At low temperatures and high stress, materials experience plastic deformation rather than creep.
Glide-controlled dislocation creep: dislocations move via glide and climb, and the speed of glide is the dominant factor on strain rate.Grain boundary diffusion ( Coble creep).Bulk diffusion ( Nabarro–Herring creep).Though there are generally many deformation mechanisms active at all times, usually one mechanism is dominant, accounting for almost all deformation. Mechanisms of deformation ĭepending on the temperature and stress, different deformation mechanisms are activated. 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. In tertiary creep, the strain rate exponentially increases with stress. Stress dependence of this rate depends on the creep mechanism. Equations that yield a strain rate refer to the steady-state strain rate. In the secondary, or steady-state, creep, dislocation structure and grain size have reached equilibrium, and therefore strain rate is constant. In class A materials, which have large amounts of solid solution hardening, strain rate increases over time due to a thinning of solute drag atoms as dislocations move. This can be due to increasing dislocation density, or it can be due to evolving grain size. In Class M materials, which include most pure materials, strain rate decreases over time. In primary, or transient, creep, the strain rate is a function of time. Strain ( ε) as a function of time due to constant stress over an extended period for a Class M materialĬreep behavior can be split into three main stages. The effects of creep deformation generally become noticeable at approximately 35% of the melting point (in Kelvin) for metals and at 45% of melting point for ceramics. Glacier flow is an example of creep processes in ice. Plastics and low-melting-temperature metals, including many solders, can begin to creep at room temperature. While tungsten requires a temperature in the thousands of degrees before creep deformation can occur, lead may creep at room temperature, and ice will creep at temperatures below 0 ☌ (32 ☏). Creep deformation generally occurs when a material is stressed at a temperature near its melting point.
The temperature range in which creep deformation may occur differs in various materials. Therefore, creep is a "time-dependent" deformation.
Instead, strain accumulates as a result of long-term stress.
Unlike brittle fracture, creep deformation does not occur suddenly upon the application of stress. For example, moderate creep in concrete is sometimes welcomed because it relieves tensile stresses that might otherwise lead to cracking. Creep is a deformation mechanism that may or may not constitute a failure mode. Creep is usually of concern to engineers and metallurgists when evaluating components that operate under high stresses or high temperatures. 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 could cause the blade to contact the casing, resulting in the failure of the blade. The rate of deformation is a function of the material's properties, exposure time, exposure temperature and the applied structural load. Creep is more severe in materials that are subjected to heat for long periods and generally increase as they near their melting point. It can occur as a result of long-term exposure to high levels of stress that are still below the yield strength of the material. In materials science, creep (sometimes called cold flow) is the tendency of a solid material to undergo slow deformation while subject to persistent mechanical stresses.
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