Two views on how materials break — the S–N curve that predicts cyclic life, and the Paris-law growth of a single crack from sub-critical flaw to final rupture.
The S–N (Wöhler) curve plots stress amplitude σa against cycles to failure Nf. Slide σa to read off life. Ferrous alloys show a true endurance limit — below it, life is effectively infinite. Aluminum and most non-ferrous alloys do not.
High-cycle fatigue: stress is below yield, life dominated by crack initiation. Reduce σa below σe for unlimited life.