What Metal Fatigue Actually Is
Metals don't get tired in any biological sense. What happens is mechanical: repeated loading causes tiny cracks to initiate, usually at stress concentrations — surface scratches, corrosion pits, machining marks, geometric notches. Once started, these cracks grow a small amount with each load cycle, often invisibly. After millions of cycles, the crack reaches critical size and final fracture is sudden and complete, at stresses that would be perfectly safe in a statically loaded structure. The material appears completely sound right up to the point of failure.
The Versailles Rail Disaster
The axle had been subjected to millions of loading cycles — each wheel rotation reversed the stress from tension to compression. At 60 km/h, the axle experienced 18,000 cycles per hour. A crack had initiated at a keyway (a machined slot that was unknowingly a severe stress concentration) and grew invisibly until it reached critical size. The German railway engineer August Wöhler, working in the 1860s, conducted the first systematic experimental study of fatigue in response to disasters like this, developing the S-N curve that remains the foundation of fatigue analysis.
The de Havilland Comet
The world's first jet airliner started falling from the sky in 1953–54. The investigation concluded that square-cornered windows were the culprit. Each pressurisation cycle caused the fuselage skin to flex; stress concentration at the window corners initiated fatigue cracks at rivet holes. After hundreds of cycles, cracks reached critical size and the pressurised cabin tore open. The Comet investigation established pressurisation fatigue testing as mandatory for all commercial aircraft — and gave us the rounded rectangular windows on every modern airliner.
Preventing Fatigue Failure
The key design interventions: eliminate stress concentrations (generous fillet radii, no sharp corners), surface finishing (polishing, shot peening), and regular inspection in safety-critical applications. Shot peening — blasting the surface with steel spheres — introduces compressive residual stresses that must be overcome before crack propagation begins, dramatically extending fatigue life. For aircraft and critical rotating machinery, structural health monitoring tracks actual cycle counts in service, allowing maintenance scheduled on actual damage accumulation rather than conservative estimates. Enter your material's tensile strength, Basquin exponent, Marin factors, alternating and mean stress. EngForge computes the endurance limit, equivalent stress, cycles to failure, and draws the Wöhler S-N curve with your operating point marked.