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Beam Cross Section (bcs) interface returns odd results
Posted 18 nov 2016, 15:03 GMT-5 Geometry, Structural Mechanics Version 5.2 4 Replies
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I was trying out the Beam Cross Section (bcs) interface in 2D to obtain some data I need for my graduation project.
I need to calculate Shear Correction Factor (ky), Max Shear Stress Factor (muy) Torsional constant (J) and Torsional Section Modulus(Wt) for a non-standard cross section.
To try out the interface I imported the geometry of a pipe with outer diameter 25.4mm and 2mm wall thickness, which is easy to calculate from formulas and so I could compare the results and check if I was doing it right. The formulas are found in COMSOLs user guide for the structural mechanics module.
However, the returned data for Torsional Section Modulus and Shear Correction Factor ar far from what is expected for that cross section: Ky should be around 1.9 but returns 0.5 and Wt should be in the 1.5e-6 order but returns 9.5e-8. Other data like area, moment of inertia, section height return correct numbers.
I attach the file I'm using if you can check if there´s any problems with the geometry, since using solid cross sections returns expected values (ex: a solid circular bar or square bar), and hollow cross sections return weird values (ex: hollow rectangular cross section or hollow round section).
Hope all this makes sense. I also attach a file with the beam interface were you can see the values COMSOL itself calculated on another study in table 3. Just in case I'm doing something worng ;) . Hope you can help me out.
I need to calculate Shear Correction Factor (ky), Max Shear Stress Factor (muy) Torsional constant (J) and Torsional Section Modulus(Wt) for a non-standard cross section.
To try out the interface I imported the geometry of a pipe with outer diameter 25.4mm and 2mm wall thickness, which is easy to calculate from formulas and so I could compare the results and check if I was doing it right. The formulas are found in COMSOLs user guide for the structural mechanics module.
However, the returned data for Torsional Section Modulus and Shear Correction Factor ar far from what is expected for that cross section: Ky should be around 1.9 but returns 0.5 and Wt should be in the 1.5e-6 order but returns 9.5e-8. Other data like area, moment of inertia, section height return correct numbers.
I attach the file I'm using if you can check if there´s any problems with the geometry, since using solid cross sections returns expected values (ex: a solid circular bar or square bar), and hollow cross sections return weird values (ex: hollow rectangular cross section or hollow round section).
Hope all this makes sense. I also attach a file with the beam interface were you can see the values COMSOL itself calculated on another study in table 3. Just in case I'm doing something worng ;) . Hope you can help me out.
Attachments:
4 Replies Last Post 21 nov 2016, 10:26 GMT-5
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Posted:
8 years ago
Hi,
A quote from the doc which I hope will solve your problem:
"If the section is not simply connected, add one Hole node for each internal hole. In that node, select all boundaries around the hole."
Regards,
Henrik
A quote from the doc which I hope will solve your problem:
"If the section is not simply connected, add one Hole node for each internal hole. In that node, select all boundaries around the hole."
Regards,
Henrik
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Posted:
8 years ago
Hi, thanks for your help.
With the suggested change, the Wt value is now correct (1596.3mm^3) consistent with User Guide formulas, but still, Ky remained the same 0.5 and should be:
Ky = 1 + di/do = 1 + 21.4mm/25.4mm = 1.84 very far away from formula value
and Shear Stress Factor uy:
uy = A*(do^3-di^3)/[12*(do-di)*Izz] = 147.03mm^2*(25.4^3mm^3-21.4^3mm^3)/[12*(4mm)*10137mm^4] = 1.9903 and the BCS returns 2.1306 which is pretty close but still makes me wonder if it's trust worthy.
From what I see in the formula, Ky could never be less than 1, am I correct?
Thanks again for your help, have a great day,
Alfredo
**NOTE: I tried attaching the file in my home computer but it gives me a "file extension error", so if you need it I could try again tomorrow in a computer with COMSOL installed in my university.
With the suggested change, the Wt value is now correct (1596.3mm^3) consistent with User Guide formulas, but still, Ky remained the same 0.5 and should be:
Ky = 1 + di/do = 1 + 21.4mm/25.4mm = 1.84 very far away from formula value
and Shear Stress Factor uy:
uy = A*(do^3-di^3)/[12*(do-di)*Izz] = 147.03mm^2*(25.4^3mm^3-21.4^3mm^3)/[12*(4mm)*10137mm^4] = 1.9903 and the BCS returns 2.1306 which is pretty close but still makes me wonder if it's trust worthy.
From what I see in the formula, Ky could never be less than 1, am I correct?
Thanks again for your help, have a great day,
Alfredo
**NOTE: I tried attaching the file in my home computer but it gives me a "file extension error", so if you need it I could try again tomorrow in a computer with COMSOL installed in my university.
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Posted:
8 years ago
Hi,
The correct (approximate) shear correction factor is (1 + di/do)^(-1), so the current expression in the theory section of the Beam interface is erroneous. Thanks for pointing that out. There are actually even better approximations, but they depend not only on the geometry, but also on the material data (Poisson's ratio).
If you look at all other cross sections in the documentation, you will see that the values are <1 as expected.
The values that you get from the Beam Cross Section interface are 'exact' according to the theory, down to discretization errors, whereas the values that you see for the Common Sections are based on approximate formulae. The difference between 1.9903 and 2.1306 shows this effect.
Regards,
Henrik
The correct (approximate) shear correction factor is (1 + di/do)^(-1), so the current expression in the theory section of the Beam interface is erroneous. Thanks for pointing that out. There are actually even better approximations, but they depend not only on the geometry, but also on the material data (Poisson's ratio).
If you look at all other cross sections in the documentation, you will see that the values are <1 as expected.
The values that you get from the Beam Cross Section interface are 'exact' according to the theory, down to discretization errors, whereas the values that you see for the Common Sections are based on approximate formulae. The difference between 1.9903 and 2.1306 shows this effect.
Regards,
Henrik
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Posted:
8 years ago
Hi,
I had initially thought the difference in uy was due to the difference in solution methods, but the huge error in ky made me wonder....
So, 1.84^-1 = 0.54 ~ 0.5 of the beam interface.
Thank you so, so, very much for your help, have a great day,
Alfredo
I had initially thought the difference in uy was due to the difference in solution methods, but the huge error in ky made me wonder....
So, 1.84^-1 = 0.54 ~ 0.5 of the beam interface.
Thank you so, so, very much for your help, have a great day,
Alfredo