NX Tutorial - Centroids and Area Moments of Inertia
ENAE 324 – Aerospace Structures – Spring 2015
NX will automatically calculate area and mass information about any beam cross section you can think
of. This tutorial will show you how to display a section’s centroid, principal axes, 2nd moments of area
and product of inertia.
1. First, sketch your cross section – the asymmetric U-channel from HW6 Problem 1 is shown below.
We sketch in the Y-Z plane according to beam analysis convention. Before finishing the sketch,
click the ‘Analysis’ option from the top menu bar and choose the ‘Section Inertia’ option.
2. Select all of the edges that define the section (individually or by clicking and dragging a box around
them) and check all of the “Annotation” and “Output” boxes in the ‘Section Inertia Analysis’ box that
appears. Additionally, set the “Section Type” to “Solid”, then click the “Show Information Window”
button
in the box.
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NX Tutorial - Centroids and Area Moments of Inertia
ENAE 324 – Aerospace Structures – Spring 2015
3. You will now see a text box containing the location of the section centroid, as well as the direction
and magnitudes of the principal axes and an equivalent rectangular section aligned with the principal
axes. Within this rectangular section is a Sketch Note providing the same information. This is the
fastest way to double check your calculation of the location of the section centroid you found by hand.
4. Finish the sketch.
5. To find the 2nd moments of area Iy and IZ, as well as the product of inertia, Iyz, of this section, we’ll use
a blend of the simulation processes detailed in the previous NX Simulation Tutorial – Static Loads.
First, extrude the beam section along the X-axis to the desired length. For this example we chose 1m
(1000 mm).
6. Click the ‘Start’
button at the top left of the screen, and select ‘Advanced Simulation’ to
open the Advanced Simulation tools. Click the ‘New FEM and Simulation’
icon to create a new Simulation (*.sim) and FEM (*.fem) file set. In the ‘New
FEM and Simulation’ window that pops up, either click on your box beam
part to choose the ‘Bodies to Use’ or choose “All Visible” from the drop down
menu. Notice that the ‘Solver’ is “NX NASTRAN” and the ‘Analysis Type’ is “Structural”; we’re doing
basic static structural analysis, but NX can do lots more. Click ‘OK’.
7. Name your ‘Solution’ using your name and UID so that when you review the results this information
will be displayed and an easy screenshot is available. Note: If you want to go back to your drawing
at any point, use the Window menu (on the menu bar at the top) to view the solid model part *.prt.
8. In the Simulation Navigator window on the left, the “YourNameAndUIDHere_fem1.fem” file should
be highlighted in blue as the ‘Work’ part; if it isn’t, right-click on it and select ‘Make Work Part’. You
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NX Tutorial - Centroids and Area Moments of Inertia
ENAE 324 – Aerospace Structures – Spring 2015
can ‘Assign Materials’
to the beam now, but it’s not necessary FOR THIS EXERCISE since
Iy, Iz, and Iyz are geometric properties (and not material properties).
9. We’ll mesh the part by creating a custom beam section mesh element based on our shape. This
process will work with whatever section you have sketched, and is a very easy and powerful way to
get high-quality simulation results in very little time (i.e. a good thing to know for your real job).
10. Save your work.
11. With the *.fem file as the Work Part (see step 8), click the ‘Mesh Type’
Element Section’ option.
icon and choose the ‘1D
12. A window will appear that lets us define a customized shape
for our meshing elements. We’ll define this shape to be the
cross section of our asymmetric U-channel as follows:
a. From the ‘Beam Section Manager’ window that appears,
click “Create Section”
.
b. In the ‘Beam Section’ window that appears, choose “Face
of Solid”.
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NX Tutorial - Centroids and Area Moments of Inertia
ENAE 324 – Aerospace Structures – Spring 2015
13. Click the asymmetric U-channel face to select this face and define it as the custom beam section. Then,
click the Y-axis vector to choose this as the reference horizontal vector.
14. Click the ‘Preview’ check box at the bottom to make sure that your section is defined properly. It
should look like this:
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NX Tutorial - Centroids and Area Moments of Inertia
ENAE 324 – Aerospace Structures – Spring 2015
15. Now click the
button, and all of the
pertinent geometric information about your cross section will appear in a window like this:
Note: The Y- and Z-Centroid information is not correct in this case, and you should use steps 1-3 for
this information. I think this has to do with how NX redefines the origin when setting up the custom
beam cross section. It also looks like NX’s calculation of the shear center might be incorrect or not
relevant to the method we describe in class.
Note also: The Moments of Inertia (this is an inaccurate term, it should read “2nd Moment of Area” or
“Area Moment of Inertia”) are given in mm4 since that’s what units we originally defined for our
model: 1 cm4 = 104 mm4, so these values agree well with what you should have found by hand analysis
of HW6 Problem 1.
Double note also: You can now use this custom beam element to perform a 1D simulation of the
beam, as demonstrated in the NX Simulation Tutorial as before.
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