Determination of Dynamic Fracture Toughness Properties of Rail Steels

摘要

Motivated by the occurrence of a long-running rail web fracture in service, dynamic fracture mechanics research was undertaken to (1) quantify the crack driving force due to the residual stresses induced by roller straightening operations, (2) determine dynamic fracture toughness values for rail steel, and (3) assess current analysis procedures for calculating dynamic fracture propagation in rail. As a first step, elastodynamic finite element analyses were made of full-section rail impact experiments conducted in Japan. These analyses revealed that the fracture propagation event in these experiments has an unexpectedly complex character. This behavior complicates the use of the raw crack jump length to measure the ability of the combination of a given rail steel and straightening operation to resist catastrophic fracture. Further analyses using a beam-on-elastic foundation model of a split rail provided a relation between the rail web residual stresses and the crack driving force. It was concluded from preliminary work that a near balance can exist between the crack driving forces arising from the residual stresses induced by conventional straightening procedures and the dynamic fracture toughness values of some rail steels. Consequently, a major safety issue possibly exists that may require the development of screening procedures to assure the selection of fracture-safe rail for service.