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Based on the axisymmetric and 3-D finite element models, the axial-load and stress distributions on each thread of the threaded connection have been studied. The effect of helix and the elastic-plastic behavior of the material at the thread roots have been explored. Simulation results indicate that the axial-load and stress distributions obtained from the axisymmetric finite element model show good agreement with the results of 3-D analysis in both elastic and plastic states with the same mesh density. So the axisymmetric model can be used to study the axial-load and the stress distributions of the threaded connection subjected to axial-load. The axial-load and stress distributions in threads are very sensitive to the applied load. With increasing the applied forces, the axial-load and stress distributions over teeth become homogeneous eventually.

In this paper, the influence of different machining methods (including rough turning, finish turning, and longitudinal polishing after finish turning) on rotating bending fatigue properties of TC21 which belonged to a new ultra high strength titanium alloy was studied. The influence of machining methods on surface integrity of TC21 titanium alloy was measured by using surface profile measurement, scanning electron microscopy, metallography microscope, micro-hardness instrument and X-ray diffraction residual stress analyzer. And fatigue fractography of specimens was further investigated. Then the mechanism of fatigue resistance which was affected by machining surface integrity was discussed. The results indicated that the fatigue life of finish turning and longitudinal polishing after finish turning was increased 3.96 times and 17.34 times compared with rough turning, respectively. The machining surface integrity had important influence on fatigue property of TC21 titanium alloy, which caused by the differences of surface roughness and texture as the dominant factors, and then the variation in surface micro-hardness, metallographic microstructure and the surface residual stress were not the main factors on three above-mentioned machining methods. By using longitudinal polishing after finish turning processing method for preparation of TC21 titanium alloy parts could ensure good surface integrity and excellent fatigue performance.