Study On Damage Failure Mechanism Of CRTS Ⅱ Slab Ballastless Track

When the service time of ballastless track increases gradually,the main components of ballastless track may be damaged or even fail,such as cracks in track slab and supporting layer,concrete damage,interface damage between track slab and CA mortar layer,etc.With the accumulation of ballastless track damage,the service performance of ballastless track will continue to deteriorate.When the ballastless track damage accumulates to a certain degree,the operation of high-speed train might be obviously affected.Therefore,it is of great significance to explore the damage failure mechanism of ballastless track for high-speed railway to improve the maintenance technology and intelligent operation and maintenance theory of ballastless track for high-speed railway in China,and improve the operation safety guarantee system of high-speed railway.In this paper,based on the existing research results,taking CRTSⅡslab ballastless track as the research object,a vehicle-substructure coupling dynamic model considering typical ballastless track damages is proposed.The damage mechanism and evolution law of typical ballastless track damages under the coupling action of train dynamic load and temperature load are analyzed.The main research work of this paper is as follows:(1)The stiffness matrix of cracked beam element can be obtained by using fracture mechanics and energy principle.A refined ballastless track element model is proposed,and a vehicle-ballastless track coupling dynamic model with consideration of concrete cracks is established,which is very useful in the analysis of the effect of train dynamic load on the dynamic response of ballastless track with cracks in detail.(2)On the basis of the vehicle-ballastless track vertical coupling dynamic model with consideration of concrete cracks,the effects of track irregularity,crack depth,CA mortar stiffness and subgrade stiffness on the crack-tip stress intensity factor(CTSIF)are analyzed in detail.The results indicate that the short wave irregularity has a significant effect on the CTSIF of track slab and supporting layer,while the medium and long wave irregularity has a relatively small effect on the CTSIF;When the crack depth ratio is greater than or equal to 0.3,the stress intensity factors of cracks in track slab and supporting layer increase rapidly;With the increase of CA mortar stiffness,the vertical displacement and CTSIF of track slab decrease gradually,while the influence of CA mortar stiffness on the vertical displacement and CTSIF of supporting layer is relatively small;Lower subgrade stiffness will obviously increase the CTSIF,which will accelerate the crack growth rate of track slab and supporting layer.(3)Based on the fatigue crack growth equation(Forman formula)and the dynamic model of wheel-rail system with concrete cracks,an iterative prediction method is proposed to predict the crack growth of concrete under long-term train dynamic load.The propagation law of supporting layer crack under different initial crack depths and local void of subgrade is discussed.The results show that when the initial crack depth is closer to the critical crack depth,the crack propagation rate is faster;When the initial crack depth is 10 mm and the iterative calculation is carried out until the fracture of the supporting layer,the crack depth presents a linear growth trend at the initial stage of the iterative calculation.When the number of trains passing through is more than 3.9989 million times,the growth rate of the crack depth increases rapidly,showing an exponential growth trend,that is,the crack enters the unstable growth stage;The crack damage of supporting layer with 70% residual life is evaluated as level I,and the crack damage of supporting layer with 50% and 30% residual life is evaluated as level II and level III;Based on the analysis results,the corresponding crack depth limits of level I,level II and level III are 16 mm,23mm and 43 mm respectively;The subgrade void will accelerate the propagation of concrete cracks,and reduce the time needed for the crack growth to the instability stage,that is,the fatigue life of the supporting layer will be significantly reduced.(4)On the basis of the principle of vehicle-track interaction,a vehicle-ballastless track coupling dynamic model considering concrete damage is established by introducing concrete elastic damage constitutive model and layered beam element model.The effects of structural parameters,bearing failure,temperature gradient load and prestress loss on the concrete damage mechanism are investigated.The results show that the concrete damage factors of track slab and supporting layer are mainly distributed on the upper and lower surfaces,and the extreme values of damage factors all appear on the upper surface;Larger fastener stiffness will obviously aggravate the concrete damage of track slab and supporting layer.The fastener stiffness currently used can not only ensure the reasonable rail vertical displacement,but also effectively reduce the cumulative concrete damage rate of track slab and supporting layer,which shows the rationality of fastener stiffness used in existing high-speed rail lines;The CA mortar stiffness influences the damage factor of upper surface and vertical displacement of track slab significantly,while the CA mortar stiffness has a relatively little influence on the lower surface damage factor of track slab;Lower subgrade stiffness will significantly increase the concrete damage caused by a single train;Compared with the interval failure of fasteners,the continuous failure of fasteners has more adverse effects on the concrete damage of ballastless track structure;The CA mortar layer void has a significant effect on the concrete damage factors;The influence of subgrade void on the concrete damage of supporting layer is far greater than that on track slab;Positive temperature gradient plays a significant role in restraining the concrete damage of track slab,and the coupling effect of negative temperature gradient and train dynamic load as well as the loss of longitudinal prestress in track slab will obviously increase the concrete damage accumulation rate of track slab.(5)The model of ballastless track-bridge element is proposed by using the finite element method,and the discrete cohesive model is introduced into the train-track-bridge model.The coupling dynamic analysis model of vehicle-ballastless track-bridge considering the interface damage of ballastless track is established.The influences of driving speed,wheel-rail adhesion coefficient,void length of CA mortar layer and cohesive model parameters on the interface damage mechanism and evolution law are investigated in detail.The main conclusions are as follows: within the scope of one span simply supported beam bridge,the longitudinal relative displacement between track slab and CA mortar layer presents a sine curve trend,and the minimum and maximum values of the longitudinal relative displacement are located at 5.2m and 27.95 m respectively;Under the coupling influence of dynamic load and bridge temperature load,the interface damage between track slab and CA mortar layer mainly occurs in the range of 25 ～ 30m;With the increase of wheel-rail adhesion coefficient,the mean value and extreme value of wheel-rail longitudinal force and the longitudinal relative displacement between slab and CA mortar layer increase gradually,which leads to the corresponding increase of interface damage factor;When there is CA mortar layer void,the interface between the ballastless track layers near the CA mortar layer void is under the mixed stress state of tension and shear;From the point of view of slowing down the cumulative rate of interface damage of ballastless track,it is suggested to repair the CA mortar layer before its void length is extended to 0.65m;As the tangential stiffness of the interface decreases,the interface damage factor increases gradually;In the range of one span simply supported bridge,the interface damage factors mainly appear in the range of 3 ～ 8m and 24 ～ 30m.