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The propagation behavior of a circumferential crack in a cylindrical barof a stainless steel was studied under various combinations of cyclic torsion and axialloading. The J-integral was found to be the most appropriate fracture mechanicsparameter for characterizing flat-type fatigue crack propagation under mode I and IIIloading. In order to estimate the J-integral value under mixed mode I and III loading,elastic-plastic stress analysis of a circumferential crack under mode I, III and mixed-mode loading, was conducted by the finite element method. The J-integral value undermixed-mode loading obtained by the simple method was in good agreement with thatestimated from the energy method. The relation between the J-integral value undermixed-mode loading and various fracture mechanics parameters, for example crack tipopening displacement and crack tip sliding displacement, was examinedThetransition condition from a factory-roof type to a flat type of the mode III fatiguefracture surface was proposed on the basis of the crack sliding displacement at thecrack tip.