Endurance limit
The maximum cyclic stress range that a material can withstand for an infinite number of cycles without breaking. It is critical for designing permanently dynamically loaded parts.
Weld transitions and notches are the crux under cyclic loading. Every weld is a built-in weak point; pores, slag, or a too-hard transition act like crack starters. The thing is: if you grind the weld smooth and taper the transitions softly, you've already gained a lot. Always draw grooves in the direction of force, never across it. And if you treat the weld transitions with a needle hammer or shot blaster, you compress the surface and create compressive residual stresses – that lasts much longer. For critical parts, stress-relief annealing is also worthwhile; it removes the tensile residual stresses from shrinkage.