*3.1.5 Viscoelasticity*

PE is a viscoelastic material. It exhibits elastic and viscous behavior when stress is applied. The application of this force results in an instantaneous elastic strain followed by a viscous, time-dependent strain [21]. This makes the material display and behave partly as a crystalline metal and partly a very high viscosity fluid. This viscoelastic behavior of polyethylene polymer makes the curve of the stress–strain of any test sample of polyethylene be divided into three essential segments.

The viscoelastic nature of PE provides for creep and stress relaxation, which are two unique engineering characteristics employed in the HDPE design [22]. This property of the polymer materials is fundamental because the functionality and applicability of the plastic can change after a while and may lead to defects and loss of functionality over sometime [23].
