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[Solved] BB model Constants  


purush537
Posts: 22
(@purush537)
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Hi,

I know that I can extract the BB model constants from two cmpression test with different strain rate, though I have a couple of questions:

- what would be the strain rate for the unloading part of the compression tests?

- Is there any way to get the constants wih tensile tests? I have all standard hyperelastic test data, stress relaxation test data and uniaxial test data with different strain rates. The strain limit for all mentioned tests are limited to our application strain (30%)

Thank you,

13 Replies
Jorgen
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- You can use any two strain-rates
- You can use either compression, tension, or both
- You can also use stress relaxation data

The MCalibration software, that I have written, makes it very simple to determine the required material parameters for the BB-model (and other models). Send me a [URL= https://polymerfem.com/sendmessage.php]private message[/URL] if you want a free trial license to this software.

-Jorgen

13 Replies
purush537
Posts: 22
(@purush537)
Eminent Member
Joined: 11 years ago

Thanks Jorgen,

- Is there any standard procedure for determining the constants with the tests you mentioned? I only saw the procedure for the compression test.

- In your article CONSTITUTIVE MODELING OF THE LARGE STRAIN TIME-DEPENDENT BEHAVIOR OF ELASTOMERS (in the Appendix A), you wrote C(B) R can be determined from [B]the instantaneous unloading slope [/B](i.e. tangent modules Et = d sigma/d epsilon) at strain reversals. What would be the strain rate for the instantaneous unloading?

Thank you,

Jorgen
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1) There is no standard general procedure available.

2) The text that I wrote in that paper provides some rules of thumb for how to determine the material parameters. As such, the unloading strain-rate can be any common rate.

In practice, it is much easier, faster, and more accurate to use an automatic algorithm for determining the material parameters. The MCalibration program is excellent at this task.

-Jorgen

RavikumarG
Posts: 1
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Joined: 9 years ago

In your paper listed above, in the appendix, you have a figure displaying what appears to be a single cycle compression test. The figure also contains something calls an equilibrium locus. It looks to be an average until it surpasses the compression curve altogether. Can you explain how to plot this curve?

Jorgen
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The equilibrium curve corresponds to the behavior that would be obtained at infinitely slow strain rate. You can estimate it from tests with multiple stress relaxation segments. It also be estimated (with some experience) from the loading-unloading curve.

-Jorgen

dvhudson08
Posts: 11
(@dvhudson08)
Active Member
Joined: 8 years ago

Hi Dr Jorgen,

Thanks for the helping us out! I have two questions about the procedure given in Appendix A of your paper mentioned above:

1) In the expression of CR(A) and CR(B), is the stretch mentioned is the stretch_equilibrium or something else? The expression mention in you paper do not have any suffix which is bit confusing.

2) Is there any rule of thumb for calculating limiting stretch or Na? Or does it require additional test at very low strain rate?

Thanks !

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