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.

Driving forces for localized corrosion-to-fatigue crack transition in Al-Zn-Mg-Cu

“…A programmed loading sequence was periodically interspersed to create microscopic marks on the fracture surface, as shown in Fig. 2 21,22,72–75 . Marker loads were applied with the baseline σ max , but R = 0.1 and f = 10 Hz.…”

Fatigue as a process of cyclic brittle microfracture

“…The possibility that moderation of environmental action can serve as an operative mechanism for the residual stress effect in metal fatigue is a logical extension of the understanding of its well‐known effect on stress‐corrosion cracking. Experiments involving specially designed load sequences and high‐resolution fractography appear to provide compelling evidence in support of this theory 30–32 . The studies were performed on Al alloys, IN‐100 nickel base superalloy tested at elevated temperature and on a Ti alloy.…”

“…Experiments involving specially designed load sequences and high-resolution fractography appear to provide compelling evidence in support of this theory. [30][31][32] The studies were performed on Al alloys, IN-100 nickel base superalloy tested at elevated temperature and on a Ti alloy. The salient feature of these experiments was that they were all conducted at high stress ratio in order to preclude crack closure.…”

“… Crack growth in air and high vacuum in 2014‐T6511 under repeated application of load sequence shown at right 30 . All three steps are of the same amplitude and duration, but at different mean stress.…”

Recent Developments in Assessing Microstructure-Sensitive Early Stage Fatigue of Polycrystals (Postprint)

“…An increasing range of experimental techniques is being introduced and developed to assess the propagation of cracks with size on the order of microstructure scale. Marking of the crack front at a given number of cycles by applying periodic blocks of ''underload'' cycles [23,24] has successfully facilitated estimation of MSC contours over several grains. This technique is limited to material systems that display different deformation behaviors at low versus high strain amplitudes (e.g., aluminum alloys) and the crack front can only be marked after a certain number of cycles, which limits the resolution of the inferred crack growth rate.…”