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Indentation damage is a particularly important limitation in the use of brittle materials in structural applications. Some nominally brittle materials have been shown to exhibit plastic type yield in indentation testing.1–10 These materials are characterized by a relatively coarse grain structure and weak grain boundaries. A particularly convenient material for study is a mica-containing glass-ceramic. As shown in Fig. 1, the microstructure consists of circular mica platelets in a glassy matrix. The size of the platelets can be modified by simple heat treatment during processing. The average platelet diameter and the aspect ratio increase with increasing crystallization temperature. During indentation with a spherica indenter, a subsurface region of accumulated damage is observed to form in preference to the characteristic Hertzian cone crack normally associated with this type of loading. An indentation stress-strain response shows a shape similar to that obtained on ductile materials. [ABSTRACT FROM AUTHOR]