Characterization of Mechanical Damage Mechanisms in Ceramic and Polymeric MatrixComposite Materials

摘要

The role of microplasticity in the compressive strength of strong ceramics isexplored, emphasizing a representative oxide (Aluminum oxides) and non-oxide (Silicon carbides). Relevant results from studies of hardness, compressive behavior, and impact response are considered. The combined evidence indicates that microplasticity is a vital factor in the compressive failure of even these very hard materials under essentially all conditions (temperature, strain rate, confinement). It is shown that uniaxial compressive strengths achieved under perfect test conditions approach one-third of the hardness as a limit. Under most conditions, dislocation activity appears to be detrimental to compressive strength. Since only an extremely limited number of slip systems is available for stresses below H/3, relaxation of intrinsic stress concentrators is ineffective; instead, the highly localized discrete slip bands themselves constitute intense grain boundary stress raisers, and thereby contribute significantly to prefailure compressive microfracture. Compressive failure of a 0 deg/90 deg glass fiber-reinforced amorphous thermoplastic is characterized. It is found that the critical event is the nucleation within 90 deg laminates of multiple shear crazes, which become shear microcracks, transition to axial cracks, and permit the specimen to fail by the flexure of 0 deg elements. Further, it is shown that the apparent kinetics of this process provide a rationale for the dramatic strain rate strengthening of these composites at high loading rate.