25 Fatigue Resistance
It is recognized that a metal subjected to a repetitive or fluctuating stress will fail at a stress much lower than that required to cause fracture on a single application of load (bend a wire back and forth until it breaks off).
In materials science, fatigue is the initiation and propagation of cracks in a material due to cyclic loading. Once a fatigue crack has initiated, it grows a small amount with each loading cycle, typically producing striations on some parts of the fracture surface. The crack will continue to grow until it reaches a critical size, which occurs when the stress intensity factor of the crack exceeds the fracture toughness of the material, producing rapid propagation and typically complete fracture of the structure.
Video
Watch this 3:21 video explaining Toughness Vs Fracture Toughness, by E-PHID Lectures, November 22, 2021.
Fatigue has traditionally been associated with the failure of metal components which led to the term metal fatigue. In the nineteenth century, the sudden failing of metal railway axles was thought to be caused by the metal crystallising because of the brittle appearance of the fracture surface, but this has since been disproved. Most materials, such as composites, plastics and ceramics, seem to experience some sort of fatigue-related failure.
Video
Watch this 8:22 video Understanding Fatigue Failure and S-N Curves by The Efficient Engineering, June 24, 2019.
Aircraft manufacturers are very interested in the fatigue properties of their wing spars and other parts due to the constant cyclic forces applied to them.
Derived from Fatigue (material) – Wikipedia accessed and available online 5 March 2024, and The Virtual Machine Shop (http://www.jjjtrain.com/vms/eng_metallurgy/eng_metallurgy_09.html retrieved from the Wayback Machine 15 January 2024.