Transient thermal fatigue of ceramic composites

Arvind K Sinha, Purdue University

Abstract

Ceramic composite materials have been developed for use at high temperatures and are subjected to cyclic thermal loads. These loads are an important factor in determining the reliability and mechanical load carrying capability of the composite. Specifically, the effect of cyclic thermal loads is to cause thermal stresses at the interface between the fiber and the matrix and cause debonding between these components. In turn, such debonding results in a change in the mechanical and thermal properties of the composite. Consequently, it is important to understand the mechanics of fiber-matrix interaction under thermal loads in order to optimize the behavior of composites at high temperatures. The changes in stiffness for a Nicalon/CAS-II unidirectional composite resulting from cyclic heating and cooling were investigated. The experimental measurements were made using an ultrasonic through-transmission method. It is shown that slower cooling rates result in degradation of stiffness mainly in the $C\sb{22}$ and $C\sb{33}$ direction through interface debonding. For a higher cooling rate, a decrease in $C\sb{11}$ is observed due to surface matrix cracks. The behavior of Nicalon/CAS-II composite under transient thermal load is studied using the various thermomicro-mechanical models. These models are then used to calculate thermo-mechanical stresses produced thermal load application. Using three-phase thermo-mechanical models, it is shown that the behavior of the matrix and the fiber-matrix interface is dependent on the transversely isotropic material surrounding the matrix. The finite element modeling done on the Nicalon/CAS-II composite verified the results obtained during thermal cycling experiments. To calculate the stiffness degradation, the concept of equivalent homogeneity was applied. The behavior of an interface and a matrix crack subjected to transient heating and cooling were studied using the single cell fiber-matrix model. Based on this model, the nondimensional strain energy release rate was calculated. Results show that when the critical strain energy release rate for the interface is similar or lower than that of the matrix, the interface crack will propagate before the matrix crack. Critical parameters affecting the behavior of the fiber-matrix interface were analyzed using three-phase thermo-mechanical models. These parametric studies include the effects of stiffness ratios between the fiber and the matrix, the effects of thermal expansion ratios and the effects of orthotropic properties by varying $v\sb{f}$.

Degree

Ph.D.

Advisors

Kokini, Purdue University.

Subject Area

Mechanical engineering|Materials science

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