A fracture is the (local) separation of an object or material into two, or more, pieces under the action of stress.

The word fracture is often applied to bones of living creatures, or to crystals or crystalline materials, such as gemstones or metal. Sometimes, in crystalline materials, individual crystals fracture without the body actually separating into two or more pieces. Depending on the substance which is fractured, a fracture reduces strength (most substances) or inhibits transmission of light (optical crystals).

A detailed understanding of how fracture occurs in materials may be assisted by the study of fracture mechanics.

Brittle fracture

In brittle fracture, no apparent plastic deformation takes place before fracture. In brittle crystalline materials, fracture can occur by cleavage as the result of tensile stress acting normal to crystallographic planes with low bonding (cleavage planes). In amorphous solids, by contrast, the lack of a crystalline structure results in a conchoidal fracture, with cracks proceeding normal to the applied tension.

The theoretical strength of a crystalline material is (roughly)

$\backslash sigma\_\backslash mathrm\{theoretical\}\; =\; \backslash sqrt\{\; \backslash frac\{E\; \backslash gamma\}\{r\_o\}\; \}$

where: -

$E$ is the Young's modulus of the material,

$\backslash gamma$ is the surface energy, and

$r\_o$ is the equilibrium distance between atomic centers.

On the other hand, a crack introduces a stress concentration modeled by

$\backslash sigma\_\backslash mathrm\{elliptical\backslash \; crack\}\; =\; \backslash sigma\_\backslash mathrm\{applied\}(1\; +\; 2\; \backslash sqrt\{\; \backslash frac\{a\}\{\backslash rho\}\})\; =\; 2\; \backslash sigma\_\backslash mathrm\{applied\}\; \backslash sqrt\{\backslash frac\{a\}\{\backslash rho\}\}$ (For sharp cracks)

where: -

$\backslash sigma\_\backslash mathrm\{applied\}$ is the loading stress,

$a$ is half the length of the crack, and

$\backslash rho$ is the radius of curvature at the crack tip.

Putting these two equations together, we get

$\backslash sigma\_\backslash mathrm\{fracture\}\; =\; \backslash sqrt\{\; \backslash frac\{E\; \backslash gamma\; \backslash rho\}\{4\; a\; r\_o\}\}.$

Looking closely, we can see that sharp cracks (small $\backslash rho$) and large defects (large $a$) both lower the fracture strength of the material.

Recently, scientists have discovered supersonic fracture, the phenomenon of crack motion faster than the speed of sound in a material.Fact: date=February 2007 This phenomenon was recently also verified by experiment of fracture in rubber-like materials.

Ductile fracture

Because ductile rupture involves a high degree of plastic deformation, the fracture behavior of a propagating crack as modeled above changes fundamentally. Some of the energy from stress concentrations at the crack tips is dissipated by plastic deformation before the crack actually propagates.