Several components made of metals do not fail due to high static loads, but when they accumulate damage under repeated cyclic loads that are much less than their ultimate strength. This is called fatigue, and it is of serious concern in structures like bridges, aircraft, pressure vessels, and rotating machines. The fatigue testing offers the necessary information on the life cycle of metals in cyclical loading, whereby engineers can design more reliable and safer structures.
Test Principle and Loading Modes
Fatigue testing is a repetitive test of stress or strain applied to a metal specimen until it fails or the number of cycles is reached. Routine load modes are axial tension, compression, bending, and torsion. The tests are a simulation of the actual service conditions when components are subjected to variable loads in the long run.
Fatigue Testing Standards
Fatigue testing is typically conducted using ASTM E466 under axial fatigue, ASTM E468 under presentation of fatigue data, and ASTM E606 under strain-controlled fatigue testing. These standards also guarantee uniform preparation of the specimen, loading conditions, and interpretation of the data.
S–N Curves and Fatigue Life
Fatigue testing results are commonly displayed as stress-life (S-N) curves, or the relationship between the magnitude of applied stress and the number of cycles. In certain metals, especially ferrous alloys, there can be an observed endurance limit, below which fatigue failure cannot readily take place. Metals that are not ferrous normally do not have a distinct endurance threshold and must be designed on a life basis.
Crack Initiation and Propagation
Fatigue failure normally occurs due to crack initiation at the stress concentrators, which include notches, surface defects, or welds. After starting, cracks spread very slowly with each loading cycle until a discontinuous fracture takes place. Fatigue testing is used to determine the important levels of stress and the surface conditions that determine the behaviour of cracks.
Factors Affecting Fatigue Performance
Fatigue resistance is influenced by the material composition, surface finish, heat treatment, residual stress, and environmental conditions. Controlled environmental testing helps engineers evaluate how these factors affect the structural safety in the long term.
Applications and Industry Use
Fatigue testing is also commonly employed in structural design, material qualification, and failure analysis. It is important in aerospace, automotive, construction, and energy industries, where cyclic loading and long-term reliability are important issues in design.
