Study On Very High Cycle Fatigue Mechanism Of Gcr15 Steel

Fatigue failure has always been one of the important failure modes for engineering components.The traditional concept of fatigue design believes that metal materials are expected to have an infinite life when the number of cyclics exceeds 10⁷ without failure.However,with the development of modern machinery towards high speed and aging,fatigue life of many mechanical and engineering structures,such as engines,automobiles and railways,is usually required to exceed 10⁸ cycles,sometimes even to reach 10¹⁰ cycles.Therefore,the traditional fatigue design criteria is far from meeting the application requirements of modern mechanical equipment in the field of very high cycle fatigue.The study of very high cycle fatigue behavior has become an important subject in the field of fatigue,which is of great significance to the strength design and life prediction of Engineering components.In this paper,GCr15 bearing steel was taken as the research object,and the very high cycle fatigue behavior with a life range of 10⁴¹0⁹ cycles was studied by using the experimental method of rotating bending fatigue test and ultrasonic fatigue test.The main content are as follows:?1?The S-N curve of the material was obtained by experiment.The fracture morphology was observed by scanning electron microscopy?SEM?.The fatigue fracture characteristic parameters were calculated respectively,it was found that there is a certain linear relationship between them and fatigue life.Then the stress intensity factor amplitude?K at the crack initiation position was calculated,and the fracture mechanics conditions of crack initiation and propagation were analyzed.?2?The fine granular area?FGA?around inclusion was investigated by using Transmission Electron Microscopy?TEM?and Transmission Kikuchi Diffraction?TKD?.The result showed that three regions can be distinctly observed around the inclusion:nano-sized grains region,partly refined grains region and tempered martensite matrix region.The FGA is composed of nano ferrites and high density of dislocations.Compared to the matrix,the FGA has much higher percentage of high-angle grain boundaries.?3?An elastic-plastic finite element modelling?FEM?was performed to investigate the accumulation of plastic strain at the vicinity of an inclusion in VHCF for both R=-1 and 0.1.The result showed that plastic deformation will occur around the inclusion in the sub-model of inclusion,regardless of the positive stress ratio or negative stress ratio.The shear stress increased with number of cycles,with much higher magnitude at R=-1 than that at R=0.1.The rate of plastic strain accumulation at R=-1 was twice as much as that at R=0.1.?4?In this paper,the formation process of FGA is explained for the first time from the viewpoint of dislocation motion and shear strain accumulation by combining the microscopic evidence with the finite element results.It is inferred that the formation of FGA may possibly be caused by the accumulation of cyclic plastic strain caused by shear stress.dislocation is induced by plastic deformation at inclusions,dislocation entanglement forms dislocation cells,dislocation cells gradually evolve into dislocation walls.The original large grains were subdivided into subgrains by these cells.As a result of accumulation plastic deformation,a fine grain area was formed.The formation process of FGA was named quasi-dynamic recrystallization.