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Crazing

[kreyz]

Crazing is a network of fine cracks inside a surface or glaze of a surface.

Crazing 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.

Polymers

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 per square meter 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, stress concentrations and molecular inhomogeneities. Crazes generally propagate perpendicular to the applied tension. Crazing occurs mostly in amorphous, brittle polymers like PS, PMMA and polycarbonate; it is typified by a whitening of the crazed region. The white colour is caused by light-scattering from the crazes.

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.

Rubber toughening

Crazing occurs in some thermoplastics such as ABS plastic when stressed. It is a typical response rubber toughening where crazes are initiated at the surfaces of the rubber particles added to toughen the material.

Ceramics

Crazing is also seen with some glazes used on pottery, on single ply roofing membranes, and on concrete when good concrete practices are not followed.