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Fissuration par fatigue en mode mixte non proportionnel des rails de chemins de fer : De l’étude expérimentale à la mise en œuvre d’un modèle

Abstract : Rails are submitted to Rolling Contact Fatigue due to repeated passages of train wheels, which induces several types of cracks, such as Squat-type cracks. Those cracks undergo non-proportional mixed-mode I + II + III loading, including compression phases, in variable proportions along the crack front, making the prediction of their paths and growth rates a challenge.Mode I crack growth kinetics, for positive and negative R ratios, were first determined in R260 steel, as well as friction-corrected crack growth kinetics for fully-reversed combined mode II and III. The effective Stress Intensity Factors (SIFs) were deduced from the measured in-plane and out-of-plane crack face sliding displacements. From those kinetic laws, it was deduced that neither pure mode I, nor pure shear mode loadings can explain the crack growth rates observed in rails. A combination of those three loading modes, according to complex loading paths had thus to be prospected.Non-proportional mixed-mode I + II fatigue crack growth tests were then performed, following representative loading paths. Stereo digital image correlation was used to measure the near-tip displacement field. Post-treatment methods generally used to deduce the effective SIFs from these fields were inappropriate because of contact and friction stresses along the crack face. New methods were thus developed. The crack paths and growth rates were analyzed, using the effective SIFs. Crack path prediction by the maximum tangential stress criterion was found not to be very reliable, but substantially improved when crack tip plasticity and the presence of contact and friction stresses along the crack faces were taken into account. The measured crack growth rates correlated well with a combination of the three effective SIFs in a Paris-type law.From these experiments, it appears that due to crack face roughness, asperities interlocking and friction substantially reduce the effective SIFs, even without any normal compression, which cannot be captured by a simple Coulomb’s law. Besides, crack faces wear also has a large influence on the effective SIFs. The challenge for structural applications is thus not only to choose the most appropriate bifurcation criterion and crack growth law, but also to take crack face roughness and wear into account, in order to estimate the correct effective SIFS to use in these models.For industrial applications, a simple engineering approach was proposed to integrate roughness-induced friction in the estimation of the effective loading path from the nominal one. This approach was validated on sequential mixed-mode I + II & III experiments.
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Thomas Bonniot. Fissuration par fatigue en mode mixte non proportionnel des rails de chemins de fer : De l’étude expérimentale à la mise en œuvre d’un modèle. Mechanics of materials [physics.class-ph]. Université Paris-Saclay, 2019. English. ⟨NNT : 2019SACLX096⟩. ⟨tel-02443488⟩

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