Evaluating the mechanical behavior of notched SiC/SiC composites using thermoelastic stress analysis

Ceramic matrix composites (CMCs) are potential materials for high temperature applications in gas turbines. Currently, the main areas of consideration include combustor liners and turbine vanes. Because these components contain stress concentrations, it is imperative that candidate materials be fully assessed concerning behavior related to stress risers. In this study, the stress fields adjacent to machined notch roots were examined for woven SiC fiber reinforced, melt infiltrated SiC matrix composites with a BN interphase, utilizing either Hi-Nicalon(TM) fibers or the stiffer Sylramic(R) fibers. The double-edge notched tensile test approach was used for multiple notch sizes. In addition, the materials ability to diffuse stress concentrations by inelastic damage mechanisms was considered. The damage was induced by applying predetermined tensile stresses known to cause matrix cracking. The thermoelastic stress analysis technique (TSA, also recognized as SPATE: Stress Pattern Analysis by Thermal Emission) was utilized to define the stress profile on the specimen surface. TSA is based on the fact that materials experience small temperature changes when compressed or expanded. When subjecting a structure to a cyclic load, a related cyclic temperature profile results. The surface temperature profile, measured with an infrared camera, is directly related to the surface stresses on the structure. The amplitude of the TSA signal is defined to be linearly dependent on the cyclic stress amplitude. Here, results were provided concerning the stress concentration behavior as a function of notch length, damage level, as well as the mean stress during TSA measurements.