Some fatigue behaviour of structural steel components fatigue crack closure

One of the major problems associated with the design againstfatigue is to predict the safe life of components orstructures (1). Fatigue is the progressive, localized,permanent structural change that occurs in materialssubjected to fluctuating stresses. Fatigue may result incracks or fracture after a sufficient number of cycles.Cracking, normally, results from cyclic stresses that arewell below the static yield strength of the material.Fatigue cracks initiate and propagate in regions where thestrain is high, typically structural defects.Under cyclic loading, a plastic zone develops at the defecttip. This zone becomes an initiation site for a fatiguecrack, which propagates under the applied stress through thematerial until fracture occurs.In 1871, the first major contribution to fatigue researchwas made by Wohler (2) by carrying out a systematic study onrailway axle steels. He showed that the greater the appliedfluctuating stress the less the lifetime. This work wasfollowed by several researchers as discussed in reference(3), to describe cumulative fatigue damage and subsequentfailure.