Fractographic reassessment of the significance of fatigue crack closure

Experiments were performed on Al-alloy test coupons under specially programmed variable amplitude load sequences to assess crack closure as well as actual microscopic crack extension. Quantitative fractography of the fractures suggests that closure can account for only a fraction of observed load interaction effects. Much of the observed retardation may be attributed to the shielding effect of the overload plastic zone, which can occur even if the crack is fully open. The study also indicates that load sequence effects attributed to notch root stress-strain hysteresis can occur only if closure is absent.