Young's modulus

[wanbot]

Hi all,

I have this confusing thought for a quite long time.

Is there any relation between the time-independent Young's modulus with the time-dependent Young's modulus (instantaneous and equilibrium moduli or loss and storage moduli)?

If yes, what is the equation to relate them?

So many terminologies which make this confusion!

Please help.

Wanbot

 

[Demon3]

The WLF equation is an equation that allows you to interchange time and temperature for polymers. It would not work for every system but may give you a way to guess at the time dependent behavior.

http://people.ccmr.cornell.edu/~cober/mse521/WLF.html

Chris DeArmitt

"Knowledge has no value except that which can be gained from its application toward some worthwhile end."
Think and Grow Rich - Napoleon Hill

 

[wanbot]

Chris,

Thanks for the comment.

However, I am pretty sure that the time-independent Young's modulus does not depend on temperature either. So, the WLF equation does indeed not applicable in this situation.

To make myself clear, the time-independent Young's modulus is also called as simply Young's modulus. While for the time-dependent Young's modulus, at least 4 terminologies relate to it which depend on type of measurements being conducted. For static measurement, we will get instantaneous and equilibrium moduli, while for the dynamic test, we will obtain the loss and storage moduli.

So, my inquiry was, is there any relationship with Young's modulus (time-independent) with instantaneous or equilibrium or loss or storage moduli (time-dependent)?

Thank you in advance for your attention and further comments.

Best,

Wanbot

 

[Demon3]

Now I am more confused than before. Polymers don't really have a completely time independent modulus because they are viscoelastic and flow when measured. The amount of flow depends on the temperature and the duration or frequency of the measurement.

Or, from a more pragmatic standpoint. If the properties are always time-dependent, why bother about any theoretical case where the modulus is not time-dependent?

Chris DeArmitt

"Knowledge has no value except that which can be gained from its application toward some worthwhile end."
Think and Grow Rich - Napoleon Hill

 

[wanbot]

Chris,

Sorry to make yourself in a confusion state which has not being my intention.

From the first place, I am not trying to focus on the polymer materials per se.

I am just curious and playing with my thinking as what if there is a material which may have a duality property characteristic. At one time behaves as a polymer(where time-dependent stiffness property applies) and at another time behave as a non-polymer (where time-independent becomes dominant).

Is so, and if there is a relationship between the time-independent and the time-dependent mechanical properties, we may be able to make prediction of all properties from a single measurement.

Best,

Wanbot

 

[Demon3]

For very fast measurements (or low temperatures), the polymer has no time to flow. It will behave elastically. For longer measurement times or higher temperatures, the flow will occur more and more.

So, we are back to the WLF equation that allows you to predict the effect of changing the time or temperature.

Chris DeArmitt

"Knowledge has no value except that which can be gained from its application toward some worthwhile end."
Think and Grow Rich - Napoleon Hill

 

[GilPolymers]

Hi there,

Is difficult to understand this post. As far as I understand from here I agree with Demon.

Which kind of material are you looking for? Independently of the type of the material (polymers, metals, ceramics), their behaviour can be defined according to 3 big groups: (1) pure elastic; (2) pure viscous and (3) visco elastic.
On viscoelastic materials you have storage and loss modulus which is the case of the polymers, so doesn’t really matter what kind of material are you working with, if it is a viscoelastic material can be roughly described by WLF equation.
Other point is the definition of Young's modulus which is different than elastic modulus but lots of times these terms are mixed up.

Viscoelastic materials are able to have this "dual" behaviour that you are looking for, if you check these videos you are going to see people running on a liquid but if they just walk, they will not be able to go through the pool.
This happens because the material has elasticity for quick loads but shows viscosty (flows) if the load is applied in a large period of time.

http://www.youtube.com/watch?v=f2XQ97XHjVw&feature=related

http://www.youtube.com/watch?v=Lz8VWZY0iOE

The same happens for example with a mattress made of viscoelastic materials. It's the relation between the viscous and the elastic effect that makes you comfortable.

This is a very complex behaviour, I would advise you to go through a viscoelasticity book.

Hope I could help,

Gil

 

[wanbot]

Dear Chris and Gil,

Thank you for your comments.

I am quoting from Chris's comment:

"For very fast measurements (or low temperatures), the polymer has no time to flow..."

And from Gil's comment:

"...This happens because the material has elasticity for quick loads but shows viscosty (flows) if the load is applied in a large period of time..."

From this notion, can I assume that the Young's modulus (for time-independent material) is same with the instantaneous modulus (for time-dependent material)?

Best

Wanbot