Fatigue Fracture Analysis Of Brake Disc Bolts Under Continuous Braking Condition

As one of the significant components of the brake disc,the bolt not only bears a high axial preload,but also receives the thermal load and external vibration excitation during braking process.Since the opening of the Lanzhou-Xinjiang(LX)Line,there have been several accidents of bolt broken or missing in the brake disc.The bolt of EMU brake disc which running on the LX line were researched to study the fracture reason.And the damage of the bolt was analyzed under the long ramp.The main tasks as follows:(1)The broken bolts were tested by physical and chemical properties such as chemical composition,hardness and tensile strength.The morphology and metallographic structure of the bolt fracture was observed through the scanning electron microscope to determine whether the root material of the bolt is defective.It can be seen from the analysis results of the bolt material that its physical and chemical properties meet the relevant technical requirements.The section shows that the bolt propagation area is small and it was suffered overload to low-cycle fatigue fracture during the service.(2)According to the fracture position of the bolt,a hexahedral mesh finite element model considering the thread is established.The thermal-mechanical coupling method is using to analyze the mechanical and thermal characteristics of the bolt.In order to analyze the influence of high temperature on the brake disc bolts,the temperature and stress fields with different initial braking speeds(350 km/h,300 km/h,and 250 km/h)were calculated.The temperature level of the bolt is lower than the surface of the brake disc.The maximum temperature is located near the middle of the bolt of the brake disc,and spreads around.There is a temperature hysteresis between the brake disc surface and the bolt.The temperature in the middle of the bolt and the thread root gradually increases and tends to be stable.(3)The working conditions of the long ramp in the LX Line were taken as heat flux density to the finite element analysis.Due to differences in the thermal properties and structure of the brake disc materials,the temperature field of the bolts is distributed unevenly during continuous braking.In the preload and loosening phases,the stress distribution of the bolt is more symmetrical,and the maximum Misses stress is at the first engaged thread.Due to the loadbearing characteristics of the bolt structure and the small radius of the thread root,the first engaged thread root causes severe stress concentration.With increasing of thermal expansion during continuous braking,the bolt preload gradually raises and there is a torque along the X axis.The change of preload and torque makes the bolt stress distribution asymmetrical,which aggravates the stress concentration at the thread root.The stress concentration position is the same as the actual fracture position of the bolt.(4)The critical surface method combined with the SWT fatigue criterion is used to analyze the crack initiation and propagation characteristics of bolts.The damage parameters on the critical plane at different thread depths are obtained through coordinate transformation.The maximum damage parameter is located on the surface of the root of the thread and decreases significantly with increasing depth.The distribution characteristics of damage parameters are roughly the same as the geometric characteristics of the root area of the thread.The additional thermal load on the root of the thread under continuous braking has a greater impact on the life of the bolt.Because the bolts are affected by the preload and thermal stress,the fatigue life of the bolts estimated by the SWT criterion is larger than the actual situation.