The role of exlrinsic crack-tip shielding on toughness and fatigue-crack growth behavior in intermetallics and ceramics is contrasted with intrinsic damage mechanisms, both at ambient and elevated temperatures. Examples are presented from behavior in an in situ toughened SiC ceramic and in ductile-phase toughened γ-TiAl intermetallics. In SiC ceramics, marked R-curve toughening is achieved under monotonic loading by grain bridging, which is diminished under cyclic loading through progressive wear at the sliding grain interfaces. As a result, cyclic crack-growth rates are extremely sensitive to K_(max) and less dependent on △K, although at elevated temperatures, behavior is influenced by extrinsic and intrinsic effects associated with deformation and damage of grain-boundary amorphous films. In contrast, fatigue-crack growth in TiNb-reinforced γ-TiAl is a function of both intrinsic and extrinsic mechanisms, intermediate to behavior in metals and ceramics, such that propagation rates are sensitive to both △K and K_(max). However, the marked K-dependence of growth rates in these materials necessitates the use of design and life-prediction procedures based on crack initiation, small crack or threshold philosophies.
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