Learning Forum

[Karel Van Wambeke]

Hello all, 

I am trying to solve a static structural analysis of a beam constrained by means of a double pin connection:

The pin connections are modeled with frictionless contacts:

The model is fixed at the bottom and a symmetry b.c. (zero normal displacement) is used to reduce simulation time: 

I am running into the following errors:

"An internal solution magnitude limit was exceeded. (Node Number 89366, Body New_structure\Lever, DOF UY) Please check your Environment for inappropriate load values or insufficient supports.  You may select the offending object and/or geometry via RMB on this warning in the Messages window.  Please see the Troubleshooting section of the Help System for more information."

" The value of UY at node 89366 is 1.870118145E+12.  It is greater than the current limit of 1000000 (which can be reset on the NCNV command). This generally indicates rigid body motion as a result of an unconstrained model.  Verify that your model is properly constrained "

The error suggests that there is rigid body motion in my model, which I think is strange, because there are no initial gaps at the pin-hole connections (the pins are tangent to the holes). 

I have tried playing with the properties of the pin-hole connections (contact type, symmetric behavior, small sliding assumption) to see what might cause the error. Changing the connection type to rough or frictional (friction coefficient 0.2) results in the same errors. Only when the connection type is set to bonded, I get a solution. This is however not properly representing the physics as I am interested in the contact stresses at the pin connections. 

Any tips on how to solve this error? 

Best regards,
Karel

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[Akshay Maniyar]

Hi Karel,

As per the error message, it looks like your model is undergoing rigid body motion. Can you recheck the pin contacts? I guess, you have not selected all the surfaces. Also, you can check the initial contact information to see if any contact is in far open status. 

Thanks,

Akshay Maniyar

[Karel Van Wambeke]

Hello Akshay, 
thanks for the swift reply. 
After spending some more time trying to figure out the issue, I came across an Ansys course on preventing rigid body motion in contacts. 
Following the course, I used the contact tool to check the initial contact status of my frictionless contacts. 
Indeed, there was an initial gap in these contacts, due to the discretization of the pin and hole geometry after meshing.
I used "Adjust to touch" on these contacts and was then able to get my first results from the simulation! 

Overall, the stress results look logical: 

• At the pin-hole contact, expected stress concentrations are captured.
• In the shaft, expected bending stresses are present. 

However, when taking a section view at the shear plane of the shaft, I am experiencing weird stress results:

In the shaft, there are two local maximum stress spots that I cannot quite understand. The stress profile does not look like shear stress nor bending stress. 

Is this stress profile expected? Or is there something wrong with my analysis that might cause this behaviour? 
Thanks again!

Karel

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