Time-dependent irreversible plastic deformation.įigure 2.Time-dependent viscoelastic deformation.Instantaneous (reversible) elastic deformation.The deformation of polymer components subjected to external stress is composed of three elements (see figure1): Because a polymer behaves in a manner that seems to combine elastic solid and highly viscous behaviour, they are termed 'viscoelastic'. For example, a large number of design formulae have been derived for beams, plates and columns dependent on the elastic modulus as a fundamental measure of the response of the material to stress, the implicit assumption being that the material behaves in a linear, elastic manner until yielding.įor most metals, this is true for polymers, the stress-strain curve is rarely linear, there is no true proportional limit and the behaviour is greatly affected by strain rate and temperature. It must be made clear that polymers respond to applied loads very differently to metals, making direct comparisons of strength and stiffness of limited use.
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