Characterization of Interphase Environmental Degradation at Elevated Temperature of Fibre-Reinforced Titanium Matrix Composites

Fibre reinforced metallic composite materials are being considered for a number of applications because of their attractive mechanical properties as compared to monolithic metallic alloys. An engineered interphase, including the bond strength between the composite's constituents, contributes to a large extent to the improvement of strength and stiffness properties of this class of materials. However, in high temperature applications, where combination of cyclic loading with environmental effects is expected, consideration should be given to interphase degradation, especially in the vicinity of stress risers, such as notches and holes. The applicability of damage tolerance analysis in structural components made of titanium matrix composite materials designed to operate under high temperature environments would depend on the availability of adequate characterization methods for the evaluation of interfacial degradation. The objective of this paper is to provide a basic understanding of interfacial degradation mechanisms due to oxidation in environmentally exposed titanium-based composites subjected to cyclic stresses. A non-destructive method has been developed enabling high-resolution monitoring of interfacial damage initiation and accumulation as well as surface/subsurface cracking behaviour during interrupted fatigue tests. This nondestructive technique is based on surface acoustic wave propagation in the composites and can detect minute changes in elastic properties of the interfacial region due to elevated temperatures as well as oxygen effects.