paper: Kitbot Bending Analysis

[Tristan Lall]

Originally Posted by kramarczyk

I agree that the FEA rough, but adequate. What would you change about it?

I'm not totally clear on how you constrained the system—did you pick the middle hole in the channel, and constrain its surface to ground? I don't think that's going to give you good results in the region of that hole...but far away from it, you're probably alright. (It's not that it's a singularity—as indicated in slide 18—so much as it's just a constraint that isn't representative of the real structure and loads.)

Also, the load of 300 lb_f in an arbitrary direction is a critical assumption. How was that arrived at? I can't help but think there needs to be some consideration of the other forces transmitted through the wheel and the frame in a collision—even in a static case, isn't the loading significantly more complicated in real life? Even neglecting other components of the loading, and focusing on this principal one, what was the reasoning behind this particular magnitude?

And because the material isn't actually homogeneous, you've got to be careful with your conclusions about the areas of peak stress which lie in strain-hardened areas (like corners that have been bent on a break). You're probably going to have stronger material in that area—counteracting some of the stress—but with more crack initiation sites that can fail (maybe in low-cycle fatigue). Since the safety factor calculation assumes homogeneous material, there's significant uncertainty here as to when failure (either by yielding or fracture) will actually occur.

Now of course, if you just picked this as a worst-case scenario, that seems quite reasonable as a rough estimate. But then it implies that you're not going to put too much stock in the safety factor calculations, because they're necessarily imprecise. It gives you a good order of magnitude study, tests relative performance of several options and highlights points of failure, but won't allow you to say with confidence exactly what the failure loads are.

So basically, it's a useful exercise, as long as the students are clear on its limitations. The key is to treat this as an estimate, rather than "the solution" (as if it was a deterministic math problem). I think that's basically what you guys noted in your conclusions—good work!