Simultaneous Fatigue and Combustion Exposure of a SiC/SiC Ceramic Matrix Composite

Abstract

A melt-infiltrated (MI) woven ceramic matrix composite consisting of a silicon carbide matrix reinforced by boron nitride coated Hi-Nicalon type STM SiC fiber, Hi-Nic-S/BN/SiC, was tested under tension-tension fatigue loading in combination with combustion conditions representative of those experienced by hot-section components such as turbine blades and vanes in modern gas turbine engines. The burner rig fatigue data and fracture surfaces were analyzed for the effects of oxidation on life, failure, and damage mechanisms. These test results were then compared with those obtained from similar fatigue tests performed in a standard furnace under laboratory air environment. Fatigue life in the combustion condition was lower by an order of magnitude in comparison to the isothermal furnace results across the range of applied stress, and so demonstrates the importance of representative combined environment testing in conjunction with fundamental load testing. The observed difference in fatigue performance is attributed to the thermal gradient stress and increased rate of oxidation due to a high moisture level in the combustion rig test condition. The former was verified using finite element analysis and the latter from microscopic analysis of the fracture surfaces.