Viscoelasticity
From Free net encyclopedia
A viscoelastic material is one in which:
- 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
Viscoelastic material models are frequently used to describe the behaviour of (soft) human tissue, plastics, soil, etc. Commonly used viscoelastic models are the Kelvin material and Maxwell material. Each model can be represented by springs and dashpots set in combinations of series and parallel elements.
Linear viscoelasticity is when the function is separable in both creep response and load. All linear viscoelastic models can be represented by a Volterra equation connecting stress and strain. Like:
- <math>\epsilon(t)= \frac { \sigma(t) }{ E_{inst,creep} }+ \int_0^t K(t-t^\prime) \sigma(t^\prime) d t^\prime</math>
or
- <math>\sigma(t)= E_{inst,relax}\epsilon(t)+ \int_0^t F(t-t^\prime) \epsilon(t^\prime) d t^\prime</math>
where t - time, <math>\sigma (t)</math> - stress, <math> \epsilon (t) </math> -strain, <math>E_{inst,creep}</math> and <math>E_{inst,relax}</math> - instantaneous elastic moduli for creep and relaxation, K(t) - creep function, F(t)- relaxation function.
Linear viscoelasticity is usually applicable only for small deformations.
Nonlinear viscoelasticity is when the function is not separable. It is usually happens when the deformations are large or if the material changes its properties under deformations.
Background
Early scientists defined matter as a solid, liquid, or gas. They found that some materials fit in more than one of these phases. Honey is a good example of a material with the physical properties of both a solid and a liquid.
Allow honey to flow from a container and then quickly tip it up. A portion of the honey will be pulled back into the container. Liquids don’t typically respond to tensile force or pulling force.
Scientists defined this material as viscoelastic because it exhibited the characteristics of a viscous liquid and an elastomeric solid. Silly putty, chewing gum, and polyurethane memory foam are also examples of viscoelastic materials.
Almost all polymers exhibit viscoelastic behavior. Polymers (and other viscoelastic materials) behave more like solids at low temperatures and at fast deformation speeds. They are more like liquids at high temperatures and slow deformation speeds.