NX Tutorial - Centroids and Area Moments of Inertia ENAE 324 Aerospace Structures Spring 2015

Transcription

1 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, 2 nd moments of area and product of inertia. 1. First, sketch your cross section the asymmetric U-channel from HW6 Problem 1 is shown below. Sketching in the X-Y plane is the most straightforward way to get the cross section s geometric properties, since the current version of NX projects sketches onto the X-Y plane to calculate these values (Sketching on any other planes gives incorrect values unless further steps are taken; we ll do the easy way first). 2. From the Analysis menu, select Advanced Mass Properties -> Area Using Curves. Click the Boundary (Permanent) button ( Boundary (Temporary) should work fine). 3. When the 2D Analysis window pops up, select the sketch edges to define the profile boundaries. 4. Click OK. Click OK again to set the tolerance (0.01 is fine). Click List All. 5. An information window will open giving centroid location, I x, I y, I xy, and principal axes directions. 1

2 1. Alternatively, if we sketch in the Y-Z plane according to beam analysis convention, we can get the same information along the way to our full simulation. 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. 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. 2

3 5. To find the 2 nd moments of area I y and I Z, as well as the product of inertia, I yz, 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 can Assign Materials to the beam now, but it s not necessary FOR THIS EXERCISE since I y, I z, and I yz 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. 3

4 11. With the *.fem file as the Work Part (see step 8), click the Mesh Type icon and choose the 1D Element Section option. 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. 4

5 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: 5

6 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 2 nd Moment of Area or Area Moment of Inertia ) are given in mm 4 since that s what units we originally defined for our model: 1 cm 4 = 10 4 mm 4, 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 prior to this one. 6