Study On The Derailment Mechanism And Running Safety Of Trains

Safety is the eternal subject of railway transportation, the rapid development of high-speed and heavy-load railways brings new challenges for the running safety of trains. On the one hand, the derailment of high-speed or heavy-load train will cause more serious damage, especailly, if a high-speed train derailed, a catastrophe may be brought. And on the other hand, the running principles, performance, and dynamical enviroment of high speed train are essentially different with those of the ordinary train. However, due to the complexity of derailment problem, the knowledge on the derailment mechanism still stays at elementary stage; even there is no exact evaluation criterion for quasi-steady derailment. Nadal's derailment criterion, founded in 1896, and its amendents laterward are the fundamental criteria for derailment evaluation nowadays. These criteria have considerable limitations for assessing dynamical derailment since they are based on the most conservative equilibrium condition for quasi-steady flange-climbing derailment. Therefore, to propose quantitative evaluation criteria with wide applicability based on thorough unstanding of derailment mechanism is the fundamental problem for the running safey research of trains.Aiming at derailment mechanism and running safety assessment, the following aspects are extensively studied based on the wheel/rail interactions:(1) The geometrical constraint and multi-point contact between wheel and rail are thoroughly studied based on the wheel/rail contact characteristic and profile shapes. It's found that the probability of two-point or multi-point contact is determined by the curvatures of track-projected lines of wheel and rail profile, while the wheel/rail constraints are determined by the dynamical equilibrium of wheelset and rail. A wheel/rail constraint model which takes the multi-point contact and contact lose into account is set up, and the non-smooth effect and contact impact between wheel and rail are analyzed. The main curvatures of wheel and rail profiles determine the effective area of Hertz contact theory during flanging. An exact dynamical model with unilateral contact between wheel and rail which can take the multi-point-contact and contact lose into account is established, and the new model is verified by passing the IAVSD benchmark model.(2) The quasi-steady derailment mechanism of a single wheelset is investigated, the changing law between the ratio of lateral creep force to normal force and angle of attack is analysed. The improved derailment criteria are proposed, which maintain the simplicity of Nadal's criterion, while take the effects of angle of attack, friction coefficient and flange angle into account. The new criteria are suitiable for evaluating quasi-steady flange-climbing and flange-sliding derailment, and the general applicality is verified by the comparision between experimental results of JNR and of TTCI and the results of the proposed criteria.(3) The wheel/rail lateral collision equation is set up, and the jumping derailment mechanism induced by the wheel/rail lateral impact is studied exploringly. The parametric study of wheel jumping derailment indicates that the wheel load, friction coefficient and lateral pre-collision velocity of wheelset have significant influence on the jumping derailment.(4) The derailment mechanism is explained in the viewpoint of the system dynamics. A novel dynamic derailment evaluation method based on the function of wheel/rail lateral displacment is suggested, and can applies to evaluating quasi-steady, dynamic and jumping derailment. The proposed dynamic derailment evaluation method includes derailment safety coefficient and its first and second time derivatives, of which the derailment safety coefficient can determine the degree of derailment risk directly, while the first and second time derivatives of derailment safety coefficient can reflect the relative lateral velocity and acceleration between wheel and rail in lateral direction. The geometrical contact of two wheel/rail profile pairs and the dynamic derailment simulation of a single wheelset are carried out to validate the dynamic derailment evaluation method.(5) The dynamic behaviours of a single wheelset during derailment are explored, including the dynamic derailment process of a single wheelset with yaw stiffness and external loads and the hunting derailment mechanism of free wheelset. The studies on the former case show that the lateral force to derail a wheel in positive angle of attack is much less than that to derail a wheel in negative angle of attack. And the simulation results of hunting derailment indicates that there is a derailment critical velocity which corresponds to the maximal stable limit cycle during hunting for wheel-rail system, any weak excitation will induce a derailment if the running velocity exceeds the derailment critical velocity. Furthermore, the comparsions between the dynamic derailment evaluation method and the traditional derailment criteria are performed through the hunting derailment simulation of single wheelset. It is found that the dynamic derailment evaluation criteria judges all of the cases correctly, while the derailment coefficient and wheel unloading ratio, although reflect the derailment risk to some extent, misjudges some of the cases.(6) From the point of view of the wheel/rail contact geometry, wheel/rail kinematics and force equilibrims, the derailment conditions of a wheel are discussed. The traditional derailment criteria are analyzed systematically, and their upper and lower limits and the relationship with derailment are studied. The phenomenon of no derailment occurring are explained even when derailment quotient and wheel unloading ratio exceed their critical values. The application of the proposed derailment criteria including the improved quasi-steady derailment criterion which takes the angle of attack into accout, the jumping derailment criterion for wheel/rail collision and the dynamic derailment criterion are introduced in detail, and the feathers and applicable conditions of these criteria are analyzed, the comprehensive derailment evaluation method are suggested.(7) The dynamic model of vehicle system is established, the running safety of curve passing and hunting are analyzed by virtue of the comprehensive derailment evaluation method, the dynamic process of vehicle hunting derailment is analyzed, and the correctness of the dynamic derailment criterion is further verified.