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Handling of input and output units

[ Edited ]

We are processing a linear static structural problem using NX Nastran within NX 8. We were given the NAST and OP2 files for the problem. The data in the NAST file is stated in the consistent units of N, mm, s, and Mg. The question I have is this: Are the following steps correct? When we import the NAST file, we should select (N)(mm)(tonne) under the "Input File Units" entry. When we proceed with "Imported Results," after selecting the OP2 file, we should, under the heading of "Units," select "User Defined."  Then, we should change the units to:

 

Mass       t

Length  mm

Time      sec

 

The strange outcome is that the displacements computed as above are same as those obtained with the default unit settings. The stresses however differ by a factor of 1000.

 

2 REPLIES
Solution
Solution
Accepted by topic author EcoCAR
‎08-26-2015 04:32 AM

Re: Handling of input and output units

The default metric unit system in NX is (mN)(mm)(kg)(s) so if you import a .op2 file into a metric .sim part, it will assume those units unless told otherwise. The user can manually override this as you describe.

 

When you post-process, displacements will be in (mm). Forces will be in (mN) and stresses are force/length^2, so they will be in (mN)/(mm^2), which is (KPa).

Pay attention to the units shown in the post header. NX will divide the values by 1000 and present them as MPa. You can edit your PostView and set the results units to (mN/mm^2(KPa)) to see values consistent with what is imported.

 

When you import results and set the units to user-defined and specify (N)(mm)(t), the length units are still (mm), so the reported displacement values will not change. Force units have changed from (mN) to (N), so force results will differ by a factor of 1000. Stresses would also change to (N)/(mm^2) which is (MPa).

 

Re: Handling of input and output units

Thank you for this write up...its a little too comples for me, is there a simpler way to explain it?

I import a model into Femap

I measure the cross sectional area...because the geometry is developed in Solidworks I already know the area in advance....so this is a kind of confirmation

Now I am sure length is in mm

And from what I have seen so far, if length is in mm, force is N and mass is tonnes (a scale factor of 1000) is automatically displayed when importing the model,

So if length is still mm, does it mean the force is mN?

Can you explain with screen shots?

 

Thank you