The influence of microstructural damage on the thermal properties of ceramic matrix composites

An experimental investigation of the effects of microstructural damage on the thermal properties of ceramic matrix composites (CMCs) has been conducted. For this purpose, a periodic photothermal technique was developed with the capability to make measurements in-situ under load. The technique was assessed through measurements on a stainless steel standard as well as a series of model systems comprising stainless steel sandwich specimens.; The study on CMCs focused on two prototypical damage modes: (a) delamination cracking and (b) transverse matrix cracking. The former damage mode was studied through measurements of the thermal conductance of delamination cracks in a unidirectionally reinforced CMC. The measurements indicate a strong dependence of the conductance on crack opening displacement. A model for crack conductance which takes into account the contributions from the air and the bridging fibers was developed and assessed through comparison with the experimental measurements.; The effects of transverse matrix cracking and interfacial debonding on thermal diffusivity were probed through experiments on two CMCs, both having the same matrix and fibers, but with different fiber architectures (unidirectional and cross-ply). Correlations were established between the thermal diffusivity, the crack spacing, the crack opening displacement, the thermal properties of the constituents (fibers, matrix and interface) and the fiber architecture. Furthermore, connections between the properties of the unidirectional and cross-ply materials were made using appropriate averaging schemes, taking into account the nature and extent of damage. A key conclusion is that cracks have a pronounced effect on the thermal diffusivity parallel to the loading direction while the material is subjected to stress and hence the cracks are open; upon unloading, the cracks largely close, allowing recovery of the longitudinal diffusivity to near-pristine levels. Furthermore, the associated debonding and sliding that occur along the interfaces adjacent to matrix cracks have a significant effect on the transverse diffusivity, a consequence of degradation in the interface conductance.