Mode III Fatigue Crack Propagation in Low Alloy Steel

摘要: To provide a basis for estimating fatigue life in large rotating generator shafts subjected to transient oscillations, study is made of crack propagation Mode III (anti-plane shear) torsionally-loaded spheroidized AISI4340 steel, and results compared analogous behavior I. Torsional S/N curves, determined on smooth bars containing surface defects, showed surprisingly close expected unnotched I data, with lifetime increasing from 104 cycles at nominal yield 106 half yield. Fatigue growth rates III, measured circumferentially-notched samples, were found be slower than I, although still power-law related the alternating stress intensity(△K III) small-scale yielding. only small fraction (0.002 0.0005) cyclic tip displacements(△CTD per cycle, contrast where was much larger (0.1 0.01). A micromechanical model proposed, advance considered take place by II coalescence cracks, initiated inclusions ahead main front. This mechanism consistent increment being △CTDIII cycle.