Strange behaviour of viscoelastic materials

In summary, the original poster is seeking an explanation for the behavior of viscoelastic materials at different strain rates, particularly in the case of uniaxial compression. They have found two papers reporting similar behavior but with no explanation, and are wondering if there is a function that can be used to model both low and high strain rates for such materials. The response suggests looking into hypo-elasticity and notes that constitutive equations for viscoelastic materials cannot be derived from a more basic theory.
  • #1
pike13
3
0
Dear Physics Forum,

I posted this in the Mechanical Engineering Forum a while ago without any responses, but by looking at the other threads I suspect that it was the wrong place so I am posting it again here where it looks more at home (so apologies if i was wrong!).

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 (the stress relaxation behaviour): my results show that at higher loading velocities, I am seeing a more rapid decay of 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 no 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? Any ideas would be much appreciated!
 
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  • #2
pike13 said:
<snip>

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

I am particularly interested in the uniaxial compression of a bulk solid to a constant level of strain (the stress relaxation behaviour): <snip>

I'm not sure anyone can explain *why* a complex material behaves the way it does; OTOH, there are lots of models that reproduce real materials (over limited ranges of physical parameters). As you mention, a viscoelastic material has both a dissipative (viscous) component and an elastic component- AFAIK, Rivlin and Eriksen, J. Rational Mech. Anal. 4 (1955) first wrote a general and invariant theory of viscoelasticity. It's important to note that constitutive equations cannot be derived from a more basic theory; they are phenomenological in nature and require experiment to determine any coefficients.

As to your second sentence, that appears to be a measurement of creep? Creep requires irreversible thermodynamic considerations and is not viscoelasticity. OTOH, you may have some luck looking into hypo-elasticity.
 

What is viscoelasticity?

Viscoelasticity refers to the ability of a material to exhibit both elastic and viscous properties. This means that when a force is applied to the material, it will deform and then return to its original shape like an elastic material, but it will also have a time-dependent response like a viscous material.

What is the cause of strange behavior in viscoelastic materials?

The strange behavior of viscoelastic materials is caused by the internal molecular structure of the material. The molecules in these materials are able to slide past one another, causing a time-dependent response to applied forces.

How does temperature affect the behavior of viscoelastic materials?

Temperature has a significant impact on the behavior of viscoelastic materials. As temperature increases, the molecules in the material have more energy and are able to slide past each other more easily, resulting in a more viscous response. At lower temperatures, the material becomes more elastic.

Can viscoelastic materials be accurately modeled and predicted?

Yes, viscoelastic materials can be accurately modeled and predicted using mathematical models such as the Maxwell, Kelvin-Voigt, and Burgers models. These models take into account the material's elastic and viscous properties and can accurately predict its behavior under different conditions.

What are some real-world applications of viscoelastic materials?

Viscoelastic materials have a wide range of applications in various industries such as automotive, aerospace, and biomedical. Some examples include shock absorbers, tires, adhesives, and artificial cartilage. These materials are also used in the production of everyday items such as mattresses and shoe soles.

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