Simple Creep/Slump Testing of a Cylindrical Part

[tffy]

Hello, I'm trying to figure out how a part made from a viscoelastic thermoplastic material will deform with time. Is there a simple method to do this?

The problem can be simplifid to a 2-d state. I can obtain relevant material values for an analytical test, or if you can suggest a way to relate temperature to slump rate I could do a test at elevated temperature.

The part has the shape of a thick-walled pipe, adhered to an outside sleeve of a perfectly rigid support material. This part will be stored laying on its side, subjected only to the force of gravity. The problem is, the inner diameter part will tend to "float up" with the passage of time - and I was hoping to be able to predict the extent of this flow behavior at a given storage temperature and time span.

Thanks!

 

[Demon3]

The normal method is to test at elevated temperature and then adjust the results to the actual temperature you are in terested in using the WLF Equation. You can find more on MLF on the net as it's a well known polymer equation.

Just on a feeling basis I would be surprised to find the hole in a polymer pipe moving up due to gravity. I don't think that should be fast enough to be noticeable for any high molecular weight polymer.

 

[tffy]

Thanks for your help. I've been trying to get the value for a_T as described by the WLF equation, and from the way the WLF equation is defined, it looks like that value of a_T will always be less than unity.

Now, that makes no sense, if the way to use a_T is divide the time ordinate in the curve fit of your original experimental data. The temperature of interest plot ends up "compressed in time" WTR to the elevated temperature test, while the expected behavior would result in a "stretched in time" plot.

Am I using a_T correctly? Thanks!

 

[Demon3]

I see your point but it was 15 years ago when I last calculated anything using the WLF equation so I can't help with the specifics. There are some really sharp people here in the forum so I hope one can jump in with some sage advice (hint).

Can you do a google search for using the WLF? I am sure there are links to polymer courses that give examples to make things clearer.

 

[tffy]

To bring some closure to this question - I did some more research and found a paper at...

http://www.composites.wsu.edu/publications2/NTpooler.pdf

which describes how to modify the constants c1g and c2g used in the WLF equation to reflect a specific reference temperature other than glass transition temperature. Page 48 of the document refers to Neilsen and Landel (Mechanical Properties of Polymers and Composites, Marcel Dekker, 1994) to get:

c1 = (c1g*c2g)/(c1g+Tref-Tg)
c2 = c2g+Tref-Tg

Thanks for your help again!

 

[Demon3]

Great that you found the solution! The WLF works pretty well for temperatures within +/- 50°C of Tg if I remember correctly. Outside of that range I think it's normal to use the Arrhenius equation instead.