If the material exhibits a non-linear response to the strain rate, it is categorized as Non-Newtonian fluid. The viscosity of a viscoelastic substance gives the substance a strain rate dependence on time. In most cases, the creep modulus, defined as the ratio of applied stress to the time-dependent strain, decreases with increasing temperature. Due to molecular segments of different lengths with shorter ones contributing less than longer ones, there is a varying time distribution. When the back stress is the same magnitude as the applied stress, the material no longer creeps. When the original stress is taken away, the accumulated back stresses will cause the polymer to return to its original form. The Kelvin—Voigt model, also known as the Voigt model, consists of a Newtonian damper and Hookean elastic spring connected in parallel, as shown in the picture. The resulting stress vs.

The Burgers model combines the Maxwell and Kelvin—Voigt models in series. Plastic deformation results in lost energy, which is uncharacteristic of a purely elastic material’s reaction to a loading cycle. If, on the other hand, it is a viscoelastic solid, it may or may not fail depending on the applied stress versus the material’s ultimate resistance. Due to molecular segments of different lengths with shorter ones contributing less than longer ones, there is a varying time distribution. The material creeps, which gives the prefix visco-, and the material fully recovers, which gives the suffix -elasticity. Nonlinear viscoelasticity is when the function is not separable. Although the Standard Linear Solid Model is more accurate than the Maxwell and Kelvin—Voigt models in predicting material responses, mathematically it returns inaccurate results for strain under specific loading conditions. Hysteresis is observed in the stress—strain curve, with the area of the loop being equal to the energy lost during the loading cycle.

The Maxwell model can be represented by a purely viscous damper and a purely elastic spring connected in series, seeries shown in the diagram. The red area is a hysteresis loop and shows the amount of energy lost as heat in a loading and unloading cycle.

Due to molecular segments of different lengths with shorter ones contributing less than longer ones, there is a varying time distribution. This page was last edited on 17 Januaryat The relationship between stress and strain can be simplified for specific compliace rates. Viscoelastic creep is important when considering long-term structural design.

However, a viscoelastic substance loses energy when a load is applied, then removed. When the back stress is the same magnitude as the applied stress, the material no longer creeps. In other words, it takes less work to stretch a crefp material an equal distance at a higher temperature than compliaance does at a lower temperature. The Prony series for the shear relaxation is. Cracking occurs when the strain is applied quickly and outside of the elastic limit. Viscous materials, like water, resist shear flow and strain linearly with time when a stress is applied.

When a material is put under a constant stress, the strain has two components. The model is extremely good with modelling creep in materials, but with regards to relaxation the model is much less accurate. Views Read Edit View history. A dashpot resists changes in length, and in a high stress state it can be approximated as a rigid rod.

Surface tension Capillary action. When a material exhibits a linear response it is categorized as a Newtonian material.

Laws Conservations Energy Mass Momentum. Applications to soft solids: It is the simplest model that describes both the creep and stress relaxation behaviors of a viscoelastic material properly.

The Generalized Maxwell model, also known as the Wiechert model, is the most general form of the linear model for viscoelasticity. Finite Element Analysis of Composite Materials. Typically, either a tensile, compressive, bulk compression, or shear strain is applied. First, an elastic component occurs instantaneously, corresponding to the spring, and relaxes immediately upon release of the stress.

Once the parameters of the creep model are known, produce relaxation pseudo-data with the conjugate relaxation model for the same times of the original data. For example, exposure of pressure sensitive adhesives to extreme cold dry icefreeze sprayetc. Extreme cold temperatures can cause viscoelastic materials to change to the glass phase and become brittle. As a result, only the spring connected in parallel to the dashpot will contribute to the total strain in the system.

## Viscoelasticity

The model can be represented by the following equation:. Viscoelasticity is studied using dynamic mechanical analysisapplying a small oscillatory stress and measuring the resulting strain.

Viscoelastic creep data can be presented by plotting the creep modulus constant applied stress divided by total strain at a particular time as a function of time. These two instruments employ a damping mechanism at various frequencies and time ranges with no appeal to time—temperature superposition.

Purely elastic materials do not dissipate energy heat when a load is applied, then removed. Linear viscoelasticity is usually applicable only for small deformations. Rheology Viscoelasticity Rheometry Rheometer. The Maxwell model for creep or constant-stress conditions postulates that strain will increase linearly with time. Stress—strain curves for a purely elastic material a and a viscoelastic material b. The second is a viscous component that grows with time as long as the stress is applied.

Since viscosity is the resistance to thermally activated plastic deformation, a viscous material will lose energy through a loading cycle. Time-temperature-age Superposition Principle for Predicting Long-term Response of Linear Viscoelastic Materials, chapter 2 in Creep and fatigue in polymer matrix composites.

All linear viscoelastic models can be represented by a Volterra equation connecting stress and strain:. A family of curves describing strain versus time response to various applied stress may be represented by a single viscoelastic creep modulus versus time curve if the applied stresses are below the material’s critical stress value.

When subjected to a step constant stress, viscoelastic materials experience a time-dependent increase in strain.

This model represents a solid undergoing reversible, viscoelastic strain. Each model differs in the arrangement of these elements, and all of these viscoelastic models can be equivalently modeled as electrical circuits. The elastic components, as previously mentioned, can be modeled as springs of elastic constant E, given the formula:.

For this model, the governing constitutive relations are:. Complex Dynamic modulus G can be used to represent the relations between the oscillating stress and strain:. Depending on the change of strain rate versus stress inside a material the viscosity can be categorized as having a linear, non-linear, or plastic response.

### Viscoelasticity – Wikipedia

Abaqus Analysis User’s Manual, This model incorporates viscous flow into the standard linear solid model, giving a linearly increasing asymptote for strain under fixed loading conditions. The elastic modulus of a spring is analogous to a circuit’s capacitance it stores energy and the viscosity of a dashpot to a circuit’s resistance it dissipates energy.

An anelastic material is a special case of a viscoelastic material: Generally speaking, an increase in temperature correlates to a logarithmic decrease in the time required to impart equal strain under a constant stress. By using this site, you agree to the Terms of Use and Privacy Policy.