In materials science, there is no room for guesswork. Every capability and specification behind a material needs to be taken into account if an optimized end product is to be produced. Given the advances of technology, it is now easier than ever to assess the quality of a product through the use of cutting-edge, high-resolution computer-aided engineering tools.

One of the most pertinent challenges of this approach, is modeling material behavior that is complex and is affected by a variety of external conditions. For one, modeling viscoelastic behavior of materials requires a nuanced understanding and application of CAE that distinguishes it from solid materials modeling.

Viscoelastic materials are more commonplace than you may first realize. In terms of sports protection equipment, the behavior of viscoelastic materials matters when you are testing the durability and effectiveness of foam materials and helmet padding. Luckily, our team at Windpact has designed this outline of modeling viscoelastic materials that will tell you everything you need to know if you decide to incorporate them into your design pipeline.

Viscoelastic key terms

Viscoelasticity is the property of some materials to respond differently under varying mechanical loading conditions. These loads induce slight change in the response of the material. The material can begin to behave like a viscous liquid for fragmentary periods of time (the visco – part) but this change can be undone by removing the outside variables and allowing the material to return to normal (the -elastic part).

Because viscoelastic materials behave so differently, they have unique properties and terms that are needed to properly describe their unique behaviors. These terms include:
  • Model creep: describes how when subject to constant stress, viscoelastic materials experience strain increase.
  • Time dependence: this creep is time dependent, meaning that the longer it is subjected to stress conditions, the greater the increase in strain over time.
  • Hysteresis: describes the dependence of a materials current state on its history. This can be observed when looking at the stress-strain curve, where the area of the given loop is equal to the energy lost after the previous load is removed.
  • Stress relaxation: refers to the decreasing stress of the material in response to the constant strain seen in the model creep behavior.

Viscoelastic Material Properties

These behaviors are what set viscoelastic materials apart from elastic materials, or materials that do not change in response to changes in load. Elastic materials will return to their original state after loading occurs. This means that there is no energy change within the material or system. Viscoelastic materials behave differently. They will dissipate energy out of the system, sometimes resulting in a change in shape. Depending on the product design, viscoelastic materials can significantly increase the life cycle of the product.

Let’s look at it this way: if you apply force on a plastic relatively quickly, and then remove that force in a short span of time, the resulting deformation will be small. If, on the other hand, you apply that force, continually, for a long period of time, you start to see greater deformation when the plastic is allowed to return to its original shape. Beyond plastics, conventional metals will experience long-term deformation in response to prolonged high temperature exposure, which can influence how engineers select parts for devices and products meant to be used under these conditions. Viscoelasticity is crucial to understand in the fields of engineering, architecture, medicine, geology and kinesiology for all of these reasons.

For example, in civil engineering, structural engineers need to model the potential viscoelastic conditions that would induce this elastic behavior, and this has major ramifications for which materials will be included in the final design. When constrained in this way, the options of viable, long-lasting materials can become limited.

List of viscoelastic materials:

  • Polymer foams
  • Spinal discs
  • Guitar strings
  • Wood
  • Tectonic plates
  • Skin tissue
  • All rubbers
  • All plastics
  • Metals at high temperatures*

*Typical metals experience creep at significantly high temperatures, resulting in structural deformation. This needs to be accounted for when designing things that will be used for purposes that subject them to high temperatures and high pressure. This is the case for turbine blades in airplanes, which are often made of superalloys which contain some or all of nickel, cobalt, chromium, aluminium, titanium, tungsten and molybdenum.

Applications for viscoelastic materials:

Viscoelastic materials are more common in the world than may be immediately obvious, and have far-reaching applications in a variety of industries. For instance, because viscoelastic materials absorb vibrational energy and dissipate it, they make excellent sound dampeners and are often used in sound-proofing panels.

Sometimes, however, using soft polymers may be inappropriate or impractical. In those cases, you need access to versatile materials that can satisfy your design goals. Fortunately, viscoelastic materials are easily accessible for product developers. For example,work gloves are lined with a viscoelastic material intended to reduce the vibrations felt in the hands transmitted by high powered drills. By lining with an appropriate viscoelastic material, the strain on the hands and the small impacts over time are reduced.

Most relevant in Windpact’s pipeline is the fact that viscoelasticity can be seen in the types of plastics used in helmets. Plastics are commonly used because they have desirable properties that give them an edge over other materials, namely their sturdiness combined with viscoelastic behavior. They are also cost-effective to produce, and can be produced very quickly in comparison to other types of materials.

Because viscoelastic plastics do not completely snap back — due to energy loss in the model creep — this gives designers a great deal of flexibility in choosing how much elastic and viscous properties a material will exhibit. However, the resulting consequences of this small elastic change needs to be taken into account when considering the influence of the viscoelastic part on the final behavior of the finished product.

Thankfully, this is easier than ever to accomplish through the application of computer-aided engineering tools (CAE) that give predictive metrics by simulating materials behavior under different conditions.

CAE Softwares For Modeling Viscoelastic Materials

As the field looks more towards cost-saving computerized solutions, the breadth of tools available for modeling material performance is vast. Trusted software for CAE methods and modeling unique viscoelastic properties of materials include:

  • LS-DYNA
  • Ansys
  • Abaqus
  • Fusion 360
  • Solid Edge
  • Solidworks
  • Simscale

Different Models/methods

There exist multiple models for accurately describing the behavior of viscoelastic materials, and they are useful for modeling the stress and strain, or time dependent change as well as environmental dependencies. These models include:

Maxwell model: Usually represented by a combination of a spring and a damper connected in a linear series. It represents the relationship between constant strain and stress, and shows that stresses relax over time. This model assumes that strain will increase over time, limiting its ability to accurately capture creep. This makes this model a poor approximation method for soft polymers. Instead, it is particularly useful for representing soft solids and high-temperature metal behavior.

Kelvin-Voigt model: Represented by a spring and damper in parallel. States that stress applied over time will cause gradual strain decrease, and models the relaxation of the material back to equilibrium. However, it is limited due to the fact that it does not give an accurate representation of relaxation compared to other models. Useful for modeling organic polymers, rubber and wood behavior when the stress is not extreme.

Standard linear model: Represented by a combination of the components of both Maxwell and Kelvin, but incorporates two springs and a damper represented in parallel. States that a constant stress will produce instant strain and deformation, which gradually returns back towards its original formation. It describes both the creep and total deformation quite well. Limited when modeling a material under different load weights.

Quasi-linear viscoelastic (QLV) model: Represented by a stress relaxation that can be expressed in terms of the instantaneous elastic response and a reduced relaxation function.

Burgers model: Consists of two Maxwell components in parallel, models the viscous flow behavior.

Generalized Maxwell model: Derived from the Maxwell model, but specifies that the relaxation occurs across time, and is not instantaneous.

Windpact can utilize these models alone or in combination to create predictive CAE models. Windpact’s pedigree material data allows for versatility as to which model is utilized. Each of the above models can be used for specific applications and with good data and proper selection, these models will output extremely accurate simulations.

Conclusion

As techniques become more sophisticated, our ability to model specific viscoelastic behavior of materials will increase in accuracy and efficiency. Windpact’s access to an expansive database of materials ensures that every parameter and option is thoroughly explored. Whether it be for helmet lining, protective padding in gear, automobiles, or military equipment, our team understands the amount of precision and thoroughness needed to produce a product that is durable and effective.