Growth Mechanism of Shear Mode Fatigue Crack of Age-Hardened Al Alloy

Abstract

In order to investigate the growth mechanism of a shear mode fatigue crack in an extruded bar of an age-hardened Al alloy 7075-T6, rotating bending fatigue tests were carried out using plain specimens of the alloy in environments of controlled relative humidity of 25%, 50%, 75% and 85%, distilled water and nitrogen gas. Fatigue strength was decreased by high humidity. The growth mechanism of a fatigue crack was different depending on environment and stress level. Although most of fractures occurred by the growth of a tensile mode crack, a crack propagated in a shear mode accompanying with slip planes and voids at high stress levels in high humidity and at all stress levels tested in nitrogen gas. The shear mode crack was related to the marked texture of the alloy. Growth rates of the shear mode crack were higher in high humidity than in nitrogen gas. Growth mechanisms of the shear mode crack were different between environments in nitrogen gas and in high humidity. That is, the reason for the growth of a shear mode crack in nitrogen gas was the suppression to growth of a tensile mode crack by reversible slip due to absence of oxide film. On the other hand, in high humidity, the growth of a shear mode crack was promoted by the formation and coalescence of voids, suggesting that the acceleration of the growth rate of a crack may be assisted by hydrogen accumulated around precipitated particles on glide planes.

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.…”

“…With increasing cyclic number such as VHCF or UHCF, the fatigue crack growth mode was shifted from mode II with many slip planes to mode I with many granular facets despite the same mechanism of HELP. Fatigue crack propagation as mode II was in a crack propagation path, with an angle of approximately 55° to the normal direction of the specimen axis ( Figure 5B-1) in high humidity, and the fracture surface had been confirmed as (100) plane by the etch pit method of Kawagoishi et al (2010). On the other hand, the aluminum alloy had a marked texture of (111) at the cross section of specimen and showed the cyclic softening property.…”

“…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.…”

Tensile Properties and Fatigue Strength in High Humidity in Extruded 7075 Al Alloys with Different Aging Conditions

“…The details of experimental procedures had been reported in the previous papers [2][3][4], so a brief explanation is given here. Commercial rods of extruded 7075 T6 Al with a diameter of 20 mm were used as the as-received specimens (hereafter called AT6-specimens).…”

“…However, it is well known that their mechanical properties are deteriorated prominently in corrosive environments. In our previous studies [2,3], the fatigue strength of extruded 7075 T6 Al alloy was decreased markedly, as the relative humidity (RH) was increased to more than 50%. The decrease in fatigue strength of 7075 T6 Al alloy under high humidity was believed to arise from the hydrogen-enhanced-local-plasticity (HELP) mechanism of which operation was closely related with the extrusion texture as well as dislocation density [3].…”