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Negative effective strain (solid.epe)

Posted 11.06.2013, 20:46 GMT-4 Structural Mechanics Version 4.2a 4 Replies

Alberto Torres
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Hello!

I'm doing a thermo-mechanical transient process, however, when I evaluate the effective plastic strain versus time at certain points, I get negative values ​​for effective plastic strain (ts.epe or solid.epe), and not only that, it also decreases at certain intervals so which is inconsistent with the theory.


Thanks a lot and best regards,
Albeto T
4 Replies Last Post 13.06.2013, 23:22 GMT-4
Henrik Sönnerlind COMSOL Employee

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Posted: 1 decade ago 12.06.2013, 03:01 GMT-4
Hi,

This is a rather common question.

The reason for this behavior is that the constitutive law is only exactly fulfilled at certain internal points in the elements (the Gauss points). During result presentation, the results are extrapolated to cover the whole element, and during that extrapolation procedure it is possible that results like this appear. The same behavior can be seen in an ideally plastic material, where stresses larger than the yield stress can be displayed in elements which are partially elastic and partially plastic.

These type of effects can sometimes be reduced by changing the settings under Quality in the result nodes for plots and graphs. The default settings try to improve presentation of 'smooth' fields, but the results from an elastoplastic analysis are usually not that well behaved in regions where there is partial yielding.

Regards,
Henrik
Hi, This is a rather common question. The reason for this behavior is that the constitutive law is only exactly fulfilled at certain internal points in the elements (the Gauss points). During result presentation, the results are extrapolated to cover the whole element, and during that extrapolation procedure it is possible that results like this appear. The same behavior can be seen in an ideally plastic material, where stresses larger than the yield stress can be displayed in elements which are partially elastic and partially plastic. These type of effects can sometimes be reduced by changing the settings under Quality in the result nodes for plots and graphs. The default settings try to improve presentation of 'smooth' fields, but the results from an elastoplastic analysis are usually not that well behaved in regions where there is partial yielding. Regards, Henrik
Alberto Torres
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Posted: 1 decade ago 12.06.2013, 14:30 GMT-4
Hi!

Thanks for answer!

I understand the 'smoth' is for 2D graphics, but what about 1D graphs? I'm plotting a single point (e.g. solid.epe vs t) which exactly matches a node, there is also extrapolation or what could be the reason why the solid.epe decreases?

Regards,
Alberto T
Hi! Thanks for answer! I understand the 'smoth' is for 2D graphics, but what about 1D graphs? I'm plotting a single point (e.g. solid.epe vs t) which exactly matches a node, there is also extrapolation or what could be the reason why the solid.epe decreases? Regards, Alberto T
Nagi Elabbasi Facebook Reality Labs
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Posted: 1 decade ago 12.06.2013, 17:00 GMT-4
Hi Alberto,

What Henrik described applies to your case too. Stresses and strains at the node you are examining are extrapolated from the Gauss point values at the surrounding elements. They can therefore take on negative values even when all the Gauss point values are positive. It’s best illustrated with a figure, but short of that here is a simple example. If an element has two integration points and the one closest to the node of interest has a value of 0.0 and the one further away has a value of 0.1 then when you extrapolate to the node you will certainly get a negative value.

Nagi Elabbasi
Veryst Engineering
Hi Alberto, What Henrik described applies to your case too. Stresses and strains at the node you are examining are extrapolated from the Gauss point values at the surrounding elements. They can therefore take on negative values even when all the Gauss point values are positive. It’s best illustrated with a figure, but short of that here is a simple example. If an element has two integration points and the one closest to the node of interest has a value of 0.0 and the one further away has a value of 0.1 then when you extrapolate to the node you will certainly get a negative value. Nagi Elabbasi Veryst Engineering
Alberto Torres
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Posted: 1 decade ago 13.06.2013, 23:22 GMT-4
Hi!

Thank you very much for your answer Henrik and Nagi.

Best regards
Hi! Thank you very much for your answer Henrik and Nagi. Best regards

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