Crazing

Crazing is the phenomenon that produces a network of fine cracks on the surface of a material, for example in a glaze layer. Crazing 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.

Crazing occurs in polymers, because the material is held together by a combination of weaker Van der Waals forces and stronger covalent bonds. Sufficient local stress overcomes the Van der Waals force, allowing a narrow gap. Once the slack is taken out of backbone chain, covalent bonds holding the chain together hinder further widening of the gap. The gaps in a craze are microscopic in size. Crazes can be seen because light reflects off the surfaces of the gaps. The gaps are bridged by fine called fibrils, which are molecules of the stretched backbone chain. The fibrils are only a few nanometers in diameter, and cannot be seen with a light microscope, but are visible with an electron microscope.

A craze is different from a crack in that it cannot be felt on the surface and 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 polystyrene (PS), acrylic (PMMA), and polycarbonate; it is typified by a whitening of the crazed region. The white colour is caused by light-scattering from the crazes.

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