Week 4: Computer Lab

The TAs and I will be on hand to help you design your sustainable chair. To make this lab useful, you should have completed a large portion of the chair on your own time before the lab begins. By the end of class, you should complete the design of your chair, ensure that your chair is under the weight limit, and complete the stress analysis of your chair. The density of the foam board is approximately 0:136g/cm3. 

Think about what your desired mass should be.  
  • Does it make sense to have your CAD part be 498g?  How accurate do you think the density of the foam core board is?  Do you think there are tight tolerances on the manufacturing of foam core board that ensure that the density is always less than 0.136 g/cm3?  What would you do if your CAD part says it is 498g, but the finished product comes out at 505g? 
  • What about a mass of 420 g?  Does having a mass so low give you any advantage over a design that is 480 g?  Does it potentially put you at a disadvantage?


Stress Analysis The point of the stress analysis is to simply help you visualize problem areas in your chair. The actual chair will be made out of foam core board, which is a composite material. Autodesk Inventor cannot easily handle composite materials. We also do not have the a lot of the material properties of foam core board.
  1. Open one of your chair parts. I suggest starting with a support piece (e.g. a leg) and then moving on to a full assembly later.
  2. Assign a material to the part. It is not terribly important which material you choose. Again, the point is for you to visualize any problem areas (e.g. stress concentrations, members under tension) that might exist in your design, not determine failure.
  3. Go to the Environments tab.
  4. Click Stress Analysis on the far left of the ribbon
  5. Click Create Simulation on the far left of the ribbon.
  6. Click OK. For our purposes, the default parameters are adequate.
  7. Click Force to apply a force. Click on all of the appropriate faces and input a value. I will be sitting on the chair and weigh 80kg. That translates to approximately 800 N. However, that value is not that meaningful since your material properties are wrong. In the end, it does not really matter. Practically any reasonable value should work.
  8. Click on a ’Fixed’ Constraint and then select the surfaces with which your part would be in contact with the floor.
  9. Click ’Mesh View’ to see how Inventor creates the mesh. Creating a proper model typically requires good knowledge of how to create a proper mesh. For the purposes of this class the mesh that Inventor creates automatically is good enough.
  10. Click ‘Simulate’.
  11. Click ’Run’.
  12. Under ’Display’ click on the adjusted x1 tab and change its value if needed.
  13. Examine the results. Look for areas of high stress. Are there any critical members that are in compression? You may need to strengthen those areas. Are there any parts that do not seem to be taking any load? You may be able to remove those or at least lighten them.
  14. Now try your chair assembly. Repeat the same steps. The simulation will take several minutes to run.