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Frequency analysis with Contact constraint



I am trying to analysis a model in ABAQUS with a plate bolted to the Bracket top surface at 4 ends. I have to define contact between them (and also bolted at 4 ends) to get the Modal frequency. But When I searched for it, I got the answer that contact doesn't work with Modal analysis as contact is a nonliner problem and Modal Analysis only solves linear problem.


Anyway I tried it, and I got good looking results with contact, but the only problem is that when I don't specify any friction between two plates it gives LOW 1st and 2nd (and other as well) mode frequency but when I specify any value of Friction coefficient it gives me 15-20Hz higher frequency for 1st and 2nd mode (and some higher number for subsequent modes). Even when I use TIE constraient it gives me similar High frequency. 

I am not sure which answer I should trust, the lower frequency because it's more conservative or the higher? But I surely don't want them to be Tied to gether and contact with some frictio value gives same result as TIE so dont know if I should trust that?

Please help me with that.




elabbasi's picture

Hi Siddharth,

Contact as you noticed should work in a linear perturbation analysis (like modal analysis) with some restrictions. The contact conditions at the start of the analysis (the base state) will be maintained, so nodes in contact will remain so, and open nodes will also remain open. It is not as straightforward when it comes to friction in my opinion. All nodes in contact will be assumed sticking if there is friction and sliding if there isn’t. That is why you’re getting this increase in frequency when you have friction. I would take both solutions into account if possible. It gets even trickier if the system vibrates with high amplitudes. In this case, some of the nodes that were in contact may separate and vice versa!

Nagi Elabbasi

Veryst Engineering

Hi Nagi,

 Thank you for your reply, I really appriciate that. And it makes more sence after your explanation.


FEA analyst

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