Learning Forum

[sriparna.2021rce15]

I have modeled a 3-D beam with dimensions: 100mm x 100 mm x 500mm in ANSYS workbench. ANSYS by default, is considering SOLID186 as an element type. The ANSYS modal results are based on Timoshenko beam theory, which does not match analytical results based on Euler theory. Can anyone suggest how I can force ANSYS to provide results based on Euler theory?

[peteroznewman]

Read some of these discussions.

[sriparna.2021rce15]

i tried changing element type to beam4 but I am getting an error as shown below

[Erik Kostson]

Beam4 is a beam (line element), so you can not use it on 3D solid bodies .

You need to define a line body.

See here how to define such a body (line) and how to analyse beams.

All the best

Erik

https://www.youtube.com/watch?v=5Ef5piegVh4

 

Erik

[sriparna.2021rce15]

thank you for your response. Sir can you please tell me which real constants I need to insert in commands. After going through several posts, I have found to use the follwing commands:

et, matid, beam4

r,2,.....................

after r,2, I am unable to understand what parameters do i need to enter

beam cross section : 100mm x 100mm x 500mm 

[Erik Kostson]

 

Hi

 

See this post for the real constants and help manual if needed:

/forum/forums/topic/euler-bernoulli-beam-theory/

use r,matid,AREA,IZZ,IYY,TKZ,TKY

and input the above (AREA=0.01*0.01 in your case I assume, etc).

I would suggest to learn the Mechanical (Getting started course),

https://innovationspace.ansys.com/product/get-started-with-ansys-mechanical/

and then also look into the APDL course as you need to learn what these commands mean when using them:

/courses/index.php/courses/intro-to-ansys-mechanical-apdl-scripting/lessons/overview-of-mapdl-lesson-1/

All the best

Erik

[sriparna.2021rce15]

http://mae.uta.edu/~lawrence/ansys/el_beam/el_beam.htm

Thank you for your response. In the above article, it shows that it can be used for 3D element. So, is it possible or not.

[Erik Kostson]

 

 

Hi

 

So beam4 is a 2 node beam element. 

In mechanical one then needs a line body ( see here how to define such a line body

https://www.youtube.com/watch?v=5Ef5piegVh4 )

You have a 3D solid body which is meshed with HEX8 or HEX20 3D solid elements so hence the error.

To sum up the geometry created needs to be a line body not a 3D solid like you have, in order to use a beam4 element – look at the video again to see how to define/create a line body.

All the best

Erik

 

 

[sriparna.2021rce15]

thank you for clarifying my doubt. I have serious doubts regarding Ixx, if you could help me with that, it would be of great help.

 

[Erik Kostson]

Hi

Please see the doc./help:

Search for beam4 (...the torsional moment of inertia (IXX)).

BEAM4 (ansys.com)

See here hwo to open the help link (/forum/forums/topic/how-to-access-the-ansys-online-help/#:~:text=Open%20Ansys%20Workbench%20(or%20any,A%20browser%20window%20will%20open.)

All the best

Erik

[sriparna.2021rce15]

thank you for your help. I will work on it

 

[sriparna.2021rce15]

My problem was that I was not able to match the numerical frequencies corresponding to the bending mode shapes with the ansys result. I read about BEAM4 element, but the results are still not accurate. I have attached the properties of the model and the natural frequencies corresponding to the 20 mode shapes have been shown in the table along with the ANSYS result. It would be very helpful if you can help me to identify that whether there is some limitation in the ANSYS software for this geometry or something else. Dimensions of 3D beam: 150mm x 150mm x 700mm

 

 

[sriparna.2021rce15]

just to clarify I have tried this problem in 3D model and also as line body for BEAM4 element

[sriparna.2021rce15]

The theoritical natural frequencies do no match with the frequencies obtained from ANSYS and the error between them keeps on increasing. Till 50th mode shape only the error is 50% and if I want even more higher mode shapes the error will be greater than 50%

[sriparna.2021rce15]

please reply if someone knows about this topic. It would really help a lot

 

[sriparna.2021rce15]

Please help me with this problem

 

[peteroznewman]

To get accurate results, you need at least 12 beam elements for each sinusoid along the beam.

For example, mode 2 has 1 sinusoid so 12 elements along the length is sufficient while mode 10 has 5 sinusoids so it needs at least 60 elements along the length. The higher modes need progressively more elements.  It's okay to have more than the minimum number of elements.

How many elements do you have along the length for the tables you are showing above?

[sriparna.2021rce15]

Thank you for your reply sir.

I am unable to understand "elements". I mean are you talking about meshing?

Kindly tell me how to check about the number of elements, so that I can share the details with you.

 

[sriparna.2021rce15]

From what I am able to understand:

I modelled a 3D beam of 0.04 x 0.04 x 1 (meter) and then extracted 2D profile from it. The meshing size was 0.001m therefore, there are 1000 elements in the beam

 

[peteroznewman]

Okay, 1000/12 = 83 so accuracy would start to degrade for modes with more than 83 sinusoids along the length.  Mode 24 had 12 sinusoids.

Another source of error is whether the beam element is using a consistent mass matrix or a lumped mass matrix.  Here is a YouTube video on that topic.

I know about this for the Nastran solver.  I haven't looked up how the Ansys solver treats the mass in the BEAM4 element.

[sriparna.2021rce15]

Thanks for your reply and your valuable time.

I would just like to clarify......I extracted 55 mode shapes only and in 55 modes the error has crossed 50% from the theoritical. So, according to the 12 elements for a sinusoid....upto mode 166 there should be no error. Please comment whether my conclusion is right or wrong.

 

I would like to ask which mass matrix is accurate:consistent or lumped?

[peteroznewman]

I was able to count the sinusoids in the deformation plots that only go up to mode 24.  I don't know how many sinusoids there are at mode 55 but it's probably less than 83 so yes, the element count per sinusoid is probably fine.  It was a potential source of error when I didn't know the element count.

The consistent mass matrix is more accurate.  The lumped mass matrix is more efficient to compute solutions to dyanamic analyses.

[sriparna.2021rce15]

Sir, please guide me in checking whether lumped mass matrix is being used or not.