Interface measurements and applications in fiber-reinforced MMCs

Interface tests were performed to evaluate the shear and normal strength characteristics of the fiber/matrix interface in fiber reinforced titanium matrix composites. The mechanisms of interface shear failure were investigated using the push-out test, and a fracture mechanics analysis was performed to analyze the bottom-face crack initiation problem. This latter mode of failure is more common for metal matrix composites. A cruciform specimen design was used to obtain the normal (transverse) interface strength. This design circumvents problems associated tensile stress singularities at free surfaces, and tests on a multi-ply composite showed that conventional specimen designs with uniform widths can significantly under predict the normal strength of the interface. The relationships between interface strength and longitudinal properties of composites with ductile matrices are currently not clear. Preliminary results are provided on such effects for a single-ply multi-fiber SiC/Ti-GAl-4V composite. A strong interface with SCS-0 fibers produced a rather homogeneous fiber crack distribution, without any apparent crack coplanarity. Conversely, a weak interface with SCS-6 fibers produced a highly localized zone of fiber fractures, that was largely dominated by matrix slip bands. Thus, local load sharing also occurs with weak interfaces, but the mechanism appears to be different from current concepts of elastic stress concentrations.