Study On Fatigue Property Of5083Aluminum Alloy Under Torsional Loading

In recent years, the speed of most engineering mechanical components increased rapidly, and the endurance life was prolonged as well. During the life time of these mechanical components, they had to endure more than1010cycles of loading without failure. Researches show that the metallic material can damage in the regime of105to109cycles. In this point of view, the studies of fatigue properties in the very high cycles are of great importance.Nowadays, studies about the very high cycle fatigue almost focus on symmetric and asymmetrical tension-compression loading, symmetric and asymmetric bending loading, environment influenced fatigue. The contexts of these researches are mainly about the characteristics of S-N curves, the fracture surface of failure specimen and the mechanism of fatigue fracture. In many shafts and springs, the metallic material endures the torsional loading in different levels. On the other hand, the fatigue property in the very high cycles with torsional loading was ignored by domestic researchers. Foreign scholars have investigated the torsional fatigue property of spring steel and cast aluminum, and such researches should be studied further. On the basis of foreign researches, the experimental method of ultrasonic fatigue test has been developed, and the fatigue prosperity of5083aluminum alloy was investigated. Follows are the research result in detail:In this investigation, the axial-torsional transducer, key of torsion fatigue system, was re-designed. And the system was introduced in detail on the basis of tension-compression one. A method was given in this paper which can probe the torsional displacement at the end of axial-torsional transducer. The development of this system and the probe method make contribution to further investigation in this field.In this research, the tube specimen and solid one was applied with the help of one dimension torsional vibration analysis result and simulation software. Furthermore, a constant section specimen was introduced in the paper in order to obtain evenly distributed shear stress. At this part, the influence to the vibration frequency, the ratio of stress and displacement caused by machining error was studied, and the study results can contribute to further researches. The S-N data of5083aluminum in the regime of105to109cycles were obtained with torsional ultrasonic fatigue testing equipment. The very high cycle fatigue strength under torsional loading of5083aluminum alloy was investigated. At the same time, the endurance data of axial tension-compression loading and torsion loading was compared, and the internal relation was discussed in this chapter. On the other hand, both fatigue data were analyzed in figure by using Mises and Tresca criterion. Fatigue crack surfaces were observed by SEM. The results show that Fatigue crack initiated from the specimen’s surface or the subsurface in the shear mode and the crack surface was rubbed heavily. The crack branched approximately45°inclined to mode II crack path in mode I manner at both sides of crack till it grew for certain length. In this stage, the crack surfaces show the characteristics of mode I and mode II propagation. At the end of fatigue life, the plastic deformation leads to instant propagation. The fracture surface was ductile mode.Mixed failure criterions were applied to investigate the length of mode II crack path and branched angle. Investigation results show that the factor KⅡ, which dominant the mode II crack length, increase with the incline of fatigue endurance. The measurement value of branched angle was different from theoretical one. Two possible reasons were given in this paper. For one thing, the criterions that study the branched angle are different in experiment and theoretical. For another thing, the shear stress of theoretical modal is uniform, but the shear stress condition in real specimen changes with the propagation of crack.