A Failure Assessment Diagram (FAD), developed for rolling contact, was specifically formulated for railway wheel steels. The FAD allows combining a multiaxial fatigue criterion with the linear elastic fracture mechanics concept; in particular, it was used to assess the fatigue failure in wet contact, related to the pressurization of the fluid entrapped in surface cracks. The approach was applied to the results of previous bi‐disc tests carried out on three railway wheel steels, subjected first to a dry rolling‐sliding contact step, then to a wet contact step. The crack size distribution after the dry contact step was evaluated by a statistical approach; subsequently, the stress intensity factor during the wet contact step was estimated by finite elements. The results of the FAD agreed with the damage observed on the cross section of the specimens at the end of the tests. Furthermore, the FAD was used to determine the maximum allowable crack to prevent rolling contact fatigue, this way showing its potential as a damage tolerant approach.
The alternation of dry and wet contact can cause severe damage on railway wheels, because ratcheting occurring in dry contact leads to the formation of surface cracks, which can propagate if water is subsequently added, owing to the entrapped fluid pressurization mechanism. In a previous work, damage in alternated dry-wet contact was studied by means of twin-disc tests, aided by an innovative vision system including two devices: one for assessing the surface status by elaborating lights and shadows due to a diffused laser light projected on the specimen surface (2D analysis), the other for quantifying the local damage depth by elaborating the deformations of a projected laser blade (3D analysis). In that work, the 2D analysis resulted effective in revealing the onset of diffused fatigue crack propagation before it could be detected by vibration increment or visible spalling, but it failed when the damage was local; on the contrary, 3D analysis was effective in evaluating the damage severity only when the final failure had occurred. In this paper, the 2D analysis method was improved, including localised analyses in addition to the analysis of the whole contact surface. Two tests were considered: one with alternated dry-wet contact sessions of 50000 cycles each, the other with a one million cycle dry session followed by wet contact up to failure. The 2D analysis results were compared with the 3D analysis ones; furthermore, at the end of the tests the specimens were cut at the locations where the vision systems indicated severe damage. In the test with short alternated dry-wet sessions the local 2D analysis results showed the same trend as the global ones: indeed, in this test the damage was diffused along
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