The velocity and attenuation of acoustic emission waves in SiC/SiC composites loaded in tension

The behavior of acoustic waves produced by microfracture events and from pencil lead breaks was studied for two different silicon carbide fiber-reinforced silicon carbide matrix composites. The two composite systems both consisted of Hi-NicalonTM fibers and carbon interfaces but had different matrix compositions that led to considerable differences in damage accumulation and acoustic response. This behavior was primarily due to an order of magnitude difference in the interfacial shear stress for the two composite systems. Load/unload/reload tensile tests were performed and measurements were made over the entire stress range in order to determine the stress-dependence of acoustic activity for increasing damage states. It was found that using the extensional wave velocities from acoustic emission (AE) events or AE produced from pencil lead breaks performed outside of the transducers enabled accurate measurements of the stiffness of the composite. The extensional wave velocities changed as a function of the damage state and the stress where the measurement was taken. A significant increase in attenuation of AE waveforms produced by pencil lead breaks occurred for the composite possessing the lower interfacial shear stress and only at significantly high stresses compared to the attenuation in undamaged material. At zero stress after unloading from a peak stress, no change in attenuation, compared to undamaged material, occurred for this composite because of crack closure. For the high interfacial stress composite no change in attenuation, compared to undamaged material, was discernable at peak or zero stress over the entire stress-range of the composite. From these observations, it is believed that attenuation of AE waveforms is dependent on the magnitude of matrix crack opening.