The fatigue limit and its elimination

TLDR

The classical fatigue limit of ferrous metals is defined by testing at a constant cyclic stress range and represents the stress level below which cracks cannot propagate beyond microstructural barriers. This paper examines how transitory cyclic‑dependent mechanisms and time‑dependent processes eliminate that fatigue limit by allowing cracks to grow across LEFM, EPFM, and MFM threshold states. These mechanisms arise from varying loading and environmental conditions in long‑life plants, leading to premature failures, new crack‑initiation zones, and a shift from surface‑generated to internal‑generated cracks that dominate fatigue failure. The elimination of the classical fatigue limit requires updates to Design Codes of Practice, especially for the extremely long‑lifetime regime (10^7–10^12 cycles) relevant to modern engineering plants.

Abstract

The classical fatigue limit of ferrous metals is a consequence of testing materials at a constant range of cyclic stress and determining the cyclic stress range below which fatigue failures do not occur. This classical fatigue 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 mechanics (EPFM) and microstructural fracture mechanics (MFM). These transitory mechanisms and processes include different loading and environmental conditions, which in a long‐life engineering plant (e.g. 30 years lifetime) can lead to apparently premature failures. Of greater concern is the creation of a new crack‐initiation zone, i.e. a transfer from a surface‐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 are 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.