Keywords: squat, rail failure mechanism, rail crack, white etching layer, shear stress, rolling contact fatigue.

Macroscopic and microscopic analyses of cross-sections over squat defects reveal metallurgical mechanisms governing internal crack front propagation. The leading crack plane of branched squats in general propagates over longer lengths (of the order of tens of millimeters) and greater depths (of the order of millimeters) before bending downward as compared to the more superficial trailing crack plane. The leading crack is therefore critical. A favorite depth of crack propagation occurs in the subsurface at 2-3 mm, which is related to the residual longitudinal stress profile exhibiting a strong gradient at that depth, giving rise to a shear stress peak 'shielding' the deeper material. This may lead to the formation of an internal crack 'plateau'. Further, oxidation processes along with branching are found to play a vital role in crack growth, especially when the crack front reaches the undeformed pearlitic microstructure and growth processes are no longer entirely mechanically driven.

Surface modification during squat growth is important, as it modifies the wheel-rail contact and therefore the rail loading. The decline of the surface can be distinguished into a phase of transient, local stress redistribution within the contact patch and one of dynamic wheel-rail contact.

Maturing squats show decoloured and hardened surface areas bordering the surface-breaking cracks and overlying the internal crack planes. If, in line with the hypothesized, shear-induced transverse failure mechanism, the V-shaped wedge loses its load bearing capacity, a redistribution of normal stresses within the contact ellipse occurs with hardening along the surface-braking crack pattern. This however only occurs if the actual contact patch for a passing wheel matches the 'failure envelope' (which is possible for initiating squats). If the actual contact patch does not match the failure envelope (especially for larger squats), the surface overlying the crack planes may settle slightly due to the Poisson effect, giving rise to reduced contact. As a result, corrosive products, 'pumped' from inside the cracks often filled with fluid, may accumulate on the surface, as confirmed by a SEM/EDX-analysis. This is a second reason for the surface decolouration along the surface-breaking cracks. The occurrence of both essentially different types of contact: with reduced contact at the wedge portion and with reduced contact at both 'lobes', has been verified in the field.

During progressive squat growth the hardened and decoloured envelope and the wedge-shaped portion are pressed into the deeper elastic material, accompanied by a gradual expansion of the contact band and a bilateral bridging of the defect. The resulting settlement of the wheel trajectory triggers a dynamic wheel-rail interaction with high-frequency impact, which causes rapid internal crack growth which may result in rail fracture if the global stress response is affected.

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