Method to calculate interface forces in random vibe analysis?

[JaviSanchez]

I have a question for you: I would like to know how to get interface forces in random vibration analysis, in order to check bolted joints survival. What I use to do is to place 6 CELAS elements (1 per DOF) in the interface (between an RBE2 and a node) and then I get the 3xRMS forces in the CELAS for a random vibration analysis; but I think I am getting too large forces by this method. ¿Do you know any other method to get the interfaces forces in random vibration analysis?

Thank you very much

 

[BlasMolero]

Dear Javier,
Historically, bolts modeled in this way were represented with DOF springs such as the nastran CELAS element. As long as the nodes of the spring connected were coincident this representation was acceptable. However, when the nodes were separated by some distance, or where not oriented in a coordinate direction, DOF springs were unacceptable due to internal grounding issues. A better alternative exist today, the CBUSH elements. The CBUSH element works equally well with coincident nodes and separated nodes, and only requires a single element to represent all six DOF.

I will reprint here for others interested in this matter what I explained before in other forum:
"I see usually this way of modeling bolting joints using CELASi elements in some users creating FEA models for space/aeronautic industry, but I do not longer use CELAS elements, instead the CBUSH element has many advantages. For example, if you use CELASi elements and the geometry isn’t aligned properly, internal constraints may be induced. The CBUSH element contains all the features of the CELASi elements plus it avoids the internal constraint problem. The CBUSH element is a perfect replacement for scalar elements for any static/dynamic analysis to avoid unintentional grounding issues.

To recover loads in mounting bolts, a CBUSH element can be used to represent the bolted connection, each one connected to the end of the rigid RBE2 element. The stiffness of the CBUSH will be estimated to be roughly the stiffness of the bolt in the axial direction. In the shear directions, and arbitrary higher value will be used, reflecting the high stiffness of a bolted joint in the shear direction.

CBUSH elements are easier as there is only one element to handle ALL directions. Stiffness is provided in three translational directions. Rotational stiffness, unless significant, usually has little effect on an analysis.

In many cases, bolts can be represented with CBAR/CBEAM elements. The technique has a couple of advantages, but also a couple of pitfalls."

Best regards,
Blas.


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Blas Molero Hidalgo
Ingeniero Industrial
Director

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