The Crack Propagates: Structure fatigue failures

Fatigue is a process in which damage accumulates due to the repetitive application of loads that may fall below the yield point. Fatigue is the initiation and propagation of microscopic cracks into macro cracks through repeated application of stresses. All structural steel materials contain metallurgical or fabrication-related discontinuities, and most also include severe stress concentrators. The fatigue begins as an internal or surface flaw where the stresses are concentrated and consist initially of shear flow along slip planes. Over a number of cycles, this slip generates intrusions and extrusions that begin to resemble a crack. A true crack, running inward from an intrusion region, may propagate initially along one of the original slip planes but eventually turns to propagate transversely to the principal normal stress until observing a sudden fracture of the remaining cross-section. The phenomenon may be problematic because a single application of the load would not produce any sign of defect, and a conventional stress analysis may lead to an assumption of safety that does not exist. The history of fatigue covering a time span from 1837 to 1994 was reviewed in an extensive paper by Walter Schütz [1]. Historical milestone papers were collected by Hanewinkel and Zenner [2] and Sanfor [3]. John Mann [4] compiled 21075 literature sources on fatigue problems covering the period from 1838 to 1969 in four books. Since that time the number of publications on fatigue has still considerably increased and it may be estimated to be around 100,000 in the year 2000. Fortunately, consulting the literature on specific topics can now be done with computerized literature retrieval systems.