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Units of measure

Creator
Creator
I'm very new with Femap and was wondering where the unit of measure is viewable. I'm using Solid Edge. Does this mean that the units are taken from the part file when imported. The numbers in the Stress Coutours on the coloured bar graph on the right hand side. What units are they displaying? Is this Mpa ? Please help

Kind Reagrds
Nev
14 REPLIES

Re: Units of measure

Solution Partner Valued Contributor Solution Partner Valued Contributor
Solution Partner Valued Contributor
Femap is "unitless" - but this means you must use a consistent set of units. eg. SI = N, m, kg, Pa. Most engineers prefer mm. If you also prefer to use N, then your consistent set of units would be N, mm, mm/sec2 (for accel), MPa, and tonnes (because F=ma). Thus, if you prefer to use N and mm, then material density must be in Tonnes/mm3. For example, steel would be 7.8e-9. SolidEdge might be using something different.

It might seem a little inconveneient, but it is simple once you know what you normally use. And I have repeatedly seen systems that claim to look after all of this for you to get it wrong once it comes to FEA!

Hope that helps.

Re: Units of measure

Siemens Phenom Siemens Phenom
Siemens Phenom
Solid Edge, like most CAD packages, stores the solid internally in meters. Parasolid works within a +/-500 box, where the algorithms tolerances have been written such that everything works. In a +/- 500 meter box, one can fit most engineered systems. When you bring geometric data in to FEMAP, FEMAP has a default scale factor, it's set at installation to be in inches (39.37). If you're a metric person, one can go in to File - Preferences in FEMAP, and on the Geometry Tab, change the scale factor to 1000.0, this will display geometry 1000 x the internal value, or for meter based geoemetry, in mm. After you set this preference, stop and restart FEMAP and bring in your SE geometry. I would always double check and measure a thickness in your model and make sure everything consistent. Once you have a length unit defined, you just need to be consistent from then on with your materials, loads, etc. so you know what units your stresses are in.

Mark.

Re: Units of measure

N/A
I was actually wondering this same thing myself. So, I imported a model that was created in inches, and entered the material as 6061 which correctly gives yield to be 35000 psi. I applied a load which I defined as a force applied to an edge and entered my value in lbs. I was assuming that the Femap was going to output stress concentrations in units of psi. Is this correct? The outputed values seem high.

Thanks,
Mike

Re: Units of measure

Siemens Phenom Siemens Phenom
Siemens Phenom
It sounds like you have everything in consistently, so yes, the results should be be in PSI. If you want me to take a look at your model, e-mail it to me and the FEMAP team can give it the once over and let you know if we think everything looks set up correctly -

sherman.mark@siemens.com

Mark.

Re: Units of measure

Solution Partner Valued Contributor Solution Partner Valued Contributor
Solution Partner Valued Contributor
Mike,

It sounds like you probably have everything correct, and results would thus be showing be in psi. However, please note that if you have any gravity loads, then consistent units would have your acceleration as inches/sec2 and density would be in "snails/inch3". This is because pound is NOT a unit of mass (never was, never will be), it is a unit of force. A "snail" (sometimes "slinch") is the amount of mass which accelerates at 1/in/sec2 when a 1lb force is applied. A snail weighs about 386.1 lbs force on earth (32.1 ft/sec2 * 12in/ft).

Thus, to summarise if you are using inches (but note that you can get Femap to scale the geom to metric when you import it!!!) and lbs, then pressures should be applied in psi, and stress results will be in psi. Deflection results will be in inches, masses and densities should use snails and snails per inch3. A gravity load would be 386.2 in/sec2.

Hope that helps!

Re: Units of measure

Creator
Creator
Wow, Thanks very much to you all for your reply's this helps very much. I now have a much better understanding and will give this a try.
Thanks again
Nev

Re: Units of measure

N/A
Femap is "unitless" - but this means you must use a consistent set of units. eg. SI = N, m, kg, Pa. Most engineers prefer mm. If you also prefer to use N, then your consistent set of units would be N, mm, mm/sec2 (for accel), MPa, and tonnes (because F=ma). Thus, if you prefer to use N and mm, then material density must be in Tonnes/mm3. For example, steel would be 7.8e-9. SolidEdge might be using something different.

It might seem a little inconveneient, but it is simple once you know what you normally use. And I have repeatedly seen systems that claim to look after all of this for you to get it wrong once it comes to FEA!

I can (almost) understand unitless CAD systems where all you are dealing with are: position (points), paths, areas (surfaces), and volumes. However, the moment you add mass, force, and gravity accelerations, the reasoning becomes a bit more specious in my estimation. A clearly defined default unit set seems to be an advantage here.

So, if my preference is to work with: inches, pounds-force, pounds-mass, and gravities, how do I convert unitless values to those I prefer?

Re: Units of measure

Solution Partner Valued Contributor Solution Partner Valued Contributor
Solution Partner Valued Contributor
A "default" system might be a problem, particularly when the world has not yet agreed on a universal set. And I have personally observed high quality CAD-based FEA systems getting niche details wrong when they attempt to insulate the user from this work.

But to answer your question, a consistent set of imperial units if you like to use inches is: inches, pounds force, slinches (pound is never a real mass, and is not consistent with F=ma for your other chosen units) and inch/sec2 for gravity/acceleration. The by-product of this is pressures in psi, stress results are in psi, densities in slinch/inch3 . As most reference sources do not provide information in such consistent imperial units, then conversion is required from those sources. Conversion factors are readily accessible via google search.
If you are adamant about using lb-mass, then your choices are: (a) don't use seconds for time; or (b) don't use inches for length or (c) use PARAM WTMASS in your Femap/Nastran runs, which I would recommend only for highly experienced FEA analysts. It internally multiplies entered "pseudo-mass" units (like pounds) by a factor (often 386 lb-mass/slinch or 32 lb-mass/slug) so real consistent mass units are actually used.

I hope that helps.

Re: Units of measure

N/A
Let's start with the basics. I load a model that was constructed using INCHES as the base dimension. Because the material information is NOT converted during importation into Femap, I select a value from a (limited) list. The analysis is completed, but the values presented to me DO NOT HAVE UNITS DEFINED. How am I to know what units are being used?
Now, 1 lb = 1 lbm * 1 g (386.09 in/sec²). My "point" is that ALL of these values are merely "masks of convenience." I understand that SI-KGS units are more convenient for calculation and are the generally accepted international standard. That is fine, but the unit Newton (and everything that derives from it) is really poorly "meshed" for real world applications. (I can go on my diatribe about German, French, and Japanese engineers blowing up test equipment back when I was the chief mechanical engineer on the program that developed automotive airbag restraint systems when calculating pressure loads in Pascals, if you wish.) The "issue" is presenting INFORMATION in the manner and format with which the USER is most comfortable!
MY question is, when a MODEL is imported in inch-based units, what are the REST OF THE UNITS that are DISPLAYED by Femap?