Study On Failure Analysis And Improvement On Fastening Bolts Of Axle Mounted Brake Disc For High-speed Train

Brake disc plays an important role in securing high-speed trains’ safty when realizing speed regulation and stop.Brake disc and disc hub are connected by fasteners.Although the bolts have been strictly checked in the design and manufacture process,the fracture accidents of the fastening bolts in the brake disc occur from time to time due to the complex state of force,which results from alternating thermal and mechanical loads during service.In this paper,the causes and other contributing factors resulting in bolt breaking in axle mounted brake disc for EMU are explored through test and simulation,and further suggestions to improve bolt structure are put forward.The main research contents and conclusions are as follows:(1)Research on the macro and micro failure mechanism of fractured bolt has been carried out.Through physical and chemical analysis of the broken bolt,it is found that the fracture form is fatigue fracture.Then the material composition,grain size,surface decarburization layer and non-metallic inclusion of the bolt are tested.The results show that the material characteristics of the bolt meet the requirements of GB/T 3098.1,so the bolt fracture is not caused by the defect of the material.(2)The temperature field and stress field of the brake disc are analyzed under severe braking conditions.The study found that under emergency braking and continuous braking on long ramps,the bolt bears radial bending load because of the disc body’s,disc hub’s and pressing ring’s uneven thermal expansion and the relative slippage it causes.The maximum stress at the bottom of the first and second thread engagement exceeds the yield limit of the bolt material,and cracks are easily generated in the high-stress area to release the excessive stress.Along with the use of the brake disc,the cracks continue to expand until the bolt broken.(3)Using ultrasonic bolt axial force measuring equipment to measure the axial force of brake disc bolts.The axial force of the measured bolts has increased by 17.6 %,which proves that the axis of the bolt has deviated from the initial position during service,and the bolt is in a state of bending and elongation,which increased with service time.While the bending elongation increase,the amplitude of the stress fluctuation that the bolt can withstand decreases,and the risk of fatigue crack initiation and bolt fracture increase gradually.(4)The simulation method is used to study the influence of pre-tightening force and impact load on the maximum stress of thread engagement position.It is found that as the pre-tightening force increases,the maximum stress at the thread engagement position also increases.When the pre-tightening force is 48 k N,the maximum stress is804 MPa.In accordance with the ultimate load set forth in the IEC 61373,the lateral impact load have greater influence on the maximum stress at the thread engagement position than the vertical impact load,but it will not affect the safety of the bolt.(5)Based on the VDI 2230 standard,the bolts were checked for safety and found that under severe braking conditions the fatigue safety factor of the original structure is less than 1,and the bolts may undergo fatigue fracture.Theoretical calculation,simulation analysis and test results show that increasing the thread length and increasing the bolt length can improve the flexibility of the bolt,so the fatigue resistance of the bolt can be improved.Through physical and chemical analysis and experimental research on broken bolts,with the help of finite element simulation,theoretical analysis and experimental verification,the main causes for bolt fracture are found and suggestions for improvement of bolts are proposed.The paper has great guiding significance for the design and safe use of fastening bolt for brake disc.