QUANTITATIVE STRESS MAPPING IN ALUMINA COMPOSITES BY OPTICAL FLUORESCENCE IMAGING

Internal residual stresses exist in almost all polycrystalline materia ls and in many cases exert a profound influence on mechanical and phys ical behavior. In inherently brittle ceramics and ceramic composites, toughening mechanisms often depend sensitively on localized residual s tresses. However, despite their prominence few studies are available w hich provide a detailed characterization of the local spatial variatio n of residual stress fields in ceramics and their composites. In this study, we report on the spatially resolved mapping of local residual a nd applied stresses in polycrystalline alumina and a variety of Al2O3- SiCw composites. Stress mapping is based on the well established metho d of measuring shifts in the characteristic optical fluorescence lines produced by Cr3+ impurities in the alumina matrix. Novel illumination and imaging optics are used to collect up to 15,000 stress measuremen ts from an area approx. 1 mm(2). Detailed stress maps are generated wh ich produce a vivid and quantitative indication of the complex residua l stress fields present in these materials. The additional effect of a non-uniform applied stress field ahead of a blunt notch is also measu red. Results are discussed in the context of thermal expansion anisotr opy within the alumina phase, thermal expansion differences between th e SiC-reinforcement and Al2O3 matrix, and the non-uniform distribution of whiskers. The mean value and fluctuation of the resultant residual stress fields are estimated by extension of recent stochastic mechani cs models and compared to measured values.