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The purpose of this study was to characterize crack initiation and growth in notched titanium matrix composites at room temperature. Double-edge-notched or center-open-hole SCS-6/Ti-15-3 specimens containing 0° or both 0° and 90° plies were fatigued. The specimens were tested in the as-fabricated (ASF) and in heat-treated conditions. A local strain criterion using unnotched specimen fatigue data was used to estimate fatigue crack initiation with varying success. The initiation stress level was accurately predicted for both a double edge notched unidirectional specimen and a cross-plied center-hole specimen. The fatigue produced long multiple cracks growing from the notches. These fatigue cracks were only in the matrix material and did not break the fibers in their path. The combination of matrix cracking and fiber/matrix debonding appears to greatly reduce the stress concentration around the notches. The laminates that were heat treated showed a different crack growth pattern. In the ASF specimens, matrix cracks had a more tortuous path and showed considerably more crack branching. For the same specimen geometry and cyclic stress, the [0/90/0] laminate with a hole had far superior fatigue resistance than the matrix-only specimen with a hole.
metal matrix composite, silicon carbide fibers, fiber/matrix interface, composite materials, fatigue (materials), fracture
Research scientist, Analytical Services and Materials, Inc., Hampton, VA
Senior research engineer, NASA Langley Research Center, Hampton, VA