Normal Modes - Large Displacement

[janelson33]

Hello, I have an issue with a rather ridiculously large displacement after simulating the Normal Modes of my assembly. My first thought is that it's un-dampened, so thus that could be attributed to a crazy displacement alone, but I just want a sanity check.

The model aims to simulate a clamp design and it's intended hardware. This simulation is giving me new design insights about our bracket that holds the other part as well, but that's not the focus here. The tube is 4130, the clamp and ported part are 6061-T6, the bracket is 5052-H32 and the cap screw connectors are Alloy Steel pulled from the material library. All of the mechanical properties have been filled in from the Autodesk library; I also have inputted some S-N data for this study to evolve later, but that's not relevant for this post either.Meshed Model, Un-deformed

Total Displacement, Actual

I am basically following the Muffler and Pipe Tutorial on Autodesk's NASTRAN website with respect to the general setup and frequency range that I am allowing the solver to exercise on my model. The only thing I have changed from the tutorial would be the Contact Type; I've used a Bonded Contact with what I would estimate to be adequate Penetration Surface Offset and Max Activation Distance. There is also friction accounted for.

Can anyone give insight into why the tutorial used that contact type? I am guessing that it's accounting for welds/brazes? I also seem to remember something from my Siemens NX's NASTRAN training wherein for some cases, offset contact might be a better contact type to use, even if it is considered bonded in the real world. Lastly, I thought gravity was always supposed to be defined for a simulation study, of which that tutorial does not do until later?Contact Specification (all)

[John_Holtz]

Hi @janelson33

A normal modes analysis (natural frequency) only calculates the frequency and shape. The magnitude of the displacement has no real meaning because there are no loads that would cause a meaningful displacement. If the shape is realistic, the analysis is okay. If the shape is not realistic, it may be related to the material properties or (lack of) contact between the parts.

All assemblies require some type of contact to define how adjacent parts are connected together. For natural frequency analysis, the choices are bonded or offset bonded (because there is no real displacement, so separation contact does not make sense). Offset bonded contact is generally used for shell models where you need to transmit a moment from one plate part to another. You would use it in a solid model when there is a gap between parts. You are correct that the tutorial model would use bonded contact to simulate the welding/brazing between the parts.

For bonded contact, the maximum activation distance is the largest distance between a node on part A to the nearest node on the adjacent part B. Therefore, this value should be 10% to 20% larger than the mesh size on the contacting faces (plus any gap that exists between the parts). Therefore, I suspect your maximum activation distance of 0.001 m (1 mm) is much too small for your model.

Lastly, gravity is only needed when gravity is an important load. Modal analysis does not need gravity because the results are based on the mass which you entered as the mass density (and the stiffness which it gets from the modulus of elasticity). If I am remembering the muffler tutorial (which I could not locate offhand), it uses an enforced acceleration motion in units of g's, so the gravity constant is entered to convert it to acceleration units (in/sec^2 or similar).

[janelson33]

I've copied in paraphrased versions of your responses and then responded to those:

“A normal modes analysis (natural frequency) …l properties or (lack of) contact between the parts.”

          Ya I was tripped-up since I knew that it should be a dimensionless number yet the software was still showing meters. That extreme, yet non-significant value, led me to believe that it was an unrealistic in general, and something might be wrong.

          I believe I have the right types of contact applied for this study. Being that it’s a fixed tubular clamp, it’s for sure going to be a bonded contact between the clamp and my 4130 tube. It would be the same thing for the bracket that attaches to the clamp and ported part.

“All assemblies require some type of contact … that the tutorial model would use bonded contact to simulate the welding/brazing between the parts.”

          The only one I realized I might have forgotten/not sure if it needs to be included would be for the spaces circled below. I am not sure if the cap screw connector is accounting for that in some way?Considering Contacts: Offset Bonded?“For bonded contact, the maximum activation distance … of 0.001 m (1 mm) is much too small for your model.”

          I have my node spacing at 6 mm currently, thus I have corrected the Maximum Activation Distance to be .0072 meters now. On a side note; is Maximum Activation Distance more crucial to the study than the Penetration Surface Offset parameter, with respect to say the rational, expected outcome?

“Lastly, gravity is only needed when ... constant is entered to convert it to acceleration units (in/sec^2 or similar).”

          Ok, that makes sense to me, that seemed like a familiar detail I sort of remembered from lab when carrying-out frequency response studies.

[John_Holtz · Accepted solution]

Hi,

"The only one I realized I might have forgotten/not sure if it needs to be included would be for the spaces circled below. I am not sure if the cap screw connector is accounting for that in some way?"

Because the cap screw head is connected to the part on the right, and the cap screw threads are connected to the part on the left, the two parts are connected together through the bolt. But of course, the question is what effect does the direct contact between the faces of the parts have on the results, and what happens when it is not included. There is only one way to know for sure (run both analyses).

[janelson33]

Thanks for the clarification, that all makes sense to me!