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[Kaiden Bedell]

I am attempting free-free modal analysis on this structure, so no loads are applied. I am looking for frequencies in 6 modes. When I use 'program controlled' formulation for all connections, the mode frequencies are way too high. I have tried using MPC for all contacts relating to the 4 highlighted rods in the design, but this led to overconstraint since the connections all share a face. Despite the overconstraint, the frequency values were in the expected range. Next I tried only using MPC for one contact per rod to eliminate the overconstraint, but this made the frequencies too high again and a warning that the max contact stiffness is too big.

Could I use MPC for all the rod contacts and ignore the overconstraint warning, or is there another solution?

Case 1: All rod contacts with MPC and the according frequencies and warning popup. 

Case 2: MPC used only once per rod, according frequency results and warning popup. 

Overall I am looking for insight on how accuracy differs between these two cases and further solutions/troubleshooting. Thanks in advance for your input. 

[Aniket]

Are the first 6 modes with all MPC contacts showing 3 rotations and 3 translations? If yes, you can use the results. When using MPC, there are a few things that can cause this warning message to appear. One possibility is that two contact regions or boundary conditions are overlapping each other. This can happen if the contact regions or boundary conditions are not defined carefully. Another possibility is that a contact region or boundary condition is overlapping a symmetry region. This can happen if the symmetry region is not properly defined. If you receive this warning message, you should first check to see if there are any overlapping contact regions or boundary conditions. If there are, you can try to move them so that they do not overlap. If this does not work, you may need to change the definition of the symmetry region. In some cases, it may be possible to ignore this warning message. However, if you are concerned about the accuracy of your results, you should try to resolve the conflict. Here are some tips for avoiding this warning message:

Carefully define the contact regions and boundary conditions in your model.
Make sure that the contact regions and boundary conditions do not overlap each other.
Make sure that the contact regions and boundary conditions do not overlap any symmetry regions.
If you follow these tips, you can help to ensure that your Ansys Mechanical models are accurate and that they converge properly.

-Aniket

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[Kaiden Bedell]

Hi Aniket, 

I was wondering what it is about MPC vs other options that creates such a drastic change in results. I understand the idea that it is a 'multi point contact' and it doesnt take into account maxiumum contact stiffness. Do you have any further knowledge on why this formulation works over others for this model? 

Thank you, 

Kaiden Bedell 

[Kaiden Bedell]

Thank you for the advice. Since I am doing free-free analysis, I don't have any loads or supports. This would mean I have no boundary conditions, correct?