Ph.D. Thesis Defense by Samuel Graham
Tuesday, July 6, 1999

(Dr. David McDowell, advisor)

"Effective Thermal Conductivity of Damaged Composites"

Abstract

Ceramic matrix composites (CMCs) are susceptible to matrix cracking, fiber-matrix debonding, and oxidation processes are as result of their application environments.  These damage mechanisms act to degrade the thermal conductivity of CMCs which is a concern of both designers and life prediction analyst.  In order to adequately predict the effect of damage on thermal conductivity, an understanding of the damage-thermal conductivity coupling is necessary along with the development of accurate material models.

This research was conducted to investigate the degradation of thermal conductivity in CMCs.  In addition, an assessment was made of current micromechanics models for predicting thermal conductivity degradation. Experiments were performed on unidirectional reinforced Nicalon-LAS II glass-ceramic composites.  Thermal conductivity was determined indirectly through the flash diffusivity method, which was extended in this work to treat orthotropic composite materials.  Samples were subjected to mechanical loading-, oxidation-, and thermal shock-induced damage.  The results showed that mechanical loading-induced damage resulted in no change in thermal conductivity transverse to the fiber axis and up to a 3.5% change parallel to the fibers.  Oxidation of the samples after mechanical loading resulted in a degradation of thermal conductivity up to 26% and 10% transverse and parallel to the fibers, respectively. These data show the importance of the fiber-matrix interface in controlling both the longitudinal and transverse thermal conductivities of damaged composites.  Predictions of thermal conductivity degradation parallel to the fiber direction were made with a shear-lag type micromechanics model.  Results were in excellent agreement with experimental data.  Thermal diffusivity data from isothermal oxidation and thermal shock experiments also showed that these tests are an effective nondestructive method for monitoring progressive material degradation in CMCs.

An assessment of transverse thermal conductivity micromechanics models was made through comparison with numerical solutions of heat conduction for random fiber inclusions with random-periodic boundary conditions. These results indicate that micromechanics solutions which account for limited fiber interaction may be used to model composites with imperfect thermal interfaces. The condition of imperfect thermal contact reduces the level of heterogeneity in the material response and the level of interaction between inclusions.  Based on the typical imperfect thermal interface found in CMCs, the dilute concentration Hasselman-Johnson model may be applied to many of these materials to determine the interfacial thermal conductance.  These values may then be used to assess the local behavior of composites with imperfect interfaces.