Quantitative stress mapping in alumina composites by optical fluorescence imaging

Internal residual stresses exist in almost all polycrystalline materials and in many cases exert a profound influence on mechanical and physical behavior. In inherently brittle ceramics and ceramic composites, toughening mechanisms often depend sensitively on localized residual stresses. However, despite their prominence few studies are available which provide a detailed characterization of the local spatial variation of residual stress fields in ceramics and their composites. In this study, we report on the spatially resolved mapping of local residual and applied stresses in polycrystalline alumina and a variety of Al2O3SiCw composites. Stress mapping is based on the well established method 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 measurements from an area approx. 1 mm2. Detailed stress maps are generated which produce a vivid and quantitative indication of the complex residual stress fields present in these materials. The additional effect of a non-uniform applied stress field ahead of a blunt notch is also measured. Results are discussed in the context of thermal expansion anisotropy within the alumina phase, thermal expansion differences between the 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 mechanics models and compared to measured values.