The heart of crystalline materials

There are several key dates in the history of fatigue study. In the mid 19th century in Germany (1860), premature fractures in railway wagon axles, despite these being designed and manufactured to work within the elastic domain, led August Wöhler to develop a specific rotating bending test. Using this test, which was reasonably representative of axles under operating conditions, he was able to assess lifetimes and lifetime dispersions for structures as a function of the maximum stresses imposed on them, in the form of the stress-number of cycles to fracture, or SN, curves, also known as Wölher curves.

In 1920, the British aeronautical engineer Alan Arnold Griffith presented his brittle fracture theory. In the 1940s, Liberty Ships, the famous American cargo vessels manufactured during the Second World War and used to resupply allied troops, suffered serious failures that led to the loss of 362 hulls. The cause of severe failures is today well understood. It should be considered in relation to the low toughness of the steels used and assembled by poor quality welding, provoking cracks to initiate at hatch corners.

In 1956, American George Rankin Irwin, who led a research group at the US Naval Research Laboratory, noted the singularity of stress fields at the crack tip and developed his theory of fractures with localised plasticity or confined plasticity.

During the 1950s, repeated fracture-related accidents in the fuselage of American de Havilland Comet jetliners, caused by propagation of cracks initiated at the corners of the (overly) square windows, made headlines around the world. In 1960, Paris proved and (with trouble) convinced the scientific community that it was possible to use cracked materials, introducing the notion of damage tolerance as part of which he posited a crack propagation model that remains widely used to this day.

In 1968, Rice developed the \(J\)-interval concept as a way to better describe the distribution of stresses in confined plastified zones of a material. This parameter would also be used to characterise the toughness of materials. From 1970 on, the scientific community examined fatigue cracking as a result of complex loads, the so-called complex fatigue.

Currently, new approaches aim to link local behaviour with overall behaviour at issue when fracturing occurs. This local approach to cracking is needed when the limits of the now well-established considerations described above are reached.