EFFECT OF TEMPERATURE AND FIBER DISTRIBUTION ON MATRIX CRACKING IN CERAMIC MATRIX COMPOSITES

In this paper, the matrix cracking of Nicalon-fiber/SiC-matrix ceramic composites under high temperature is studied. First the fracture toughness of the matrix material (SiC) is obtained using monolithic (SiC coated on a graphite base) and composite (Nicalon/SiC) specimens at various temperatures ranging from room temperature to 800-degrees-C. It is observed that the matrix fracture toughness is reduced significantly at elevated temperatures. The experimental results also indicate that it is easier to generate microcracks in regions with higher local density of fibers and thus the "apparent fracture toughness" which includes the effect of fiber distribution is reduced with increasing local volume fraction of fibers. A theoretical model to predict this behavior has been developed and the results agree qualitatively and quantitatively well with the experimental findings. Finally, debonding along the fiber/matrix interface is studied and the shear strength of the interface is obtained by a combination of experimental data and finite element calculations.