Fatigue Performance Assessment Of S38C High-speed Railway Axle With Foreign Object Damage

Fatigue fracture is one of main mechanical problems of components.As the crucial left part of high-speed train,axles are prone to be damaged by hard objects during service,resulting in surface damage,leading to fatigue fracture,endangering driving safety.In order to simulate foreign object damage（FOD）and evaluate the influence of damage on axle fatigue performance,the research work is as follows:In this thesis,the compress gas gun was used to launch tungsten projectile on the surface of four-point bending specimens taken from S38 C axle,to simulate actual FOD.Laser confocal microscope,microhardness tester,tensile testing machine,stereoscopic microscope,scanning electron microscope and high frequency fatigue testing machine were used to characterize the basic properties of the materials before impact and the macroscopic morphology and microscopic damage of the defects after impact.The fatigue strength of FOD specimens were tested by the step-loading method to study the relationship between FOD and fatigue performance.From the view of damage tolerance method,the defect size of the axle was estimated,and trying to provide some references for the damage evaluation,the formulation of maintenance regulation of axle in the whole country.The research mainly draws the following conclusions:（1）The surface of S38 C axle has undergone induction hardening treatment,which is a gradient material.The surface material（0～2.5 mm）is tempered martensite structure,with the microhardness of 560 HV and the tensile strength of 1794 MPa,and the interior（>7 mm）are pearlite and ferrite structure,with the microhardness of 200 HV and the tensile strength of 620 MPa.（2）The damage,impacted by spherical projectile under different projectile velocities and different angles,became severe with the increase of impact velocity.The grains at the bottom of the crater were refined and elongated,and distributed in orientation along the crater profile.At the lower speed（200 m/s）impact,only a tiny proportion of craters generated Adiabatic Shear Bands（ASB）and cracks.The fatigue strength of damaged specimens decreases linearly with the increase of the impact angle.The fatigue crack initiated from the place where material failed around the crater,and most of them were single-source cracking.At the higher speed（300m/s and 400m/s）impact,a large proportion of craters formed ASB and cracks.The fatigue strength of damaged specimens fluctuates from 300 MPa to 400 MPa.In the fatigue fracture,multiple-source cracking and obvious fatigue stripes were found in some damages.（3）The damage,impacted by cube projectile under vertical condition,can be divided into surface damage,edge damage and corner damage.Lamellar structure and material melting appear on the damage surface.The grains at the bottom of the crater were refined and elongated,and without obvious orientation.At the lower impact energy（0.5 J）,only surface damage formed,and the fatigue strength of damaged specimens decrease with the increase of defect depth.At the higher impact energy（8 J）,three types of damage will come into being,and the fatigue failure is mainly caused by the corner damage,the fatigue strength value fluctuating around 375 MPa.At other impact energy,the fatigue strength of damaged specimens fluctuates from 350 MPa to 550 MPa.Fatigue crack initiate from the bottom and edge of the crater.（4）Taking the square root of projected area of defect on the plane perpendicular to the principle stress into consideration,based on the linear elastic fracture mechanics method and the modified Kitagawa-Takahashi model,the damage tolerance size is area^1/2 = 689μm,area^1/2 = 570 μm,respectively.Taking the defect depth as the measurement index,the damage tolerance size of S38 C axle is d = 600 μm at the 95% confidence level after data fitting.