Damage Determination in Ceramic Composites Subject to Tensile Fatigue Using Acoustic Emission.

Acoustic emission (AE) has proven to be a very useful technique for determining damage in ceramic matrix composites (CMCs). CMCs rely on various cracking mechanisms which enable non-linear stress⁻strain behavior with ultimate failure of the composite due to fiber failure. Since these damage mechanisms are all microfracture mechanisms, they emit stress waves ideal for AE monitoring. These are typically plate waves since, for most specimens or applications, one dimension is significantly smaller than the wavelength of the sound waves emitted. By utilizing the information of the sound waveforms captured on multiple channels from individual events, the location and identity of the sources can often be elucidated. The keys to the technique are the use of wide-band frequency sensors, digitization of the waveforms (modal AE), strategic placement of sensors to sort the data and acquire important contents of the waveforms pertinent for identification, and familiarity with the material as to the damage mechanisms occurring at prescribed points of the stress history. The AE information informs the damage progression in a unique way, which adds to the understanding of the process of failure for these composites. The AE methodology was applied to woven SiC fiber-reinforced melt-infiltrated SiC matrix composites tested in fatigue (R = 0.1) at different frequencies. Identification of when and where AE occurred coupled with waveform analysis led to source identification and failure progression. For low frequency fatigue conditions, damage progression leading to failure appeared to be due to fiber failure at or near the peak stress of the stress cycle. For higher frequency fatigue conditions, significantly greater amounts of AE were detected compared to low frequency tests a few hours prior to failure. Damage progression leading to failure included AE detected events which occurred on the unloading part of the fatigue cycle near the valley of the stress cycle. These events were associated with 90 tow longitudinal split and shear cracks presumably due to local compressive stresses associated with mating crack surface interactions during unloading. The local region where these occurred was the eventual failure location and the "valley" events appeared to influence the formation of increased local transverse cracking based on AE.