Investigation Of Tensional And Torsional Fatigue Of Stainless Steel With Nanocrystallized Surface

Fatigue,a very common phenomenon,is one of the most important causes of serious accidents in aviation,mechanical and civil engineering,etc.In most of the materials,fatigue crack originates from the exterior layer of work pieces.In most engineering applications,people only need to improve surface properties of materials to increase the material’s overall service performance and fatigue life.Therefore,the material with the nanostructured surface and coarse grained interior,because of its higher yield strength,hardness and wear resistance,has attracted great attention.This also promotes the development of surface nanocrystallization technology,which has a broad prospect of engineering application.In this dissertation,304 stainless steel(SS)specimens with surface nanograined layer(NGL)are used in tensional,torsional and combined tension and torsion fatigue tests.By comparing the fatigue life of the untreated specimens,2-mm-diameter balls and 3-mm-diameter balls treated specimens,it is found that the surface nanocrystallization can greatly improve the tensional,torsional and combined fatigue life of the 304 SS.Different diameters of the balls in the process of surface nanocrystallization have a remarkable effect on the fatigue life.Generally,the tensional fatigue life of the specimens treated by 3-mm-diameters balls is longer than that of the specimens treated by 2-mm-diameters balls,and the opposite can be found in lower stress levels.The torsional and combined fatigue life of the specimens treated by 3-mm-diameters balls improve more pronounced in all stress levels.The cohesive finite element method and the Monte Carlo approach are employed to investigate the crack initiation and fatigue life under the tensional and torsional fatigue loading.It is shown that the heterogeneous model can predict more realistic fatigue crack patterns.For axial fatigue simulation,the NGL plays an important role in improving the fatigue life;as the layer thickness increases,the axial fatigue life of the nanostructured metal decreases,especially at low stress levels.For torsional fatigue simulation,it is found that the flow stress of the NGL can be used to distinguish the different treatment;as the layer thickness increases,the torsional fatigue life of the nanostructured metal also increases;when the thickness of NGL is 60 μm,the fatigue crack initiation modes changes,crack initiation starts from the internal surface of the specimens with a larger probability and it is more beneficial to improve the fatigue life.