Strain vs Creep in polymers

Hi,
I would like to know if there is a way to determine the creep rate of a polymer under strain.
Is it something that can be calculated or is it something that needs to be
measured?
I am looking for very low level creep (um and sub-um).
Not being a materials specialist, any pointers would be helpfull.
Regards,
Eric Girard
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gagir wrote:

Start by measuring strain vs. time at constant stress or stress vs. time at constant strain.
ISO 899 should be a useful standard.
Michael Dahms
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There is no universal technique for measuring the rheological characteristics of polymers, which is not surprising given the very wide range of mechanical properties that you can get from polymers (extremely soft to extremely strong). Ferry's book has 3 chapters alone on measuring these types of properties, and there are new techniques coming all the time.
John
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Be careful to define your terms. To most people "creep" means deformation that is not recoverable; ie. inelastic.
However many materials exhibit slow but fully elastic, recoverable deformation often called "delayed elasticity".
I think that many people include "delayed elasticity" in their definition of "creep" because in many practical cases the consequences are the same. However that is not true in all cases.
For example, say you are marketing straight glass rods. If those rods are stored by leaning them against a wall, they may appear to be permanently deformed after some time has passed. If the deformation was due to creep they are indeed permanently deformed. On the other hand, if the deformation was due to delayed elasticlty the rods will slowly straighten themselves out after the stress is removed.
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Well Dave, you got me, I meant delayed elasticity.
When applying axial strain (less than 0.5%) on acrylate coated optical fiber I want to know if it is possible the delayed elasticity dispplacement to the micron level.
Sorry for being vague.
Eric Girard

be
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gagir wrote:

It is possible.
Michael Dahms
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Oops, bad phrase.
Is it possible to predict/calculate the delayed elasticity to the micron level?
Any pointers ? Has this been done before for SMF-28?
I could not find any specific data, I assume most of it to be proprietary.
Thank you,
Eric Girard

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gagir wrote:

You must measure it.
Michael Dahms
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Optical fiber is after all a composite composed of some secret materials. I guess there is no realy good way besides experimenting.
I have tried Corning, but so far service is better from this NG.
There is good information out there on how to conduct such an experiment and I have access to a MTS so it is just a matter of fixturing and perhaps procuring an extensiometer.
Thank you,
Eric Girard

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It should be easy to estimate by experiment.
Bend a coated fiber into a circle such that the maximum strain is 0.5% (this is a circle with a diameter of about 100 fiber diameters.) This is a lot of strain by the way.
Hold the fiber in the circular shape for some length of time then release it.
If the fiber does not return to its perfectly straight original state then the system is not perfectly elastic. You can monitor its return to the original straight state to see how much of the deformation is permanent and how much is due to delayed elastic effects.
This is done by measuring the radius of curvature of the fiber as a function of time. strain ~ (fiber radius)/(Radius of curvature of the bent fiber).
Do the same experiment with an uncoated fiber to see if the fiber itself shows any non-ideal behavior.
The difference in behavior is due to the coating.
Remember the fiber's modulus is much greater than that of the polymer so a little residual curvature may imply a lot of polymer strain.
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0.5% may be high for polymers, but for optical fiber 3% would be considered as high. I breaks at around 6.2%. Corning provides some calculators at http://www.corning.com/opticalfiber/products__services/technical_papers /
I wills see from there. Uncoated fiber is quite a bitch to manipulate....
Thank you for your help,
Eric Girard

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