A new test to study the cyclic hardening behaviour of a range of high strength rail materials

Plastic ratcheting plays a key role in wear and rolling contact fatigue crack formation at the wheel-rail interface. Tests to examine the wear and rolling contact fatigue behaviour of rail materials over a wide range of service conditions are expensive and can be impractical. A physical simulation of the deformation behaviour associated with ratcheting is an attractive replacement for such tests. In this work, the Plane Stress Local Torsion (PSLT) test is proposed as a novel mechanical testing method to physically simulate near-surface deformation in rails and to characterize the cyclic deformation behaviour of rail materials. Contrary to the orthodox mechanical tests, the proposed method is capable of producing a nonlinear strain gradient in test samples which resembles the real gradient in the rail-wheel system. The PSLT testing was performed on specimens of the nominated rail steels in a strain-controlled fashion to simulate the unidirectional as well as the fully reversing strain cycles. The test was used to examine the cyclic hardening behaviour and ratcheting characteristics of a range of high strength rail materials under cyclic loading at room temperature. The effect of the cyclic strain amplitude under symmetrical strain cycling on the cyclic hardening behaviour was investigated. Experimental torque-twist data were used to compare the plastic flow behaviour of commonly used rail materials in heavy haul applications under cyclic loading. The cyclic and ratcheting strain accumulation behaviour in the test samples was characterized based on the torque-twist data to allow a comparative study of the mechanical properties. Optical microscopy of the tested samples was also performed to compare the microstructures at the flow localization zone for different materials subjected to cyclic strain accumulation.