Viscoelasticity: Difference between revisions

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{{Continuum mechanics|cTopic=rheology}}

'''Viscoelasticity''' is a material property that combines both viscous and elastic characteristics. Many materials have such viscoelastic properties. The[https://www.viscoelasticity.info/what-is-viscoelasticity/ onlyEspecially requirement ismaterials that the material consistsconsist of longlarge flexiblemolecules fiber-likeshow particlesviscoelastic or longproperties]. [[macromoleculePolymer|Polymers]]s are viscoelastic because their macromolecules can make temporary entanglements with neighbouring molecules which causes elastic properties. After some time these entanglements will disappear again and the macromolecules will flow into other positions (viscous properties).

ViscoelasticityA hasviscoelastic beenmaterial studiedwill sinceshow theelastic nineteenthproperties centuryon byshort researchers such as [[James Clerk Maxwell]], [[Ludwig Boltzmann]],timescales and [[Williamviscous Thomson,properties 1ston Baronlong Kelvin|Lordtime Kelvin]]. Viscoelasticity is particularly relevant for materials like polymers, metals at high temperatures and biological tissuesscales. These materials exhibit behavior that depends on the time and rate of applied forces, allowing them to both store and dissipate energy.

Viscoelasticity has been studied since the nineteenth century by researchers such as [[James Clerk Maxwell]], [[Ludwig Boltzmann]], and [[William Thomson, 1st Baron Kelvin|Lord Kelvin]].

Several models are available for the mathematical description of the viscoelastic properties of a substance. Constitutive models of linear viscoelasticity assume a linear relationship between stress and strain. These models are valid for relatively small deformations. Constitutive models of non-linear viscoelasticity are based on a more realistic non-linear relationship between stress and strain. These models are valid for relatively large deformations.

Several models are available for the mathematical description of the viscoelastic properties of a substance:

The viscoelastic properties of polymers are highly temperature dependent. From low to high temperature the material can be in the [[Amorphous solid|glass phase]], rubber phase or the melt phase. These three different phases influence the mechanical properties and application range of polymers strongly.

* [[Constitutive equation|Constitutive models]] of linear viscoelasticity assume a linear relationship between [[Stress (mechanics)|stress]] and [[Strain (mechanics)|strain]]. These models are valid for relatively small [[Deformation (physics)|deformations]].

Unlike purely elastic materials that do not lose energy during loading and unloading, viscoelastic materials lose energy in the form of heat during a stress cycle. This is visible as a hysteresis loop in the stress-strain diagram.

Several models are available for the mathematical description of the viscoelastic properties of a substance. Constitutive models of linear viscoelasticity assume a linear relationship between stress and strain. These models are valid for relatively small deformations.* Constitutive models of non-linear viscoelasticity are based on a more realistic non-linear relationship between stress and strain. These models are valid for relatively large deformations.

The viscoelastic properties of polymers are highly temperature dependent. From low to high temperature the material can be in the [[Amorphoushttps://www.viscoelasticity.info/5-glass-rubber-and-melt-phase/ solid|glass phase]], rubber phase or the melt phase]. These three different[[Phase (matter)|phases]] have a very strong effect influenceon the mechanical properties and applicationviscous rangeproperties of polymersthe stronglypolymers.

Under constant load, viscoelastic materials exhibit [[Creep (deformation)|creep]]: an increase in strain over time. This behavior is crucial for long-term structural design, as it determines how materials behave under prolonged loads.

Typical viscoelastic properties are:

The viscoelasticity properties are measured with various techniques, such as [[tensile testing]], [[dynamic mechanical analysis]], shear rheometry and extensional rheometry.

* A time dependant stress in the polymer under constant deformation (strain).

* A time dependant strain in the polymer under constant stress.

* A time and temperature dependant [[stiffness]] of the polymer.

* Viscous energy loss during deformation of the polymer in the glass or rubber phase ([[hysteresis]]).

* A [[strain rate]] dependant [[viscosity]] of the molten polymer.

* An ongoing deformation of a polymer in the glass phase at constant load ([[Creep (deformation)|creep]]).

The viscoelasticity properties are measured with various techniques, such as [[tensile testing]], [[dynamic mechanical analysis]], shear rheometry and extensional rheometry.

==Background==