metric scaling issues 1

[darcyo]

Hi there. i am new to Nastran, and i am importing large assemblies from Pro/E, using iges format (the parasolid woudl not import), and i have been successful creating meshes, applying contact surfs, etc. i would like to be working in mm\N\s\Kg (metric). i am using 72e9 for E of AL7005. my stress result look somewhat as expected, though my displacements are way too small, in the 1e-6 range. i understnad that Nastran does not understand units, but can someone point me in the direction of getting my disp's as expected. So i'm assuming that it is a scaling issue, but where do i change it - is it Solid geometry scale factor, in the prefs menu?

oh yeah, i made a 100unit(mm?) long bar, with the same mat prop as above, and applied a 1000N force to the end of the bar, and my disps were still 1e-5mm.

thanks for your help.

 

[johnhors]

darcyo

You are using the SI value (Newtons per square metre) of Aluminium, in your chosen metric system it is 72000 Newtons per square millimetre. A factor of million out!

 

[JakeLiv]

With nastran, units consistency is the major pitfall of ALL beginners.
In all calcualtions, write down the units used for input - be careful with density when doing modal analysis, E and G in general properties, and note the units of force/pressure used.
If you are doing vibration analysis (MSC.RANDOM) check ALL your units, or you might get obscene results.

If your units are consistent, you can begin searching for structural model problems using the results, but if the units are garblesd then you can't draw any conclusions from the model.

 

[darcyo]

thanks guys. so Nastran then uses Standard International (S.I.) and standard Imperial units then? because, SI units are N m s, not N mm s.. guess that is because 1MPa is 1N/mm^2...

so is there a reference page that either of you know of with Nastran's system of units?

much appreciated,
darcyo

 

[JakeLiv]

nastran has no units.
you can do lb/mm^2 if you want...
thats the pitfall.

 

[johnhors]

Darcyo,

As JakeLiv says Nastran in common with ALL FE software that I know of is unitless. It is 100% the responsibility of the user to be working in a consistent set of units. However dynamics software like MSC/Adams allows the user to choose from a list of consistent set of units.

 

[Aerodesign]

The key is to use consistent unit, for metric units, adopt mm, N, s, Mg (yes thats megagrams), you will get out N, mm, MPA (N/mm^2) and masses in Mg. This becomes particularly relevant if you are using an impact code with constraint staht include velocity (mm/s) or accelerations (mm/s/), so 1g (gravity) is 9810 (mm/s/s). Now all you have to do is sort out the signs.
Hope this helps.