Strange behaviour of viscoelastic materials

[pike13]

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!

 

[Andy Resnick]

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