Ce projet de doctorat est aux prises avec un problème de ruptures en fatigue de un alliage de titane aéronautique en raison de haute fréquence chargement. Matériel pour cette enquête a été prise de compresseur du moteur disque de l'avion réel. Essais de fatigue à ultrasons ont été réalisées jusqu'à dépasser la limite de 1010 cycles. Cette région de la durée de vie est connu comme Gigacycle ou fatigue très grand nombre de cycles. Ce projet de thèse montre pour la première fois les résultats des tests de fatigue sur l'lliage de titane aéronautique VT3-1 dans la région Gigacycle. Les propriétés de fatigue de l'alliage de titane ont été déterminées à 109 cycles pour les conditions de chargement différentes: traction-compression, tension-tension et de torsion. Mécanismes d'initiation des fissures typiques ont été identifiés et des défauts critiques de microstructure ont été trouvés. L'effet de l'anisotropie en raison de processus de fabrication sur les propriétés de fatigue de l'alliage de titane VT3-1 forgé a été étudiée. Une influence du processus de fabrication sur les propriétés de fatigue a également été étudiée par comparaison les résultats sur extrudé et forgé VT3-1 alliage de titane. La nouvelle machine de torsion à ultrasons a été conçu et installé pour la longue durée de vie (jusqu'à 1010 cycles) de tests de fatigue en rotation. Les premiers résultats sous la chargement en torsion ultrasons ont été obtenues pour l'alliage de titane réalisé par extrusion et technologies forgés. / This PhD project is dealing with a problem of fatigue failures of aeronautical titanium alloy due to high frequency loading. The material for investigation was taken from the real aircraft engine compressor disk. Ultrasonic fatigue tests were carried out up to outrun limit of 1010 cycles. This region of lifetime is known as Gigacycle or very high cycle fatigue. This PhD project shows for the first time the results of fatigue tests on the VT3-1 aeronautical titanium alloy in the Gigacycle region. The fatigue properties of the titanium alloy were determined at 109 cycles for different loading conditions: tension-compression, tension-tension and torsion loading. Typical crack initiation mechanisms were identified and critical defects of microstructure were found. The effect of anisotropy due to fabrication process on the fatigue properties of the forged VT3-1 titanium alloy was studied. An influence of technological process on fatigue properties was also studied by comparison the results on extruded and forged VT3-1 titanium alloy.The new ultrasonic torsion machine was designed and installed for the long life (up to 1010 cycles) fatigue tests under rotation. The first results under ultrasonic torsion loading were obtained for the titanium alloy made by extrusion and forged technologies.

The behaviors of short fatigue crack (SFC) propagation through grain boundaries (GBs) were monitored during high cycle fatigue in an Al-Li alloy AA8090. The growth behaviors of SFCs were found to be mainly controlled by the twist components (α) of crack plane deflection across each of up to first 20 GBs along the crack path. The crack plane twist at the GB can result in a resistance against SFC growth; therefore SFC propagation preferred to follow a path with minimum α at each GB. In addition to the grain orientation, the tilting of GB could also affect α. An experiment focusing on quantifying GB-resistance was conducted on an Al-Cu alloy AA2024-T351. With a focused ion beam (FIB) and electron backscatter diffraction (EBSD), the micro-notches were made in front of the selected GBs which had a wide range of α, followed by monitoring the interaction of crack propagation from the notches with the GBs during fatigue. The crack growth rate was observed to decrease at each GB it had passed; and such growth-rate decrease was proportional to α. The resistance of the GB was determined to vary as a Weibull-type function of α. Based on these discoveries, a microstructure-based 3-D model was developed to quantify the SFC growth in high-strength Al alloys, allowing the prediction of crack front advancement in 3-D and the quantification of growth rate along the crack front. The simulation results yielded a good agreement with the experimental results about the SFC growth rate on the surface of the AA8090 Al alloy. The model was also used to predict the life of SFC growth statistically in different textures, showing potential application to texture design of alloys. Fatigue crack initiation at constituent particles (β-phase) was preliminarily studied in the AA2024-T351 Al alloy. Cross-sectioning with the FIB revealed that the 3-D geometry, especially the thickness, of fractured constituent particles (β-phase) was the key factor controlling the driving force for micro-crack growth. The resistance to micro-crack growth, mainly associated with crack plane twist at the particle/matrix interface, also influenced the growth behaviors of the micro-cracks at the particles on the surface.

La course à l'allègement des aéronefs constitue aujourd'hui l'un des enjeux principaux de l'industrie aéronautique. Le remplacement des rivets par des soudures FSW sur les voilures permettrait de gagner jusqu'à 20% de la masse totale de la voilure. Ce travail réalisé en collaboration avec Dassault-Aviation s'inscrit dans le cadre du projet ADSAM (Assemblage de Structures Aéronautiques Métalliques) visant à fabriquer un démonstrateur de caisson de voilure soudé par FSW. Cette étude traite du comportement mécanique en traction monotone quasi-statique et en fatigue oligocyclique (10⁴ à 10⁵ cycles) d'alliages d'aluminium (séries 2000 et 7000) soudés par FSW. L'objectif est d'étudier l'effet des hétérogénéités microstructurales induites par le soudage sur les hétérogénéités de comportement mécanique dans les joints soudés. La corrélation d'images numériques a été utilisée pour définir la réponse mécanique (σnominale – εvraie) dans chacune des zones constituant ces soudures. En complément, des essais sur des mini-éprouvettes prélevées dans l'épaisseur des joints ont permis d'établir le comportement local (σvraie – εvraie) dans chacune des zones du joint. Sous chargement de traction monotone, la rupture des soudures FSW a été observée dans la zone de localisation des déformations anélastiques située aux minima des profils de microdureté. En revanche, les fissures de fatigue ne démarrent pas nécessairement dans ces zones de localisation. Des analyses microscopiques des faciès de rupture ont permis d'identifier deux mécanismes principaux d'amorçage de fissure : (i) rupture ou (ii) décohésion de particules intermétalliques. D'autre part, les essais cycliques montrent une adaptation élastique des matériaux dans les différentes zones du joint (noyau, ZAT et ZATM) après stabilisation autour d'une déformation moyenne non nulle. En terme de durée de vie, les résultats d'essais sur éprouvettes entaillées traitées OAC (représentative de zones critiques sur structure) montrent que le soudage par FSW réduit jusqu'à 50% la tenue en fatigue de ces alliages. L'amorçage des fissures de fatigue est dû aux piqûres créées par le traitement d'OAC. Les résultats obtenus dans le cadre de ce travail ouvrent des perspectives intéressantes pour modéliser le comportement mécanique des joints soudés par FSW afin d'estimer leur durée de vie en fatigue oligocyclique. / Mass reduction of aeronautic structures is an important challenge for aeronautic industries. FSW is an efficient joining process to replace rivets : the mass reduction using this welding process could reach 20% for aircraft wings. This work carried out in collaboration with Dassault-Aviation was done in the framework of ADSAM project. The FSW process significantly modifies the microstructure and the local material properties, the overall mechanical response of the joint is largely governed by the response of the different zones of FSW joint. In the present study, the global and local mechanical responses under monotonic quasi-static tension and cyclic tension-tension (R=0.1) in low cycle fatigue regime (10⁴ to 10⁵ cycles) were carried out on FSW joints made of aluminum alloys (2000 and 7000 series). Full field strain measurements were carried out on FSW welded specimens using digital image correlation techniques assuming an iso-stress configuration (σlocal = σnominal ). Additionally tension tests parallel to the welding direction have been performed on micro specimens machined in each representative zone of FSW joints. These tests allow us to determine the local mechanical response (σlocal − εlocal) of the joint. Local mechanical properties were determined for each representative weld zone (nugget, TMAZ and HAZ). The full field anelastic strain localization scenario prior to fracture shows a good correlation with the microhardness profile along the FSW joints. Under monotonic loading, fracture was observed at the strain localization zones unlike under cyclic loading. In fatigue, the fracture zones are very dispersed for each weld configuration. SEM observations led to the identification of two crack initiation mechanisms:(i) fracture of intermetallic particles or (ii) decohesion between intermetallic particles and the matrix. Digital image correlation under cyclic loading shows an elastic shakedown of the studied materials in each zone of the joint around a non null mean strain. In terms of number of cycles to failure, fatigue test results on ACO treated notched specimens (representative of structure critical locations) show that the FSW process reduce up to 50% the faigue life of the base materials. The crack initiation was associated to ACO pits. The results of this study open interesting prospects for modelling the mechanical behavior of FSW joints and to assess their fatigue life.

The goal of this research was to investigate the tensile fatigue behavior of a carbon fiber / epoxy composite material. Specifically, the stress levels at which cracks initiated in static and fatigue loading in the 90 degree plies of a "quasi-cross ply layup" [0/905]S was investigated. For layups which contain them, cracks in composite laminates initiate and propagate from 90 degree plies (including the ubiquitous "quasi-isotropic layup" 0/±45/90). Thus, this work provides valuable insight into the fatigue behavior of the plies which originate fatigue damage. Unidirectional off-axis 90 degree and 10 degree specimens were also tested, but the bulk of testing was done on the cross-ply laminates. The project sponsors, Boeing, were in the process of extending a failure model to the case of fatigue. The body of work presented here provided empirical data for that effort. Several different inspection techniques were used to investigate for cracking in the 90 degree plies, including: x-ray images, edge replicates, dye penetrants, and optical microscopy. Plots of the stress level at which crack initiation occurred will be presented, as well as images illustrating damage development in these layups. Comparisons are made to the experimental results of other investigations of this type of layup. Explorations of the effect of R-ratio (including R = 0.1 and 0.5), loading frequency (including 3, 10, and 30 Hz), and surface roughness (hand polished specimen edges to 1500 grit smoothness) on fatigue crack initiation were also performed. For the most damaging case (10 Hz, R = 0.1, no polishing), the crack initiation strain (0.00276) was one half of the strain at which cracks initiated in static monotonic loading (0.0054), and was 16% of the cross-ply specimen's (0 degree fiber dominated) ultimate strain value of (0.018).

In this dissertation the work done in the projects KME-410/502 will be presented.The overall objective in these projects is to evaluate and develop tools for designingagainst fatigue in single-crystal nickel-base superalloys in gas turbines. Experimentshave been done on single-crystal nickel-base superalloy specimens in order toinvestigate the mechanical and fatigue behaviour of the material. The constitutivebehaviour has been modelled and veried by FE-simulations of the experiments.Furthermore, the microstructural degradation during long-time ageing has been investigatedwith respect to the material's yield limit. The eect has been includedin the constitutive model by lowering the resulting yield limit. Moreover, the fatiguecrack initiation of a component has been analysed and modelled by using acritical plane approach in combination with a critical distance method. Finally, asan application, the derived single-crystal model was applied to all the individualgrains in a coarse grained specimen to predict the dispersion in fatigue crack initiationlife depending on random grain distributions. This thesis is divided into three parts. In the rst part the theoretical framework,based upon continuum mechanics, crystal plasticity, the critical plane approachand the critical distance method, is derived. This framework is then used in thesecond part, which consists of six included papers. Finally, in the third part, detailsof the used numerical procedures are presented.

H υποβάθμιση της δομικής ακεραιότητας των αεροπορικών κατασκευών με το χρόνο και η αποτίμηση της αποτελεί ένα σημαντικό τεχνολογικό πρόβλημα της αεροναυπηγικής. Το πρόβλημα αυτό σε συνδυασμό με το ότι εξακολουθούν να χρησιμοποιούνται αεροσκάφη, τα οποία έχουν ξεπεράσει τον αρχικά σχεδιασμένο χρόνο της επιχειρησιακής τους λειτουργίας, οδήγησε τη διεθνή επιστημονική κοινότητα και την αεροπορική βιομηχανία να ασχοληθεί με το πρόβλημα του “γηράσκοντος” αεροσκάφους (aging aircraft). Το πρόβλημα της γήρανσης της δομής ενός αεροσκάφους οφείλεται κυρίως στην εμφάνιση ευρείας έκτασης βλάβης κόπωσης (wide spread fatigue damage) σε διάφορες περιοχές της κατασκευής, η οποία αυξάνει την πιθανότητα εμφάνισης καταστροφικών αστοχιών. Οι ηλωτές συνδέσεις των ελασμάτων που συνθέτουν την άτρακτο ή τις πτέρυγες ενός αεροσκάφους αποτελούν περιοχές συγκέντρωσης τάσεων και εστίες διάβρωσης με συνέπεια την εμφάνιση και εξέλιξη μικρών ρωγμών σε πολλαπλά σημεία της δομής. Η κατάσταση της ύπαρξης πολλών αλληλεπιδρουσών ρωγμών στο ίδιο δομικό στοιχείο ονομάζεται πολλαπλή βλάβη (Multiple Site Damage – MSD) και οδηγεί σε σημαντική μείωση της εναπομένουσας αντοχής του δομικού στοιχείου και σε υποβάθμιση της δομικής του ακεραιότητας. Η μελέτη του προβλήματος της πολλαπλής βλάβης και η αντιμετώπισή του αποτελεί σημαντικό πεδίο έρευνας, καθώς περιλαμβάνει πλήθος επιμέρους φαινομένων, τα οποία αλληλεπιδρούν μεταξύ τους. Τέτοια φαινόμενα είναι η έναρξη, η διάδοση και η συνένωση πολλαπλών ρωγμών, η τελική αστοχία δομικού στοιχείου σε συνθήκες πολλαπλής βλάβης, καθώς και η επίδραση περιβαλλοντικών παραγόντων όπως η διάβρωση στη συμπεριφορά των υλικών. Σκοπός της παρούσας εργασίας είναι η ανάπτυξη μιας ολοκληρωμένης μεθοδολογίας για την εκτίμηση της εμφάνισης και εξέλιξης της πολλαπλής βλάβης κόπωσης σε αεροπορικά δομικά στοιχεία, έτσι ώστε να είναι εφικτή η εκτίμηση της δομικής τους ακεραιότητας σε οποιαδήποτε στιγμή της διάρκειας λειτουργίας τους. Τα τεχνολογικά προβλήματα, που αντιμετωπίζονται στην παρούσα εργασία με ανάπτυξη πρωτότυπων μεθοδολογιών, περιλαμβάνουν: • Ανάλυση τάσεων σε πολλαπλά ρηγματωμένες κατασκευές και υπολογισμό συντελεστών έντασης τάσεων στις ρωγμές • Πρόβλεψη έναρξης και διάδοσης ρωγμών • Πρόβλεψη συνένωσης ρωγμών και εναπομένουσας αντοχής Για την ανάλυση τάσεων και τον υπολογισμό των συντελεστών έντασης τάσεων σε πολλαπλά ρηγματωμένα δομικά στοιχεία με ή χωρίς ενισχύσεις (stiffeners) αναπτύσσονται στη βάση της τεχνικής της υπο-μοντελοποίησης της μεθόδου των πεπερασμένων στοιχείων (ΠΣ) κατάλληλα υπερ-στοιχεία (super-elements) για τη μοντελοποίηση των κρίσιμων τμημάτων της κατασκευής. Η μεθοδολογία υπερ-στοιχείων επιλέγεται στην παρούσα εργασία, με σκοπό τη σημαντική μείωση του υπολογιστικού κόστους σε σχέση με τις κλασικές μεθόδους των πεπερασμένων στοιχείων, έτσι ώστε να καταστεί πρακτικά δυνατή η επίλυση του πολύπλοκου αυτού προβλήματος. Το πρόβλημα της δημιουργίας νέων ρωγμών λόγω κόπωσης σε κάποιο δομικό στοιχείο παρουσιάζει έντονα πιθανοθεωρητικό χαρακτήρα. Για την αντιμετώπισή του αναπτύσσεται μεθοδολογία σύμφωνα με την οποία η πιθανότητα να εμφανιστεί μια ρωγμή σε κάποιο σημείο μετά από ορισμένο αριθμό κύκλων φόρτισης εξαρτάται τόσο από την τάση που αναπτύσσεται στο σημείο αυτό όσο και από την πιθανότητα ύπαρξης τοπικών ατελειών του υλικού στο υπό εξέταση σημείο. Για την εφαρμογή της μεθοδολογίας χρησιμοποιούνται στατιστικά δεδομένα που προέρχονται από πειράματα κόπωσης απλών δοκιμίων. Για την εκτίμηση της εναπομένουσας αντοχής ενός πολλαπλά ρηγματωμένου δομικού στοιχείου αναπτύσσεται ένα ενεργειακό κριτήριο συνένωσης ρωγμών. Το κριτήριο βασίζεται της μεταβολής της ενέργειας παραμόρφωσης που λαμβάνει χώρα κατά τη διάρκεια της αστοχίας του υλικού που συνδέει δύο παρακείμενες ρωγμές. Τέλος, τα μοντέλα έναρξης, διάδοσης και συνένωσης ρωγμών συνδυάζονται σε μια ολοκληρωμένη μεθοδολογία, η οποία μπορεί να χρησιμοποιηθεί για την πρόβλεψη της εξέλιξης της πολλαπλής βλάβης κόπωσης. Η αποτελεσματικότητα της μεθόδου επαληθεύεται με τη βοήθεια πειραμάτων κόπωσης πολλαπλά ρηγματωμένων δοκιμίων. / Structural integrity degradation of aeronautical structures due to fatigue, as well as its assessment, represents a major problem of aeronautics. This problem, combined to the fact that numerous aircrafts that have already exceeded their initial design operational life are still in service, led the international scientific community and the aeronautical industry to the in-depth study of aging aircraft. The main problem of an aging aircraft structure is widespread fatigue damage at various areas of the structure, that increase the probability of catastrophic failures. Riveted joints of fuselage or wing panels are areas of high stress concentration and corrosion, therefore, sites susceptible to multiple cracks development. The state of multiple interacting cracks existence at the same structural element is called Multiple Site Damage (MSD) and leads to residual significant strength reduction and structural integrity degradation of the structure. Study and handling of multiple site damage problem is an important research field, as it involves a number of different interacting phenomena, such as crack initiation, propagation and link-up and final structural failure under MSD conditions, as well as the effect of environmental factors like corrosion on the material behaviour. The aim of the present work is the development of an integrated methodology for the prediction of MSD evolution at aeronautical structural elements, in order to make feasible the assessment of their structural integrity throughout their service life. The technological issues that are faced in the frame of the present work by the development of innovative methodologies are: • Stress analysis and stress intensity factors calculation of structures under MSD condition. • Crack initiation and propagation prediction. • Prediction of crack link-up and residual strength. For the stress analysis and stress intensity factor calculation of un-stiffened of stiffened structural elements suitable super-elements are developed for the simulation of crucial structural elements. The super-element methodology is based on finite element sub-structuring technique and, in the present work, is chosen in order to obtain a significant reduction of computation cost with compared to the classical finite element method, so that handling of such complex problems becomes feasible. Fatigue crack initiation at a structural element is a problem exhibiting a strongly stochastic character. A crack initiation prediction methodology is developed according to which the probability of crack initiation at a site of the structure depends on the local stress state combined to a stochastic (random) variation of the fatigue strength at every site susceptible to crack initiation. For the methodology application statistical data from simple fatigue tests are used. For the crack link-up and the residual strength estimation of a multiple cracked structural element an energy-based link-up criterion is developed. The proposed criterion is based on the strain energy variation during the failure of the ligament of two adjacent cracks. Finally, the initiation, propagation and link-up models are combined and implemented in an integrated methodology that can be used for the assessment of multiple site damage evolution. The effectiveness of the methodology is validated using suitable fatigue tests of multiple-site damaged panels.

En situation d’interaction pastille-gaine (IPG), les gaines de crayons combustible en alliage de zirconium sont susceptibles de rompre lors de transitoires de puissance incidentels dans les réacteurs à eau pressurisée, par un mécanisme de corrosion sous contrainte induite par l’iode (CSC-I). Cette étude traite de l’amorçage intergranulaire des fissures de CSC-I dans le Zircaloy-4 recristallisé, en milieu méthanol iodé à température ambiante, en s’intéressant particulièrement aux paramètres mécaniques critiques et à la concentration en iode. Pour cela, une approche mêlant expériences et simulations numériques a été adoptée. Un modèle de comportement mécanique macroscopique de l’alliage, viscoplastique et anisotrope, a été établi et validé sur une large gamme de sollicitations. Par la réalisation de nombreux essais de traction à vitesse de déformation imposée et de fluage en flexion quatre points, nous avons montré l’existence d’une concentration seuil I0 proche de 10-6 g.g-1 nécessaire à l’apparition du dommage de CSC-I, mais également celle d’une concentration de transition I1 proche de 2 10-4 g.g-1 au-delà de laquelle le mécanisme change, menant à un amorçage anticipé des fissures et une sensibilité réduite de l’endommagement aux paramètres mécaniques. L’importance de la concentration sur des paramètres tels que la densité des fissures, leur longueur moyenne et la vitesse de propagation intergranulaire et transgranulaire a été mise en évidence. Les résultats expérimentaux montrent que la déformation plastique macroscopique n’est pas indispensable à l’amorçage de fissures de CSC-I, pour un temps d’essais suffisamment long en présence de contrainte. Son principal effet est de précipiter l’apparition de fissures par la création de sites d’amorçage, par rupture de la couche d’oxyde et accumulation de contrainte intergranulaire. En dessous de I1 la détermination des déformations critiques à l’amorçage montre un fort effet de vitesse. Dans ce domaine, une contrainte seuil de 100 MPa a été déterminée, bien inférieure à la limite élastique. L’utilisation d’éprouvettes entaillées et de la simulation numérique a permis de mettre en évidence un fort effet protecteur de l’augmentation de la biaxialité des contraintes vis-à-vis de l’amorçage, dans le domaine élastique comme dans le domaine plastique. Des éprouvettes préalablement irradiées aux protons à une dose de 2 dpa ont été testées dans les mêmes conditions que les éprouvettes non irradiées. La sensibilité accrue du matériau irradié à la CSC-I a pu être quantifiée et nous avons constaté qu’un effet de concentration et un effet de vitesse de sollicitation subsistent après irradiation. L’irradiation induit une localisation plus importante de la déformation menant à un amorçage prématuré des fissures, mais une sensibilité chimique plus importante, tendance à la piquration et décalage de I1, apparaît comme le principal responsable de la plus forte sensibilité de l’irradié. / During the pellet-cladding interaction, Zirconium-alloy fuel claddings might fail when subjected to incidental power transient in nuclear Pressurized Water Reactors, by Iodine-induced Stress Corrosion Cracking (I-SCC). This study deals with the intergranular initiation of I-SCC cracks in fully recrystallized Zircaloy-4, in methyl alcohol solution of iodine at room temperature, with the focus on critical mechanical parameters and iodine concentration. It was carried out with an approach mixing experiments and numerical simulations. An anisotropic and viscoplastic mechanical behavior model was established and validated over a wide range of loadings. With numerous constant elongation rate tensile tests and four points bending creep tests, the existence of a threshold iodine concentration I0 close to 10-6 g.g-1 was highlighted, necessary to the occurrence of I-SCC damage, along with a transition concentration I1 close to 2 10-4 g.g-1. Above I1 the mechanism changes, leading to a sped up crack initiation and a loss of sensitivity towards mechanical parameters. The importance of concentration on parameters such as crack density, crack average length and intergranular and transgranular crack velocities was evidenced. Experimental results show that plastic strain is not required for I-SCC crack initiation, if the test time is long enough in the presence of stress. Its main influence is to rush the occurrence of cracking by creating initiation sites, by way of breaking the oxide layer and building up intergranular stress. Below I1, the critical strains at initiation show a substantial strain rate sensitivity. In this domain, a threshold stress of 100 MPa was found, well below the yield stress. Thanks to the combined use of notched specimens and numerical simulations, a strong protective effect of an increasing stress biaxiality ratio was found, both in the elastic and plastic domains. Proton-irradiated samples, up to a dose of 2 dpa, were tested in the same conditions as fresh specimens. The higher I-SCC sensitivity of the irradiated material was measured and the effects of concentration and strain rate were found to remain. Irradiation leads to a higher strain localization causing early crack initiation, but the main reason for the higher sensitivity of the irradiated material seems to be a chemical one, with higher pitting occurrence and a shift of I1.

AA6061 Al alloys modified with addition of Mn, Cr and Cu were homogenized at temperatures between 350 ºC and 550 ºC after casting. STEM experiments revealed that the formation of α-Al(MnFeCr)Si dispersoids during homogenization were strongly affected by various factors such as heating rate, concentration of Mn, low temperature pre-nucleation treatment and homogenization temperature. Through analysis of the STEM results using an image software Image-Pro, the size distributions and number densities of the dispersoids formed during different annealing treatments were quantitatively measured. It was revealed that increasing the heating rate or homogenization temperature led to a reduction of the number density and an increase in size of the dispersoids. The number density of dispersoids could be markedly increased through a low temperature pre-nucleation treatment. A higher Mn level resulted in the larger number density, equivalent size and length/width ratio of the dispersoids in the alloy. Upsetting tests on two of these Mn and Cr-containing AA6061 (Al-Mg-Si-Cu) Al alloys with distinctive Mn contents were carried out at a speed of 15 mm s-1 under upsetting temperature of 450 ºC after casting and subsequent homogenization heat treatment using a 300-Tone hydraulic press. STEM experiments revealed that the finely distributed α-Al(MnFeCr)Si dispersoids formed during homogenization showed a strong pinning effect on dislocations and grain boundaries, which could effectively inhibit recovery and recrystallization during hot deformation in the two alloys. The fractions of recrystallization after hot deformation and following solution heat treatment were measured in the two alloys with EBSD. It was found that the recrystallization fractions of the two alloys were less than 30%. This implied that the finely distributed α-dispersoids were rather stable against coarsening and they stabilized the microstructure by inhibiting recovery and recrystallization by pinning dislocations during deformation and annealing at elevated temperatures. By increasing the content of Mn, the effect of retardation on recrystallization were further enhanced due to the formation of higher number density of the dispersoids. STEM and 3-D atom probe tomography experiments revealed that α-Al(MnFeCr)Si dispersoids were formed upon dissolution of lathe-shaped Q-AlMgSiCu phase during homogenization of the modified AA6061 Al alloy. It was, for the first time, observed that Mn segregated at the Q-phase/matrix interfaces in Mn-rich regions in the early stage of homogenization, triggering the transformation of Q-phase into strings of Mn-rich dispersoids afterwards. Meanwhile, in Mn-depleted regions the Q-phase remained unchanged without segregation of Mn at the Q-phase/matrix interfaces. Upon completion of α-phase transformation, the atomic ratio of Mn and Si was found to be 1:1 in the α-phase. The strengthening mechanisms in the alloy were also quantitatively interpreted, based on the measurements of chemical compositions, dispersoids density and size, alloy hardness and resistivity as a function of the annealing temperature. This study clarified the previous confusion about the formation mechanism of α-dispersoids in 6xxx series Al alloys. Four-point bend fatigue tests on two modified AA6061 Al alloys with different Si contents (0.80 and 1.24 wt%, respectively) were carried out at room temperature, f = 20 Hz, R = 0.1, and in ambient air. The stress-number of cycles to failure (S-N) curves of the two alloys were characterized. The alloys were solution heat treated, quenched in water, and peak aged. Optical microscopy and scanning electron microscopy were employed to capture a detailed view of the fatigue crack initiation behaviors of the alloys. Fatigue limits of the two alloys with the Si contents of 0.80 and 1.24 wt% were measured to be approximately 224 and 283.5 MPa, respectively. The number of cracks found on surface was very small (1~3) and barely increased with the applied stress, when the applied stress was below the yield strength. However, it was increased sharply with increase of the applied stress to approximately the ultimate tensile strength. Fatigue crack initiation was predominantly associated with the micro-pores in the alloys. SEM examination of the fracture surfaces of the fatigued samples showed that the crack initiation pores were always aspheric in shape with the larger dimension in depth from the sample surface. These tunnel-shaped pores might be formed along grain boundaries during solidification or due to overheating of the Si-containing particles during homogenization. A quantitative model, which took into account the 3-D effects of pores on the local stress/strain fields in surface, was applied to quantification of the fatigue crack population in a modified AA6061 Al alloy under cyclic loading. The pores used in the model were spherical in shape, for simplicity, with the same size of 7 μm in diameter. The total volume fraction of the pores in the model were same as the area fraction of the pores measured experimentally in the alloy. The stress and strain fields around each pore near the randomly selected surface in a reconstructed digital pore structure of the alloy were quantified as a function of pore position in depth from the surface using a 3-D finite element model under different stress levels. A micro-scale Manson-Coffin equation was used to estimate the fatigue crack incubation life at each of the pores in the surface and subsurface. The population of fatigue cracks initiated at an applied cyclic loading could be subsequently quantified. The simulated results were consistent with those experimentally measured, when the applied maximum cyclic stress was below the yield strength, but the model could not capture the sudden increase in crack population at UTS, as observed in the alloy. This discrepancy in crack population was likely to be due to the use of the spherical pores in the model, as these simplified pores could not show the effects of pore shape and their orientations on crack initiation at the pores near surface. Although it is presently very time-consuming to calculate the crack population as a function of pore size and shape in the alloy with the current model, it would still be desirable to incorporate the effects of shape and orientation of the tunnel-shaped pores into the model, in the future, in order to simulate the fatigue crack initiation more accurately in the alloy.

Mechanical fatigue is a failure phenomenon that occurs due to repeated application of mechanical loads. Very High Cycle Fatigue (VHCF) is considered as the domain of fatigue life greater than 10 million load cycles. Increasing numbers of structural components have service life in the VHCF regime, for instance in automotive and high speed train transportation, gas turbine disks, and components of paper production machinery. Safe and reliable operation of these components depends on the knowledge of their VHCF properties. In this thesis both experimental tools and theoretical modelling were utilized to develop better understanding of the VHCF phenomena. In the experimental part, ultrasonic fatigue testing at 20 kHz of cold rolled and hot rolled stainless steel grades was conducted and fatigue strengths in the VHCF regime were obtained. The mechanisms for fatigue crack initiation and short crack growth were investigated using electron microscopes. For the cold rolled stainless steels crack initiation and early growth occurred through the formation of the Fine Granular Area (FGA) observed on the fracture surface and in TEM observations of cross-sections. The crack growth in the FGA seems to control more than 90% of the total fatigue life. For the hot rolled duplex stainless steels fatigue crack initiation occurred due to accumulation of plastic fatigue damage at the external surface, and early crack growth proceeded through a crystallographic growth mechanism. Theoretical modelling of complex cracks involving kinks and branches in an elastic half-plane under static loading was carried out by using the Distributed Dislocation Dipole Technique (DDDT). The technique was implemented for 2D crack problems. Both fully open and partially closed crack cases were analyzed. The main aim of the development of the DDDT was to compute the stress intensity factors. Accuracy of 2% in the computations was attainable compared to the solutions obtained by the Finite Element Method. / Very High Cycle Fatigue (VHCF) is considered as the domain of fatigue life greater than 10 million load cycles. Structural components that have service life in the VHCF regime include wheels and axles of high speed trains, gas turbine disks, and components of paper production machinery. Safe and reliable design, and the longevity, of these components depends on the knowledge of their VHCF properties. The overall aim of the experimental portion of this thesis was to gain in-depth knowledge of the VHCF properties of stainless steels. Fatigue test data in the VHCF regime was generated for different stainless steel grades using ultrasonic fatigue testing. The mechanisms for fatigue crack initiation and short crack growth were investigated using electron microscopes. Theoretical modelling of complex crack geometries involving kinks and branches was carried out by using the Distributed Dislocation Dipole Technique (DDDT). The main aim of this development was to compute the stress intensity factors and to analyse the stress state around the cracks. The results showed that accuracy of 2% was attainable compared to the solutions obtained by Finite Element Method (FEM). / <p>Artikel 4 publicerad i avhandlingen som manuskript</p>

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.