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Effects of small defects, nonmetallic inclusions, and hydrogen trapped by inclusions on fatigue strength are overviewed. by the authors' group and Japanese industries are introduced. Other related problems such as the quality control of materials from viewpoint of the inclusion or defect rating by statistics of extreme and the effect of small defects on fatigue lives are also discussed. Finally, the mechanism of ultralong life fatigue in the regime of N > 107 is discussed based on the particular morphology in the vicinity of the inclusion at fracture origin and it will be concluded that the hydrogen trapped by inclusions is a crucial factor which causes the elimination of conventional fatigue limit defined by N = 107 and the ultralong life fatigue failure of high strength steels at N > 107.

The scatter of bolt clamping force tightened by the conventional torque control method very often causes loosening and fatigue failure of bolted joints. Different values of frictional coefficients between mating screw surfaces and between mating bearing surfaces cause the scatter of clamping force in each tightening. In this study, a new tightening method to reduce the scatter of clamping force of bolted joints has been developed. In the new method, the frictional coefficients at screw and at bearing surface of bolt head can be separately detected during a tightening process by simultaneously application of tightening torque and compressive force to a bolt. The tightening experiments have been successfully conducted to demonstrate the efficiency and usefulness of the proposed method. Furthermore the optimum configuration of bolt head to improve the accuracy of the proposed method has been investigated.

The effect of Si-phase on the axial, low-cycle fatigue behavior of Al-Si eutectic alloys was investigated using test specimens prepared from alloys processed either by continuous casting or extrusion. Results indicate that, for continuous casting, all fatigue fractures resulted from shear-type crack initiation and propagation with an attendant shortening of fatigue life. For extruded material, fatigue cracks originated in the Si phase. In both instances, initiation and growth mechanisms were essentially identical to those observed in high-cycle fatigue. Cyclic properties obtained from phenomenological models are presented and discussed.