MULTIPLE CRACKING OF UNIDIRECTIONAL AND CROSS-PLY CERAMIC-MATRIX COMPOSITES

This paper examines the multiple cracking behavior of unidirectional a nd cross-ply ceramic matrix composites. For unidirectional composites, a model of concentric cylinders with finite crack spacing and debondi ng length is introduced. Stresses in the fiber and matrix are found an d then applied to predict the composite moduli. Using an energy balanc e method, critical stresses for matrix cracking initiation are predict ed. Effects of interfacial shear stress, debonding length and bonding energy on the critical stress are studied. All the three composite sys tems examined show that the critical stress for the completely debonde d case is lower than that for the perfectly bonded case. For cross-ply composites, an extensive study has been made for the transverse crack ing in 90 degrees plies and the matrix cracking in 0 degrees plies. On e transverse cracking and four matrix cracking modes are studied, and closed-form solutions of the critical stresses are obtained. The resul ts indicate that the case of combined matrix and transverse crackings with associated fiber/matrix interfacial sliding in the 0 degrees plie s gives the lowest critical stress for matrix cracking. The theoretica l predictions are compared with experimental data of SiC/CAS cross-ply composites; both results demonstrated that an increase in the transve rse ply thickness reduces the critical stress for matrix cracking in t he longitudinal plies. The effects of fiber volume fraction and fiber modulus on the critical stress have been quantified. Thermal residual stresses are included in the analysis.