Time dependent Properties: Creep and Stress Relaxation
If a polymeric material is subjected to a constant stress, the strain will not be constant but will increase slowly and continuously with time. This effect is due to a molecular rearrangement induced by the stress. On the release of the stress, the molecules slowly recover their former spatial arrangement and the strain simultaneously returns to zero. This effect is termed creep and is a manifestation of a general property of polymeric solids known as viscoelasticity.
For metals, creep effects are negligible at ordinary temperatures except that the very soft metals like lead. For polymers, creep is often occur significantly at ordinary temperatures and even more noticeable at higher temperatures. All plastics will creep to a certain extent. The degree of creep depends on several factors, such as the type of plastic, temperature, and stress level.
If the applied load is released before creep rupture occurs, an immediate elastic recovery will happen, equal to the elastic deformation, followed by a period of slow recovery as shown in Fig. (1), where a constant load is applied at to and removed at t1. On removing the load from a polymer, the material can recover most, or even all, of the strain through giving it sufficient time. The material in most cases does not recover to the original shape and a permanent deformation remains. This is different from metals where the strain produced by creep is not recoverable. The time taken to recover depends on the initial strain and the time for which the material was creeping under the load.
The creep data for polymers can be represented in three ways
1. Creep curve: strain-time curve at constant stress.
2. Isochronous curve: strain-stress curve at constant time.
3. Isometric (stress relaxation) curve: stress-time curve at constant strain.
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