Components used in the construction of aircraft, bridges, buildings, and other structural systems will undergo both high and low cycle fatigue, which can cause the initiation of fatigue cracking. Fatigue crack initiation and propagation is studied within the field of fracture mechanics. Fracture initiation is thought to occur at material flaws, irregularities, or as a result of local strain hardening. If undetected or untreated, fatigue cracks can lead to material failure with drastic consequences. Fatigue crack testing is conducted on a wide variety of structural materials including metallic materials, ceramics, plastics, and engineered composites. Much testing is concerned with fatigue crack propagation or growth, but many tests also deal with observing cycles to crack initiation. In either case, notched fatigue specimens are used to localize stress concentrations and promote crack initiation in a predictable location. The geometry of fatigue crack specimens is chosen based on the expected loading and stress distribution within the service component, and loading cycles are determined to replicate the various load conditions experienced by the component. Engineers often use the results of fatigue crack tests in conjunction with fracture toughness tests, to determine safe operation lifespans for structural components.
ASTM and ISO have developed standard test methods for testing the fatigue crack growth and initation of various materials. Fatigue crack testing can be very specialized for the specific end use material application, so the standards compensate for many geometries, notch styles and machining methods, and load profiles. However, the standards still provide a test method which is reproducible across laboratories for verification testing and comparison of results. Popular fatigue crack test methods are ASTM E647 for measurement of fatigue crack growth rates, ASTM E2714 for creep-fatigue testing, ASTM E2760 for creep-fatigue crack growth, and ISO 11782-2 for fatigue crack propagation in precracked metal specimens. Electrodynamic and servo-hydraulic can be used for fatigue crack testing. These machines are capable of the wide range of loads and loading frequencies needed for the whole range of fracture mechanics testing, including fatigue crack initiation and precracking, fatigue crack growth, and fracture toughness tests. Clevis grips are used for fatigue crack testing compact tension C(T) type and eccentrically loaded single edge crack tension ESE(T) specimens. Clevis and wedge grips are common for middle tension M(T) specimens.