Investigation of fatigue durability and influencing factors of coil springs: A case study for metro vehicles

Abstract

This study investigates the fatigue failure behavior of coil compression springs and various influencing factors. The intrinsic and induced causes of fatigue failure in metro steel springs were revealed by systematic tests. Macroscopic observation and metallographic analysis of the fracture indicate that decarburization, which leads to a reduction in the fatigue limit of the material, was the essential cause of the failure. The resonance of the steel spring caused by rail corrugation leads to an increase in dynamic stress, which is the induced cause of the failure. The combined effect of the two factors promoted the failure of the springs. In addition, decarburization led to a decrease in the microhardness of the spring surface from 462 HV to 151 HV and a decrease in fatigue life from over 3.6 million kilometers to 521,000 km (with the decarburization depth of 0.3 mm). The effects of different stress ratios and decarburization depths on the equivalent shear stress (ESS) were comprehensively investigated. The ESS of steel springs with decarburization and different stress ratios were calculated according to the FKM Guideline and the EN 17149 standard. The ESS decreases from 188 MPa to 77 MPa at a stress ratio of R = 0, and from 168 MPa to 76 MPa at a stress ratio of R = 0.5. If the effects of rail corrugation are not taken into account, the ESS meets the design life requirements. Furthermore, the effects of different rail fastener types and track structures on the dynamic stress were compared. It was found that the double-layer nonlinear vibration damping fasteners can significantly improve steel spring vibration. Finally, improvement measures and recommendations are proposed based on the root causes of failure and the influencing factors.