A true fluid flows when it is subjected to a shear field and motion ceases as soon as the stress is removed In contrast, an ideal solid subjected to a stress recovers its original state as soon as the stress is removed Some materials exhibit viscoelastic characteristics having some of the properties of both a solid and a liquid Two examples of viscoelastic behavior are
A liquid (or solid) with both viscous and elastic properties A viscoelastic liquid will deform and flow under the influence of an applied shear stress, but when the stress is removed the liquid will slowly recover from some of the deformation
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
The dual nature of polymers, partly viscous fluid and partly elastic solid, is referred to as viscoelasticity In flowing polymers viscoelasticity is responsible for time-dependent properties, such as stress relaxation, normal stresses, very large elongational viscosities, and numerous unusual phenomena such as extrudate swell, entry flow vortices and some flow instabilities