Effects of Matrix Cracks on the Thermal Diffusivity of a Fiber‐Reinforced Ceramic Composite

Abstract

Effects of matrix cracks and the attendant interface debonding and sliding on both the longitudinal and the transverse thermal diffusivities of a unidirectional Nicalon/MAS composite are investigated. The diffusivity measurements are made in situ during tensile testing using a phase-sensitive photothermal technique. The contribution to the longitudinal thermal resistance from each of the cracks is determined from the longitudinal diffusivity along with measurements of crack density. By combining the transverse measurements with the predictions of an effective medium model, the thermal conductance of the interface (characterized by a Biot number) is determined and found to decrease with increasing crack opening displacement, from an initial value of Ϸ1 to Ϸ0.3. This degradation is attributed to the deleterious effects of interface sliding on the thermal conductance. Corroborating evidence of degradation in the interface conductance is obtained from the inferred crack conductances coupled with a unit cell model for a fiber composite containing a periodic array of matrix cracks. Additional notable features of the material behavior include: (i) reductions of Ϸ20% in both the longitudinal and the transverse diffusivities at stresses near the ultimate strength, (ii) almost complete recovery of the longitudinal diffusivity following unloading, and (iii) essentially no change in the transverse diffusivity following unloading. The recovery of the longitudinal diffusivity is attributed to closure of the matrix cracks. By contrast, the degradation in the interface conductance is permanent, as manifest in the lack of recovery of the transverse diffusivity.

Evaluation of Ceramic Matrix Composites by Thermal Diffusivity Imaging

“…The effect of distributed porosity and voids is analyzed in “”; as shown there, the diffusivity decreases nearly linearly with the CMC volume fraction (). In many situations, microscopic matrix cracks are aligned in the through‐thickness direction 19 . Therefore, the measurement of through‐thickness thermal diffusivity may not be considered sensitive to these cracks.…”

“…However, these cracks normally have lateral extensions, and most importantly, their presence would cause debonding at the matrix/fiber interface once the component is stressed. The debonded interface will impede heat transfer through the interface, and coupled with the large volume fraction of the fibers, interface debonding may considerably reduce thermal diffusivity in the thickness direction 19 . Therefore, thermal diffusivity imaging may still be able to determine the distribution of microscopic cracks and interface debonding.…”

Crack opening behavior in ceramic matrix composites

“…Crack opening displacement (COD) is a function of the applied stress. While CODs can be calculated, most models consider unidirectional reinforcement and require constituent parameters, like interfacial shear stress, that are difficult to measure and are often approximated from other measurements. Direct experimental measurements of COD values are largely unavailable with references being the exceptions.…”

Thermal Conductance of Delamination Cracks in a Fiber‐Reinforced Ceramic Composite

“…The latter damage mode is the focus of the present paper; a study of the role of multiple matrix cracks on the in-plane properties is reported elsewhere. 5 To further motivate the current work, it is useful to consider the problem of a thin CMC plate that contains a long delamination crack along the midplane and is subject to a steady-state temperature difference ⌬T between the two surfaces of the plate. The resulting energy release rate for the crack (G) is given by 6 G ϭ EA͑␣⌬T͒ 2 32͑1 ϩ b͒ 2 (1) where E is the in-plane modulus and A is the plate thickness.…”