ACOUSTIC-EMISSION MONITORING OF DAMAGE IN METAL-MATRIX COMPOSITES SUBJECTED TO THERMOMECHANICAL FATIGUE

Acoustic emission (AE) was monitored during thermomechanical fatigue ( TMF) tests conducted on a silicon carbide fiber, titanium alloy matrix composite, SCS-6/TIMETAL(R)21S. The tests were stress-controlled with a stress Patio of 0.1, and temperature was cycled from 150 to 650 deg rees C. In-phase and out-of-phase TMF cycling were characterized by di stinct AE behavior The pare of fiber fractures as indicated by AE duri ng in-phase cycling was much higher than during out-of-phase cycling, correlating with metallography and micromechanics analyses which indic ate thar the fiber stress is higher under in-phase than out-of-phase c ycling. The fiber fracture rate was proportional to the maximum fiber stress and remained nearly constant during most of the test. The Fate was greater during the early cycling as well as near the end. AE also indicated that most of the early cyclic damage occurred during the ini tial loading.