"Work hardening" is indeed what causes failure when you take, for example, a steel rod and bend it back-and-forth, back-and-forth, back-and-forth until it breaks.

The minimal amount of bending done to handle bars to form them into different shapes comes nowhere near the rod example above. The bending of the rod above actually increases its tensile strength to a point [well before it fails], just as forming handle bars into different shapes makes them stronger.

DON