The fatigue limit and its elimination

The classical fatigue limit of ferrous metals is a consequence of testing m aterials at a constant range of cyclic stress and determining the cyclic st ress range below which fatigue failures do not occur. This classical fatigu e limit of a material is equated to the condition for which fatigue cracks can not propagate beyond microstructural barriers. This paper discusses the causes leading to the elimination of this fatigue limit, including the introduction of transitory cyclic-dependent mechanisms and time-dependent processes that will permit a previously non-propagating crack to grow across the different threshold states expressed in terms of linear-elastic fracture mechanics (LEFM), elastic-plastic fracture mechanic s (EPFM) and microstructural fracture mechanics. These transitory mechanisms and processes include different loading and env ironmental conditions, which in a long-life engineering plant (e.g. 30 year s lifetime) can lead to apparently premature failures. Of greater concern i s the creation of a new crack-initiation zone, i.e. a transfer from a surfa ce-generated crack to an internal-generated crack that eventually dominates the fatigue failure event. The impact of these conditions on the elimination of the classical fatigue limit necessitates changes in Design Codes of Practice, and such changes ar e discussed in relation to the extremely long-lifetime regime (10(7) < N-f < 10(12) cycles-to-failure) which is increasingly applicable to the modern day engineering plant.