Analysis On The Dynamic Evolving Tribological Behaviors Of Train Brake Systems Under Thermomechanical Field

Disc brake system for trains is mainly composed of brake discs and friction blocks,which are key components to ensure driving safety.During the braking process,the relative motion of the disc-block contact interface leads to friction,then the huge kinetic energy is transformed into thermal energy by friction,accompanied by complex thermomechanical tribological behaviors,such as friction heat,wear degradation,elastoplastic deformation,friction-induced vibration(FIVN),contact condition evolution,interface fatigue damage,etc.Meanwhile,the interaction mechanism and dynamic evolution of these tribological behaviors are more complex.For example,the thermal deformation of friction material caused by friction heat indirectly affects the interface contact condition.Penetration and embedding between contact pair materials may lead to sub-surface plastic strain and reduce surface ductility,eventually causing material degradation.The changed contact surface profile caused by material loss directly affects the contact pressure distribution and FIVN characteristics.In summary,the braking process of trains is accompanied by the strong thermomechanical coupling action like temperature variation,stress change,unstable vibrations,and material degradation.Experimental testing is a common method to study the interface tribological behavior under thermomechanical coupling fields.But its time and economic costs are expensive,and it is difficult to effectively study the evolution and interaction mechanism of thermomechanical coupling tribological behaviors,by only analyzing the worn surface.Meanwhile,due to the complexity in disc brake system structure and shape and non-liner contact conditions,using the analytic method to calculate thermomechanical tribology behaviors under real brake conditions becomes extremely difficult.Luckily,With the development of computer technology and finite element method,potential methods for the effective analysis of thermomechanical coupling tribological behaviors are available.Crucially,Heat transmission,mechanics,friction and wear,as well as vibration noise issues,are all tightly coupled at the brake interface.Thus,these above problems need to be solved at the same time when studying and predicting the brake process.It brings considerable challenges both in theory and technology.Therefore,this study focuses on the brake interface tribology behaviors under the effect of thermo-mechanical field.And the technical route of presenting the method,validating the method,and applying the method is applied.The main works and conclusions are as follow:(1)Based on the co-simulation by ABAQUS,FORTRAN,and MATLAB,the selfwritten subroutines(UMESHMOTION and DFLUX),Arbitrary Lagrangian-Eulerian(ALE)meshing technology,result transfer technology,restart technology,and Archard wear model considered temperature effects are applied.And the new method is proposed to study the dynamic evolving tribological behaviors in brake systems of trains under the effects of a thermomechanical field.The results show that this method can comprehensively adopt multidisciplinary knowledge of thermodynamics,tribology,dynamics,and damage mechanics to systematically investigate tribological behaviors of train disc brake systems under thermomechanical fields,taking into account the evolution and interaction of the disc interface temperature,friction block wear behavior,disc-block contact condition,friction vibration noise,etc.The dynamic evolving tribological behaviors of the braking system under the action of mechanical stress fields and thermo-mechanical coupling fields under different braking conditions can be investigated.(2)The bench test is developed to obtain the real thermomechanical coupling tribological behaviors under typical train disc braking conditions.The proposed method for predicting dynamic evolving tribological behaviors in the brake systems under the effects of a thermomechanical field is further adopted to simulate the running-in and formal test process.And the proposed method is comprehensively validated in terms of core indicators,interface temperature,wear degradation,friction vibration noise,and contact condition evolution.The results show that as this new method can consider material removal and thermomechanical coupling effects at the same time to obtain reliable interfacial tribological behavior results,it provides an effective approach for studying the thermomechanical coupling tribological behavior of train disc brakes.Also,it was discovered that during the running-in phase,wear behavior has a substantial impact on contact pressure,and material removal can alleviate the uneven contact condition.Thus,the running-in process should not be ignored when simulating the brake interface problem.The wear mechanism of the friction blocks is abrasive wear and mild adhesive wear under the operating conditions in this research.The interior surfaces of the friction blocks exhibit more severe wear degradation and increased roughness when compared to the outer surfaces.Because of competing mechanisms between the system’s structural rigidity and the length of the frictional contact arc,the maximum temperature occurs in the contact ring band somewhat closer to the inner side.The wear degradation of the friction block accelerates as the temperature rises,so the effect of temperature on wear should not be overlooked in simulation.The mode coupling mechanism may be a potential cause for the self-excited vibration phenomenon,further,the squeal noise is generated in this braking test condition.(3)The validated predictive method is applied to solve the problem of friction block shape optimizing design for trains.The brake bench tests are performed to ensure the applicability and effectiveness of the proposed method in this problem.The evolution of friction force,wear profile and wear volume,contact conditions,and stress distribution are all revealed.The coupling effect mechanism of friction block shapes on the tribological and wear behaviors are explored.Based on the above research,the intrinsic link among friction block shapes,tribological behaviors,and wear behaviors is established.The results indicate that brake block shape significantly influences the interface wear conditions.At the same time,the block shape affects the coupling mechanism between friction,contact inclination,contact area,and contact pressure,which ultimately affects the interface wear features.Among the three shapes in this work,the hexagonal block has the most obvious phenomenon that friction force concentrates in the leading edge,which leads to the poor interface contact state,increasing contact inclination,slower expansion of the contact area,and the concentration of contact pressure,which ultimately leads to more severe interface wear.The results indicate that a suitable friction block geometry can improve the service tribological feature of railway friction brake system,reduce the brake block wear,and enhance the service reliability of the brake system.This research can provide theoretical support for the design optimization of brake block shapes design of brake systems in the future.