Strain rate estimation of alloy steel frog and its impact on wheel-rail rolling contact behavior

Abstract

The passage of wheels over fixed frogs induces intense wheel-rail interactions, and the mechanical properties of frogs under wheel impact vary significantly due to strain rate effects. Considering the strain rate dependency of frog materials is critical for analyzing wheel-rail contact behavior during wheel traversal. This study establishes a three-dimensional transient rolling contact finite element (FE) model of the wheel-frog system using an explicit dynamic approach. The strain rate distributions on the surface and along the depth of the nose rail and wing rail during wheel passage are investigated. Quasi-static and impact compression experiments were carried out on the alloy steel nose rail material, revealing the strain rate strengthening mechanism of the material from a microscopic perspective. Meanwhile, based on the Johnson-Cook empirical model, its dynamic constitutive relationship was established. The explicit finite element model was used to evaluate the influence of strain rate effects on the wheel-rail rolling contact behavior. Results indicate that the strain rate of the nose rail increases abruptly under wheel impact and decreases to match the wing rail level after load transition. The equivalent strain rate in the wing rail distributes more widely along the depth compared to the nose rail, where strain rates concentrate near the surface. Speed significantly affects the strain rate magnitude in the frog, while axle load shows negligible influence. The nose rail material exhibits pronounced strain rate sensitivity, with yield strength increasing with strain rate. The strain rate strengthening effect of the nose rail material (bainitic steel) is mainly dominated by the high dislocation density and dynamic dislocation behavior in its microstructure, and the phenomenological Johnson-Cook model effectively captures this strain rate dependence. Strain rate effects marginally influence wheel-rail forces, contact stresses, and stick–slip distributions but notably alter equivalent stresses and plastic strains. At high strain rate regions, equivalent stress peaks increase markedly while plastic strain maxima decrease. This study provides reliable strain rate-dependent mechanical parameters for wheel-frog dynamic contact simulations, thereby enabling a more realistic description of contact behavior at the wheel-rail interface.