Abstract
A study has been made of fatigue crack propagation in mode III (anti-plane shear) for A469 and A470 commercial rotor steels (tensile strength 621 and 764 MN/m2 respectively) using torsionally-loaded circumferentially-notched cylindrical specimens. For crack growth under both small-and large-scale yielding conditions, radial mode III crack propagation rates are observed to be similar in both steels and to be uniquely related to the plastic intensity range 163-1 per cycle, provided friction, abrasion and interlocking between sliding crack faces is minimized by the application of a small tensile mean load. Over the range studied (i.e., ≈10-6 to 10-1 mm/cycle), mode III growth rates (dc/dN)III are found to be independent of load ratio (for R=−1.0 and −0.5) and to be a power law function of 163-1 or the mode III cyclic crack tip displacement. When compared to mode I crack growth at equivalent cyclic crack tip displacements, however, crack propagation rates in mode III are seen to be two orders of magnitude smaller than in mode I. Based on fractographic evidence of elongated voids, parallel to the crack front, at the tip of the fatigue crack, several models for mode III crack growth are proposed utilizing the concept that mode III crack advance occurs by the initiation and coalescence of voids formed at inclusions directly ahead of the crack tip. By considering the linkage of these voids to take place by mode II shear parallel to the main crack front, expressions for the mode III crack propagation rate are developed based either on considerations of the local mode II crack tip displacementsor the mode II accumulated crack tip strain (computed from the Coffin-Manson damage relationship). Whereas both types of models predict mode III growth rates to be a small fraction of the cyclic crack tip displacements per cycle, the damage accumulation model in particular is found to provide excellent agreement with experimentally measured growth rates in the present rotor steels.
Published Version
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