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Hello everyone,
I need a small information from your side. As i am performing modal analysis for my structure. My geometry is very simple made up of granite material. I would like to describe my problem in points, that would be easy to unterstand,
1. Firstly, i did the static analysis of my structure, where the outcome was acceptable. I mean to say the deformation (0.00155mm) and the stress (1.004Mpa) were under the limit.
2. Then i performed the modal analysis of my structure with same constraint and load collector, where i have selected 6 eigen mode. After the analysis I got the E1 (507.741Hz) to E6 (1358.87Hz) value.
3. Can someone explain me how to read the modal analysis results, because at each eigen value the results getting changed. Also, should I consider the deformation and the stress value from the modal analysis? Because compare to the static analysis their values are very high.
I would appreciate if anyone have a look on this concern. Thanks
SFM
Solved! Go to Solution.
It sounds like you may want to brush up on FEA basics. There are lots of training packages out there, some offered directly by PTC.
That being said, static analysis is meant for something that does not have dynamic loads. IE you put an object on a table and you want to see how much stress the table sees.
Modal analyses are for dynamics. IE you want a tuning fork to resonate at a particular frequency or you are designing a building and you want it to not have the first same frequency as an earth quake. Modal analyses tell you how flexible/stiff something is.
Dynamic analyses use modal analyses to determine stresses for an input range of frequencies. IE you have a rocket going up to space and you want to know what the vibration loads are going to do to your product.
It sounds like you may want to brush up on FEA basics. There are lots of training packages out there, some offered directly by PTC.
That being said, static analysis is meant for something that does not have dynamic loads. IE you put an object on a table and you want to see how much stress the table sees.
Modal analyses are for dynamics. IE you want a tuning fork to resonate at a particular frequency or you are designing a building and you want it to not have the first same frequency as an earth quake. Modal analyses tell you how flexible/stiff something is.
Dynamic analyses use modal analyses to determine stresses for an input range of frequencies. IE you have a rocket going up to space and you want to know what the vibration loads are going to do to your product.
I would beg to differ with Chris3 on this Modal question. If you are faced with a dynamic loading or response problem, then yes you will benefit from understanding the modes (E1-E4 or beyond in the OP's vernacular) as produced by a Modal study. BUT, I use Modal analyses for almost everything that we design or study. Ninety percent of our work is static structures or devices of some sort. Doing one right now where a steel fabricated pedestal is required to support a half-million pound turbine for maintenance. No motion...no dynamics.
Understanding the relationship between stiffness and mass (as well as the mode shape) is key to executing a good design. We aim to improve the stiffness of a structure by making changes that bump the first mode (natural freq) up from one iteration to the next. Modal studies validate our efforts.
Think of an engine stand in your garage...a post extending out of a substantial base with a Ford Cobra Jet 429 attached to it (yes, this will expose my age). Your post may not be over stressed by the 720 lb cast iron V-8, but if the assembly had a first mode (E1) of 1 or 2 or 3 Hz then you would not be as likely to lean on that torque wrench as much as you would if it were say 15 Hz. Customer perception of build quality is also impacted by these modes.
The Stress results are not really an intended or useful result quantity for Modal studies. Remember, the displacement during a Modal study is normalized to 1 unit. This means that strain and therefore stress are not really accurate. The locations and patterns of stress do represent interesting and useful quantities, just not the magnitudes.
I don't think @Chris3 was meaning to be exhaustive in his explanation. I think you answered the key question and that is modal stress results are not real or accurate numbers, They are just relative. You can get real stresses when you use the advanced vibration analysis types. (which are not available in the basic Creo Simulate license) If you look at the modal analysis definition there was nothing entered to describe the inputs or loads to compute the stress. This is done in the advanced vibration analysis types and those use the modal analysis as a basis for further results. Buckling analysis also uses the modal results but I would not think buckling was a concern for a piece of granite unless its for some tall stone building column. Below is a basic screenshot of the advanced vibration analysis types, as I do not know what shows up for lower level simulate licenses. These all require a modal analysis to be defined first.
@SM_10342271 If your granite is heavy you may need gravity self-loading in addition to any forces applied. Thanks for asking this general question. Feel free, If you can, to share specific difficulties or more details, or even model files as you go along.