Measurement Testing

TimH · Guest

Finite Element Analysis (FEA) is used for the determination of stresses and displacements in mechanical objects and systems, as applicable in vibration testing. Software programs tremendously aid the engineer or scientist when using this mathematical tool.

Open-source finite element software include:

* Z88
* SLFFEA
* YADE
* FEniCS
* deal.II
* freeFEM

Sever commercially available FEA software packages are also available, including ABAQUS, Ansys, Nastran and Marc.

* NEiNastran FEA
* ABAQUS, Inc.
* NAFEMS - The International Association for the Engineering Analysis Community
* ANSYS, Inc.
* MSC Software (Nastran and Marc)
* LSTC (provide LS-Dyna)

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So, what is the "best" vibration FEA solver software for problems related to sound, noise and general vibration? That is one that gives reliable results, even for complex materials.

And, perhaps, specifically modal and forced vibrational analysis with an emphasis on acoustics.

What modern software can integrate vibration analysis with sound?

This program should allow material description to be as accurate as possible, i.e. complex materials like resin and fibers, foams, composites, etc.

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Suggestions:

First try this free FEA software called LISA from:

http://www.freebyte.com/cad/fea.htm#fea

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You may then want to move up to MSC (Nastran) for the vibration analysis; in turn, the results can be imported to LMS Sysnoise for the noise analysis. The two software suites are purported to work well together.

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Most, COTS (commercial of-the-shelf) FEA software suites will give you "reliable" results. "Best" FEA software and is hard to quantify. Ultimately, most programs will do the job for you, but it's down to the experience of the user whether or not the results are uselss or useful.

Some vibration engineers have been using ANSYS and ABAQUS for many years, as well as a few other lesser-known programs, such as NASTRAN, PAFEC, DIANA, NISA, etc. For a low-cost package, check out NISA, which may be somewhat dated by now. If you have complex calculations, NISA has a limited geometry integration capability, so this is something to thconsider. It does however, have a good eigenvalue solver, as a few claim, along with NASTRAN. ANSYS is also reported to be excellent. With this program, you get very good functionality (forced vibration, acoustics, emag, statics, transient, coupled-field, &c.), an GUI and superb CAD integration. The flexibilty of using ANSYS programming language as well (APDL) is also very useful. Many engineers like ANSYS, and been using it for a long time. Drawback of ANSYS, NASTRAN, ABAQUS is the cost of licence. If you want just a dynamics solver, it may be worth shelling out extra for NASTRAN. Otherwise, have a look into ANSYS. You can check ANSYS capabilitites here:
http://www.ansys.com/assets/brochures/capabilities-90.pdf.

This list is certainly non-exhaustive, and only a smal piece of the stuff available. Try free demos until you find an FEA tool you like.

Does describing mechanical properties varying with frequency lead to more accurate results for some applications? In MSC Nastran, there it's possible to describe material properties. This program is not available in a demo version. Also there is Cosmosm which does not have this material properties description feature, but has creep and is avail as demo.

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In the big three -- ANSYS, ABAQUS, NASTRAN -- one can change some frequency-dependent material properties, but the properties that can be changed are very limited (and, reportedly, for good reasons). In ANSYS, however, there is an undocumented procedure for changing the other mechanical parameters (mu, E, etc.) in terms of frequency; it's a cheat, and you get a warning message, but it certainly works and, purportedly, lots of people that have used it.

One can easily set the damping characteristics as a function of frequency legitimately (as a material property) within ANSYS and ABAQUS (but unsure of COSMOS) and probably NASTRAN. These are the Rayleigh damping constants (alpha/beta damping) and the material damping.

Naturally, the above three programs also have all of the common material models (elasto/visco/plastic/etc.) such as creep as well as the capability of user materials/compound (hybrid) models and composites. Reportedly, ANSYS, in particular, covers the use of composites very well in the current version.

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You may also design ad hoc code for noise and vibration models; then, Nastran can be used to generate the linear bits, and then combine the results from Nastran together with a systems model that specifically allows for frequency (and other) dependent properties. This then allows one to drive the system with measured force or vibration spectra, and generate sound spectra.

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On an email list someone said that they use NEiNastran for complex dynamic models (aerospace) and that it works as good or better than MSC and other FEA products his company also have in-house, such as ABAQUS. One of the major strengths of Nastran in general is dynamics. NEiNastran supports residual vectors which this engineer has been using recently to help with the higher frequency modes. He reports that there should also be no problem using acceleration test data.

Digging around further on other message boards, there were suggestions for:

Vibration: ANSYS/Nastran
Acoustics: LMS Sysnoise (This has been proven and tested for several years and is the most widely used acoustic software in the world).

Vibration: NEiNastran
Acoustics: VNoise. It is a fully capable BEM code that has a direct interface to NEiNastran and FEMAP. It is a fraction of the cost of Sysnoise.

Viration FEA software: not sure, but:
Acoustics: ABAQUS also has a full set of acoustic elements which integrate seamlessly with the more conventional structural elements. People use them to analyse the noise in car interiors for example.

One final note of "caution" that many FEA engineers state is:

There is no universal "best" FEA software, and even "good" software depends on so many variables.

A more sensible question is to ask folks to recommend a FEA (or whatever) package. Better yet, ask: "I'm thinking (there's a good clue) about buying FEA package "XYZ" -- how robust is this package? What sort of problems might I encounter?" Why is this a better strategy? It shows you've taken the time to research the question a little in Yahoo or Google.