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Fracture is the separation of a material into two or more pieces under an applied stress. Fracture can result from

 Rapid growth of a crack through the material, a process described as fast fracture but often referred to simply as fracture

 Slow crack growth, a process described as fatigue (Section 4.2.7).

Unless when necessary to distinguish between fast fracture and fatigue, we shall use the term facture to mean fast fracture. Fracture typically occurs by propagation of existing cracks in a material which suddenly become unstable when the material is subjected to a sufficiently high stress which may be constant or slowly varying. Often, the magnitude of this stress is equal to a significant fraction of its measured yield strength or fracture strength. In comparison, fatigue often occurs when a material is subjected to repeated stress cycles over a long period, the magnitude of the stress often being a small fraction of its measured strength.

Fracture is classified into two types, ductile fracture and brittle fracture, which depend on the mechanical response of the solid. Ductile fracture occurs in most metals (including their alloys) whereas ceramics (including glasses and glass‐ceramics) show little or no overall plastic deformation but, instead, fail catastrophically in a brittle manner (Figure 4.6). At room temperature, some amorphous polymers, such as polystyrene for example, fracture in a brittle manner with little sign of ductility. In comparison, semicrystalline polymers and some amorphous polymers show ductile fracture to varying degrees at room temperature.

Fracture of materials is said to involve two successive steps, crack formation and crack propagation (growth), regardless of the fracture type. On the other hand, once a crack is formed, there is a marked difference in the mechanism of crack propagation and fracture between ductile and brittle fracture.