Acoustic Emission Study of Failure Mechanisms in Ceramic Matrix Composite under Longitudinal Tensile Loading

Acoustic emission (AE) measurements were correlated with macroscopic stress-strain behavior and direct microscopic observations of damage mechanisms in a unidirectional ceramic matrix composite under longitudinal tensile loading. Matrix crack initiation and the onset of AE events correspond to the proportional limit of the material. Both the crack density and cumulative AE counts vary linearly up to the point of crack saturation. Thereafter follows a strain hardening region corresponding mainly to fiber-matrix debonding and manifested by an increase of low to moderate amplitude AE activity and leveling off of high amplitude AE signals. At higher strains, beyond the strain hardening region, there are two characteristic parts in the AE output, one related to further damage (possibly debonding and fiber fracture) and the other clearly related to interfacial frictional sliding. Upon unloading further AE output related to sliding is observed. AE activity of low to intermediate amplitude continues in the last stages of loading and is believed to be associated with fiber fractures.