Crack Growth Behavior of Al Alloy under Ultrasonic Fatigue

Abstract

In order to investigate the effect of frequency on the crack growth behavior, ultrasonic fatigue tests were carried out for an extruded age-hardened Al alloy, 7075-T6, and the results were compared with those in rotating bending fatigue. Fatigue strength in ultrasonic was higher than that in rotating bending. This was mainly caused by the retardation of crack initiation. Growth direction of a crack changed from a tensile mode to a shear one in ultrasonic fatigue, though fracture occurred by the growth of a tensile mode in rotating bending. The growth direction of a shear mode crack was inclined about 55 degrees to the tensile axis. The relation between an applied stress σa and a crack depth at transition of growth direction T was expressed by a nT=C, where C and n are constants. These results were discussed from the points of view of the time dependent environmental effect and the texture of material.