Abstract
In order to investigate the effect of loading frequency on crack growth behavior, ultrasonic fatigue and rotating bending fatigue tests were carried out for an extruded age-hardened Al alloy 7075-T6 in ambient air and in N2 gas. Fatigue strength increased in ultrasonic fatigue due to the retardation of crack initiation and its early propagation. In ultrasonic fatigue, however, crack growth transition took place from tensile mode to shear mode when cracks grew faster than ∼3x10-9m/cycle. As a result, macroscopic crack paths changed from nearly vertical to stress axis to oblique ∼35 degrees against the axis. Fracture mechanism involved in ultrasonic fatigue also changed from striations featured to transgranular facets characterized with microscopic voids. The relation between the applied stress, σa, and the crack depth at the crack growth transition, bT, can be expressed as σanbT = constant. The results were discussed from the viewpoints of time dependent environmental effect and the texture of the alloy.
