Fatigue Testing

Fatigue testing is a specialized form of mechanical testing performed by applying cyclic loading to a coupon or a full structure, used to generate fatigue-life and crack-growth data, identify locations at greatest risk of failure, or demonstrate that a structure prone to fatigue is safe to operate; it is applied to components ranging from small coupons up to full-size test articles such as automobiles and aircraft. Coupon testing is typically carried out on servo-hydraulic machines capable of large variable-amplitude cyclic loads, or on simpler oscillating machines for constant-amplitude testing, and the number of cycles a coupon survives before breaking, its fatigue life, is used to build stress-life or strain-life curves, while the rate at which a crack grows can be measured during or after the test through fractography. Full-size articles instead require special test rigs with series of hydraulic or electric actuators built to reproduce the real loads a structure experiences, which for an aircraft can include maneuver, gust, buffet and ground-air-ground loading; airworthiness standards generally require large aircraft to undergo fatigue testing before certification to establish their safe operating life, while small aircraft may instead demonstrate safety through calculation, typically with larger safety margins to offset the added uncertainty.