Creep
Definition - What does Creep mean?
Creep, in materials science, is the propensity of a solid material to move slowly or deform permanently under the influence of mechanical stresses. It can happen as a result of long-term exposure to high levels of stress that are still beneath the yield strength of the material. Creep is more severe in materials that are subjected to heat for long periods, and generally increases as they near their melting point.
Creep is usually of concern to engineers and metallurgists when assessing components that operate under high stresses or high temperatures. Design considerations are very important to avoid creep.
Creep is also known as cold flow.
Corrosionpedia explains Creep
Creep is a time-dependent deformation under a certain applied load. It occurs at high temperature (thermal creep), but can also happen at room temperature in certain materials, albeit much slower.
Creep may or may not constitute a failure mode. For example, moderate creep in concrete is sometimes welcomed because it relieves tensile stresses that might otherwise lead to cracking.
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 the creep of a turbine blade may cause the blade to contact the casing, resulting in failure of the blade.
Since the creep minimum temperature is related to the material's melting point, creep can be seen at relatively low temperatures for some materials. Plastics and low-melting-temperature metals, including many solders, can begin to creep at room temperature, as can be seen markedly in old lead hot-water pipes.
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.
There are three stages of creep:
- Primary creep - Starts at a rapid rate and slows with time due to work hardening
- Secondary creep - Has a relatively uniform rate. "Creep strain rate" typically refers to the rate in this secondary stage.
- Tertiary creep - Has an accelerated creep rate and terminates when the material breaks or ruptures. It is associated with both necking and formation of grain boundary voids.
