ULTRASONIC EVALUATION OF FATIGUE DAMAGE IN METAL-MATRIX COMPOSITES

This paper describes an experimental nondestructive technique for fati gue damage assessment in metal matrix composites by measuring ultrason ic phase velocity and attenuation. A [0/90] SiC/Ti-15V-3Cr-3Al-3Sn met al matrix composite is considered as a model system. Cyclic loading at 50 and 70% of the ultimate sample strength were used until failure. T he ultrasonic phase velocities and attenuations were measured periodic ally and found to be very sensitive to fatigue damage. The fatigue-ind uced changes in the composite elastic constants were calculated from t he measured ultrasonic velocity data. For samples heat treated prior t o fatigue (815 degrees C) above the matrix beta transus (about 760 deg rees C), the dominant damage mechanism is debonding of the fiber/matri x interface. We found that when samples were fatigued for less than 50 % of the lifetime, the reduction of the composite moduli was linearly dependent on the number of fatigue cycles, which is explained by exten sion of interfacial partial debonds. This was supported by micromechan ical analysis based on a partial disbond model. The rate of decrease i n the composite moduli in the second half of the fatigue life was foun d to be lower, which may serve as a basis for estimation of the remain ing fatigue life of the composite from ultrasonic velocity and attenua tion measurements. The attenuation data was obtained in directions per pendicular to the fiber. A single-fiber scattering model has been used to explain the effect of the fiber/matrix interface on attenuation. G ood correlation between attenuation and moduli measurements was observ ed.