Crazing

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Crazing, also known as alligatoring, is a phenomenon that frequently precedes fracture in some glassy thermoplastic polymers. Crazing occurs in regions of high hydrostatic tension, or in regions of very localized yielding, which leads to the formation of interpenetrating microvoids and small fibrils. If an applied tensile load is sufficient, these bridges elongate and break, causing the microvoids to grow and coalesce; as microvoids coalesce, cracks begin to form.

A craze is different from a crack in that it can continue to support a load. Furthermore, the process of craze growth prior to cracking absorbs fracture energy and effectively increases the fracture toughness of a polymer. The initial energy absorption in a craze region has been found to be up to several hundred times that of the uncrazed region, but quickly decreases and levels off. Crazes form at highly stressed regions associated with scratches, flaws, and molecular inhomogeneities, and generally propagate perpendicular to the applied tension. Crazing occurs mostly in amorphous, brittle polymers like PS, PMMA and is typified by a whitening of the crazed region. The white color is caused by light-scattering from the fibrils.

One of the main differences between crazing and shear banding, another form of stress deformation, is that crazing occurs with an increase in volume, which shear banding does not. This means that under compression, many of these brittle, amorphous polymers will shear band rather than craze, as there is a contraction of volume instead of an increase. In addition, when crazing occurs, one will typically not observe "necking," or concentration of force upon one spot in a material. Rather, crazing will occur homogeneously throughout the material.

Crazing is also seen with some glazes used on pottery, and may be seen on concrete when good concrete practices are not followed.