Autodesk Mechanical Desktop: Can You Take the Stress?
By Bill Fane
One of the many interesting tools in the Mechanical Desktop? Power Pack is the FEA (finite-element analysis) functionality. You can use it to perform load, stress, and strain analyses on a part.
This tutorial will not be a full lesson in stress analysis. It assumes you already have a working knowledge of this topic. What you will learn is how to use the FEA tools in Mechanical Desktop software to perform stress analysis operations on Mechanical Desktop parts.
View Larger Image Figure 1: A simple truss structure.
Let’s begin with a bit of background theory.
Calculating the loads in the members of a simple truss structure is relatively easy (see Figure 1). That calculation is really just basic Statics 101.
View Larger Image Figure 2: A complex part, subject to multiple loads.
On the other hand, doing a stress analysis on a complex solid link subject to multiple loads and anchorages could get a little messier (see Figure 2). In fact, it is probably impossible, or at least extremely difficult, to analyze such a structure using traditional manual methods.
View Larger Image Figure 3: The part represented as a finite-element mesh.
On the other hand (wait a minute; how many hands is that?), what if you replaced the solid link with a bridgelike truss consisting of a great many thin members like the structure shown in Figure 3? The math for solving this problem is quite simple; it just takes a whole lot of time if you perform the calculations manually. Getting Started Luckily, you don’t have to do that. Your Mechanical Desktop software can perform thousands of calculations in a few seconds. So replacing the continuous, homogeneous part with a mesh of small finite-sized elements, which you can then analyze very easily with the FEA tools in the software, is a quite viable option. The Power Pack option of Mechanical Desktop includes both 2D and 3D FEA tools. We start this tutorial with a simple 2D example, using many of the default settings that come with the software. Next month in part 2 of this tutorial we will look at some of the other 2D options and, finally, consider more complex 3D designs. 1. Start Mechanical Desktop from the Power Pack icon on your desktop.
2. Start a new drawing (in English units) using the Start from Scratch option.
3. Start the Rectangle command by selecting Design > Rectangle from the main menu.
4. Pick a suitable starting point, then enter @10,1 at the Command: prompt.
Beam Me Up... This rectangle represents the front view of a simple rectangular beam. Now you are going to apply loads and supports to it.
View Larger Image Calculation dialog box.;Figure 4: The FEA 2D
1. From the main menu, select Content 3D > 2D to switch the software into 2D analysis mode. The menu bar changes to show Content 2D. 2. Select Content 2D > Calculations > FEA… to start the AMFEA2D command.
3. Pick a point inside the rectangle, which opens the FEA 2D—Calculation dialog box (see Figure 4).
4. Enter 1 in the Thickn. d = text box on the Default pane.
5. On the Loads and Supports pane, click the Individual Force button. The FEA 2D—Calculation dialog box closes, and you are prompted to specify an insertion point. 6. Using an object snap, pick the midpoint of the top line of the rectangle.
7. Press the Enter key to accept the default value of 1000 pounds.
8. Press Enter again to accept the default rotation angle. The dialog box reopens.
9. On the Loads and Supports section, click the Fixed Support button. The dialog box again closes, and you are prompted to specify an insertion point.
10. Using an object snap, pick the intersection point of the lower-left corner of the rectangle.
11. Press Enter to accept the default rotation angle. The dialog box reopens.
12. On the Loads and Supports pane, click the Movable Support button. The dialog box closes, and you are prompted to specify an intersection point.
13. Using an object snap, pick the intersection point of the lower-right corner of the rectangle.
View Larger Image Figure 5: A simple beam with loads and supports applied.
14. Enter –90 as the rotation angle. The FEA 2D—Calculation dialog box reappears.
15. Drag it off to one side so you can see your drawing, which should now resemble Figure 5.
I Feel Stressed... Now that you have applied the load and supports to the beam, you are ready to do the stress analysis. 1. Drag the FEA 2D—Calculation dialog box back on screen.
2. In the upper-left corner of the Results pane, click the Isolines and Isoareas button. The FEA 2D—Isolines (Isoareas) dialog box opens (see Figure 6).