I'm working on a final report for the mechanics of material course, and the project was to choose a structure or mechanical element and carry out stress analysis using solidworks and then confirm results with hand calculations. I have chosen a 4-wheel mobile robot structure and worked out the numerical analysis, but confused with the analytical requirements which are:

  1. The generated six independent stresses.
  2. The generated six independent strains.
  3. The generated three principal stresses.
  4. The generated maximum shear stress.
  5. The generated equivalent Von-Mises stress.
  6. The resulting displacement.

I have assumed a load of 500N on the upper surface of the structure and don't know how to start.

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The motor shafts are assumed to be fixed in the wheels with bearing supportenter image description here

If you have a resource for a similar problem or you have a clear intuition, I would be grateful to receive your help.


A couple of comments:

  1. As a general recommendation, you should not include components like motors in a finite element analysis. You should only include the components for which you are interested in calculating the internal stresses. If you include the entire motor bodies you are not only adding unnecessary computation time but you are most likely adding inaccuracies as well since you do not know the actual stiffness of the motor and there are multiple materials included in it which you must assign independently, it is not ok to assign a single material to the entire engine body. If the motors add substantial weight on the frame, you can replace them by their equivalent masses in SW simulation, look it up.
  2. As for the analytical requirements, you know the usual drill: Free body diagram(s) must be your starting point, this is true in college and at work. This means that you probably have to make educated assumptions, come up with a simplification of the real structure.
    1. Since this frame pretty much resembles a 3D truss you might try two different approaches: You can treat it as a 3D frame and solve for the internal forces in each member (then proceed with the stress and strain calculations) or you can go piecewise as follows:

enter image description here

  1. Section "a" has the largest unsupported span, it is probably here where you will get the maximum bending stress due to the vertical load. Apply the corresponding distributed force and solve for R1=R2 and M1=M2. Then you will be able to figure out the maximum bending stress in this portion. This bending stress corresponds to the maximum normal stress in the axis which is parallel to the member's length. This stress must be quite close to your SW simulation results, make sure you plot the normal stress corresponding to this direction in SW. The results might be off by a small percentage (10% is usually acceptable). If the results differ by too much, and you are performing an adequate FEA, then the simplification and assumptions for the analytical method might be inaccurate and need to be corrected.
  2. Move to section b. The loads here must be: the corresponding distributed load, R1 and M1. Once again, calculate the resultant forces in this section and the corresponding internal stresses and strains. Then compare your results with those from SW sim and proceed to the last section of the frame.

Recommendations to increase the accuracy of your SW simulation:

  • Strip off any unnecessary features which only add complexity to the model
  • Model your fixtures are close as possible to real life, this is absolutely critical. Do not add more unnecessary stiffness.
  • Produce a high quality mesh, for this problem i would recommend at least 3 or 4 elements per thickness to get decent stress and strain results. You can also check the quality of your mesh by creating a mesh plot to show the aspect ratio distribution, here you must aim for >90% of elements with an aspect ratio less than 3 (rule-of-thumb), the higher this percentage, the better.

As for the rest of the analytical requirements. The 6 independent stresses refer to the normal stresses X,Y and Z and the 3 shear stresses. Look this up if you've forgotten. The same goes for the strains. For the principal stresses, refer to the Mohr stress circles from mechanics of materials class. Look up the theory behind maximum transverse shear in bending. For the Von Mises stress you must figure out all 3 normal stresses and the shear stresses also, the formula requires you to input all of them in order to obtain the equivalent stress. SW will do this for you automatically so you have something to compare against. You should be able to obtain decent approximations of the displacement by either using the 3D truss method or the piece-wise analysis.

Good luck!

  • 1
    $\begingroup$ Thanks a lot, I appreciate your kindness. It really helped. $\endgroup$ – Ammar Taha Jul 8 '20 at 20:30

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