Fatigue behavior of ceramic matrix composites.

Abstract

Stress-strain behavior of unidirectional Nicalon fiber/CAS II matrix composite was examined at varying frequencies on two specimens that were pre-cracked under constant tensile loading and on a specimen that was fatigued for 10<super> 8</super> cycles prior to varying frequency experiments. Besides the effect of loading frequency, the effect of specimen temperature increase due to frictional heating on interfacial properties was studied. It was found that while loading frequency does not affect the stress-strain behavior, specimen temperature increase leads to an increase in the energy dissipation. The stress-strain behavior of Nicalon fiber/CAS II matrix composite was examined under tension-tension cyclic loading conditions. Comparison of the experimental stress-strain behavior with the predictions of a constant interfacial shear stress and an interface that is constrained only by the interfacial shear stress model revealed that this model is not satisfactory. It was shown that locking of the interface, hence a decrease in the slip-length as a function of the loading cycles should be included in the stress-strain model. It was also argued that an increasing shear stress along the slip-length may further improve the predictions. Stress-strain behavior and failure characteristics of the unidirectional Nicalon fiber/CAS II matrix composite during cyclic loading are discussed. It is shown that locking of the fiber-matrix results in higher stresses on the fibers. Failure of global load sharing assumption under these conditions leads to further increase in the fiber stress. Weibull analysis shows that these two effects acting on the fibers causes a decrease in the fiber survival probability; hence, they decrease the composite strength. Loading frequency effect on fatigue life is attributed to time dependent matrix cracking and resulting variations in the locking lengths of fibers at a given matrix crack plane.