Learning Forum

[Mans1993]

Dears

i am trying to simulate a system subjected to random vibration, i can use the random vibration analysis but the issue is that i have several screws which are prestressed and i am interested in their capability of handling the random vibration so i modeled them and applied several non-linear contact (frictional and drictionless).

i know that i can do it in transient structural, but the problem is that i have the load in as frequency based PSD.

[peteroznewman]

All, posted in this discussion: /forum/discussion/comment/132874#Comment_132874
where I replied once, but now I have found the same basic question in this new discussion, I will only answer here.
I haven't seen the model of the structure that has the bolts in it, but I would not go directly to a nonlinear transient structural simulation. Is it known that the bolt was not broken after the sine sweep?
Let's assume the bolt broke during the Random Vibration test. What was the duration of the Random Vibration? How was the duration determined? How was the PSD profile determined?
Is it known that the bolt was correctly torqued before the test began? What else besides torque was applied to prevent bolt loosening? For example, a thread lock adhesive, other mechanical device. Were the other bolts checked for loss of preload?
Let's assume that the bolt was correctly installed into the structure according to the specified torque and that there was a thread lock device/adhesive. Let's also assume that the Random Vibe profile and duration are appropriate, therefore the issue is the bolted joint design.
To design a bolted joint, you need to extract the forces and moments that go through that bolt during the Random Vibration profile. Since Random Vibe is a statistical result, you need to take 5 sigma values to design with.
Once you have the forces and moments, check that the bolt size selected and the preload created with the appropriate torque for that size will support the maximum expected forces. Specifically, that the normal forces do not allow the joint to open and the shear forces do not allow the joint to slip. For this you need to know the coefficient of friction between the faces that are being bolted together.

[Mans1993]

Dear Thank you for your helpful reply.
i will try this approach !
is it better to 3d model the bolt to get the forces and moments or should i model it as a line body?

[peteroznewman]

To extract the forces and moments going through a bolted connection, a simple approach is to use a Fixed Joint. Put the joint coordinate system at the center of the circular edge at the interface between the two bodies being fastened. In the Solution, output the Joint forces and moments going through that Fixed Joint. If you have solid elements, you would scope the mobile side of the joint to the cylindrical face of the hole in one side and the reference side of the joint to the cylindrical face of the hole in the other face. You don't model the bolt at all during force extraction.
After you have the forces and moments going through the fixed joint from the Random Vibe solution, build a Static Structural model, delete the Fixed Joint and add a 3D bolt model with pretension applied to show how the bolted connection can support the forces and moments.

[pasagadak]

Hi All,
I've been following this post since I am dealing with a similar Simulation.
I have a component bolted to a fixed planar support with the use of bolts. The goal of the simulation would be to calculate the stresses in the component after the support is subjected to a Random Vibration (I have an input PSD G acceleration for this).
I have used static structural to model the bolt pretension, then a Modal analysis to extract the natural frequencies, and finally a Random Vibration module.
When the component body is scoped for stresses, I get stress values much lower than when pretension is applied. I am not sure how to verify the model for its accuracy. Can you please help?
Please let me know if you need any additional clarification.
Thank you!

[pasagadak]

[peteroznewman]

Using Bolt Pretension in Static Structural creates a significant stress around the bolt and flange.
Using the solution to the Static Structural as the Pre-Stress environment to a Modal analysis only takes account of the stiffness of the structure due to any loads applied in Static Structural, but it does not also take the stress into the Random Vibe solution, which starts with zero stress.
Therefore it is normal and expected that the stress around the bolt and flange could be lower in the Random Vibration analysis than it was in the Static Structural analysis.

[pasagadak]

Thanks for your reply .
Is there a way I could I modify my simulation such that I can capture the max stresses occurring during random vibration while the bolt and flange are still under a pre-stress condition (instead of starting from zero stress)? TIA.

[peteroznewman]

Random Vibration is a statistical result, so there is no such thing as a maximum stress. You can request a 1-sigma stress, a 2-sigma stress, a 3-sigma stress, a 4-sigma stress etc. Each sigma level capturing a higher and higher percentage of the distribution.
You can take stress component results from the Random Vibe at a particular sigma level and add them to the absolute value of the stress component from the Static Structural. I say absolute value because the stress component in Static Structural can be positive or negative, while the stress component from Random Vibe is only positive. You should do some research on this topic yourself as it is not simple.