Viscoelasticity

When viscoelasticity is called a partially elastic, partially viscous material behavior. So Viscoelastic substances combine characteristics of liquids and solids in it. The effect of time, temperature and frequency-dependent and occurs in polymer melts and solids such as plastics, as well as other materials.

Material behavior

The elastic component basically causes a spontaneous, limited, reversible deformation, while the viscous component basically causes a time-dependent, unlimited, irreversible deformation. The viscous and elastic component is differently pronounced in different viscoelastic materials, differs also the type of interaction.

Elastic behavior in rheology by means of a spring, the Hooke - element, and viscous behavior is represented by a dashpot, the Newton - element. Visco-elastic behavior can be modeled by a combination of two or more of these elements.

The simplest viscoelastic models are the Kelvin body and the Maxwell body. In Kelvin body spring and damping cylinder is connected in parallel. Under load, for example, by stretching the deformation is braked by the damping cylinder and limited in its extent by the spring. After a relief of the body goes due to the Hooke - element back to its original position. The Kelvin body deforms so time-dependent as a liquid, but limited and reversible as a solid.

The series connection of the two elements produces the Maxwell body. Under load, the spring deforms immediately, then starts the time-dependent and unlimited viscous deformation. After discharge, only the spring moves back, the viscous component remains. It is therefore a time-dependent, unlimited, irreversible deformation as in a liquid, but it is also a time-independent and reversible spontaneously elastic component as a solid.

For a quantitative description also covers complex shear modulus and the complex modulus.

Transition between viscous and solid material behavior

All liquids and solids can be considered as viscoelastic materials by their storage and loss modulus, and can be given. In ideal - viscous liquids ( Newtonian fluid ) is the storage modulus is very small compared to the loss modulus in perfectly elastic solids which obey Hooke's law, the loss modulus is very small compared to the memory module. Viscoelastic materials exhibit both a measurable memory module as well as a measurable loss modulus. If the storage modulus is greater than the loss modulus, is referred to as solids, liquids otherwise.

Causes

The viscoelasticity of the polymer is based on a delayed equilibration of the macromolecules to one another at or after mechanical stress. The proportion of each strain components of the total strain is determined by secondary bonds ( Dipolbindung, hydrogen- bonding, van der Waals bond) and Molekülverhakungen. The time- dependent strain component is determined by stretching, Entknäuelungs and Entschlaufungsvorgänge.

The reversible elastic behavior is due to the entropy elasticity. Depending on the temperature, stress duration and speed leads to irreversible viscous Molekülabgleitungen.

In crystalline solids such as metals or ceramics mainly defects such as interstitial atoms, or displacements of delayed elongation and thus viscoelastic behavior are responsible. Usually the deviations from the ideal elasticity are considerably smaller than the case of plastics.

Viscoelastic experiments

• The oscillation experiment: measurement of stress and strain in sinusoidal loading.
• The creep ( retardation ): Measurement of the time-varying strain at constant stress.
• The stress relaxation: measurement of the time-varying voltage at / after a sudden strain.