An experimental study of the growth of surface flaws under cyclic loading

Fatigue crack growth studies were carried out using 7075-T651-aluminum alloy specimens with a semi-elliptical surface crack. Multiple experiments were conducted under constant amplitude loading with a stress ratio R = 0.1, which inherently included the effects of fatigue crack closure, and R = 0.7, which was assumed closure free. Crack length measurements on the free surface were made with a traveling microscope. To allow crack depth measurements, marker bands were used. The aspect ratio evolutions for the two stress ratios tested were compared to determine whether crack closure affects aspect ratio or crack shape evolution. It was found that closure impacts this evolution in a significant manner. In addition, the fatigue crack growth data collected from the multiple specimens was used to perform uncertainty analyses on the crack lengths, crack depths, and the aspect ratio. This was done to determine if crack growth models are best validated using the aspect ratio as the measured quantity or the individual measured crack length and crack depth. The measured aspect ratio was found to exhibit a smaller percentage of experimental uncertainty, and thus should be used for model validations. Finally, the observed crack growth rate-stress intensity factor range relationships for R = 0.7 at the free surface and the deepest point of penetration were compared to existing through-crack data, with a good correlation being observed.