The role of air in fatigue load interaction

Abstract

Natural fatigue crack formation and growth were studied in notched Al-Cu alloy coupons through high-resolution SEM fractography. The experiments were conducted under programmed loading conditions designed to induce microscopic marking of the crack formation and growth process under varying stress ratio and closure-free crack tip conditions. Control experiments were performed by switching between an air and vacuum environment. In air, varying the stress ratio from 0.74 down to 0.64 retards crack growth by up to a factor of five. This 'closure-free' stress ratio history effect totally disappears in vacuum, suggesting a significant environmental influence on stress ratio and its history. Crack-tip stress state appears to moderate environmental action, revealing a potential mechanism sensitive to residual stress. Consequently, crack closure, residual stress and crack front and plane orientation are identified as major load interaction mechanisms whose synergistic action controls fatigue under variable amplitude loading. The study also appears to suggest that as a consequence of the crack seeking the path of least resistance, load-sequence sensitive crack plane and front orientation may only induce retardation effects.