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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.
Nabarro-Herring creep has a weak stress dependence. Coble creep, or grain-boundary diffusion, is the diffusion of vacancies occurs along grain-boundaries to elongate the grains along the stress axis. Coble creep has a stronger grain-size dependence than Nabarro–Herring creep, and occurs at lower temperatures while remaining temperature dependent.
In materials that were deformed under very high temperatures, lobate grain boundaries may be taken as evidence for diffusion creep. [7] Diffusion creep is a mechanism by which the volume of the crystals can increase. Larger grain sizes can be a sign that diffusion creep was more effective in a crystalline material.
Source: [6] can be obtained by accelerated creep test in which strain is recirded, interpolating the data (,) (˙) = ˙ + (˙) When adopting the Omega Method for a remaining life assessment, it is sufficient to estimate the creep strain rate at the service stress and temperature by conducting creep tests on the material that has been exposed to service conditions.
In phase (IP) thermo-mechanical loading (when the temperature and load increase at the same time) is dominated by creep. The combination of high temperature and high stress is the ideal condition for creep. The heated material flows more easily in tension, but cools and stiffens under compression.
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If the material exhibits a non-linear response to the strain rate, it is categorized as non-Newtonian fluid. There is also an interesting case where the viscosity decreases as the shear/strain rate remains constant. A material which exhibits this type of behavior is known as thixotropic.
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