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Crack growth resistance curve
In materials modeled by linear elastic fracture mechanics (LEFM), crack extension occurs when the energy release rate G equals R, the material's resistance to crack extension. A plot of R versus crack extension is called a crack resistance curve, or R curve. The corresponding plot of energy release rate versus crack extension is called a driving force curve.
== Types of R-curve==
Generally, materials display either a flat R curve or a rising R curve. For a flat R curve, material resistance is constant with respect to crack extension. When the resistance curve is flat, it exhibits a critical value of energy release rate Gc. A material with a rising R curve, however, cannot be uniquely characterized with a single toughness value. A flawed structure fails when the driving force curve is tangent to the R curve, but this point of tangency depends on the shape of the driving force curve, which depends on the configuration of the structure. Materials with rising R curves are generally characterized by the value of G at the beginning of crack growth, but the precise moment of initiation is unclear. The R curve for an ideally brittle material is flat because the surface energy is a fixed property. Nonlinear behavior, like ductile fracture, can result in a rising R curve as the plastic zone at crack tip increases in size with extension. Falling R curves occur when a metal fails by cleavage. Cleavage propagation is unstable and very high strain rates are found near the crack tip. This suppresses plastic deformation.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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