fatigue

Bend a paperclip back and forth. The first bend does nothing dramatic, and neither does the second — but keep going and after a dozen cycles it snaps, at a force far below what it would take to pull it apart in one go. That is fatigue: failure under repeated or fluctuating loads, at stresses well below the material's static strength. It is the slow death of a part that is loaded, unloaded, loaded, unloaded, thousands or millions of times.

Fatigue works in two stages. First, initiation: even at modest stress, the back-and-forth slip of dislocations roughens the surface and a tiny crack nucleates, usually at a surface stress raiser — a scratch, a bolt hole, a sharp corner, a rough machining mark. Then, propagation: each load cycle nudges the crack forward a minute step, so it grows a little every cycle, leaving telltale ripples (striations, one per cycle; beach marks on a coarser scale). The part carries on looking fine until the crack reaches its critical length and the remaining section snaps in one final, sudden fracture.

The sobering fact is that most in-service mechanical failures — a large majority by many estimates — are fatigue, not one-time overload. And the crack hides: the part looks undamaged right up to the last cycle. Defenses are all about the surface and the stress raisers: polish it, avoid sharp corners, add generous fillets, and induce compressive surface stress by shot peening or by rolling threads instead of cutting them, since a surface held in compression resists cracking. Design either to keep stress below the endurance limit forever, or to inspect and retire the part before a crack grows critical.