The simplest way to understand work hardening for our trade is to rather understand the effects of work hardening to piano wire in the strung piano. When a metal is stretched or bent beyond its elastic limit, meaning it has deformed into a permanent shape, work hardening has taken place. Along with hardening comes brittleness; imagine taking a wooden dowel and, after rubbing it hard with a burnisher (i.e. work hardening it), magically tuning it into a piece of chalk.
Although a gross analogy, this is what happens when metal is work hardened. Most of us have restrung entire pianos, or at least replaced a broken string. You will have noticed, after the coil has been removed from the tuning pin, how easy it is to break the small becket length off of the coil. That sharp bend represents serious deformation of the wire, hence work hardening and brittleness.
Those locations where piano wire is has been work hardened and deformed exist at the tuning pin and all termination and deflection points such as the capo bar, counter bearings, bridge pins and the hitch pin. These points are clearly visible upon removing an old string. Every deflection or bend in the wire represents a point of compromise due to "atomic dislocations", meaning that the natural internal crystalline structure of the metal is beginning to separate at internal boundaries of the structure.
We sometimes repair a string broken at the tuning pin by pulling the string around the hitch pin. The bend at the hitch pin, considered a serious deformation and work hardened segment, is also pulled around but never really straightens out. Dislocations in the crystalline structure have occurred there and represent a possible future failure. But if that tight little bend, now being asked to go straight, should exist in the speaking length of the string a complete failure is almost certain.
Now, very important: Repeated flexure causes work hardening. Take the modern airplane. It must be designed to,
"evenly distribute flexure, which can lead to work hardening and in turn stress cracking [especially where the long wings attach to the fuselage, NG], possibly causing catastrophic failure. For this reason modern aluminum aircraft will have an imposed working lifetime ... after which the aircraft must be retired." (Wikipedia)
Like the aircraft, piano wire is subjected to repeated to-and-fro flexure and at very high levels of repetitions per second. Think of removing the aluminum tab from your Pepsi; you flex it back and forth causing work hardening, then brittleness, followed by "catastrophic failure". Thus higher-frequency notes in the piano scale are subjected to a more serious repetitious work hardening mechanism (primarily at the capo bar) as compared to notes lower in the scale. Failure at the tuning pin exists for similar reasons of stress and release, stress and release. Piano wire work hardening occurs less so at the bridge pin due to the yielding of the soundboard.
Notice in the above quote that an "imposed working lifetime" is assigned to an aircraft. Same is true in piano wire depending on the mileage it receives. Piano wire simply wears out, and the more that wire is pressed into service the more quickly it wears out.