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Mechanical Behavior of a Continuous Fiber Reinforced SiC/RBSN Ceramic Composite

The purpose of the following research was to obtain an understanding of both ambient and elevated temperature mechanical behavior of a SiC fiber reinforced RBSN composite. At ambient temperature, applicability of available mechanics models to describe the stress-strain curve were examined. Emphasis was also placed on fracture toughness, R-curve behavior, and toughening mechanisms as well as the applicability of available fracture mechanics models to describe toughening behavior. At elevated temperature, an attempt was made to characterize the short term and long term effects on the composite. The material used was a RBSN reinforced with large diameter continuous SiC fibers. A limited investigation of the mechanical behavior of a commercially available RBSN monolith was also performed for comparison purposes with the reinforced material. At ambient temperature the composite exhibited noncatastrophic failure and analysis of the results suggested that fiber pullout as well as elastic fiber bridging effects may both provide significant contributions to the overall toughness, with the overall potential for toughness calculated to be on the order of 54 MPa-m*. Analysis of the toughness based on these mechanisms was complicated, however, by the large amount of delammation that took place in this composite. Toughening occurred as fiber pullout, bridging, and mixed mode failure at room temperature. At elevated temperatures, crack initiation under constant load occurred, after a delay period, in the matrix at temperatures on the order of 1000° C, but occurred in the fiber itself as the temperatures increased past 1350° C. The elevated temperature cracks were normal to the reinforcing fibers and were associated with unbroken bridging fibers in the crack wake. Furthermore, it was observed that the fibers pulled out of the matrix in a time-dependent fashion, thus making bridging a time dependent process during elevated temperature exposure.
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