Why parts break well below their yield strength — and why the number of times a load is applied matters as much as how big it is.
A steel bracket can hold a one-time load ten times over and survive with room to spare. Bolt that same bracket into a machine that flexes it a small amount a few hundred times a minute, and it can snap in half within a year — at a stress level far below anything that would have bent it, let alone broken it, in a single application. That is fatigue failure: damage that accumulates from repeated, cyclic loading, not from any single load being too large. It is one of the most common causes of mechanical failure in real service, and it is governed by an entirely different set of rules than static strength.