creep in plastic materials

In material sciences, creep is sometimes referred to as cold flow. In this regard, the difference between plastics and other materials is that plastics display time-dependent viscoelastic behavior. Viscoelastic behavior is the property of a material to display both viscous and elastic characteristics when undergoing deformation. It can result from long-term exposure to high levels of stress that are still beneath the yield strength or yield point of the material. The yield strength or yield point of a material is the property defined as the stress at which that material begins to deform permanently. Deformation refers to any change in the shape of an object due to an applied force or a change in temperature. The first instance can be the result of tensile forces, sometimes called pulling forces, compressive forces (pushing forces), or shear, bending or torsion (twisting). Deformation is often described as “strain”. Prior to the yield point, the material will deform elastically and will return to its original shape when the applied stress is removed. Once the yield point is passed, some fraction of the deformation will be permanent and non-reversible.

Types of Creep
There are various kinds of creep. Nabarro-Herring creep is a form of diffusion creep that is strongly temperature dependent. Coble creep is a second form of diffusion-controlled creep. It is still temperature dependent but not as much as the Nabarro-Herring creep. Harper-Dorn creep has been observed in aluminum, lead and tin systems as well as some ceramics, ice and some solders. The first two types of creep are grain size dependent while the Harper-Dorn creep is dislocation-motion dependent.

Stages of Creep
Creep is one of the most commonly studied long-term property tests performed. Creep does not happen suddenly, but rather results as an application of stress over the long term. Creep is therefore a time dependent deformation. There are three stages of creep. In the first stage, referred to as “Primary Creep,” the strain rate is at first relatively high, but slows over time. In Secondary Creep, the creep occurs at a relatively uniform rate and is called at this point “creep strain rate”. Finally, Tertiary Creep occurs at an accelerated creep rate and ends when the material breaks or ruptures.

Rate of Deformation
There are several functions of a material that determine the rate of deformation. These include the properties of the material, exposure time, exposure temperature and the structural load that is applied. In fact, depending on the amount of stress applied, how long it is applied, the temperature and the applied structural load, the deformation may be so large that a component can no longer perform in the application for which it is designed. A turbine blade would be one example. The creep may be so great that over time the blade will make contact with the casing, causing the blade to fail. Creep may not result in a failure mode. It can be desirable in concrete because it relieves tensile stress that could result in cracking.

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