Damage to an A-N720 ceramic matrix composite under simulated gas turbine static component conditions using laser heating

Abstract

A ceramic matrix composite material constructed from a porous alumina matrix and Nextel™ 720 fibers was subjected to a novel type of experiment designed to reproduce the operating conditions of a gas turbine static component. Material specimens were exposed to cyclic laser heating on one side and active air cooling on the other side while being constrained in their bending deflection. For most specimens, accumulation of damage under the constant-amplitude heat load resulted in a temperature increase above the material limit of 1200℃ at the heated surface. Stress relaxation was observed due to inflicted sudden damage, like surface fiber buckling or ply delamination, and due to gradual permanent deformation of the material. The amount of damage was quantified by the variation of a stiffness parameter based on the measured reaction force and through-thickness temperature difference. While the level of damage was significant at the hot face, with the depth of cracking and delamination reaching up to half of the specimen thickness, the specimen back face remained intact. Stabilization of the damage was observed due to stress relaxation and decoupling of the damaged plies from the non-affected ones.