Fracture

For the feature film, see Fracture (film)

For fractures in bones, see Fracture (bone).

A fracture is the separation of a body 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 requires the study of fracture mechanics.

Types of fracture

Brittle fracture

In brittle fracture, no plastic deformation takes place before fracture. In brittle single crystals, cleavage fracture occurs as the result of tensile stress acting normal to any of a crystal's cleavage planes. In amorphous solids, by contrast, a lack of crystallinity means that any direction may be considered a cleavage plane; the result is a conchoidal fracture, with cracks proceeding normal to the applied tension. Recently, scientists have discovered supersonic fracture , the phenomenon of crack motion faster than any speed of sound in a material. This phenomenon was recently also verified by experiment of fracture in rubber-like materials.

Ductile fracture

In ductile fracture, extensive plastic deformation takes place before fracture. In metallic materials, the plastic deformation is a well understood mechanism that stems from the atomic structureslip in crystalline structures. However, such mechanism only provides explanation for the material deformation, but not for material separation that constitutes fracture. No satisfactory atomic level model has been agreed on to explain the occurrence of the material separation in a pure material without inclusion. Many ductile metals, especially materials with high purity, can sustain very large deformation of 50-100% or more strain before fracture under favorable loading condition and environmental condition. The strain at which the fracture happens obviously is controlled by the purity of the materials. Pure iron can undergo deformation up to 100% strain before breaking, while casted iron or high-carbon steels can barely sustain 3% of strain.

Bibliography

• Dieter, G. E. (1988) Mechanical Metallurgy ISBN 0071004068
• A. Garcimartin, A. Guarino, L. Bellon and S. Cilberto (1997) " Statistical Properties of Fracture Precursors ". Physical Review Letters, 79, 3202 (1997)