Strange behaviour of viscoelastic materials

[pike13]

Dear PF,

Can anyone explain why viscoelastic materials behave differently at different strain rates?

I understand the general explanation that the behaviour of viscoelastic materials is governed by a solid phase (elastic) and a fluid phase (viscous) and that at higher strain rates the elastic behaviour dominates while at lower strain rates the viscous effects dominate... however I am having trouble translating this into meaningful physical behaviour!

I am particularly interested in the uniaxial compression of a bulk solid to a constant level of strain: my results show that at higher loading velocities, I am seeing a more rapid decay in the force response then at lower loading velocities. This means that after x amount of time, there is a higher reaction force in the slowly compressed test then in the faster compressed test (both subjected to the same load). By looking at the curves, the lower strain rates produce greater degrees of damping i.e. flatter curves, then those produced by high strain rates. In fact, when trying to use a two parameter power function to fit the data, it works well for the higher loading velocity, but not very well at all at the lower velocities (which suggests different mechanisms are at play)! Does a function exist that can be used on both low and high strain rates for such materials? Does anyone have any experience with modelling viscoelastic materials?

Ive done some research and managed to find two papers reporting similar behaviour (but littel attempt at explanation!) using polymers or organics fruits... Maybe this is an agreed characteristic of a viscoelastic material which does not need explaining in scientific journals... either way my research has failed to help me on this one...

Can anyone help?

 

[Achy47]

Thank you for your question regarding the behavior of viscoelastic materials at different strain rates. This is a commonly observed phenomenon in many materials and can be explained by the underlying physical mechanisms at play.

As you mentioned, viscoelastic materials have both solid (elastic) and fluid (viscous) components. At low strain rates, the viscous effects dominate and the material behaves more like a fluid, with the ability to flow and dissipate energy. However, at higher strain rates, the elastic behavior becomes more dominant and the material behaves more like a solid, with the ability to store and release energy.

In the case of uniaxial compression, at lower loading velocities, the material has more time to dissipate the energy and therefore experiences a lower reaction force. This is because the viscous component is able to flow and dissipate energy, resulting in a more gradual and flat curve. On the other hand, at higher loading velocities, the material does not have enough time to dissipate the energy and the elastic component takes over, resulting in a more rapid decay of the force response. This is why you are seeing a higher reaction force in the slowly compressed test compared to the faster compressed test, even though they are subjected to the same load.

When it comes to fitting the data with a function, it is important to consider the underlying mechanisms at play. A two parameter power function may work well for higher loading velocities as the elastic behavior dominates, but it may not be appropriate for lower loading velocities where the viscous behavior is more significant. It is possible that a more complex function, taking into account both elastic and viscous behavior, may better fit the data for both low and high strain rates. However, this may require more advanced modeling techniques and further research.

In terms of literature, I would recommend looking into the work of WLF (Williams-Landel-Ferry) and Arrhenius, who have proposed models that can predict the behavior of viscoelastic materials at different strain rates. These models take into account the temperature and time dependence of the material properties, which can also affect the behavior of viscoelastic materials.

I hope this helps answer your question and provides some insight into the behavior of viscoelastic materials at different strain rates. Best of luck with your research!