Viscoelasticity: Difference between revisions

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Unlike purely elastic substances, a viscoelastic substance has an elastic component and a viscous component. The [[viscosity]] of a viscoelastic substance gives the substance a strain rate dependence on time. Purely elastic materials do not dissipate energy (heat) when a load is applied, then removed. However, a viscoelastic substance dissipates energy when a load is applied, then removed. [[Hysteresis]] is observed in the stress–strain curve, with the area of the loop being equal to the energy lost during the loading cycle. Since viscosity is the resistance to thermally activated plastic deformation, a viscous material will lose energy through a loading cycle. Plastic deformation results in lost energy, which is uncharacteristic of a purely elastic material's reaction to a loading cycle.<ref name=Meyers/>

Specifically, viscoelasticity is a molecular rearrangement. When a stress is applied to a viscoelastic material such as a [[polymer]], parts of the long polymer chain change positions. This movement or rearrangement is called [[Creep (deformation)|creep]]. Polymers remain a solid material even when these parts of their chains are rearranging in order to accompanyaccommodate the stress, and as this occurs, it creates a back stress in the material. When the back stress is the same magnitude as the applied stress, the material no longer creeps. When the original stress is taken away, the accumulated back stresses will cause the polymer to return to its original form. The material creeps, which gives the prefix visco-, and the material fully recovers, which gives the suffix -elasticity.<ref name=McCrum/>

== Linear viscoelasticity and Nonlinear viscoelasticity==