| Ceramic-matrix composites (CMCs) reinforced with continuous fibers exhibit important progressive damage/failure behavior. The performance of CMCs and their damage mechanisms depends on material processing techniques, external loading, and the arrangement of reinforcements. The intent of this research is to study relationships between constituent properties, fiber arrangements, and the progressive damage behavior of multi-directionally reinforced CMCs, including (1) unidirectional composites, (2) cross-ply composites, and (3) woven fabric composites. The material systems examined in this study include C/borosilicate glass and SiC/CAS unidirectional, SiC/CAS cross-ply, and SiC/SiC woven fabric composites.; For unidirectional CMCs, a partial debonding model is introduced. Thermomechanical behavior, with an emphasis on matrix cracking, is examined. The influence of debonding length on the problem is studied through the model. By using an energy balance method two closed-form solutions of the critical strains for matrix cracking have been obtained. The effects of interface properties, fiber/matrix sliding, matrix surface energy and fiber volume fraction on the damage of the composite have been investigated.; For cross-ply CMCs, two kinds of microcracking have been observed: transverse cracking in the 90{dollar}spcirc{dollar}-ply and matrix cracking in the 0{dollar}spcirc{dollar}-ply with fiber bridging. Through combinations of these two types of microcracking, five damage modes, including one transverse crack and four matrix cracks, are studied. Matrix cracking in the 0{dollar}spcirc{dollar}-ply is a unique feature of cross-ply CMCs. Using the energy balance method, closed-form equations of critical stresses for the damage modes have been obtained. It can be shown that the four matrix cracking equations for cross-ply CMCs reduce to those two for unidirectional CMCs as the thickness of the 90{dollar}spcirc{dollar}-ply vanishes. The relation between composite performance (stiffness, strength and yielding strain and nonlinear behavior) and constituent properties (thicknesses, thermal properties, surface energies, interfacial shear stress and fiber volume fraction) has been studied.; The damage evolution and nonlinear tensile behavior of 2-D woven-fabric Nicalon-fiber-reinforced SiC-matrix composites fabricated by chemical vapor infiltration (CVI) have been examined. The effect of inter- and intra-yarn pores due to CVI processing and the effect of fiber undulation have been stated. The nonlinear portion of the stress-strain responses is due to transverse cracks initiated at inter-yarn pores. Strain energy release rates of the cracks in transverse and longitudinal yarns have been calculated by a finite element method and a crack-closure approach. The results indicate that the composite stiffness reduction due to fiber undulation is insignificant for the SiC/SiC composite. However, the porosity is important to composite stiffness, due to the high-modulus ceramic matrix. The results of strain energy release rates show that using thinner fiber yarns can lead to smaller pores and higher critical stress for transverse cracking. |
