Crazing (Polymers)

thumb|Crazes in polystyrene (PS) Crazing is a yielding mechanism in polymers characterized by the formation of a fine network of microvoids and fibrils. These structures (known as crazes) typically appear as linear features and frequently precede brittle fracture. The fundamental difference between crazes and cracks is that crazes contain polymer fibrils (5-30 nm in diameter), constituting about 50% of their volume, whereas cracks do not. Unlike cracks, crazes can transmit load between their two faces through these fibrils.

Crazes typically initiate when applied tensile stress causes microvoids to nucleate at points of high stress concentration within the polymer, such as those created by scratches, flaws, cracks, dust particles, and molecular heterogeneities. Crazes grow normal to the principal (tensile) stress, they may extend up to centimeters in length and fractions of a millimeter in thickness if conditions prevent early failure and crack propagation. The refractive index of crazes is lower than that of the surrounding material, causing them to scatter light. Consequently, a stressed material with a high density of crazes may appear 'stress-whitened,' as the scattering makes a normally clear material become opaque.