A combination of viscous and elastic properties in a plastic exists with the relative contribution of each being dependent on time, temperature, stress, and strain rate. It relates to the mechanical behavior of plastics in which there is a time and temperature dependent relationship between stress and strain. A material having this property is considered to combine the features of a perfecdy elastic solid and a perfect fluid; representing the combination of elastic and viscous behavior of plastics.

In the plastic, strain increases with longer loading times and higher temperatures. It is a phenomenon of time-dependent, in addition to elastic, deformation (or recovery) in response to load. This property possessed by all plastics to some degree, dictates that while plastics have solid-like characteristics such as elasticity, strength, and form-stability, they also have liquid-like characteristics such as flow depending on time, temperature, rate, and amount of loading. These basic characteristics highlight: (a) simplified deformation vs. time behavior, (b) stress-strain deformation vs. time, and (c) stress-strain deformation vs. time (stress-relaxation).

A constitutive relationship between stress and strain describing viscoelastic behavior will have terms involving strain rate as well as stress and strain. If there is direct proportionality between the terms then the behavior is that of linear viscoelasticity described by a linear differential equation. Plastics may exhibit linearity but usually only at low strains. More commonly complex non-linear viscoelastic behavior is observed.

Thus viscoelasticity is characterized by dependencies on temperature and time, the complexities of which may be considerably simplified by the time-temperature superposition principle. Similarly the response to successively loadings can be simply represented using the applied Boltzmann superposition principle. Experimentally viscoelasticity is characterized by creep compliance quantified by creep compliance (for example), stress relaxation (quantified by stress relaxation modulus), and by dynamic mechanical response.

The general design criteria applicable to plastics are the same as those for metals at elevated temperature; that is, design is based on (1) a deformation limit, and (2) a stress limit (for stress-rupture failure). There are cases where weight is a limiting factor and other cases where short-term properties are important. In computing ordinary short-term characteristics of plastics, the standard stress analysis formulas may be used. For predicting creep and stress-rupture behavior, the method will vary according to circumstances. In viscoelastic materials, relaxation data can be used to predict creep deformations. In other cases the rate theory may be used.

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