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Question about the triaxial test

Posted Nov 4, 2012, 12:24 a.m. EDT Modeling Tools & Definitions, Parameters, Variables, & Functions Version 4.3a 4 Replies

Quan Gao
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I have a problem about the example in the model library. There is a step in the example, which i don't understand.

Integration 1
1 ) Right-click Definitions and choose Model Couplings>Integration.
2 ) In the Integration settings window, locate the Source Selection section.
3 ) From the Geometric entity level list, choose Boundary.
4 ) Select Boundary 3 only.

Variables 1
1 ) Right-click Definitions and choose Variables.
2 ) In the Variables settings window, locate the Variables section.
Fres=intop1(solid.sGpz*r*2*pi)

why do we need to calculate the axial stress using the formular in the example because normally, the axial stress is gradually increasing in the real triaxial test? I am a little bit confused by the expression.

Any one can tell me how model the viriation of water content of the soil sample and its influence on the stress-strain and strength features?

Thanks a lot!
4 Replies Last Post Nov 6, 2012, 12:49 a.m. EST
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago Nov 4, 2012, 2:54 a.m. EST
Hi

I'm not sure I understand fully your question, but in the model library example, one imposes a displacement on the top plate , hence to estimate the Force due to the stress a the plate level one must integrate the stress in "z" over the plate surface which is (2*pi*r*stress) wherer 2*pi*r is the 2D-axi loop length.

Now the aspect of water content is, from my understanding not included herein, as we need then to consider porous media and 2 component medias, this is not my domain, so for that you would need to get help from some geophysicist used to soil measurements.

I had to look up what a triaxial test is, I found already some reasonable explanation on Wiki

--
Good luck
Ivar
Hi I'm not sure I understand fully your question, but in the model library example, one imposes a displacement on the top plate , hence to estimate the Force due to the stress a the plate level one must integrate the stress in "z" over the plate surface which is (2*pi*r*stress) wherer 2*pi*r is the 2D-axi loop length. Now the aspect of water content is, from my understanding not included herein, as we need then to consider porous media and 2 component medias, this is not my domain, so for that you would need to get help from some geophysicist used to soil measurements. I had to look up what a triaxial test is, I found already some reasonable explanation on Wiki -- Good luck Ivar
Nagi Elabbasi Facebook Reality Labs
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Posted: 1 decade ago Nov 4, 2012, 9:39 p.m. EST
The extra axial loading stress calculated in this example is the average stress over the top surface since the stress is not uniform. That stress is mainly proportional to the deviatoric stress (that’s why you subtract the Pxc term).Effect of water content and water pressure is not accounted for in this material model.

Nagi Elabbasi
Veryst Engineering
The extra axial loading stress calculated in this example is the average stress over the top surface since the stress is not uniform. That stress is mainly proportional to the deviatoric stress (that’s why you subtract the Pxc term).Effect of water content and water pressure is not accounted for in this material model. Nagi Elabbasi Veryst Engineering
Quan Gao
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Posted: 1 decade ago Nov 6, 2012, 12:09 a.m. EST

The extra axial loading stress calculated in this example is the average stress over the top surface since the stress is not uniform. That stress is mainly proportional to the deviatoric stress (that’s why you subtract the Pxc term).Effect of water content and water pressure is not accounted for in this material model.

Nagi Elabbasi
Veryst Engineering


The integral operator intop1 is used for calculating the resulting strength over the whole soil sample. Since 2D axisymmetric conditions are used, integrate the z component of the stress tensor at each Gauss point times the circumference’s length: 2πr.

It is mentioned in the explaination that the axial stress is " integrate the z component of the stress tensor at each Gauss point times the circumference’s length: 2πr". what is the meaning of Gaussian point. Does it mean that the distribution of axial stress on the top plate is assumed as Gaussian distribution?
[QUOTE] The extra axial loading stress calculated in this example is the average stress over the top surface since the stress is not uniform. That stress is mainly proportional to the deviatoric stress (that’s why you subtract the Pxc term).Effect of water content and water pressure is not accounted for in this material model. Nagi Elabbasi Veryst Engineering [/QUOTE] The integral operator intop1 is used for calculating the resulting strength over the whole soil sample. Since 2D axisymmetric conditions are used, integrate the z component of the stress tensor at each Gauss point times the circumference’s length: 2πr. It is mentioned in the explaination that the axial stress is " integrate the z component of the stress tensor at each Gauss point times the circumference’s length: 2πr". what is the meaning of Gaussian point. Does it mean that the distribution of axial stress on the top plate is assumed as Gaussian distribution?
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago Nov 6, 2012, 12:49 a.m. EST
Hi

no the Gauss points are a specific way to average your element results fron the node values, check the doc, and then you integrate this stress in [Pa] to get the total force, normal to your plate (and you use "z" because the plate is normal to "z", else you would have need to use a scalar product

--
Good luck
Ivar
Hi no the Gauss points are a specific way to average your element results fron the node values, check the doc, and then you integrate this stress in [Pa] to get the total force, normal to your plate (and you use "z" because the plate is normal to "z", else you would have need to use a scalar product -- Good luck Ivar

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