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The propagation behavior of fatigue cracks in Mode III (anti-plane shear), measured under cyclic torsion, is described and compared with more commonly encountered behavior under Mode I (tensile opening) loads. It is shown that a unique, global characterization of Mode III growth rates, akin to the Paris “law” in Mode I, is only possible if characterizing parameters appropriate to large-scale yielding are employed and allowance is made for crack tip shielding from sliding crack surface interference (that is, friction and abrasion) between mating fracture surfaces. Based on the crack tip stress and deformation fields for Mode III stationary cracks, the cyclic crack tip displacement, (ΔCTD)III, and plastic strain intensity range, ΔΓIII, have been proposed and are found to provide an adequate description of behavior in a range of steels, provided crack surface interference is minimized. The magnitude of this interference, which is somewhat analogous to crack closure in Mode I, is further examined in the light of the complex fractography of torsional fatigue failures and the question of a “fatigue threshold” for Mode III crack growth. Finally, micromechanical models for cyclic crack extension in anti-plane shear are briefly described, and the contrasting behavior between Mode III and Mode I cracks subjected to simple variable amplitude spectra is examined in terms of the differing role of crack tip blunting and closure in influencing shear, as opposed to tensile opening, modes of crack growth.
fatigue (materials), cracking (fracturing), crack propagation, torsion, Mode I (tensile opening), Mode III (anti-plane shear), crack closure and sliding interference, fatigue thresholds, variable-amplitude loading
Professor materials science, University of California, Berkeley, CA