Multimodal approach for the in-situ investigation of high temperature tensile behavior and damage progression in SiCf/SiC ceramic matrix minicomposites

Ceramic matrix composites (CMCs) offer significant advantages for implementation into hot zone components of gas turbine engines when compared to superalloys. For their continued implementation into advanced engines, reliable evaluation of life limiting behavior under specific thermomechanical conditions is required. In this research, the mechanical behavior and damage within unidirectional ceramic matrix minicomposite specimens under high temperature tensile loading conditions will be investigated using acoustic emission (AE) and digital image correlation (DIC) techniques. Initial tensile experiments were performed at room temperature to establish the baseline, expected minicomposite mechanical behavior. DIC was demonstrated to provide accurate, non-contact strain measurements at room temperature. DIC strains were used to determine fundamental minicomposite properties at room temperature, including modulus and interfacial shear strength. Next, high temperature experiments were performed to evaluate the effect of temperature on mechanical properties and AE-estimated damage events of the minicomposites. Tests were conducted from 1200°C up to 1482°C using a conventional slit furnace. The decline in mechanical strength properties as test temperature was increased was related to the weakening of SiC fibers and the boron nitride interphase at high temperatures. The use of DIC during high temperature experiments would provide in-situ strain information of the minicomposite surface under thermomechanical loading. Before DIC was used during minicomposite tensile testing at high temperatures, a high temperature DIC method was first developed. A quadrupole lamp furnace (QLF) was adapted for this technique. Development of this method included the accurate determination of the sample temperature, accurate identification of the hot zone, and use of the experimental setup under mechanical loading conditions. The developed high temperature DIC method was used to measure surface strains of the minicomposites during tensile loading from 982°C up to 1200°C. The AE activity was also monitored during these tests. Results showed a decrease in mechanical properties with an increase in temperature, accompanied by an increase in fiber sliding. Comparisons between slit furnace and QLF tests revealed differences in mechanical behavior, arising from differences in the thermal profiles of the furnaces. In this dissertation, the benefits of employing a concurrent, multimodal approach to monitoring damage in ceramic matrix minicomposites at high temperatures are demonstrated. In addition, insights of high temperature damage mechanisms within SiCf/SiC minicomposites are discussed.