Notch Effect

It has been shown, in Chap. 2 in particular, that nucleation damage in fatigue is always a localized sub-microscopic damage either on persistent slip lines in some surface grain or in persistent grain boundaries somewhere in the material, depending on whether fatigue is acting in the high cycle or low cycle regime. It is evident that any factor producing a localized stress amplification favors the premature outbreaks of such a damage and the subsequent crack formation. This local stress amplification is generally caused by a component notch or discontinuity. William Rankine[aut]Rankine, W. of England was probably the first to understand this effect studying axels failures and stating in 1842 (Rankine in Institution of Civil Engineers, Minutes of Proceedings, vol 2, pp105–108, London, 1842) “the fractures appear to have commenced with a smooth, regularly-formed, minute fissure, extending all-round the neck of the journal, and penetrating on an average to a depth of half an inch. … until the thickness of sound iron in the center became insufficient to support the shocks to which it was exposed”.