Question

Explain what the viscoelastic behavior of plastic materials is.

 

Answer 1

Viscoelasticity is the property of materials that exhibit both viscous and elasticcharacteristics when undergoing deformation. Viscous material, like honey, resist shear flow and strain linearly with time when a stress is applied. Elastic materials strain when stretched and quickly return to their original state once the stress is removed.

Viscoelastic materials have elements of both of these properties and, as such, exhibit time-dependent strain. Whereas elasticity is usually the result of bond stretching along crystallographic planes in an ordered solid, viscosity is the result of the diffusion of atoms or molecules inside an amorphousmaterial.

Some examples of viscoelastic materials include amorphous polymers, semicrystalline polymers, biopolymers, metals at very high temperatures, and bitumen materials. Cracking occurs when the strain is applied quickly and outside of the elastic limit. Ligaments and tendons are viscoelastic, so the extent of the potential damage to them depends both on the rate of the change of their length as well as on the force applied.

A viscoelastic material has the following properties:

hysteresis is seen in the stress–strain curve
stress relaxation occurs: step constant strain causes decreasing stress
creep occurs: step constant stress causes increasing strain.
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 loses energy when a load is applied, then removed.
Types:::-
1.Linear viscoelasticity is when the function is separable in both creep response and load.
2.Nonlinear viscoelasticity is when the function is not separable. It usually happens when the deformations are large or if the material changes its properties under deformations.

3.An anelastic material is a special case of a viscoelastic material: an anelastic material will fully recover to its original state on the removal of load.