Defect Growth Characterization in Modern Rail Steels

The Federal Railroad Administration (FRA) has been sponsoring research on rail integrity for several decades. This research has been chiefly managed and conducted by the Volpe National Transportation Systems Center (Volpe). Particular focus has been given in this research to rail head defects, known as detail fractures, since they are the most commonly encountered defect in continuous welded rail track [1]. Testing and analyses have been performed on railroad rails manufactured without head hardening. Modern rail, however, are now heat treated during the manufacturing process to harden the rail surface to increase its resistance to wear. As such, the heat treatment and nonuniform cooling induce complex residual stress patterns in the rail that can affect microstructure and fatigue crack growth rate behavior. This paper will describe research to examine defect growth behavior of modern rail steels. This research is a collaboration among several organizations: Thornton-Tomasetti, Arcelor-Mittal, Lehigh University, Harvard University, National Institute of Standards and Technology (NIST), Fraunhofer Institute, and Volpe. Arcelor Mittal donated rails with different grades of steel: advanced head hardened, head hardened, and standard strength (i.e. non-head-hardened). Lehigh conducted laboratory tests on specimens cut from these rails to perform various tests, which include: hardness measurements, mechanical testing to measure tensile properties, fracture toughness measurements, and fatigue crack growth rate tests. All of these tests were performed in accordance with applicable ASTM International standards. NIST and Fraunhofer performed preliminary neutron diffraction measurements of residual stresses on the different rails. Moreover, this paper will present results from the laboratory testing program. Implications of these results on detail fracture growth behavior will also be discussed.