Effect of elevated temperature and cyclic loading on properties and damage modes in ceramic composites.

摘要

This report discusses the results of a ongoing program of research concerned with the investigation of the effects of elevated temperature on the properties and performance of ceramic composites. The present report is concerned with just two aspects of the general program, the changes in properties and damage modes due to the effects of exposure of two ceramic composite systems to high temperature air and multiaxial loading. Two material systems and specimen configurations are discussed. Tubular specimens fabricated at the Oak Ridge National Laboratory using a SiC/SiC combination were tested quasi-statically at elevated temperatures. Changes in stiffness as a function of time and temperature were observed, and an SEM study revealed substantial differences in the microstructure associated with that exposure. Flat Nicalon/CAS-II coupons were fabricated by the Corning Glass Company, subjected to elevated temperature soaks for various periods of time and temperature, and then tested monotonically and under cyclic loading. Changes in damage and failure modes were observed that system, and a new surface damage mode on the fiber was discovered, possibly due to frictional sliding. The present report discusses the observations, interpretations, and some micromechanical modeling associated with these activities, and discusses the consequences of these temperature effects on future designs of material systems.