Creep (deformation)
Creep is more severe in materials that are subjected to heat for long periods and generally increases as they near their melting point.
[5] In the secondary, or steady-state, creep, dislocation structure and grain size have reached equilibrium, and therefore strain rate is constant.
This can be due to necking phenomena, internal cracks, or voids, which all decrease the cross-sectional area and increase the true stress on the region, further accelerating deformation and leading to fracture.
However, since the number of nearest neighbors is effectively limited along the interface of the grains, and thermal generation of vacancies along the boundaries is less prevalent, the temperature dependence is not as strong as in Nabarro–Herring creep.
At low temperatures, the solute atoms are immobile and increase the flow stress required to move dislocations.
[11][12] Solute drag creep sometimes shows a special phenomenon, over a limited strain rate, which is called the Portevin–Le Chatelier effect.
The exponent m for dislocation climb-glide creep is 4.5 if M is independent of stress and this value of m is consistent with results from considerable experimental studies.
[13] It is characterized by two principal phenomena: a power-law relationship between the steady-state strain rate and applied stress at a constant temperature which is weaker than the natural power-law of creep, and an independent relationship between the steady-state strain rate and grain size for a provided temperature and applied stress.
where ρ0 is dislocation density (constant for Harper–Dorn creep), Dv is the diffusivity through the volume of the material, G is the shear modulus and b is the Burgers vector, σs, and n is the stress exponent which varies between 1 and 3.
[16] Twenty-five years after Harper and Dorn published their work, Mohamed and Ginter[17] made an important contribution in 1982 by evaluating the potential for achieving Harper–Dorn creep in samples of Al using different processing procedures.
The experiments showed that Harper–Dorn creep is achieved with stress exponent n = 1, and only when the internal dislocation density prior to testing is exceptionally low.
Thus, a certain magnitude of applied tensile stress is required to offset these shrinkage effects and cause void growth and creep fracture in materials at high temperature.
For a general void with surface energy γ and principle radii of curvature of r1 and r2, the sintering limit stress is[27]
Wood is considered as an orthotropic material, exhibiting different mechanical properties in three mutually perpendicular directions.
It has also been shown that there is a substantial difference in viscoelastic properties of wood depending on loading modality (creep in compression or tension).
Studies have shown that certain Poisson's ratios gradually go from positive to negative values during the duration of the compression creep test, which does not occur in tension.
[40] Refractory metals, such as tungsten, molybdenum, and niobium, are known for their exceptional mechanical properties at high temperatures, proving to be useful materials in aerospace, defense and electronics industries.
Creep in epoxy anchor adhesive was blamed for the Big Dig tunnel ceiling collapse in Boston, Massachusetts that occurred in July 2006.
If too much deformation occurs, the adjacent turns of the coil touch one another, causing 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 with small amounts of oxygen trapped in the crystallite grain boundaries is used to slow the rate of Coble creep.
Teflon insulation is resistant to elevated temperatures and has other desirable properties, but is notoriously vulnerable to cold-flow cut-through failures caused by creep.
Creep deformation is important not only in systems where high temperatures are endured such as nuclear power plants, jet engines and heat exchangers, but also in the design of many everyday objects.
The term ‘superalloy’ generally refers to austenitic nickel-, iron-, or cobalt-based alloys that use either γ′ or γ″ precipitation strengthening to maintain strength at high temperature.
They can be used for solid solution strengthening, to reduce the formation of undesirable brittle precipitates, and to increase oxidation or corrosion resistance.
Therefore, by making the proper choice, suitable ceramic components may be selected, capable of operating at various conditions of high temperature and creep deformation.
Therefore, the properties of dislocations and vacancies, their distributions throughout the structure, and their potential change due to long-time exposure to stress and temperature must be considered seriously in materials selection for components design.
To recap, materials must be selected and developed that possess low dislocation and vacancy contents to have a practical creep resistance component.
Fortunately, creep doesn't occur suddenly in brittle materials as it does under tension and other forms of deformation, and it is an advantage for designers.
Over time, creep strain develops in a material exposed to stress at the temperature of the application, and it depends on the duration of the exposure.
It can be generalized as this function ε = F(t, T, σ) that tells the designer all the three parameters, including time, temperature, and stress acting in concert, and all of them must be considered if a successful creep-resistance component is to be attained.
