Brittle fracture precursors

The short circumferential cracks apparent in Fig. 10.7 are precursors of the complete, brittle fractures shown in Fig. 10.3. Numerous fractures had occurred adjacent to the tube sheet over the past year. 

Shear yielding in the form of a quasi-homogeneous,bulk process can contribute substantially to the crack resistance of a polymeric solid. On the other hand, however, localized shear yielding in the form of shear micro-bands is befleved to be a precursor of brittle fracture in many semicrystalline and glassy thermoplas-... 

The appearance of crazes in a material is generally a precursor to brittle failure. While such a mode is normally to be avoided, the presence of crazes can provide some beneficial effects. Because the crazes contain highly drawn fibrils of material, considerable plastic deformation and hence energy goes in to their formation and this can be a major source of fracture toughness. Indeed, the deliberate inclusion of small, typically rubber, particles into inherently brittle polymers is commonly undertaken to produce tough materials (112-115), because the presence of the particles dramatically increases the craze density. 

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