A new test to study the cyclic hardening behaviour of a range of high strength rail materials
Introduction
Flow localization can be studied in two extreme contexts; as a useful process or as a damage precursor. As an example of the former, localized grain refinement processes have been considered recently to improve material strength at critical zones of a part or structure (e.g. Surface Mechanical Attrition Treatment (SMAT) [1] and the plane stress local torsion (PLST) process [2]). The latter has been proposed and studied recently to enhance the mechanical properties of materials locally. The PSLT process offers a zero shape change treatment to accumulate localized shear strain and localized grain refinement at a critical zone (e.g. a fastener hole edge). The flow localization can be devised deliberately as it develops a gradient of microstructure in the metallic parts and results in their local reinforcement. The process is characterized by variations in the structure and mechanical properties of the material similar to functionally graded materials [3] or those obtained by a method called “OPTICA” to improve wear resistance of ductile iron by locally optimizing the microstructure [4]. A large number of studies have shown that the gradient contributes as a strengthening mechanism developed by the severe plastic deformation [5], [6], compressive residual stress [7] or laser peening [8].
In contrast, there are situations in which the flow localization is harmful and has to be prevented. This is the case when due to a severe loading condition (such as contact loading) a significant amount of local plastic strain develops by time. For example, the cyclic loading in the presence of a mean compressive stress can lead to wear in rails caused by ratcheting that eventually results in component failure [9], [10].
At the wheel–rail interface, both rolling and sliding occur in the contact zone. Wheel–rail systems are inherently subject to damage due to sliding that typically manifests itself as plastic deformation, wear or rolling contact fatigue (Fig. 1). The key factors that influence the rate of damage are “material behaviour” and “the severity of loading”, i.e. normal load or contact pressure in addition to the sliding velocity or creepage. Plastic ratcheting in rails, accumulation of small increments of plastic deformation with each pass of the wheel, occurs when the loading conditions are above the plastic shakedown limit [13]. The localized deformation usually develops a large and nonlinear strain gradient in the direction of friction at the contact surface. The gradient is necessary for a continuous and smooth transition between the plastic zone and its underlying undeformed zone. Experimental observations on the microstructures developed by a twin-disk test such as those reported in [14] have confirmed the non-linear nature of the strain.
The “incremental plastic strain” and “microstructure degradation” immediately below the rail surface affect the wear and fatigue behaviour of the rail system subjected to cyclic loading or adverse service environments.
The features of cyclic deformation in rails have been modelled using “two-surface plasticity” [15], “revised two-surface model” [16], and “unified visco-plastic model” [17]. For rail steels, some results of uniaxial and multi-axial strain cycling and ratcheting have been reported [18], [19]. However, analytical calculations are not practical due to the “incremental” and “hysteresis” nature of the near surface deformation; some history-related parameters are needed to quantify the damage. A significant number of multifaceted “transient” and “history related” phenomena are actively interacting during formation of the damage, including microstructure degradation. The deformation response of the rail to the cyclic loading and un-loading depends not only on the current loading but also on its past status. This dependence arises because a lower stress is required to reverse the direction of slip on a certain slip plane of the polycrystalline metal than to continue slip in the original direction. Actual tests on the operating rail–wheel system are impractical. It is therefore desirable to develop a “physical test” that can be fully controlled under laboratory conditions during which the instantaneous damage is measured and correlated to the in-field damage using a combination of modelling and simple measurements.
Two commonly-used physical tests for simulation of ratcheting in the rail–wheel system are the compression-torsion and the twin-disc tests. The torsion test or its variants (e.g. torsion–tension) have been frequently used to simulate complex paths of deformation (e.g. [18], [20], [21], [22], [23], [24], [25]) and the flow localization in situations where shear is the prominent deformation mechanism. However, the tests suffer from serious limitations. They are limited in their ability to reproduce the exact deformation mode corresponding to actual service conditions. The torsion test produces heterogeneous deformation but its deformation gradient, even for large plastic deformations, remains linear in the radial direction. Based on some symmetry considerations, Canova et al. [25] have explained the conditions which are responsible for the linear gradient. In many cases such as the running surface in rails, the gradient is highly nonlinear and cannot be adequately represented by a torsion/tension test. Another physical simulation, twin disk test [14], requires relatively large samples and therefore could be unsuitable when the rail has a non-uniform structure such as in rail welds. High pressure torsion test has been frequently used as a grain refinement process (see for example [26]) or as a physical test [27]. The test requires small test samples, however its gradient of deformation in the radial direction and the longitudinal direction are not fully known yet; several researchers have assumed a linear gradient.
A physical simulation of the deformation behaviour with a small sample can offer a higher resolution for the non-uniform material cases and therefore is an attractive replacement for twin-disk tests or tests under actual service conditions. The test could also provide initial screening data to rank the behaviour of a range of rail material grades.
A new physical test, the Plane Stress Local Torsion, is proposed and performed in this work to avoid the limitations with the orthodox physical tests. The test is used to produce a nonlinear strain gradient in its samples and to study and compare their strain cyclic behaviours.
Section snippets
Description of PSLT
The plane stress local torsion test is an axi-symmetric (one-dimensional) test that allows the physical simulation of the flow localization. The test involves twisting the material in a constrained manner to produce a localized plastic deformation via torsional loading and measuring the torque–twist response. It creates a plastic zone adjacent to a rigid zone with a nonlinear distribution of the shear deformation in the plastic zone which is similar to the deformation of rail materials after a
The PSLT experiments and results
The PSLT specimens were made out of a range of high strength rail steels commonly used in heavy haul applications; these comprised a plain C–Mn head hardened grade (HH) corresponding to Australian Standard AS1085.1, a low alloy heat treated grade (LAHT), and a hypereutectoid carbon heat treated grade (HE). The main differences between these grades were the composition (with only the latter containing a hypereutectoid carbon level) and the heat treatment method. The HH grade is produced using an
Conclusions
The strain hardening behaviour of three grades of rail steel (HH, HE, and LAHT) has been studied using a new mechanical test method, plane stress local torsion (PSLT). The ratcheting torque–twist curves obtained from the PSLT experiments were used to study and compare the torsional stiffness behaviour of rail grades. The strain hardening behaviour of rail grades observed in the PSLT experiments confirms the usability of the PSLT test to simulate the near-surface damage associated with wear and
Acknowledgement
The authors would like to thank Rio Tinto Iron Ore and BHP Billiton Iron Ore for their support of the research activities on which this paper is based, and for provision of the sample rails.
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