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Abstract: Rail breaks, resulting from increasing heavy axle-load operations and harsh environmental conditions, remain a major cause of catastrophic derailment of vehicles in North America. The objective of this research project is to develop convenient testing methods to identify the optimum high-strength rail steels for Canadian weather, which can contribute to reducing derailment risks for the Canadian railway industry. In this research project, both destructive and non-destructive testing methods are developed to quantify the fracture toughness of high-strength rail steels. The project can be broadly divided into two phases. Phase I, which is reported in Chapters 2 and 3, involves establishing an extended strain energy density (SED) model for estimating the fracture toughness of high-strength rail steels at 23, -10, and -40oC. First, the mechanical properties, including the mode I critical stress intensity factor (KIc), constitutive equation, and Vickers hardness, of three types of high-strength rail steels are tested and compared at 23, -10, and -40oC. According to the experimental results, their KIc values and tensile properties are not correlated with each other. Further study in phase I involves investigating the influence of stress triaxiality (defined as the ratio of hydrostatic stress to von Mises stress) on the plastic deformation and fracture behaviour of rail steels. An extended SED model that considers the effect of stress triaxiality on both distortional and dilatational SEDs is proposed for assessing the fracture toughness of high-strength rail steels. The critical SED factor, determined by calculating the product of the critical SED and a characteristic distance ahead of the crack tip, is found to well correlate with the KIc values among the three types of rail steels at 23, -10, and -40oC. The results also confirm that the dilatational energy dissipation (also known as damage energy dissipation) at the crack tip is the primary component to correlate the critical SED factor with the KIc of rail steels. In Phase II, which is reported in Chapters 4 and 5, a more convenient non-destructive indentation technique is developed for estimating the fracture toughness of high-strength rail steels. First, a new constitutive model with coupled stress-triaxiality-dependent plasticity and damage is postulated to describe the mechanisms involved in the plastic deformation and ductile damage to rail steels under different levels of stress triaxiality. Based on the new constitutive model, not only is the independence of the constitutive equation from stress triaxiality explained, but a stress-triaxiality-dependent ductile damage model is also developed to estimate the critical damage parameter (Dcr) at the crack tip. With this ductile damage model, the indentation fracture toughness (KInd) of rail steels is estimated based on the Dcr at the crack tip. In addition, the study in Phase II uses a parameter κ to accommodate the potential difference of the Dcr value in the two loading modes (tensile fracture and indentation compression). The study shows that Dcr is indeed stress-triaxiality dependent and increases with the increase of stress triaxiality. The results also show that the change in KInd based on Dcr either at the crack tip or for the smooth specimen is generally consistent with the difference in the measured fracture toughness (KIc) among the three rail steels. However, for materials that show small difference in KIc, i.e., within the scattering of the measured data, KInd based on the Dcr at the crack tip may show a different trend from that based on the Dcr for the smooth specimen, which depends on the selected κ values. Such an issue needs further investigation using materials that cover a wide range of fracture toughness. Compared to the destructive testing method, the non-destructive indentation technique is more convenient, and has the potential to serve as a tool for the in-field health monitoring of rail steels and for the material evaluation, at an early stage, of the new rail steel under development.