What is Creep in Plastic Materials?

Creep in plastic materials is sometimes referred to as cold flow.

Viscosity

The viscosity of a fluid is a measure of its resistance to gradual deformation by shear or tensile stress.  Viscoelastic behavior is the property of a material to display both viscous and elastic characteristics when undergoing deformation.  The difference between plastics and other materials is that plastics display time-dependent viscoelastic behavior.  A material can deform as the result of 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 defined as the stress at which that material begins to deform permanently.

Deformation

Deformation refers to any change in the shape of an object due to an applied force or a change in temperature. Tensile Forces can be one of the primary causes of deformation.  These tensile forces are sometimes called pulling forces, compressive forces (pushing forces), as well as shear, bending or torsion (twisting). Deformation is often described as “strain”. The material will deform elastically prior to the yield point. It 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. It does not happen suddenly, but rather results as an application of stress over the long term. Thus it is a 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. Secondary 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 due to creep. 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.

Turbine Blades

This application is an example. The creep may be so great that over time the blade will make contact with the casing, causing the blade to fail. However, creep may not result in a failure mode. It can be desirable in concrete because it relieves tensile stress that could result in cracking.

Creep is only one consideration when determining the right plastic for your project.  At Craftech our staff is available to assist with the right choice of material for your application. So give us a call, fax, or e-mail.

 

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