Creep is an engineering term used to describe the slow deformation of materials under constant load. Most materials under constant load will creep and the rate of creep generally increases with temperature. Creep rates differ for different materials. For example, the creep of steel at room temperature is insignificant. On the other hand, plastics tend to have significant creep even at room temperature. Since creep rates can be quantified and used in engineering design, it is good practice when designing plastic components to avoid exposing them to situations under constant load and the subsequent effects of creep.
Stress relaxation is a property similar to creep. Both terms are used to describe the viscoelastic behavior of materials. While creep describes the rate of deformation under constant load, stress relaxation described the reduction in material stress following loading and maintaining a prescribed level of deformation. This is typically measured by elongating a material sample a fixed amount and monitoring the drop in stress over time. Like creep, stress relaxation is also a quantifiable material property that may be used in engineering design.
The common definition of creep is to move slowly with the body staying near to the ground. It is often used to describe the slow movement of insects. The engineering definition shares the idea of slow movement, but like many engineering terms creep rate can be determined as a quantifiable value in addition to describing general behavior.
