The main subject of this master’s thesis was to evalute inluence of loading cycle asymmetry and long-term thermal exposure on fatigue crack growth rate in martensitic P91 and P92 steels. Experiments were carried out in Material and metallurgical research Ostrava Ltd. and their main aim was to study the influences of different loading cycle asymmetries (R = 0,1 and R = 0,6) as well as 5000 hours/600 °C (P91) respectively 5000 hours/650 °C (P92) of thermal exposure on fatigue crack growth rate.

The main goal of this diploma thesis is the examination of the influence of structure directionality on fatigue properties of formed aluminium alloy 6082/T6. The main attention is focused on the study of the influence of structure directionality on kinetics of short fatigue cracks growth. The measurement of short fatigue cracks growth was performed on cylindrical samples. The samples were made in two different directions; one parallel with the forming direction and second perpendicular to the forming direction. Servo hydraulic machine MTS 880 was used for the cyclic loading. The samples were cycled at two different constant stress amplitudes. Cyclic loading was systematically interrupted in order to measure the length of short cracks by a light microscope. Tensile tests, measuring of cycling hardening-softening curves, observation of microstructure, observation of surface relief, measuring of microhardness and fractographical analysis of fracture surfaces were used for further examination of the influence of the structure directionality. Some of these measured characteristics did not show any influence of the structure directionality (microhardness, fatigue life curve, Young modulus). In other cases is this influence measurable, however insignificant (yield stress, ultimate stress, cyclic hardening-softening curves and kinetics of short fatigue cracks growth). It can be summarised that the material, though the directionality of its microstructure is apparent, shows relatively isotropic mechanical behaviour.

Master’s thesis deals with study of fatigue crack growth retardation models. This document presents description, evaluation, verification and finally application of retardation models demonstrated on the damage tolerant structure. Analysis was provided on the L 410 NG aircraft in the location of the rear spar lower flange joint. Output of this thesis is comparison of the damage tolerance analysis with the current method using in the Aircraft Industry a. s., i.e. linear model without using load interaction models. The comparisons are fatigue crack growth curves, threshold and intervals of inspection program. AFGROW software was used for verification and application of retardation models.

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.

L’influence des renforts nanoparticules de TiB2 (6 wt.%) sur la précipitation interfaciale de la phase (Zn1.5Cu0.5)Mg, la résistance à la traction et la fissuration sous chargement de fatigue (fatigue crack growth-FCG) des composites à matrice de Al-Zn-Mg-Cu ont été étudiées. Des échantillons de composites ont été obtenus par réaction in-situ pendant le moulage suivi d’un FSP (friction stir processing) et une extrusion à chaud. Seuls les échantillons moulés et extrudés ont été utilisés pour étude de FCG à cause de la limitation de la taille après FSP. Des observations au microscope électronique à balayage (SEM), avec la diffraction des électrons rétrodiffusés (SEM/EBSD) et au microscope électronique en transmission à haute résolution (HRSTEM) ont été réalisées pour caractériser la microstructure.Des échantillons présentent une structure des grains équi-axiaux et des nanoparticules de TiB2 sont distribuées de façon homogène dans la matrice. En état de solution solide, l’interface TiB2/Al est de nature semi-cohérente et très propre. En état de vieillissementou ou sur vieillissement, la précipitation interfacaile hétérogène de la phase (Zn1.5Cu0.5)Mg a été observée. La cinétique de la précipitation interfaciale a été discutée. Les interfaces entre Al/(Zn1.5Cu0.5)Mg/TiB2 sont quasi cohérentes et l’interface TiB2/Al a été renforcée grâce à la réduction de l’énergie de l’interface. Ce mécanisme de précipitation interfaciale peut expliquer l’effet de renforcement de l’interface contribuant simultanement l’augmentation de la résistance et de l’élongation des échatillons de composite.La majorité de nanoparticules TiB2 tentent de s’agglomérer le long des joints de grains dans des échantillons sans FSP. La vitesse de croissance de fissure a été augmentée à l’intérieur des grains avec un facteur d’intensité (ΔK) intermédiaire ou important à cause de l’affinement de grains. Cependant, la vitesse de croissance de fissure a été diminuée aux joints de grains avec (ΔK) faible ou intermédiaire à cause de la présence des clusters de TiB2 tandis que cette vitesse augmente avec (ΔK) important à cause de la coalescence des micropores. / The influences of TiB2 reinforcement nanoparticles (6 wt.%) on the interfacial precipitation of (Zn1.5Cu0.5)Mg phase, the associated tensile and fatigue crack growth (FCG) properties of the Al-Zn-Mg-Cu matrix composites have been studied. The composite samples were produced by in-situ reaction during casting followed by friction stir processing (FSP) and hot extrusion, while only casted and extruded samples were used for evaluating FCG due to size limit of the nugget zone after FSP. Scanning electron microscopy (SEM), electron backscatter diffraction (EBSD) and high-resolution scanning transmission electron microscopy (HRSTEM) were employed for the microstructure characterization.The as-processed composite samples contain the fine equiaxed-grain structure, where TiB2 nanoparticles are homogenously distributed. At solid-solution state, the TiB2/Al interfaces are featured by the clean and semi-coherent nature. At the peak-aged and overaged states, the interface precipitate determined as (Zn1.5Cu0.5)Mg phase was formed, and the underlying heterogeneous interfacial precipitation kinetics was discussed. The Al/(Zn1.5Cu0.5)Mg/TiB2 multi-interfaces were revealed to be almost coherent, and the TiB2/Al interfaces were thus strengthened due to the greatly reduced coherency strains. This mechanism was proposed as precipitation assisted interface strengthening, which has contributed to the simultaneously enhanced tensile strength and uniform elongation of the as-processed composite.The majority of TiB2 nanoparticles tend to aggregate along grain boundaries (GBs) in the composite samples without FSP. The FCG rate is increased inside grains at intermediate and high stress intensity factor (ΔK) ranges due to the refined grain size. However, the FCG rate at the GBs is decreased at the low and intermediate ΔK ranges by fatigue crack deflection and trapping due to the presence of TiB2 clusters, while it increases at the high ΔK range due to microvoid coalescence.

Nickel superalloys are used for high-temperature application in energetic and aerospace industry. They are exposed to aggressive environment at high temperatures with the interactions between fatigue and creep processes, high-temperature oxidation, corrosion and erosion. Lifetime extension of such strained parts while increasing the performance of particular machine is possible by applying protective surface coatings. The subject of this work is to investigate the fatigue failure mechanisms of superalloy Inconel 713LC at 800 °C and to compare these mechanisms between material with a protective coating based on Al-Si and material without coating. The location of initiation fatigue cracks, their propagation and the fatigue crack propagation rate in some areas were analyzed by optical microscopy, scanning electron microscopy and confocal laser scanning microscopy. Based on previous research it was found that the application of the coating AlSi has a positive effect on lifetime of alloy Inconel 713LC. These results were confirmed and estimated in the diploma thesis.

This work deals with fatigue properties and stability of microstructure of ultrafine-grained (UFG) copper prepared by severe plastic deformation by means of equal channel angular pressing (ECAP) method. The effect of different fatigue loading regimes and thermal exposition on microstructural changes was investigated and the fatigue lifetime curves were experimentally determined. The research attention was focussed on localization of cyclic plastic deformation and fatigue crack initiation in UFG structure. Experimental results indicate that after stress-controlled fatigue loading (both symmetrical and asymmetrical) the microstructure remains ultrafine; no grain coarsening was observed. Contrary to this, strain-controlled fatigue loading results in formation of bimodal structure. Grain coarsening was observed also after thermal exposition at 250 °C for 30 minutes. Annealing at lower temperatures does not result in grain coarsening or development of bimodal structure. Fatigue loading results in development of surface relief in form of cyclic slip markings. Their density, distribution and shape differ for particular fatigue loading regimes. Differences in crack initiation mechanism in low- and high-cycle fatigue region were found. Nevertheless, the characteristic feature for all loading regimes was stability of UFG microstructure in the region of cyclic slip bands and fatigue cracks.

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.

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