Originally Posted by grouch
Does that calculation take into consideration the flexing at corners or is it based upon an ideal, theoretical, unflexing shape? I would be surprised if that CAD software was doing anything more than plugging numbers into the formulas and spitting out the results. Those results don't cover everything to be considered.
I just don't think that polar moment of inertia is the whole picture. That only tells you how much the object resists. It doesn't tell what happens to the object if it is subjected to continuously varying torsion over a long period of time. It doesn't tell what happens to the steel along the edges where the planes meet after that boxed frame has been bounced down the highway a few years.
It looks to me like the greatest stress will occur at the corners. A box can flex at the corners at levels of stress far below the failure point. Isn't that what causes metal fatigue? Especially with sharp corners, isn't boxing setting the frame up to fail from little cracks, sometime down the road?
How long do you need it to last? I would venture that either design will outlast all of us.
Metal fatigue doesn't usually occur in structures that are never subjected to stresses that are less than 1/2 their yield point. I would think that the open leg of a channel would be subject to developing stress risers at the edge way before the same channel that was boxed. Even figuring in a crappy weld. Also, the opposite corner would be sharper of an angle, and would have less ability to resist development of stress risers than a 90 degree corner that you'd have with a fully boxed beam.
And making joints at an angled side of a beam would make a fabrication nightmare, Although there would be more of an interface area to spread the stresses out.
Do you have any examples of structures that use a triangular cross section type beam instead of a rectangle or square?
I'd think if it was stronger it would be in much greater use today.
I think many times metal fatigue occurs because it was crappy metal to begin with, and joint design and misapplication of the end use causes failure.
It's easy to say that metal fatigue was the cause of failure, when you've exceeded the design limits of the frame many times over by putting more weight, power and subjecting the frame to higher stresses than it was ever intended to resist..
BTW, I misunderstood your first post. I thought you were talking about the frame structure as a whole, not just the beam.