Crazing
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Crazing, also known as alligatoring, is a phenomenon that frequently precedes fracture in some glassy thermoplastic polymers. Associated with crazes are regions of very localized yielding, which leads to the formation of small and interconnected microvoids. Fibrillar bridges form between these microvoids wherein molecular chains become oriented. 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 support a load across its face. Furthermore, this process of craze growth prior to cracking absorbs fracture energy and effectively increases the fracture toughness of a polymer. Crazes form at highly stressed regions associated with scratches, flaws, and molecular inhomogeneities: in addition they propagate perpendicular to the applied tensile stress, and typically are 5 μm or less thick. Crazing occurs mostly in amorphous, brittle polymers like PS, PMMA and produces a whiteish color caused by the presence of air in the craze. Unlike cracks, crazes can be recovered under pressure or high temperature annealling.
Crazing is also seen with some glazes used on pottery, and may be seen on concrete when good concrete practices are not followed.
External links
- http://www.emmgraphics.com/pottery/orttip6.html Understanding Crazing
- http://ceramic-materials.com/cermat/education/190.html Crazing in Stoneware Glazes: Treating the Causes, Not the Symptoms
- http://www.nrmca.org/aboutconcrete/cips/03p.pdf Crazing Concrete Surfaces
- http://images.google.com/images?q=crazing Images of crazing via Google Images (look for the images of fine 'cracks')
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