Biaxial tension vs uniaxial compresstion

[kavkir]

Dear all

I am new to this forum and first what I have to say that this WEB site is
very useful for those investigating polymers.

For one of our projects we require to carry out FE simulations of a polyurethane coated sleel rollers. Our material supplier provides us only tensile uniaxial test data, but, obviously, this is not enough to tune material model with a good accuracy. Already in several sources I read that we need to perform some extended material testing which includes: tensile uniaxial test, tensile biaxial test and simple shear. However, it is rather extended testing, and for example biaxial testing machine is rather hard to find.

Does anybody know whether the uniaxial compression test is a good alternative for biaxial tension? How to eliminate friction (barreling) ?
And how to deal with the so-called shape factor of the standard specimen?
Maybe somebody knows other points of attention while performing such tests?

I would really appreciate any suggestions!

[Bigal-85]

I am pretty sure that for elastomers these tests provide the same material data. I have recently done some uniaxial compression tests on elastomers and found that using PTFE tape reduced the barralling effect, I am not sure what the friction coefficeint between the specimen and platens was but there may still have been some material shearing present.

My approach was to use the tape and then use the data and model the test and see how they compared. There is some literature out there if you hunt about.

I realise this isn't much help but thought I would let you know that someone else is in the same boat as you.

[kavkir]

...I have recently done some uniaxial compression tests on elastomers ...

Thanks Bigal-85!
How do you think will a sort of silicone grease appied additionally to Teflon tape give better result?
How do you deal then with stress-strain data? Because the stress in such compresstion test is affected by the height of the specimen... or when you eliminate the friction it is not being affected anymore?
And do you then recalculate the derived stess in true stress to tune your hyperelastic material model? How do you do this?

[Jorgen]

Here are a few more comments:

- Feel free to apply both silicone grease and PTFE tape.
- The typical rule of thum is to use compression specimens that have a height that is about half the diameter (in order to avoid buckling)
- You can recalculate the stress and strain to true-stress and true-strain if you want. It is necessary as long as you keep track of what type of stress and strain you are using.

Jorgen

[kavkir]

Jorgen,

Thank you for your answers!
I am getting back to my problem.... after a while.

In general, I can check the influence of the friction
by adding the friction in my FE model and try to vary it.
The accuracy of +/-10% is rather acceptable for me.

Ultimately, I can try measure the friction force between the PU
and Teflon tape....