# pic: FEA - Stress Analysis of Frame Rail

Trying out the FEA package built into Inventor to determine the appropriate web sizing of the rail. Good thing too, because our first guess was a little too agressive. Anybody done this? How about with a full frame?

My son was trying to use Inventor’s FEA to figure out maximum stress on a section of ramp, but could not figure out how to set up the loading to represent pinned connections at the ends, they always came out fixed, as in your picture.

so he figured out how to roughly calculate the moment of inertia of the section, and calculated stress due to bending using the good old formula, with pencil and paper. Then we tested it by building a section and supporting it freely at the ends and standing on it.

I’m a little confused by the unit representation in the upeer left corner. It looks like I have to multiply it by 10^6 to get the load in psi and 1,576,000 psi seems a bit high for a load on a FIRST bot.

Also I’ve never done FEA on a bot part. If I’m concerned about a part I break out my copy of “Roark’s Formulas” and do a hand calc.

It looks like it says max: 1.105x10^3, which is more reasonable. That’s really cool, though. I’m thinking we should try it with our ramp platform.

Regarding pinning, I vaguely remember some trick where you can model a (symmetrical) beam pinned at both ends by using a cantilevered half-beam with one end fixed and one end free, then applying the pin reaction force to the free end. Anyone else know this for sure?

Very nice, and its a great experiment … on the surface …

However looking at the deflected plot shape I am a bit concerned about how you have your model constrained. When using FEA, in order to have meaningful results, it is critical to know a lot of background about structural analysis and to understand how your applied loads and boundary conditions can influence your outcome. Here at R-R we have several entire departments dedicated solely to their own areas of FEA due to the intricacies involved with properly utilizing these tools and the criticality and safety concerns inherent in our product.

It is amazing and encouraging for me to see FEA software available to HS students. It makes me wonder how talented our new engineering graduates will be over the next 5-10 years.

As a mechanical design engineer I use these tools for preliminary analysis to help compare different concepts. Our final designs are always put through extensive analysis by our specialists.

I agree that boundary conditions are everything in FEA. That’s why we didn’t use the absolute numbers for our decision, we looked at relative differences instead. This shot is one of a series that we conducted trying to determine an appropriate web width. We started with a tube without any weight reduction and noted the stress and deflection then proceeded to take out weight. We got lucky with the geometry and didn’t have any significant singularities pop up and the models convereged pretty quickly for us.
We plotted max stress vs. weight and max deflection vs. weight and looked for the knee in the graph. We had a safety factor of 10 throught the analysis, but I’m not sure if the software was basing it on ultimate or yield strengths though, so I can’t draw much of a conslusion from it. Some design choices should keep the rail from taking a point load hit (i.e. bumpers) so I am pretty confident that we’ll have dropped about 40% of the frame weight with no loss in performance.