Damage characterization of two-dimensional woven and three-dimensional braided SiC-SiC composites

A comprehensive investigation of the room temperature behaviour of two-dimensional woven and three-dimensional braided SiC-SiC composites fabricated by the chemical vapour infiltration route has been conducted. A morphological study of the residual porosity in the composites revealed the existence of primarily two populations of pores: small intrayarn pores and larger interyarn pores. The sizes and the shapes of the two types of pores depended largely on the fibre architecture; the two step braided composite in which the majority of the fibre yarns were orientated along the axial direction exhibited the smallest pore size. The pore size and shapes in turn influenced the onset of damage in the composites under tensile loading. Damage was found to be initially matrix dominated, thus being essentially independent of the fibre architecture. At higher stress levels, however, fibre dominated damage prevailed. Unlike the tensile behaviour, where damage led to non-linearity in the stress-strain curve, the compressive behaviour of the composites was linear elastic almost up to failure. The off-axis tensile properties as well as compression after tension behaviour of the two-dimensional woven composites were also investigated. The information obtained from these tests provides the basis for the modelling of damage in these materials.