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Fracture

From Academic Kids

Mechanical failure modes
Buckling
Corrosion
Creep
Fatigue
Fracture
Melting
Thermal shock
Wear

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.

Contents

Types of fracture

The fracture and fracture mechanics deal with the behavior of existing cracks under external loading. Such cracks can be internal defects when the structures or materials were manufactured, or arise from other causes during service such as fatigue, corrosion, etc. In fact, the most commonly used method to initiate a crack in standard fracture mechanics test procedure is to fatigue the specimen. The essence of fracture is the high stress concentration at the crack tip, due to severe geometry discontinuity. If the crack tip is assumed to be infinite sharp as converging to a point, the elasticity theory gives singular solution at that point. The solution predicts the stress at any point near the crack tip is proportional to the inverse of the distance from this point to the crack tip. When the distance approaches zero, the stress approaches infinite. In reality, such infinite stress obviously can not happen, but instead, the high stress causes irreversible damage around crack tip that prevents stress from increasing beyond certain limit. Two mechanisms are generally considered as the irreversible damage: the breaking of atomic or molecular bonds to form a new surface and the irreversible deformation of the material (without forming a new surface). Both of these mechanisms absorbs energy during the fracture process and requires addition energy input to drive the propagation of the cracks. The roles of these two mechanisms are different in different materials. If the atomic/molecular bond breaking happens early, without significant material irreversible deformation, the material is considered as brittle material, or the fracture process as brittle fracture. If the new surface formation occurs after significant volume of material sustaining irreversible deformation, the material is considered as ductile material, or the fracture process as ductile fracture. Although materials are generally categorized as brittle or ductile as observed under room temperature and simple loading condition, they do not always exhibit the same behavior under different environments or loading conditions. Rubbers can become very brittle under low temperature, while glass can be very ductile near its melting temperature.

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.

Ductile fracture

In ductile fracture, extensive plastic deformation takes place before fracture. Similarly, shear fracture arises from the action of shear stress and slip in crystals.

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)de:Bruch

es:Fractura fr:Rupture (matériau) nl:Breuk (mechanica) ja:骨折 pl:Przełam

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