properties of virgin material of the same alloy. Similar comparisons were performed for the tensile stress-strain properties. The primary objective was to determine whether damage tolerant analyses for older aircraft should employ updated properties which more accurately represent the current state of the material, or if the virgin material properties properly characterize the aged/corroded alloy.