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.
Facts
Core PrincipleFatigue testing rests on Wohler's finding that the stress amplitude in cyclic loading, not the peak stress, decides whether the cyclic loading destroys the cohesion of a material, which is why a material can fail from repeated loads well below its static strength. 1 FAQs
Can a metal part fail under a load lower than its strength?
Yes. Repeated stress below static strength can still cause failure; the stress amplitude decides.
Yes. Wöhler's laws, state that materials can be induced to fail by many repetitions of stress, all of which are lower than the static strength, and that the stress amplitude, or stress range, is decisive for the destruction of the material. Fatigue testing records this on S-N curves. On such a curve, the point where the slope reaches zero is called the endurance limit, and materials subjected to stresses below it will never fail under cyclic loading.
Connections
Sources
1. Stress-Life Fatigue Testing Basics (Quality Magazine)
Wohler's laws sectionQuote, Wohler's laws section
Stress amplitudes (or stress range) are decisive for the destruction of the cohesion of the material.
Fatigue testing (Wikipedia)
In Group: Interface Definition and Verification Testing, IntroQuote, In Group: Interface Definition and Verification Testing, Intro
Fatigue tests are used on a range of components from coupons through to full size test articles such as automobiles and aircraft.
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