CHARACTERIZATION AND MODELING OF MICROCRACKING AND ELASTIC-MODULI CHANGES IN NICALON CAS COMPOSITES

The damage progression in silicon carbide fiber (Nicalon) reinforced calcium alumino-silicate (CAS) glass-ceramic composites subjected to monotonic uniaxial tensile loading has been studied. A unidirectional composite and three cross-ply laminates were investigated. Microcrack densities were recorded at regular intervals of applied stress. Young's modulus and the major Poisson's ratio were also measured at the corresponding stress levels. Both longitudinal and transverse stress/strain relations show significant nonlinearity with the onset and development of damage. Matrix microcracking in a direction perpendicular to the applied stress occurs in the unidirectional composite when the applied load reaches a critial level, and the Young's modulus and Poisson's ratio start decreasing. On further loading, cracking increases and reaches a saturation level. A shear lag model was used to predict the variation of Young's modulus with applied stress based upon the experimentally measured crack densities. Model predictions suggest that at the saturation of matrix cracking, load transfer from the fiber to the matrix occurs only through frictional stresses. In the cross-ply laminates the two major modes of damage are transverse cracking in the 90-degrees plies and matrix cracking in the 0-degrees plies. The initiation strain and saturation density of transverse cracks increase as the 90-degrees layer thickness decreases. The initiation strain and the saturation density of matrix cracks in the 0-degrees plies of the cross-ply laminates are comparable to those of matrix cracks in the unidirectional composite. A shear lag model was adapted to calculate the reduced Young's modulus of the cracked 90-degrees plies. The shear lag model for the unidirectional composite was used to calculate the reduced Young's modulus of the 0-degrees plies. The reduced moduli of both 0-degrees and 90-degrees plies were input to the rule of mixtures for predicting the Young's moduli of the cross-ply laminates. Here again, model predictions suggest that at the saturation of matrix cracking, load transfer from the fiber to the matrix occurs only through frictional stresses.