Damage Accumulation in a SiCf/SiC Ceramic Matrix Composite under Conditions of Cyclic Constraint

Abstract

This paper will consider the effects of constraint on fatigue behaviour in a SiCf/SiC CMC under two distinct experimental scenarios. In the first instance, bulk fatigue performance will be quantified (ε–N curve) employing traditional strain controlled mechanical testing of plain coupons. Monitoring of constitutive stress-strain loops (σ–ε) and progressive stress-life response (σ–N) resulting from the fatigue loading will be presented indicating that the CMC displays increases in compliance as damage ensues. A basic strain-controlled fatigue database of this form would underpin future designs of thick, plain section engineering components. Secondly, focus will be placed upon the initiation of damage in more complex engineering designs, i.e., in the vicinity of stress raising features. A series of tests will be described using coupons that contained a circular or elliptical hole (offering fatigue stress concentration Kt factors of 2.4 and 4.5 respectively). Advanced monitoring techniques were employed for these particular tests, including the combination of acoustic emission (AE) and digital image correlation (DIC) in order to detect the earliest possible onset of damage. Both temporal and spatial damage progression was then defined during the course of stepped fatigue cycling. Supplementary experimentation and data quantifying the rate of sub-critical crack growth in this CMC system will demonstrate that physically “long” cracks (i.e., many millimetres in scale) retain “small” crack characteristics through the interactions between the crack tip and the fibre reinforcement architecture.