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Hi, Sorry to respond so late. However, despite my sincere efforts of reading, I still have not managed to understand how the authors was able to do it, as it is acknowledged as a common problem. Due to the formulation of differential stiffness matrix, we get only 3 rigid modes, the remaining 3...

I am using linear solver. The linear solution followed by modal analysis will have the effect of preload. The solver calculates first the element forces, then by using them, it calculates differential stiffness matrix. In the end, the modal solver uses total stiffness matrix.

@dmapguru Thank you for your suggestion. Unfortunately, I use Optistruct and this card does not exist in the solver. @jball1 The question is not about pure free-free, but a preloaded free-free analysis. In other words, you apply a loading to structure and then perform modal analysis to...

After spending my months to this problem, my research and trials lead me some conclusion. The differential stiffness matrix is definitely grounding the rigid body rotational modes. This can be easily verified by simple differential stiffness matrix of a beam. When you give rigid displacements...

I tried again to check my constraints. They were improper. After properly constraining the structure by 3-2-1 rule, it worked perfectly fine. I have now 6 rigid modes and they are all very close to zero. Edit: I was wrong. Essentially, this gave similar results with Inertia Relief. I still...

I tried this and ended up with the 4-6 modes are finite, although they seem rigid.

Thanks for your reply. I am using Hypermesh as pre/postprocessor and Optistruct as solver. I requested 10 modes. I am attaching .fem file too. It is a tank cyclindrical tank as follows: Standard steel properties with 50 mm diameter, 0.4 mm wall thickness, 6 mm cap thicknesses, 250 mm length...

Hello, It might sound weird but I would like to make a free-free preloaded modal analysis. Imagine that a rocket flies with its pressurized propellant tanks. I want to include the stiffening effect of the pressure. I tried two things: 1. Restrain the structure during static (preload) load...

Could you please elaborate what you mean by frequency response requirements? Do you need random vibration or sine vibration input curves?

Physically, what I learnt from Tom Irvine's notes is the following: These numbers give you a measure, how much you can excite the systems CoM by applying base excitation. In other words, it gives you a measure of which modes might play important role for your analysis/test. That's why there is...

I would highly recommend to read Tom Irvine's notes on this issue to gain insight and learn how each and everything calculated.

The levels given in SMC is acceptance levels. To achieve the qualification levels, you need to add 6 dB, which is 4 times the values you have it in the figure.

Have you tried to notch down the input around those frequencies? I would first do a low level sine test and get the transfer function. Then, do the notching in such a way that my vibration will still be in the limits of my desired application and its tolerances.