Propagation of short fatigue cracks

TLDR

Fatigue crack propagation has been extensively studied, yet most research focuses on long cracks, leaving short crack behavior relatively unexplored despite its engineering importance. The review examines the mechanics and micromechanisms of subcritical growth of short fatigue cracks, contrasting their propagation with long cracks in terms of fracture mechanics, microstructure, and environment. Short cracks are defined by their length relative to microstructural dimensions, local plasticity scale, or physical size, and the review critically examines how local plasticity, microstructure, crack‑tip environment, growth mechanisms, driving force, and premature closure influence their propagation. Short cracks propagate at least as fast as long cracks under the same driving force, meaning defect‑tolerant fatigue designs based on long‑crack data can overestimate lifetimes.

Abstract

AbstractFatigue crack propagation in engineering materials has been the subject of considerable research, and extensive review articles have appeared over the past several years. Most of these investigations focused on the behaviour of ‘long’ fatigue cracks, even though the characteristics associated with the extension of small cracks in metals and alloys remain relatively unexplored, despite their unquestionable importance from an engineering standpoint. In this review, the mechanics and micromechanisms of the subcritical growth of short fatigue cracks are examined, and aspects of their propagation behaviour are contrasted with those of long cracks in terms of fracture mechanics, microstructure, and environment. Cracks are defined as being short (i) when their length is small compared to relevant microstructural dimensions (a continuum mechanics limitation), (ii) when their length is small compared to the scale of local plasticity (a linear elastic fracture mechanics limitation), or (iii) when they are simply physically small (e.g. ≤ O. 5-1 mm). Since all three types of short flaw are known to propagate faster. than (or at least at the same rate as) corresponding long fatigue cracks subjected to the same nominal driving force, current defect tolerant fatigue design procedures which utilize long crack data can, in certain applications, result in overestimates of lifetimes. The characteristics of the short crack problem are critically reviewed in the light of the influences of local plasticity, microstructure, crack tip environment, growth mechanisms, crack driving force, and the premature closure of the crack.