Effect of Environmental Corrosion on Fatigue Crack Growth Morphology: A Detailed Investigation of the Boundary between Fatigue and Corrosion Control Regimes

Abstract

Legacy 7XXX series aluminum alloys were developed primarily for their high strength with less regard for their fatigue properties, corrosion resistance and fracture toughness. The constituent alloying elements in these materials (used to achieve high strengths) markedly increased their corrosion susceptibility. Consequently, aircraft structures made from these alloys have exhibited fatigue and corrosion damage. In the present work, we have investigated a crack finding in a fuselage skin of AA7XXX series alloy. This investigation revealed the crack propagated by a combination of fatigue and corrosion. Through the use of extensive metallography, mechanical analysis and laboratory experiments, we have separated the contributions to the damage growth due to corrosion and fatigue. We have also confirmed that in-service mixed-mode failures like this, observed in these alloys, can be reproduced reliably in the laboratory. Furthermore, it was observed that the presence of corrosion can actually change the propagation of a fatigue crack from mode I, the preferable orientation for fatigue crack propagation, to mode II, the preferable orientation for corrosion propagation. Even though the mechanical driving force is enough to grow the crack in mode I, the presence of corrosion can change it to mode II by electrochemical degradation of the material. Using electrochemical measurements, we relate the change in failure mode to the frequency of cyclic loading. At slow enough cyclic frequency the electrochemical energy released due to galvanic corrosion degrades the material such that the crack turns and propagates in the orientation which has only one third the mechanical driving force as compared to the original crack propagation path. This is the first time such phenomenon has been successfully replicated in the laboratory and modeled with finite element analysis.