[Structral 3.5a] How do you validate your models ? (2024)

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Hello everybody

First, I would suggest that we all start our subject line with a short summary of the application mode and the version we use or to which we relate our discussion, this would help us others to sort things and to give better replies.
Either by mentioning the specific application mode i.e [smsld 3.5a], or at least the global title in the subject line, knowing that the more precise we are more and better replies we could expect.

" Can we all survive with this ? "

Back to my specific question, (by the way Niklas I'm sure you are out here reading our comments, this could also be a subject for your Blog) :
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I'm interesting to know and to exchange means how you verify and validate your structural models, how are you sure that the model is set up correctly?

Aspects such as: no over-constrained BC's, you are asking for sufficient many modes, i.e. not overlooking something, then only you might "solve" and validate your model against your measurements.

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Update: following NAFEMS definition see:

www.nafems.org/downloads/working_groups/amwg/4pp_nafems_asme_vv.pdf

Definitions: for me "verification" of the model is checking that I'm coherent with the physics and I'm respecting the global hypothesis and limitations of the application mode. I start with this even before I solve the model. Thereafter, I "validate" my model that is: I compare my solved model results with my measurements to check that my "validated and solved model" is truly representative of my existing device being modelled.

The COMSOL documentation talks about how to do multiphysics for the Scientists, I'm still searching the specific chapter(s) on model verification methodology for us Engineers ;)

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My first steps are systematically:

upd: 0) I import generally my CAD geometries from SolidWorks or another CAD tool to have independent volume, mass, centre of gravity (CoG) and inertia tensor calculations.

1) set up the model, the BC's, the material etc, mesh it and solve for "Get Initial Values" (just to fill the matrices).

2) check the mass against my CAD model values by: "Postprocessing Subdomain Integration" insert the density expression "rho_smsld" (or whatever application mode you use, but if you have several you must make a global variable i.e. "rho_all" and set it (Options Expressions Subdomain Expressions) and give the specific densities for the different subdomains, respectively shell, beam ... domains and do not forget any masses added to specific "points", if applicable). Then select all subdomains and "apply".
My tolerance here is typically < 5% difference, but mostly I'm around 1% or lower.

3) check the Inertia tensor against my CAD model values by "Postprocessing Geometric Properties", insert (again) the density expression "rho_smsld" (or whatever ... as above) ).
My tolerance here is typically < 5% difference, but mostly I'm around 1% or lower here too
Often you have to play with the coordinate system in the CAD, as COMSOL does not allow you to choose any coordinate system for the inertia, its all in the default coordinate system, w.r.t. the CoG (would be easy to update though in COMSOL).

4) Check the CoG, (and total Volume and Surfaces if applicable) here too I expect < 1% difference, otherwise I have misplaced a material or my meshing is so gross that I have lost significant items.

Note: the beam and shell modes do no allow you to calculate by COMSOL directly the inertia; you must go back to the subdomain integration and apply the full inertia formulas, just as if you want the results for a specific coordinate system).

5) I run a modal eigenfrequency analysis over the first >6 modes, none should be close to 0 Hz (typically none <1Hz if I expect my model to be fully constrained in 3D)
By the way, an additional question: how many modes to consider, how to, in COMSOL without any mass participation factors available, should one decide that most relevant modes stop at X Hz or after N modes ?

upd: 6) Static load check: by applying an acceleration of 1m/s^2 in x,y,z one should get reaction forces corresponding to the total mass of the model.

7) required for thermal analysis, but I mostly do it systematically: I go to the Subdomain Settings, I select all subdomains and set the thermal expansion to the same value i.e. 1.2E-5 [m/m/K], then in the "Load" tab I set include thermal expansion and put 100°C temperature difference between "Temp" and "Tempref". I solve and check the stress build-up, there should be none if my model is correctly constrained.
Typical tolerance: strain energy < 1e-3 [J].

8) ... more for specific needs, but let’s stop here, just now, and get your feedback.

Now I'm finally ready to solve, analyse my results and to validate my model.

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What about your ways?

Expecting to read you soon
Ivar