Effect of Humidity on Fatigue Strength of Age-Hardened Al Alloy

Abstract

Ultrasonic and rotating bending fatigue tests were carried out using plain specimens and specimens with a small blind hole for an extruded and age-hardened Al alloy 7075-T6 in different environments in order to investigate the effect of humidity on fatigue strength and fracture mechanism. Fatigue strength was decreased by high humidity under both tests. The effect of humidity on fatigue strength was larger in ultrasonic fatigue. The humidity affected both of crack initiation and propagation processes. Crack propagated in tensile mode then changed to shear mode macroscopically in all environments under ultrasonic fatigue, though it was only in tensile mode under rotating bending fatigue. These differences in fracture mechanism related to the difference in environmental effect on fatigue strength in both tests.

Environment-induced fatigue cracking behavior of aluminum alloys and modification methods

“…In addition, the fatigue crack growth as the tensile mode (mode I) with facets can be also explained as follows. Facets were observed in the long-life region (VHCF or UHCF) where diffusion of hydrogen into matrix can increase (Fukudome et al, 2010;Kawagoishi et al, 2010;Nakamura et al, 2014;Sofuku et al, 2008). In addition, the fraction of facets increased with increasing loading cyclic numbers, suggesting that dense hydrogen was related to the formation of facets.…”

“…The angle between plane (111) and plane (100) was approximately 55°. Therefore, fatigue crack propagated preferentially in the specified direction macroscopically, just like a growth of a crack in a single crystal due to its texture and cyclic softening properties (Fukudome et al, 2010;Kariya et al, 2012;Kawagoishi et al, 2010Kawagoishi et al, , 2011Kawagoishi et al, , 2012Nakamura et al, 2014). Fatigue crack growth as the shear mode (mode II) was a phenomenon related to the marked texture.…”