The fracture behavior of multilayered and fibrous ceramic/metal composites

Layered materials comprising one brittle and one ductile constituent exhibit crack growth characteristics that depend on the formation of cracks in each brittle layer. For the case where the crack must sequentially renucleate in each layer, the results of experimental investigations of two aspects of the problem are presented: crack renucleation across intact single metal layers and subsequent crack extension. Aluminum/alumina and copper/alumina multilayered composites are investigated. Crack renucleation criteria are determined and compared with predictions of micromechanical models. Several high resolution strain mapping techniques, including moire interferometry, optical stereoscopy and a high-resolution strain mapping procedure (HASMAP) are employed to determine the surface strain fields surrounding cracks. Optical fluorescence spectroscopy is also used to probe the crack tip stresses. Good agreement is found between these measurements and a small scale yielding model for multilayers with strongly bonded interfaces. Plastic deformation of the metal layers in the vicinity of the crack tip does not alter significantly the crack tip stress fields in the next brittle layer from those expected for an elastic, homogeneous body. Crack progression subsequent to crack renucleation occurs either by the extension of a dominant, nearly planar crack or by the formation of a zone of periodically spaced cracks. Both patterns are analyzed. The cracking behavior is found to depend on the volume fraction and the yield stress of the metal.; In contrast to the strong interfacial bond observed during monotonic loading of Al/Al{dollar}sb2{dollar}O{dollar}sb3{dollar} multilayered composites, cyclic loading results in controlled debonding of the interface. The growth characteristics of cracks at the interface between sapphire and aluminum under cyclic loading conditions are analyzed and found to be qualitatively similar to those of fatigue cracks in monolithic aluminum alloys. The effects of such interface debonding on the associated crack tip stress fields are determined and compared with the predictions of micromechanics models.; In-situ observations of crack growth and direct measurements of elastic/plastic strains in the regions surrounding a crack in an Al/Al{dollar}sb2{dollar}O{dollar}sb3{dollar} metal matrix composite are presented. From the strain measurements and bridging zone observations, estimates are made of the separate contributions to the measured toughness from bridging and process zone mechanisms. Both mechanisms are found to have important effects on the overall fracture behavior. (Abstract shortened by UMI.)...