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Simple Tensile Test

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Hopefully the fix can easily be managed. I am working with a 1mm^3 cube and want to back out a poisson ratio after applying a known displacement.

Property values include Young Modulus (3000 MPa), density 1380 (kg/m^3), and poisson ratio (.4).

I am applying a displacement of 1% (to one side, the other opposite side cannot move in the x direction). In the Y directions, the faces can only move in the Y and X direction while in the Z direction it can only move in the X and Z directions.

The most dense mesh I have used is on the order of 100000 elements with element qualities of 1 (I am using cubes).

My issue is that the displacments in the Y and Z directions provide maximum displacements that would back out a poisson ratio with error on the order of 10-15%. As this is a simple unit problem it should be closer to 1%. What common issues might be responsible for the skew?



5 Replies Last Post 28 nov 2017, 12:06 GMT-5
Jeff Hiller COMSOL Employee

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Posted: 7 years ago 16 nov 2017, 16:02 GMT-5
Updated: 7 years ago 28 nov 2017, 10:39 GMT-5

Your BCs are suspicious. I am not familiar with an analytical solution for the problem as you set it up. Perhaps you meant to set up the model as attached. Even with a much coarser mesh (a single element!), you can "back out" a Poisson ratio of .4 with accuracy of 14 decimal places, i.e. numerical noise.

Jeff

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Jeff Hiller
Your BCs are suspicious. I am not familiar with an analytical solution for the problem as you set it up. Perhaps you meant to set up the model as attached. Even with a much coarser mesh (a single element!), you can "back out" a Poisson ratio of .4 with accuracy of 14 decimal places, i.e. numerical noise. Jeff


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Posted: 7 years ago 28 nov 2017, 10:37 GMT-5

Thanks for the help. It seems the files you attached are for a later edition of Comsol than I have (I have 5.2a). Is there anyway to work around this for my edition?

Thanks for the help. It seems the files you attached are for a later edition of Comsol than I have (I have 5.2a). Is there anyway to work around this for my edition?

Jeff Hiller COMSOL Employee

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Posted: 7 years ago 28 nov 2017, 10:44 GMT-5

Sorry, Jeremy, but in the Discussion Forum I can only offer to build model files in the current version of the software. Please consider upgrading to the current version.

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Jeff Hiller
Sorry, Jeremy, but in the Discussion Forum I can only offer to build model files in the current version of the software. Please consider upgrading to the current version.

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Posted: 7 years ago 28 nov 2017, 11:10 GMT-5

Thats understandable. In any case, would it be possible to explain what kind of boundary conditions used in the model? This may enable me to get to the results I am looking for.

Thats understandable. In any case, would it be possible to explain what kind of boundary conditions used in the model? This may enable me to get to the results I am looking for.

Jeff Hiller COMSOL Employee

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Posted: 7 years ago 28 nov 2017, 12:06 GMT-5
Updated: 7 years ago 28 nov 2017, 12:18 GMT-5

Bottom, at x=0[mm], face: no x-direction displacement

Top, at x=1[mm], face: .01[mm] x-direction displacement

Vertex at (0,0,0): fixed constraint

Vertex at (0,0,1[mm]) : zero y-direction displacement

Jeff

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Jeff Hiller
Bottom, at x=0[mm], face: no x-direction displacement Top, at x=1[mm], face: .01[mm] x-direction displacement Vertex at (0,0,0): fixed constraint Vertex at (0,0,1[mm]) : zero y-direction displacement Jeff

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