Effect of Large Shear Deformations on the Fracture Behavior of a Fully Pearlitic Steel
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INTRODUCTION
MOTIVATED by the exceptional mechanical and physical properties of ultrafine-grained and nanocrystalline materials, severe plastic deformation (SPD) has gained great scientific interest especially recently as a route to produce such materials.[1–4] Although this route requires great technical efforts to be taken to synthesize these materials, they can occur also unintentionally in different fields, for instance during machining,[5] wire drawing,[6] and on the surface of rails.[7,8] In the last example, the material changes are promoted during railwheel contact because of the loading conditions, which can be characterized by high contact forces and large shear stresses. These contact characteristics lead to strong plastic deformations of the rail surface and often form the initiation point of roll cycle fatigue (RCF) related crack-like defects such as headchecks and squats.[9,10] In general, rail steels are tested regarding their RCF behavior, for instance, through two-disk roller tests.[11,12] These tests can rank materials by their crack formation tendency but cannot provide information about the local fracture properties of the strongly plastically deformed surface layers. Classic testing methods for evaluating the fracture properties are not applicable because the thin deformation layers do not meet basic requirements for subsequent testing. However, the mechanical, in particular, the fracture properties of these strongly deformed surface layers, would be of great interest in understanding damage development A. HOHENWARTER, Post-Doc, and R. PIPPAN, Senior Researcher and Vice Director, are with the Erich Schmid Institute of Materials Science, Austrian Academy of Sciences, A-8700 Leoben, Austria, and with the CD-Laboratory for Local Analysis of Deformation and Fracture, A-8700 Leoben, Austria. Contact e-mail: anton. [email protected] A. TAYLOR, PhD Student, is with the Erich Schmid Institute of Materials Science, Austrian Academy of Sciences. R. STOCK, Staff Researcher, is with voestalpine Schienen GmbH, A-8700 Leoben, Austria. Manuscript submitted December 21, 2009. Article published online November 19, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A
in such materials while also being helpful in future material design strategies. High-pressure torsion (HPT) is a SPD deformation technique that can apply large plastic strains to hard and brittle materials. It provides the possibility to simulate the same microstructural conditions as can be found in deformed rail surface layers, and at the same time, suitable specimen sizes for subsequent material characterization, e.g., fracture toughness measurements, can be obtained. Here, the technique was used as an approach to simulate comparable microstructural conditions as also found in rails in service by deforming a fully pearlitic rail steel to well-defined degrees of deformation. The fracture toughness after predeformation in different crack propagation directions was tested to obtain insight into the local fracture properties of deformation layers, which
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