Composites composed of a precipitation-hardened 2124 aluminum (Al) alloy matrix reinforced by silicon-carbide (SiC) whiskers, which are fabricated by powder metallurgy, are susceptible to fatigue damage due to microcracking along SiC-Al interfaces, especially at the external surfaces of a body. The microcracks that are produced in the crack initiation process are generally too small to be detected and characterized by conventional ultrasonic techniques. Since they may link up to produce macrocracks and rapid final fracture, a simple experimental technique is required to assess the extent of the microcracking. This paper reports on an ultrasonic surface wave technique that was developed for that purpose. In this technique, the integrated effect of the damage is established by careful measurements of the changes of the phase velocity and the coefficient of attenuation as the number of cycles of fatigue loading increases.
The specimens, 6.6 mm thick, 13 mm wide, and 105 mm long in the gage section, were fatigued in pull-pull under load control. Surface waves were generated at 5 MHz with conventional surface wave transducers. A special self-calibrating ultrasonic bridge was employed to measure surface wave motion by means of two dry contact point pickups separated by a known distance along the direction of surface wave propagation. Wave speed measurements were obtained by the use of a variation of the overlap technique. To measure the attenuation, the data were deconvolved from the response functions of the point contacts by the use of data for surface wave generation in two opposite directions.