The high frequency fatigue behavior of continuous-fiber-reinforced ceramic matrix composites

Many potential applications for continuous fiber ceramic matrix composites (CFCMCs), such as gas turbines and heat exchangers, will involve high frequency cyclic loading (75 Hz or higher). While most of the work in the area of fatigue of CFCMCs has concentrated on low frequency behavior, it has been shown that fatigue at high frequencies can exacerbate the accumulation of microstructural damage and significantly decrease fatigue life.; "Soft" matrix composites with strong interface bonding provided superior resistance to high frequency fatigue damage. Nicalon/SiCON composites with strong interfacial bonding between the fibers and matrix exhibited very little internal heating during high frequency fatigue loading. This composite system exhibited excellent fatigue life, with fatigue runout at 10{dollar}sp7{dollar} cycles occurring for stresses close to 80% of the ultimate strength (at a loading frequency of 100 Hz).; Thick fiber coatings may be more effective in reducing the amount of fiber wear and damage which occur during high frequency fatigue. More effective lubrication at the fiber/matrix interface was achieved with thicker carbon coatings in Nicalon/C/SiC composites subjected to high frequency fatigue loading. Composites with thicker coatings exhibited substantially lower frictional heating and had much higher fatigue lives.; The effect of laminate stacking sequence had a significant effect on the high frequency fatigue behavior of CFCMCs. In SCS-6/Si{dollar}rmsb3Nsb4{dollar} composites, frictional heating in angle-ply laminates ({dollar}pm{dollar}45) was substantially higher than that in cross-ply laminates (0/90). Since the angle-ply had a lower stiffness, matrix microcracking in this composite was more predominant.; Finally, preliminary fatigue damage mechanism maps for CFCMCs were developed. These maps provided a means to identify which fatigue mechanisms were operating at a given stress level and number of cycles.