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Bolt pre-load in modal analysis

Hi everyone!

I'm using Siemens NX 10.0 to perform a finite element analysis on a single-bolted joint. My goal is to get the pre-stressed natural frequencies (with changing bolt pre-load) and, at last, the frequency response functions.

Unfortunately, I haven't been able to even get the natural frequencies with changing bolt pre-load. I'm using a model with random dimensions based on an example I watched on Youtube by Matthew Jackson:

I'm modeling the bolt with a CBEAM element connected to the edges of the holes with RBE2 rigid links (see picture below). I've also defined a surface-to-surface contact between the two plates.FEA model of the bolted jointFEA model of the bolted joint

I've uploaded the NX .prt, .fem and .sim files to this discussion.


When I perform a SOL 103 REAL EIGENVALUES analysis with a first statics subcase, the natural frequencies keep constant even when I use bolt pre-loads as different as 1N, 10N, 100N or 1000N.


Can anyone please help me? Thank you so much!



Re: Bolt pre-load in modal analysis

Well, modal analysis disconsider loads inputs. You may try to use displacements instead induced by your desired load. I´m not sure if it works in NX.



Re: Bolt pre-load in modal analysis

The differential stiffness affect of the bolt preload on this geometry is only going to affect the structure immediately surrounding the hole. It is not going to significantly affect the global stiffness of the structure, so the modal frequencies are only going to change slightly.


If I run your model with and without preload affects included, I do get a small change in frequency:


No Preload     Preload
 272.4328     272.4325
 582.9326     582.7915
 888.0516     888.0097
 972.6120     972.6116
1616.2330    1616.2320
1687.2600    1687.2600
1853.9370    1853.9280
1923.0700    1923.0430
2058.8290    2058.7660
2065.3780    2065.3630


In the attached model, in the "All Subcase Preload" solution, the preload affects the stiffness of the entire structure, so the change is frequency is much more evident.

Re: Bolt pre-load in modal analysis

I´ve tried to run a structural analysis with bolt pre-load and then run modal analysis with those pre-stress and got a little more sensivity results. I don´t know to run with Nastran, I used Ansys. But for small preload range the results do not change significantly.