Most materials under continuous load will slowly deform. Typical examples are metals, window glass, plastics, ice and rock. Ancient glass windows in churches have deformed under gravity in such an way that at the bottom the glass has become thicker than at the top. The ice of the glaciers slowly flows downwards and the basalt rocks in a mountain become plastically deformed by gravity forces. So plastics is just one group of the very many materials that show slow deformation once subjected to a force for a prolonged time. The common name for this slow deformation under load is creep.

Description of the creep process

When a load is put on a body made from a polymer it will be deformed . This deformation is time dependant, as shown in the figure below and consists of two parts:

1. An elastic deformation that occurs immediately after applying the load.
2. A plastic deformation that grows in time for the duration of the load

After removing the load, the elastic deformation will disappear immediately. The plastic deformation, however, will take a long time to disappear. Under practical circumstances time will be too short to remove all plastic deformation. Some plastic deformation will be left in the product.

The load on the body has changed the shape of the polymer molecules by rotation of the chain segments. A part of this deformation will be recovered after some time. This is due to the rubber stress that is induced in the body due to the deformation. This rubber stress forces the polymer molecules back into their original shapes. This recovery process will take a long time however, because the rubber stress is relatively low (see Recovery of a deformed plastic body).

Creep or plastic flow?

The speed with which the dimensions of the polymer body under load change (the strain rate) is strongly dependant on the height of the load (stress) and on the temperature of the body. Increasing loads and increasing temperatures will increase the speed of this process.

1. Under low stresses, it may take hours or days before some noticeable dimension changes can be observed. This slow process of changing dimensions is called creep.
2. Under high stresses, noticeable dimension changes can be seen already after seconds or minutes. This fast process of changing dimensions is called plastic flow.

Although the names are different, the underlying process is the same. In both cases the polymer molecules are deformed due to the rotation of the chain segments under load. The main difference between the two is the speed of the process.

In the figure above a schematic representation of the deformation of a product under stress changing with time is shown. This is called a creep curve. In this example three different creep curves are shown for three different stresses (s₁ is the lowest and s₃ is the highest stress on the body).

In case of the lowest stress the load (or stress) becomes balanced with the elastic rubber stress caused by deformation of the body. The dimensions of the body stop changing. In case of the two higher stresses such a balance is not obtained. Instead, the speed of the deformation increases until finally rupture of the body occurs.

Failure due to creep occurs as soon as the deformation exceeds a certain limit. This limit is the strain at which the polymer yields in a tensile test.

Creep is an important mechanism for failure of plastic products under long-time load. An example of such a failure in a plastic pipe is shown in the figure below.

Summary

• Creep is a common feature of many materials. Examples are basalt, ice, window glass, metals and plastics.
• Creep is the process of slow deformation of a body under a small continuous load.
• Plastic flow is the process of fast deformation of a polymer body under a high continuous load.
• The deformation of a polymer body during creep or flow is mainly caused by deformation of the polymer molecules due to chain segment rotation.
• The speed of deformation during creep strongly increases with the temperature and the level of the load.
• After removing the load, the dimensions of the plastic body are not fully restored.
• Failure due to creep occurs as soon as the plastic deformation exceeds the elongation at yield.

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