Fatigue limit
The maximum stress amplitude that a material can withstand for theoretically infinite load cycles without failing. Many steels show no progressive fatigue below this limit.
Fatigue strength means: a component withstands alternating load practically indefinitely without breaking. In welding, this is important because the weld and the heat-affected zone are often the weakest points. If the fatigue strength is poor, the part will eventually crack under cyclic loading, even if the load itself is not high. When welding, make sure you avoid notches and sharp transitions – they eat away at fatigue strength. Cooling is also critical: too fast cooling makes the structure hard and brittle, too slow cooling makes it soft and coarse-grained. You want to hit the right middle ground so the structure stays tough and fatigue-resistant.