# How to get around the problem of contact interaction in modal analysis?

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Dear Abaqus users,

I need to perform a modal analysis of a compressor stator blade in an industrial gas turbine. The context of my analysis is the effect of fouling and corrosion in the blade attachment on vibration characteristics of the whole blade. They will basically affect the contact stiffness (and the damping). In order to do so, I would like to take into account the contact at the attachment in my frequency analysis on Abaqus. I know that contact, which is non-linear, is ignored in modal analysis because it is linear. But basically I want to simulate the difference between a clean attachment with a clearance and a dirty one where some contact areas are steel-steel and other are steel-dirt-steel by changing the stiffness of the contact. For the clean one, because of the clearance, the contact will vary depending on the mode shape (it is not clamped because it is a stator, no centrifugal force). For the dirty one, I will consider that the fouling makes the blade root clamp, thus the contact will remain the same with different contact stiffness along the root. I tried to simulate it by an elastic foundation but the clearance cannot be represented, that is why I ask you if you know a way to simulate this contact on Abaqus in order to find the natural frequencies of the blade, please. I read in a forum that performing a transient analysis will take into account the contact and applying an FFT on the response will give us the natural frequencies, so I tried the step modal dynamic but the contact interaction was still ignored.

I have already tried to model the fouling as a 3D part which is tied to both surfaces but the problem of this simulation is that it introduce a tension stiffness into the system which does not exist. Basically, the contact stiffness is only a compression one (and also tangential normally due to friction but not taken into account for now).

I have attached a picture of the attachment.

What I am thinking to do is to perform a dynamic analysis of the system, which will include the contact, on which an impulse of force (like a hammer tap) will be applied so the displacement response signal will give me the natural frequency after an FFT (as in a basic hammer test). But I don't know which step I should use. I tried with a Dynamic, Implicit but I've got the following error message:

"Error in job Job-5: Too many increments needed to complete the step

Error in job Job-5: THE ANALYSIS HAS BEEN TERMINATED DUE TO PREVIOUS ERRORS. ALL OUTPUT REQUESTS HAVE BEEN WRITTEN FOR THE LAST CONVERGED INCREMENT."

So if anyone has an idea help me, please.

Thank you.

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