| Metro vehicles must be kept in constant braking during operation.Metro vehicles using tread braking generate a large amount of frictional heat during braking,which causes the temperature to rise on the wheel contact surface,and causes the local thermal stress and the wheel-rail mechanical load to be coupled.It is easy to lead to transverse thermal cracks on the surface of wheel treads.With the increase of operating mileage and braking times,thermal cracks may expand under the repeated action of wheel-rail dynamic loads and frictional heat of braking,thereby affecting the safe operation of metro vehicles.Aiming at the above thermal cracks,a 3D elastic-plastic finite element thermal-mechanical coupled wheel model with cracks was established based on the real geometry of metro wheels using the finite element software ANSYS.The temperature rise of wheel-rail friction and braking,as well as the effect of thermal convection were considered in the model.The thermal-mechanical coupling stress field in the crack region was obtained by the finite element model calculation.The crack-tip stress intensity factor was used to determine the crack propagation,and the expansion behavior of the thermal crack on the tread was studied.The conclusions are obtained as follows:(1)When the rolling speed of the wheel is 80 km/h and the longitudinal creep rate is 1.5‰,the friction between the wheel and rail can cause a temperature rise of about 43~°C on the surface of the contact patch.The temperature rise of friction basically has no effect on the crack propagation.The expansion of the crack in the contact patch is driven by the mechanical load and its expansion mainly belongs to sliding type.When the speed is 20km/h and wheel slip occurs(longitudinal creep rate is 13.5%),the temperature caused by friction on the surface of the wheel-rail contact patch is up to 483°C.The temperature field is progressively transmitted from the surface of the contact patch to a depth of 0.2 mm in the radial direction of the wheel.That’s to say,when the crack tip depth is less than 0.2 mm,the crack propagation is driven by the thermal stress and the mechanical stress in the contact patch.Compared with the results considering only the mechanical load under the same working conditions,it is found that the thermal stress can increase the crack tip stress intensity factor by about 23%.When the crack depth is above 0.2 mm,crack propagation is mainly affected by mechanical loads.(2)The contact area between the wheel tread and the brake shoe has a large effect on the expansion of thermal cracks.The smaller the area is,the higher the temperature generated on the surface of the wheel is when the train is braking.If the contact area is reduced to 30%,40%and 50%of the brake surface of the brake shoe,the temperature of contact zone of tread in the braking can be up to 502~°C,384~°C and 313~°C.When the contact area is less than 50%,the crack growth increases with the decrease of the contact area.When the contact area is greater than 50%,tread braking basically has no effect on the expansion of the wheel surface cracks.(3)The greater the axle load of the train is,the higher the surface temperature of the wheel is when the tread is braking.If the trains with the axle weight of 11,14 and 16 t are braked under the same conditions,the maximum temperature of the tread caused by friction is 355,443 and 502~°C,respectively.The crack tip stress intensity factor of wheel tread increases with the increase of temperature.When the wheel with high temperature contacts with the rail,the cooling of the contact patch by the rail reduces the stress intensity factor of the crack in the contact patch by about 9%. |
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