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Encyclopedia > Creep (failure mode)
Mechanical failure modes
Thermal shock

Creep is the term given to the material degradation that occurs as a result of long term exposure to levels of stress that are below the yield or ultimate strength. The rate of this damage is a function of the material properties and the exposure time, exposure temperature and the applied load (stress). Depending on the magnitude of the applied stress and its duration, the deformation may become so large that a component can no longer perform its function - for example creep of a turbine blade will cause the blade to contact the casing, resulting in the failure of the blade. Creep is usually a concern to engineers and metallurgists when evaluating components that operate under high stresses and/or temperatures. Creep is not neccessariliy a failure mode, but is instead a damage mechanism. In engineering, buckling is a failure mode of a structural member characterised by a failure to react to the bending moment generated by a compressive load. ... Corrosion Corrosion is deterioration of useful properties in a material due to reactions with its environment. ... In materials science, fatigue is a process by which a material is weakened by cyclic loading. ... A fracture is the separation of a body into two, or more, pieces under the action of stress. ... Physics In physics, melting is the process of heating a solid substance to a point (called melting point) where it turns liquid. ... Thermal shock is the name given to cracking as a result of rapid temperature change. ... In materials science, wear is the errosion of material from a solid surface by the action of another solid. ...

Rather than failing suddenly with a fracture, the material permanently strains over a longer period of time until it finally fails. Creep does not happen upon sudden loading but the accumulation of creep strain in longer times causes failure of the material. This makes creep deformation a "time-dependent" deformation of the material. Strain, in any branch of science dealing with materials and their behaviour, is the geometrical expression of deformation caused by the action of stress on a physical body. ...

Creep deformation can be obtained in reasonable time frames under very high temperatures i.e. temperatures around half of the melting temperature. Plastics and low-melting-temperature metals creep at room temperature, and virtually any material will creep upon approaching its melting temperature. This deformation behaviour is important in systems for which high temperatures are endured, such as nuclear power plants, jet engines, heat exchangers etc. Since the relevant temperature is relative to melting point, creep can be seen at relatively low temperatures depending upon the alloy; it occurs at room temperature in solders, and can be seen markedly in older lead hot-water pipes.

An example of an application involving creep deformation is the design of tungsten lightbulb filaments. Sagging of the filament coil between its supports increases with time due to creep deformation caused by the weight of the filament itself. If too much deformation occurs, the adjacent turns of the coil touch one another, causing an electrical short and local overheating, which quickly leads to failure of the filament. The coil geometry and supports are therefore designed to limit the stresses caused by the weight of the filament, and a special tungsten alloy that creeps less than pure tungsten is used.

Cooling systems of power plants with superheated vapour work under high temperature and high pressure. In a jet engine temperatures may reach to 1000 degrees Celsius, which may initiate creep deformation in a weak zone. Because of these reasons, understanding and studying creep deformation behaviour of engineering materials is very crucial for public and operational safety.

  Results from FactBites:
Creep (deformation) - Wikipedia, the free encyclopedia (1786 words)
Creep is the term used to describe the tendency of a material to move or to deform permanently to relieve stresses.
Creep is not necessarily a failure mode, but is instead a damage mechanism.
Creep arises from this because of the phenomenon of dislocation climb.
LJ Gibson Trabecular Bone (1308 words)
In an attempt to determine the phase (collagen or hydroxy-apatite) responsible for these similar creep behaviors, we investigated the creep behavior of demineralized cortical bone, recognizing that the organic (i.e., demineralized) matrix of both cortical and trabecular bone is composed primarily of type I collagen.
The creep behavior was characterized by the three classical stages of decreasing, constant, and increasing creep rates at all applied normalized stresses and temperatures.
Creep failure was also studied using a single cell analysis, in which a closed-form solution was obtained after prescribing the creep properties of the trabeculae.
  More results at FactBites »



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