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Jagon (Aeronautics) (OP)
4 Apr 08 5:59
Hi,

I must say I am quite a newbie using Femap (9.2 with nx nastran). I have to understand the whole functionning of this soft all by myself. And of course I encounter some trouble.
My request is simple ; it's about the unit system.

-What are the base units of this soft?
-How can I change units (I'd like to switch to international system (m, N, Kg,...))?

Thanks in advance

julien
Jagon (Aeronautics) (OP)
4 Apr 08 7:54
Just simply : how to display units ?

Julien
Helpful Member!  ThomasH (Structural)
4 Apr 08 10:30
Hi

The units is up to you. Basically meaning that you need to be consistent.

If your beam is 1 "something" long you have to choose meters or feet or whatever.

Personally i always use the SI-system meters/Newtons/kg etc.

I'm not sure how to explain it clearly but Femat as such does not have any "preset" units.

Feel free to ask more

/Thomas
Jagon (Aeronautics) (OP)
4 Apr 08 11:07
Thanks.

But the thing is when I import an .igs or a .model (drawn in milimeters on another soft), Femap turns it into inches.
i.e. : A 1000mm long beam on Catia V5 becomes a 39.37 "something" long on Femap.
So I have to set a 25.4 mult. factor to lengths in the converting tool in order to have a 1000 "something" long beam.
This seems not to produce good results. For example, I apply a 100(N) bearing load on a 10x10(mm) section beam. The stress I find should be F/S=100/100=1MPa, but it is not.

quoting from you : If your beam is 1 "something" long you have to choose meters or feet or whatever
-> how can I choose meters or feets ...?

And finally, how do you proceed usually?


Regards,


Julien


(Sorry about my bad english)
brkica (Mechanical)
4 Apr 08 12:44
Jaqon,

If you import something which has been created in mm's in CAD, you should indeed in Femap apply length multification factor 25.4 in order to arrive at mm's in Femap. And applying an axial load of 100 N to a beam with cross section 100 mm^2, should indeed give a stress of 1 N/mm^2 = 1 MPA. On the other hand, you can when you import the .igs or .stp file in Femap replace the factor 39,37 with 1. In this case you get units in meters, provided that the units in the CAD file were mm's.

In the above case, if you had applied a force in kN (thus 0.1) you would get stress with unit kN/mm^2 = 1 GPa.

So you don't have the option to specify your units in i.e. mm-N-kg or whatever. You only need to make sure you are consistent with your units.

I will post you the .mod file of a 3D solid beam tonight.

Regards.


 
ThomasH (Structural)
4 Apr 08 12:45
Ok

First of all, I don't work with Catia so this is partially guesses.

When I import a geometry model there is a scale factor. I set it to an appropriate value. Sometimes I just import and se "what happens" then I can set an appropriate value and do a second import.

Practically, the people I work with draw or model in millimeters. I import the model and usually get the 1000 mm long beam to be 1000 "something" long. Then I scale it down by 1/1000. That means that the beam is 1.0 "someting" long.

Then when I apply loads, masses etc and assume that the "something" is meters. So I don't tell the software that the length unit is meters. But based om my assumptions with length in meters and forces in newtons the resulting stresses will be in Pascals.

There are softwares where you use a specific unit system but Femap/Nastran is not one of them. That is why it is so important to be consistent. Especially when you work with dynamics.

As for your calculated stress I don't se anything wrong in your statement. Have you tried "measuring" the model?

Good Luck and feel free to ask more.

Thomas
brkica (Mechanical)
4 Apr 08 14:01
Jagon,

Here is the .step and .mod of beam. The geometry was created in Pro/Engineer (units: mm-N-s). Using these units, you can verify that the mass is in tonne, but is not important as it is static analysis in this case.

Geometry beam: L x W x H = 150 x 10 x 10 mm (in Pro/E).

When inported in Femap, the lentgh is manually scale with factor 25.4 (1 inch = 25.4 mm), so Femap shows also 150x10x10.

Force applied: 100.

-> Calcualated stress: 1. The stress has units N/mm^2 as you assume length units in mm and force units in N.

I usually work with Marc&Mentat, and Mentat neither has preset units, but when consistent units are used, the results are the same of course.

Regrads.

 
brkica (Mechanical)
4 Apr 08 14:04
Here is the .step file
brkica (Mechanical)
4 Apr 08 14:05
STEP file
brkica (Mechanical)
4 Apr 08 14:07
For some reason I am not able to upload the step file. Here is  an attepted with the step file zipped.
Jagon (Aeronautics) (OP)
7 Apr 08 3:13
Thanks a lot for your quick and precise answers.
You rock.

Julien

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