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doghak
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Hello all,

I have test results (force, displacement) form a test like this one:

[url] [/url]

How can I get the true stress/strain?

I go to eng. shear strains of 200% and it seems the eng vs true stress is giving weird results after 150% or so.

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jamal.mousavi
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[QUOTE=Settler,13456]Hello all,

I have test results (force, displacement) form a test like this one:

[url] [/url]

How can I get the true stress/strain?

I go to eng. shear strains of 200% and it seems the eng vs true stress is giving weird results after 150% or so.[/QUOTE]

Hello,

I have test results, too. My test bench same as below link. [url] [/url]

My question is same with yours. We can find nominal stresses and strains but about true stresses we do not have any thing. Can anyone help us?

Thanks

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doghak
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My biggest problems start when I want to incorporate test data with 200% shear strain.

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jamal.mousavi
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[QUOTE=Settler,13458]My biggest problems start when I want to incorporate test data with 200% shear strain.[/QUOTE]

What about your thickness? My height is 80mm and thickness is 2mm. My max strain aprox. 90%.

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Jorgen
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Why do you need true stresses?

You can calibrate directly to the engineering shear strain-strain using [URL= http://www.veryst.com/what-we-offer/polymer-modeling ]MCalibration[/URL].

-Jorgen

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saleh
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Hello Jorgen,

I'm trying to use Mcaibration to calibrate a hyperelastic model based on simple shear test data on rubber. I would be then able to use the calibrated model in Abaqus for characterizing the hysteretic behavior of rubber in shear.

The simple shear test data that I have is based on Gamma (shear angle) and continues up to Gamma equals 2. For such relatively large shear strains, Gamma is not equal to engineering strains (or Nominal strain in Abaqus) and true strain.

Mcalibration has the model ABAQUS-PRF-2Net-Yeoh-BB which seems to be the most useful one for my purpose. This model incorporates 2 networks (Yeoh model and BB model).

The very important point is that the calibrated model under such levels of shear strains would be totally dependent on the type of shear strain used for calibration. The major problem here is that the calibration of a material model in simple shear should be done using "Gamma" rather than True strain on Engineering strain in order to be applicable in Abaqus. Again, although the Abaqus reports Nominal shear strains and True shear strains, the calibrated model needs to be based on Gamma. I have checked this.

However, after reading the existing discussions in PolymerFEM (e.g., watching the end summary of the video found in the following link) and also doing my analyses in Mcalibration, I noticed that the calibration in Mcalibartion only works when the shear strain data is based on engineering shear strain or true shear strain.

Again, this is very important to know that the calibrated models (or determined model constants) in simple shear and under such relatively high levels of shear strains would be completely dependent on the type of shear strain in your experimental test data.

Therefore, I found that the calibrated models in Mcalibartion based on simple shear test data and under such level of shear strain wouldn't be applicable in other software such as Abaqus. I mean at least the current version of Mcalibration is not useful for this specific case because it requires the shear test data to be in Engineering shear strain or True shear strain.

More information about the differences between these types of shear strains and their applications in Mcalibration be found in the following link created by Jorgen Bergstrom.

https://polymerfem.com/5-common-mistakes-when-analyzing-simple-shear/

Please correct me if you think otherwise.

Thank you so much.
Saleh,

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Jorgen
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Hello Saleh,

I agree that at large shear strains there is a big difference between gamma, engineering strain, and true strain. You can read more about this in this article

You are incorrect, however, when you say that "The major problem here is that the calibration of a material model in simple shear should be done using Gamma rather than True strain on Engineering strain in order to be applicable in Abaqus". That is NOT true. In fact, it does not matter if you use gamma or one of the strains, as long as you are consistent in how you interpret the data.

The Load Case dialog in MCalibration contains the following image:

I recommend that you convert your gamma data into engineering shear strain using the equation shown in that figure, and then read in the converted data in MCalibration for use with your calibration.

What do you think?

/Jorgen

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saleh
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@jorgen

Thank you so much for your response.

I have actually read that article and have also converted the gamma to engineering shear strain for calibration. It works well in Mcalibration, and I get a good fit for the experimental data.

However, my problem is with the applicability of the calibrated model or the determined model constants in Abaqus.
I think I understand your point about being consistent and also interpret the model correctly.

However, we don't have so many choices about how to interpret the calibrated model in Abaqus for the analysis. What we can do in Abaqus is that we enter the material constants. I have already check this issue in Abaqus. The Abaqus itself interprets the shear strain in the calibrated model as Gama for the analysis. In Abaqus, when I apply a shear displacement (equivalent to Gamma 2) in a simple model with the adopted calibrated model for rubber, I get very very high shear stresses or forces. This illogical response exactly comes from the issue mentioned above. The Abaqus itself apply gamma in the adopted model for the calculation of stresses in that model. However, I had calibrated the model based on engineering shear strain.

Please see the attached figure which shows two exactly the same experimental shear test data but one based on gamma and the other based on engineering shear strain.

As can be seen in the top figure, the shear force (at high shear strains) goes up with a much higher slope for a little increase of engineering shear strain. This is fine as long as it is interpreted correctly for the analysis. However, Abaqus interpret it as Gamma for the analysis.

If you agree with my point, I wonder whether there is any way to resolve this issue. For example, is there any way to calibrate the model based on Gamma in Mcalibration?

I really appreciate your thought and help on this.

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saleh
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Dear Jorgen,

Please note that the attached figure in my previous comment or my reply to your comment about the comparison of the engineering shear strain and gamma curve isn't accurate.

However, my raised issue about the applicability of the calibrated model in Abaqus still exists.

Based on my analyses, Abaqus doesn't interpret the shear strain as the Engineering shear strain which is used for calibration of the models in Mcalibation. I do appreciate it if you read my previous comment for more details and let me know what you think.

Thank you so much.

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