Finite Element Simulation Of Fretting Fatigue Crack Initation In Press-fitted Part Of Railway Wheel-axle

The railway axle is one of the most important structures in the railway system and nearly bears the whole loading of the vehicle.The failure of an axle usually results in a derailment,which may cause further disastrous consequences.The fretting fatigue at the press-fitted part such as the wheel seat is the main damage for axle.A fail safe design is not available for axle,therefore the fretting crack initiation at the wheel seat has been increasingly emphasized.During the operation,micro-slip occurs inevitably near the contact edge under cyclic bending,and the fretting damage occurs.Due to the fretting wear,the contact profile changes cycle by cycle,and as a result,the distribution of contact parameters and the stress and strain near contact surface changes significantly,which will influence the crack initiation properties of fretting fatigue.Nevertheless,the investigation about the fretting damage of press-fitted shaft is rare at home and abroad,especially for the study about the effect of fretting wear on the fretting fatigue properties.For the full-scale railway wheel-set,the fretting fatigue is obviously influenced by the shape parameters of the wheelseat,such as the stress relief groove,the interference grip,and the overhanging of hub,etc.However,the range of the shape parameters is wide according to railway standard.The difference of fretting fatigue is great when the selected shape parameters change within the range of values.Therefore,it is necessary to propose a prediction model of fretting crack intiation for the press-fitted part of full-scale railway wheel-set,which will provide a scientific guidance for the optimization of shape parameters of the wheelseat,and provide a powerful research tool for the investagition of fretting fatigue in press-fitted shaft.This work was supported by the National Natural Science Foundation of China named ‘Research on fretting fatigue damage behavior and fagtigue life evaluation method for press-fitted high speed train axle-wheel'(No.51375406).In this dissertation,the interrupted rotating–bending fatigue tests were conducted by using the small-scale press-fitted shaft.The corresponding models for predicting the fretting wear and the crack initiation properties of fretting fatigue were proposed,after the necessary amendent,the prediction model of fretting fatigue initiation for full-scale railway axle was created.The main contents and conclusions of this dissertation as follows:(1)Study on the evolution of surface appearance and the initiation characteristics of fretting fatigue cracks in press-fitted specimensThe evolution of surface appearance and the initiation characteristics of fretting fatigue cracks in press-fitted specimens were studied by interrupted rotating–bending fatigue tests.The mechanism of the different fretting area was studied in detail with the combination of simulation results and experimental results.The results shown that the peak value of wear depth appeared at the contact rim,and then the wear depth decreased gradually with the locations towards to the internal of contact zone,the depth and width of fretting wear scar increased with the fretting cycles gradually.The fretting fatigue cracks initiated from the contact area 50–250 ?m away from the contact edge at approximately 30% of the total fretting fatigue life.The orientation of the fatigue micro-cracks was approximately 20° relative to the radial direction of the shaft.Three characteristic fretting zones(zone I,zone II,and zone III)were identified,zone I located the nearest to the contact edge was narrow and clean,with little wear debris present.Severe abrasive wear was found in zone I.The simulation result showed that the whole zone I area located in the open zone.The width of zone I increased with the fretting cycles.Heavy delamination and oxidative wear were found in zone II,and the simulation result showed that the zone II boundary was consistent with the stick-slip boundary under compression.With increased fatigue cycling,the depth of the stick-slip boundary under compression(line B)was decreased by the fretting wear,as a result,the depth of the zone II boundary decreased gradually.Slight delamination and oxidative wear were found in zone III,the zone III boundary was consistent with the stick-slip boundary under tension,and the depth of which remained unchanged.Meanwhile,it was found that the width of wear scar and the open zone was almost equal.(2)The development of predicted models for the simulation of fretting wear and fretting crack initiation propertiesThe optimization of the mesh size,cycle jumps value and increments per fretting cycle was necessary to reduce the computatinal time.It was found that the optimum mesh size should be less than 3% of the final longitudinal wear width to give a good discretization of the profile.The computatuinal efficiency increased with the increasing of cycle jumps value,while when the cycle jumps value was too large,the wear depth increased rapidly.Meanwhile it shown that the depth of predicted wear scar decreased with the increasing increments very slightly,the computational time was almost proportional to the increments per fretting cycle.Therefore the increments should be as small as possible.It was necessary to use the mean correction SWT to avoid overvaluing the inhibition of compressive stress.The validation results shown that the scar width and the peak value of wear depth predicted by the wear model were in agreement with those of experiment.While the predicted wear depth inside of contact zone was slightly larger than that of the experimental results.The intiation angle,location and life predicted by the fretting crack initation model were accurate.(3)The influence of fretting wear on the evolution of surface profile,contact parameters and the initiation properties of fretting crackWith the increase of fretting cycles,the depth and width of wear scar increased and the axial relatively slip decreased gradually.Meanwhile,an increasing peak value of contact pressure appeared at the boundary of wear scar.For the contact rim,the severe fretting wear significantly suppressed the fretting crack initiation at the contact edge by greatly reducing the stress concentration and grinding off the surface material with high accumulated fatigue damage.The fretting wear greatly promotes the fretting crack initiation at the inner surface of the contact area by introducing the large stress concentration near the edge of fretted wear scar.(4)Qualitative simulation about the influence of stress relief groove on the fretting fatigue by the fretting crack initiation prediction model of the full-scale wheelsetIt was shown by the validation results that the fretting crack initiation prediction model of the full-scale wheelset was able to qualitatively simulate the influence of stress relief groove on the fretting fatigue.With the increase of groove depth,the stress concentration near contact rim decreased,and the corresponding contact pressure and the axial slip decreased,as a result,the depth and width of wear scar,the total equivalent strain amplitude and the accumulated damage decreasd accordingly.For the inner surface of contact area,the new stress concentration induced by fretting wear increased with the increase of groove depth,and the location of which moved to the contact rim gradually.The total equivalent strain amplitude and the accumulated damage in it decreasd gradually.With the increase of groove radius,the contact pressure and the axial slip near contact rim increased slightly,as a result,the depth and width of wear scar increased.For the contact rim,the stress concentration,total equivalent strain amplitude and accumulated damage increased gradually with the increase of groove radius.For the inner surface of contact area,the location of new stress concentration moved towards to the internal of contact zone,and the total equivalent strain amplitude and accumulated damage increased gradually with the increase of groove radius.