1. Junior Member
Join Date
2008-05
Posts
20

[1]
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that is a good point I overlooked that. Nevertheless, the effective strain rate would be different for 3:

All three use the following formula for the strain rate:

gamma_dot=gamma0_dot*((lambda_b^p)-1)^c*(tau_b/tau_base)^m

but tau_b for 3 (BB 1998) is different then tau_b for 1 and 2 (BB 01, Bergstroem). So in my view that would result in two different gamma_dot (one for 1,2 and one for 3).

[2]
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that would be great if you could get back to me on that one. Thanks!

[3]
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I do. I was just trying to clarify that I do not have to multiply B_b^p with (J_b^p)^(-2/3), like it is done for the chain stretch in T_A [lambda^(*): from B_(*)=J^(-2/3)*B] and in T_B [lambda_b^(e*): from B_b^(e*) =(J_b^e)^(-2/3)* B_b^(e)] but I just realized that J_b^p=1....or is it is this only in the very beginning when lambda+0

Thanks

2. Junior Member
Join Date
2009-04
Posts
2

## Strain Energy

I'm a college student and recently we did biaxial tension (and uniaxial tension) experiments on Natural Rubber. It was using a simple tester we just designed. It generated stresses using weights (by potential energy). Basically, we loaded weights of specific increments on a platform and measure the changes in lengths of the specimen and the fall of the platform.

We were supposed to plot Engineering Stress against Engineering Strain, modelling Stress through the YEOH model. We just needed an expression for the Strain Energy (for the Cauchy Stress). We assumed that this Strain Energy was equal to the Potential Energy but the results weren't that good (compared to PolymerFEM experimental data).

I hope you could enlighten me on this matter. Perhaps a guide to a better expression for Strain Energy or a comment on the assumptions or the procedure. Thanks.

Mario

3. The Yeoh model typically works relatively well. How bad was your prediction?

I think your approach sounds fine. Are you sure you don't have other experimental errors, for example from friction.

-Jorgen

4. Junior Member
Join Date
2009-04
Posts
2

## Strain Energy

Thanks for your response Dr. Jorgen.

The experiment was within a limited range of Engineering Strain [it was intended just for instructional purpose]. The errors were some 5% to 20%, but then increases to as much as 150% with increased Engineering Strain (between 0.15 and 0.30). I am inclined to think that the errors are more likely due to friction or maybe from the length measurements. Our instructor, however, thought that the expression of Strain Energy [assumed to be equal to the Total Energy, mass x g x height displaced by platform] could be a mistake in the first place.

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