Comsol Tutorial 1 – A simply supported beam modeled in 3D

In this tutorial you will create a fully 3-dimensional beam model using Comsol’s drawing tools. You may also import 3-D modeling files from Solidworks (IGES) .igs files.. For the projects of this course you may use either Comsol or Solidworks to create the geometries, however you should use Comsol for generating the mesh and you must use Comsol for the analyses.

Start Comsol 3.5a and open a New model > Comsol MultiPhysics > Structural Mechanics > Solid Stress Strain > Static Analysis. Notice the dependent variables are now u, v, and w. A 3D drawing grid will appear. Under Options, Preferences, Modeling, set the Default Unit System to IPS (inch Pound, Second).

In the left-hand drawing tool bar select the Block tool. Set the Corner Base-Point at 0,0,0 and the length to 4.0, 0.75 , and 0.75. This creates a solid square beam. Create a second square beam with Base-Point at 0,.75/4, .75/4. and length 4,.75/2, .75/2. In the graphics window, select both blocks then select the Difference icon from the left-hand drawing window. This should create a hollow box beam with wall thickness .75/4.

The beam is still prismatic. Modify it by cutting holes along its length. Select the Cylinder tool from the drawing bar. Choose R=.75/4, H=.75, and set the Axis Base Point to .5, 0, .75/2. Also set the Axis Direction Vector to 0,1,0. Hit the XZ-Plane icon from the drawing tool bar for a better view. Next make sure the cylinder is still selected then select the Array tool from the drawing toolbar. Set the displacement to 1,0,0 and the Array Size to 4,1,1. This creates four pretty big cylinders. Using Edit>Select All from the pull down, highlight everything, then select the Difference icon to cut holes in the beam. Return to the isometric view.

Finally, we need to create a place to apply our load. Choose the Block icon and place a box at 2-1/16,0,.75 with lengths 1/8,.75,.1.

Next set the boundary conditions. Under Physics>Subdomain set the material properties to Structural Steel (you may need to Load the property library). Select Physics>Boundary Settings and highlight the top of the loading section (boundary # 33) and apply 1000 psi downward. Next choose Physics>Edge Settings (new to 3D) and set edge #2 to a Constraint of Rx=0, Rz=0 and hit Apply. Select edge # 144 and set Rz=0; hit Apply. These edges are the contacts with the supports.

This recreates a pin condition on the left and roller on the right but leaves the y-direction free which is not good. An unrestrained direction allows a Free Body Mode (i.e. translation). Correct this by choosing points #1 and #81 and setting Ry=0 for both. Hit OK. This allows the beam to naturally contract due to the Poisson effect in the y-direction on the boundary.

Choose the Equals Sign (Solve) from the top menu and go to Postprocessing. Under General select Subdomain and Deformed Shape. Turn off Slice. Choose the Subdomain tab and examine the Sxx and von Mises Stresses.

Next select Postprocessing and Boundary Integration . From the Predefined pulldown select Face Load in Global z. Hit Apply and look for the “Value of Integral” in the information box at the bottom of the window. Remember that we put 1000psi on 1/8 in. x 3/4in. which yields 93.75 pounds as expected.

Finally, from Postprocessing>Subdomain select the Max/Min tab. Turn the Marker On and select z-displacement from the pulldown. Note that since there are no free points at the center line, there was no way to select a point for the max/min as down in the 2D model.