Fiber-matrix interface study of carbon-carbon composites using ultrasonics and acoustic microscopy

This paper addresses the use of ultrasonics in the characterization of carbon-carbon (C/C) composites during pyrolysis in three ways: (1) to monitor in situ the evolution of the fiber-matrix interface by acoustic emission (AE); (2) to determine the effect of microcracks on the composites by measuring the elastic stiffness; (3) to examine the evolution of microstructure as a function of temperature by scanning acoustic microscope (SAM). Analysis of the anisotropic wave propagation in C/C composites is presented for two cases: (1) the determination of local mechanical properties by the use of bulk waves; (2) the determination of global mechanical properties by the use of guided plate waves. Other measurements, such as those of mass spectrometry, bulk porosity, weight loss and cross-ply thickness shrinkage were carried out to support the ultrasonic measurements. AE results show that the majority of the cracks were found to form in the temperature range from 400 to 600°C. This observation was supported by the measurements of porosity, weight loss, thickness shrinkage, mass spectrometry, and surface morphology with SAM. The stiffness measurements showed a decrease of 72.21 GPa for in-plane stiffness along the fiber direction and only 5.06 GPa for out-of-plane stiffness. The greater decrease in the in-plane stiffness is attributed to the large number of transverse cracks which make the composite "acoustically soft". The analysis of the dispersion curves for plate waves suggests the viability of monitoring the global mechanical properties of C/C composites during the first carbonization.