Polymer Youngs Modulus much different in Compression?

[Airfix]

I am designing a test fixture that needs to be of a certain overall axial stiffness to test a linear actuator. The test stand sees a linear load from the actuator that puts a compressive load and bending load into the test fixture.

To get the test stand to the correct stiffness I made a very rigid steel frame and the planned to use a polymer of the correct youngs modulus, xsectional area and length to give the correct stiffness. A kind of rigid elastomer.

We ordered 2 polymers for test. A polyethelyne and a polyurethane. We are finding the actual youngs modulus in compression is significantly different from the published tensile youngs modulus. Also by changing the area or the length of the polymer we are getting a different youngs modulus than at the other length/area. I don't get it. I'm I just being dumb? I have no polymer experience whatsoever.

We are using E=(F*L)/(A*dL) to determine what length/area of material to use to give is the stiffness (F/dL) we want where:
E = Youngs Modulus (either calculated from experiment or used published tensile data)
F = applied load to the polymer
L = overall length of the polymer under compression
A = cross sectional area of the polymer
dL = the change in length due to the load F

Essentially F/dL is stiffness K. So we have:
E=KL/A

That equation is just derived from E=stress/strain.

We can measure F, L, A and dL. In test for a given F at L1 and A1 we get a different E to when we test with L2 A2. Why?

So here are my questions to all you polymer guru's:

1) Is my equation E=(F*L)/(A*dL) or E=KL/A the correct one to use for stiffness?
2) Is the youngs modulus of a polymer much different in tension than it is in compression. If so why?
3) Is the youngs modulus of a polymer in compression linear (as long as we keep the shear stress under the yield allowable)?

Help appreciated.

Cheers

Steve

 

[MikeHalloran]

You know how mild steel does odd things when you take it past the yield point? I.e., it behaves 'plastically'? That particular word is used because, in that region, steel behaves like plastic does in every region. Plastic is not elastic.

If you want elastic behavior, you need steel springs.

You can make very stiff coil springs by milling or sawing helical slots through thick steel tubes. Three slots gives you interleaved springs that don't react eccentrically under axial load.





Mike Halloran
Pembroke Pines, FL, USA

 

[Demon3]

Normal thermoplastics are only elastic at very low extension and for short testing times. At longer testing times the polymer will creep (cold flow). You can find some information here. It shows that the stress / strain curve is not linear for thermoplastics, that's why you are measuring a different "modulus" at different strains.

http://web.mit.edu/course/3/3.11/

http://www/modules/ss.pdf

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