Hi there Hubcap!

Mechanics is more related to classical physics. As such, it involves the interaction of energy, mass, and the physical dynamics that involves them. Therefore, when I say mechanical here, I'm referring to some component that is essential to the proper function of getting the bicycle to translate human energy into motion.

Structural, involves the form of the bicycle, the way it appears, and the material from which it's made. Therefore, when I say structural here, I'm referring primarily to the frame. Structural concerns itself more with form. Mechanical concerns itself more with function.

If you're riding down the street and your chain snaps, that's mechanical. It involves a component that is housed by the frame and is necessary as an integral part of bicycle motion. If you're riding down the street and your chain stay snaps. That's more structurally related, because it deals primarily with the frame and the actual form of the bicycle, that aids in its structural integrity.

There's mechanical engineering and there's structural engineering...

Structural engineering deals more with housing the parts that produce a load. Mechanical engineering deals more with transferring a load or a force to a component to affect its operation.

So to answer your question...No. I would not consider a failed weld as a mechanical problem. It's more related to a structural problem.

- Slim

PS.

When your car's engine fails your take it to the "mechanical" guy, the mechanic, for repair.

When your car's fender gets smashed your take it to the "structural" guy, the autobody repairman.