Creep tests are designed to determine the long-term effects of loads that are below the elastic limit of the material being tested. Therefore, creep is the plastic deformation that results from this long-term loading. Even though the load is less than the elastic limit of the material, long-term loading effects tend to create plastic deformation. One can see these effects in shelving that has held a heavy load over time and started to sag in the middle. Elevated temperatures, severe service conditions, and other factors may accelerate creep.
Creep is an important design and service factor in structural applications such as bridges, building construction, and other support structures which contain members that are subjected to large static loads over long periods of time in varying conditions. In general, the higher the temperature, the faster the creep rate will be. For metals, creep strength is generally expressed as the stress necessary to produce a 0.1% strain over 1000 hours under specified temperature. The same general criteria are used for other materials, but varies according to the material being tested.