This Ph.D. thesis deals with methodology for determination of residual fatigue lifetime of railway axles based on damage tolerance approach. This approach accepts an existence of potential defect, which could lead to fatigue failure of whole axle. The behavior of crack in railway axle is described by approaches of linear elastic fracture mechanics. There are plenty of factors, which more or less influence determined residual fatigue lifetime. The aim of this thesis is to quantify effects of these factors. The first part of Ph.D. thesis represents overview of studied problems relating to fatigue damage of railway axles. This part is focused on parameters, which influence fatigue crack growth in railway axle materials. The second part of thesis shows procedure for determination of residual fatigue lifetime, which was developed at the Institute of Physics of Materials of the Academy of Sciences of the Czech Republic. The main aim of this thesis was to improve current procedure for more precise estimation of residual fatigue lifetime. Significant part of this work is determination of significance of studied factors, which influencing calculated residual fatigue lifetime of railway axles (e.g. effect of threshold value, load spectrum, retardation effects, residual stress, axle geometry, material of axle etc.). The procedures described and results obtained can be also used for determination of residual fatigue lifetime of general mechanical component (not only railway axles). Therefore, results obtained in this Ph.D. thesis can be used e.g. for assessment of regular inspection intervals of cyclically loaded general mechanical parts.

This Ph.D. thesis deals with methodology for determination of residual fatigue lifetime of railway axles based on damage tolerance approach. This approach accepts an existence of potential defect, which could lead to fatigue failure of whole axle. The behavior of crack in railway axle is described by approaches of linear elastic fracture mechanics. There are plenty of factors, which more or less influence determined residual fatigue lifetime. The aim of this thesis is to quantify effects of these factors. The first part of Ph.D. thesis represents overview of studied problems relating to fatigue damage of railway axles. This part is focused on parameters, which influence fatigue crack growth in railway axle materials. The second part of thesis shows procedure for determination of residual fatigue lifetime, which was developed at the Institute of Physics of Materials of the Academy of Sciences of the Czech Republic. The main aim of this thesis was to improve current procedure for more precise estimation of residual fatigue lifetime. Significant part of this work is determination of significance of studied factors, which influencing calculated residual fatigue lifetime of railway axles (e.g. effect of threshold value, load spectrum, retardation effects, residual stress, axle geometry, material of axle etc.). The procedures described and results obtained can be also used for determination of residual fatigue lifetime of general mechanical component (not only railway axles). Therefore, results obtained in this Ph.D. thesis can be used e.g. for assessment of regular inspection intervals of cyclically loaded general mechanical parts.

Le but de la thèse est de déterminer l’historique de chargement d’une pièce rompue ou fissurée en service, par analyse des surfaces de fissuration. Pour les analyses de défaillances, les enjeux sont de définir le rôle du chargement dans la défaillance et de déterminer la fraction de durée de vie atteinte lors de la détection de la fissure. Un enjeu complémentaire est d’enrichir la base de données de chargements en service. L’organe choisi pour cette étude est l’essieu-axe ferroviaire. Ainsi, la sollicitation étudiée est en flexion rotative et les matériaux considérés sont les aciers A4T (25CrMo4) et A1N (C40). Suite à une revue bibliographique, quatre méthodes d’analyse quantitative des surfaces de fissuration ont été ciblées et testées, deux ont été retenues et développées. La première est basée sur la quantification des faciès de rupture (stries de fatigue, cupules, fissures secondaires…). La seconde s’appuie sur l’analyse des contraintes résiduelles (fractographie X) en profondeur pour déterminer l’épaisseur de matériau plastifié sous la surface de rupture. Cette profondeur correspond au sillage plastique induit par la propagation de la fissure et est fonction du chargement recherché. / The aim of the thesis is to determine the loading history of a broken or cracked piece in service, by analysing the cracking surfaces. For failure analyses, the issues are to define the role of the loading in the failure and to determine the fraction of lifespan reached when the crack is detected. An additional issue is to expand the in-service loadings database. The component chosen for this study is the railway axle. Thus, the studied stress is in alternate bending and the considered materials are the steels A4T (25CrMo4) and A1N (C40). Following a bibliographic review, four methods for quantitative analysis of cracking surfaces were targeted and tested, two of which were selected and developed. The first one is based on the quantification of fractographic features (fatigue striations, dimples, secondary cracks...). The second is based on the in-depth analysis of the residual stresses (X-ray fractography) to determine the thickness of plasticized material below the fracture surface. This depth corresponds to the plastic wake induced by the crack propagation and is a function of the sought loading.