Study On The Adhesion Enhancement And Damage Behaviours Of Wheel/Rail In The Application Of Hard Particles

The adhesion between wheel and rail is crucial for the safe and efficient operation of trains.In order to cope with the low adhesion problems induced by water,oil,leaves or other surface contaminants presenting in the wheel/rail contact,sanding is generally adopted in the railway to enhance the degraded adhesion.During enhancing wheel/rail adhesion,sanding could also lead to the deteroriation of wheel/rail wear and damage.In order to optimize the application of sanding,by which the benefits on enhancing adhesion could be taken full advantage of sanding while the side-effects on wheel/rail damage could be reduced,it is necessary to explore the mechnasims of sand particles on improving adhesion and aggrevating wheel/rail damage.In this paper,the wheel-rail rolling contact simulation tests were carried out to study the low adhesion behaviors of wheel and rail under contaminated conditions(wet,oil and leaf conditions).Then,the effects of particle parameters,including the applying quantity,paricle size and different types of particles,on the adhesion improvement and wheel/rail damage were explored when simulating wheel/rail sanding under low adhesion conditions.Furthermore,the fracturing characterstics of enhancer particles and the abrasion behaviors of the crushed sand fragments at the wheel/rail contact interface were studied;and the roles of them on enhancing adhesion and inducing wheel/rail damage were investigated respectively.Based on the results,an optimization of railway sanding rates was proposed and a new kind of adhesion enhacing particles was developed.The main reslusts and conclusions of the thesis are as follows:(1)The wheel/rail adhesion could be significantly impacted by the 3rd-body contaminantion.A quite small amout of contaminants(e.g.,30?l water,10?l oil or a peciece of leaf of 10mm×10mm)was able to induce low adhesion(?<0.2).The adhesion coefficient descended with the increase of contaminantion content until the wheel/rail contact reaching a steady lubricating state,after which the adhesion coefficient kept stable in this low-state.Inceaseing the content of contaminantion could increase the low-adhesion duration in the running of wheel/rail.Besides the drop of maximum adhesion level,surface contaminats would also cause the left shift of the peak position(wet or oil conditions)in the wheel/rail adhesion-creep curve and reduce the curve's initial slope(oil or leaf conditions).(2)When applying hard particles to enhance adhesion,among the particle parameters,the particle-laying density had the most significant effect on the adhesion improvement.In addition,there was a clear threshold value(about 0.607g/m)of the particle distribution density:the adhesion improvement increased sharply with the increase of particle-laying density until the particle-laying density threshold was reached,after that the adhesion improvement kept stable.The effect of particle size on the adhesion enhancement was only obvious when the particle-laying density was lower than the threshold value.In this case,smaller particles were more efficient in entering the wheel/rail contact;thus,performed better on enhancing adhesion.There was little of difference among the adhesion enhancing performances of different types of sand particles,while the alumina particles performed better than sand.(3)The application of solid particles into wheel/rail contact could significantly aggravate the wheel/rail wear and plastic deformation,coupled with the sereve surface damage with indentation and embedding-particles features.More than that,the fluild-driven rolling contact fatigue(RCF)damge occoured when sanding under wet condition,which eventually resulted in severe surface delamination damage on the driven discs.The wheel/rail wear rates were dramatically improved with the increase of sanding rates until the over-dosing of sand particles happened.After a sharp decrease when particles were over dosed,the wheel/rail wear rates continued to increase on a slow growth trend with the further increase in sanding rates.The effect of particle size on wheel/rail wear and damage depended on the the chosed sand rates,but in most of cases,larger particles could induce more significant surface indentation damage while samller particles could cause sever RCF damage.Compared to sand particles,the application of aluminum particles caused more serious wheel/rail wear and damage problems(the wear rates could be doubled).(4)The normal load to fracture a slica sand particle in the size range of 0.2?2.0mm was smaller than 350 N.Therefore,sand particles in wheel/rail sanding would be crushed into smaller particles during entering into the wheel-rail contact.During being compressed between wheel and rail materils,the particles was able to cause the yielding of wheel/rail materials before fratucring;thus,lead to wheel/rail surface damage.The surface damage induced in the particle compression was similar to that in wheel/rail sanding simulation tests.The fracturing strength of particles increased with the decrease of particle size.Based on this law and the compliance curve of the indentation of a particle on the wheel/rail materials,the ultimatize size of silica sand and alumina was evaluated to be 6.5?13?m and 17?24?m,respectively.(5)Compared with normal sanding(standand sized sand),the application of sand fragements(micro particles)colud almost achieve the same improvements on adhesion coefficient under different low adhesion conditions(wet,oil and leaf).The adhesion coefficient increased with the content of micro-particles in wheel/rail contact,so did the wheel/rail wear and damage.However,the abrasion of the micro particles between wheel and rail induced much milder negative-effects on the wear,surface damage,and plastic deformation of wheel/rail materials than the normal sanding.(6)In the railway sanding,the applied sand particles would be fractured into micro fragements in the inlet zone of wheel/rail contact.The size distribution of those particles that finally enterd into in the contact interface concentrated around the ultimatize size of sand.The adhesion enhancement by sanding was determined by the ploughing of crushed fragments in the wheel/rail contact and the improvement of adhesion coefficient higly depended on the contents of acting particles.Although the crushing process of initial sand particles had no visible promotion on improving adhesion,it was recognized that this process dominated in inducing sereve wheel/rail wear and damage problems.(7)Based on the results,an empirical formula was proposed to evaluate the adhesion improvement in railway sanding.In addition,for a better optimizing stragety,it was suggested to use an adjustable sanding flow rate based on the trains speed rather than a fixed one.To reduce the negative effects of the particle crushing process on wheel/rail wear and damage,a new type of particles with lower crushing strength than sand was developed.The new particles showed the equivalent capabilities on improving adhesion coefficient as sand,while induced slighter wheel/rail wear and damage.