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 acoust ic emission (AE); (2) to determine the effect of microcracks on the co mposites by measuring the elastic stiffness; (3) to examine the evolut ion of microstructure as a function of temperature by scanning acousti c microscope (SAM). Analysis of the anisotropic wave propagation in C/ C composites is presented for two cases: (1) the determination of loca l mechanical properties by the use of bulk waves; (2) the determinatio n of global mechanical properties by the use of guided plate waves. Ot her measurements, such as those of mass spectrometry, bulk porosity, w eight loss and cross-ply thickness shrinkage were carried out to suppo rt the ultrasonic measurements. AE results show that the majority of t he cracks were found to form in the temperature range from 400 to 600 degrees C. This observation was supported by the measurements of poros ity, weight loss, thickness shrinkage, mass spectrometry, and surface morphology with SAM. The stiffness measurements showed a decrease of 7 2.21 GPa for in-plane stiffness along the fiber direction and only 5.0 6 GPa for out-of-plane stiffness. The greater decrease in the in-plane stiffness is attributed to the large number of transverse cracks whic h make the composite ''acoustically soft''. The analysis of the disper sion curves for plate waves suggests the viability of monitoring the g lobal mechanical properties of C/C composites during the first carboni zation.