Blasting plays a crucial role in several engineering applications, from mining and tunneling to demolition projects. One of the remaining challenges of this process is that it can significantly affect the integrity of the rock mass by inducing damage in the form of cracks. Broadening the understanding of the behavior of the blast-induced cracks is essential for predicting the damage. One way of investigating this issue is through small-scale blasting experiments focused on crack propagation behavior. Controlled blasting experiments were conducted on rock-like cylindrical samples charged with Pentaerythritol tetranitrate (PETN) cords. Different blast designs were tested and a method for integrating a Digital Image Correlation (DIC) technique in the analysis was developed. The DIC system was composed of an Ultra High-Speed Camera (UHSC), a light system, and a data acquisition system. The setup was tested in a laboratory and underwent different calibrations before implementing it in the mine, where using explosives during the tests is allowed. The UHSC captured the blasting process regarding crack propagation. To analyze the development of the cracks, DIC technique was employed and results in terms of displacement versus time were measured from the sample surface. The described experiments integrate a novel analysis approach to the results from the DIC technique and propose a way of interpreting the outcomes regarding crack development in terms of velocity. While developing the methodology, the pre-processing of the data (UHSC images) was shown to enhance the DIC analysis and affect the further post-processing of the results. The presented methodology proposes a human-independent procedure of analysis that can help to differentiate the displacement of the crack along its time. Nevertheless, a visual analysis of the results was performed to complement the results and try to broaden the understanding of the crack development process. The DIC results indicated a nonconstant crack propagation velocity while the development patterns were interpreted to match previous literature. The experimental studies confirmed the radial propagation behavior surrounding the blasthole in the single borehole test, while the two borehole configurations show to influence the crack propagation direction and interconnection. This work describes small-scale experiments that provide meaningful insights in crack propagation and how the different blast design parameters can affect their development. The findings of this study could be useful as an input of a predictive tool to assess blast-induced crack initiation and development. / BeFo (Rock Engineering Research Foundation, Sweden) project number 427, “Experimental and Numerical modeling of blast-induced damage around rock tunnel using LS-DYNA”

This master thesis project was performed in collaboration with Scania CV AB, Engine Materials group. The purpose with the project was to investigate different ceramic TBC (Thermal Barrier Coating) thermal insulation properties inside the combustion chamber. Experimental testing was performed with a Single-Cylinder engine with TBC deposited on selected components. A dummy-valve was developed and manufactured specifically for this test in order to enable a water cooling system and to ease the testing procedure. The dummy-valve consists of a headlock, socket, valve poppet and valve shaft. Additionally, a copper ring is mounted between the cylinder head and the valve poppet to seal the system from combustion gases. Thermocouples attached to the modified valve poppet and valve shaft measured the temperature during engine test to calculate the heat flux. The TBCs consisted of three different materials: 7-8% yttrium-stabilized zirconia (8YSZ), gadolinium zirconia and lanthanum zirconia. The 8YSZ TBC was tested as standard, but also with microstructural modifications. Modifications such as pre-induced segmented cracks, nanostructured zones and sealed porosity were used. The results indicated that the heat flux of 8YSZ-standard, 8YSZ-nano and 8YSZ-segmented cracks was in level with the steel reference. In the case of 8YSZ-sealed porosity the heat flux was measured higher than the steel reference. Since 8YSZ-standard and 8YSZ-sealed porosity are deposited with the same powder it is believed that the high heat flux is caused by radiative heat transfer. The remaining samples have had some microstructural changes during engine testing. 8YSZ-nano had undergone sintering and its nanostructured zones became fewer and almost gone after engine testing leading to less heat barrier in the top coat of the TBC. However, for 8YSZ-segmented cracks and gadolinium zirconia lower heat flux was measured due to the appearance of horizontal cracks. These cracks are believed to act as internal barriers as they are orientated perpendicular to the heat flow. During long-time (5 hour) engine tests the 8YSZ-standard exhibited the same phenomena: a decrease in heat flux due to propagation of horizontal cracks. One-dimensional heat flux was not achieved and the main reason for that was caused by heating and cooling of the shafts outer surface. However, the dummy-valve system has proven to be a quick, easy and stable to perform tests with a Single-Cylinder engine. Both water-cooling and long-time engine tests were conducted with minor issues. The dummy-valve has been further developed for future tests. Changes to the valve shaft are the most remarkable: smaller diameter to reduce heat transfer and smaller pockets to ensure better thermocouple positioning. Another issue was gas leakage from the combustion chamber through the copper ring and valve poppet joint. The copper ring will be designed with a 1 mm thick track to improve sealing, hence better attachment to the valve poppet.

This diploma thesis deals with the use of glass fiber geogrids and geocomposites in road construction. The reinforcement elements are inserted between the wearing and binder asphalt layer. This diploma thesis summarizes the knowledge connected with reinforcing of asphalt roads using glass fiber. It examines the effect of reinforcing fiberglass elements on the resistance of asphalt layers loaded with traffic. At the same time, it deals with the application of a various amount of tack coat and subsequent behavior of the asphalt layers. In this work, laboratory tests of bond strength between layers on cores and four-point bending were performed.

Thesis (MScEng (Civil Engineering))--University of Stellenbosch, 2011. / ENGLISH ABSTRACT: The southbound screener lane of the Heidelberg Traffic Control Centre received structural improvements by means of an ultra thin continuously reinforced concrete pavement (UTCRCP) overlay. This experimental overlay forms part of the South African National Roads Agency Limited’s innovative highway repair strategy on existing pavements that have exceeded design life. The primary objective of this study was to characterize the UTCRCP overlay with regard to crack spacing formation under accelerated pavement testing (APT). Characterization comprised of empirical modelling techniques, statistical analysis, non destructive testing and software simulations. Pavement deflection responses were empirically and linear elastically converted to input parameters. These parameters were used in the mean crack spacing prediction model of the Mechanistic-Empirical Pavement Design Guide (M-E PDG). Observed cracking under APT was recorded and analyzed by means of descriptive statistics. The outcome of the M-E PDG’s mean crack spacing and the statistics of the observed cracking were evaluated against cncPave simulations. Initial shrinkage crack formations ranged from 500 mm to 900 mm, with a mean spacing of 695 mm. Subsequent secondary cracking reduced the segments, delineated by initial cracking, to intervals consisting of 100 mm to 350 mm. A statistical analysis of the observed cracking indicated that traffic had little effect on the transverse crack spacing formation. The observed cracks yielded a mean spacing of 296 mm, compared to the 186 mm of the M-E PDG mean crack spacing calculation. cncPave simulations indicated that the expected range of cracking would fall between 237 mm and 350 mm with a probability of 50% that crack spacing would exceed 265 mm. The 50th percentile of the observed cracks resulted in a spacing of 233 mm. The APT project was limited to a single test section. No pavement failures occurred during the APT project. A total of 2.8 million 80 kN load repetitions were applied to the UTCRCP. However circular crack formations regarded as a punchout development have formed on the UTCRCP test section. Circular cracks formed around weaknesses in the pavement system. The prediction of these punchout formations incorporates the mean crack spacing result. Occurrence of mean crack spacing forms part of a crack spacing distribution defined by a range. Therefore designing a punchout prediction model for UTCRCP should include the characteristics and range of the crack pattern and not merely the mean crack spacing value. Key Words: UTCRCP, APT, Mean Crack Spacing, Punchout, Descriptive Statistics, cncPave, M-E PDG, Transverse Cracks, Dense Liquid Foundation, Elastic Solid Foundation. / AFRIKAANSE OPSOMMING: Die suidwaartse moniteerings laan van die Heidelberg Verkeersbeheer Sentrum, het strukturele verbetering ondergaan deur die konstruksie van ‘n ultradun aaneen-gewapende betonplaveisel (UDAGBP) wat dien as ‘n deklaag. Hierdie eksperimentele deklaag is deel van die Suid-Afrikaanse Nasionale Paaie Aggentskap Beperk (SANRAL) se vernuftige deurpadherstelstrategie vir bestaande deurpaaie waarvan die ontwerplewe verstryk het. Die primêre doel van hierdie studie is om die UDAGBP te karakteriseer, met betrekking tot kraakspasiëring, deur middel van Versnelde Plaveisel Toetsing (VPT). Die karakteriseringsproses het bestaan uit empiriese moduleringstegnieke, statistiese ontleding, nie-destruktiewe toetsmetodologieë en sagtewaresimulasies. Die plaveiseldefleksiegedrag is empiries en linieêr elasties ontleed en omgeskakel na invoerparameters. Hierdie parameters is gebruik in die peilingsmodel vir gemiddelde kraakspasiëring van die Meganisties-Empiries Plaveisel Ontwerpsgids (M-E POG). Waargenome kraakspasiëring na die afloop van VPT is opgeteken en deur middel van beskrywende statistiek ontleed. Die resultate van die M-E POG se gemiddelde kraakspasiëring en die statistiese ontleding van die waargenome krake is geëvalueer teenoor cncPave simulasies. Aanvanklike krimpingskrake het gevorm met wisselende kraakspasiëring tussen 500 mm en 900 mm met ‘n gemiddelde spasiëring van 695 mm. Daaropvolgende krake het die aanvanklike segmente, wat gevorm het tydens die aanvanklike krimpingskrake, verkort na intervalle van 100 mm tot 350 mm. ‘n Statistiese ontleding van die waargeneemde krake het aangedui dat verkeer weinig ‘n aandeel het in die dwarskraak-vormingsproses. Die waargenome krake het ‘n gemiddelde spasiëring van 296 mm in vergelyking met 186 mm van die M-E POG se gemiddelde kraakspasiëring berekening. cncPave simulasies het aangedui dat die verwagte kraakspasiëringsgrense tussen 237 mm en 350 mm is en ‘n 50% waarskynlikheid dat die kraakspasiëring meer as 265 mm is. Die VPT projek is beperk tot ‘n enkele toetsseksie. Geen plaveiselfalings is waargeneem gedurende die VPT projek nie. In totaal was 2.8 miljoen as-lasherhalings aangewend op die UDAGBP. Daar het egeter sirkelvormige kraakformasies, wat beskou word as ponsswigting, ontwikkel op die UDAGBP toetsseksie. Sirkelvormige kraakpatrone het gevorm rondom swak plekke in die plaveisel. Die peilingsmodelle van hierdie ponsswigting maak gebruik van die gimiddelde kraakspasiëringsresultaat. Die verskynsel van gemiddelde kraakspasiëring in hierdie studie is deel van ‘n kraakspasiëringsverdeling, gedefinieerd deur ‘n spasiëringsgrens. Daarom moet die kraakspasiëringskarakteristieke en spasiëringsgrense in ag geneem word in die ontwerpsproses van ‘n UDAGBP ponsswigting-peilings-model, nie slegs die waarde van die gemiddelde kraakspasiëring nie. Sleutel woorde: UDAGBP, VPT, Gemiddelde Kraakspasiëring, Ponsswigting, Beskrywende Statistiek, cncPave, M-E POG, Transversale Krake, Digte Vloeistof Fondasie, Elasties- Soliede Fondasie.

L'étude de l'endommagement des matériaux à comportement pseudo-fragile fait l'objet denombreuses études et la caractérisation du réseau de fissures constitue une étape nécessairepour une meilleure compréhension de leur comportement. L'objectif principal est ici d'identifierde manière la plus fine possible cet espace fissuré en trois dimensions grâce à la techniqued'imagerie nommée microtomographie X. Pour ce faire, une machine d'essai in-situ a étédéveloppée et une procédure d'analyse des images 3D a été validée. L'objectif du dispositif insituest de maintenir l'échantillon dans différents états fissurés pour rendre possible lesacquisitions microtomographiques. Une fois les images 3D reconstruites, la procédure detraitement est appliquée et l'espace fissuré est identifié. Des mesures sont alors réalisées surl'évolution du réseau de fissures au cours de l'endommagement. Ce travail constitue la premièreétape d'un traitement plus général qui a pour objectif de simuler numériquement lecomportement mécanique de ces matériaux en se basant sur leur géométrie réelle. / Materials displaying a pseudo-brittle behavior have been well studied over the past decade andthe characterization of the cracks network has become nowadays an important step for theunderstanding of their damaging behavior. The aim of this work is to characterize, in the finestavailable way, this crack space in 3D using X-ray computed microtomography. This wasachieved: 1) by designing an in-situ compressive device which maintains a sample in a crackedstate during microtomographic data acquisition and, 2) by processing the images with relevantimage filtering techniques for a better cracks network characterization. Two parameters ofchoice are then measured: the cracks network surface and volume. This work is the first step ofa global procedure which aims to numerically model the mechanical behavior of pseudo-brittlematerials by using real 3D crack geometry.

Le contrôle non destructif (CND) par Courants de Foucault (CF) de défauts de fissuration est l’une des techniques les plus répandues dans de nombreux secteurs industriels. L’utilisation d’outils de modélisation adaptés permet d’améliorer les procédés de contrôle et la compréhension des données expérimentales observées. Ce travail de thèse, réalisé au CEA LIST et sous la direction de D. Lesselier (Laboratoire des Signaux et Systèmes), a pour objectif de proposer une approche de modélisation semi-analytique dédiée à la simulation du CND CF de défauts fins ou très fins dans une pièce plane conductrice composée de plusieurs couches. Il a fait l’objet d’une collaboration, dans le cadre du projet CIVAMONT, avec l’équipe Meander de l’University of Western Macedonia (Grèce), dirigée par le professeur T. Theodoulidis.Du point de vue de la simulation, la complexité du problème à traiter est liée aux particularités des défauts de fissuration : une ouverture très fine, un profil complexe et la possibilité d’avoir des ponts de conductivité entre les deux faces latérales du défaut. Ces caractéristiques expliquent la difficulté qu’ont les méthodes de simulation classiques, semi-analytiques ou purement numériques, à traiter efficacement ce type de configuration. Pour ces raisons, une approche dédiée aux défauts fins, fondée sur la méthode des éléments de frontière, a été développée. Elle présente l’avantage majeur de ne requérir qu’une discrétisation surfacique du défaut, en traitant analytiquement le calcul dans la direction de son ouverture. Après la résolution, avec la Méthode des Moments, de l’équation intégrale décrivant les interactions entre le champ d’excitation et le défaut, la réponse de la sonde est calculée en appliquant le théorème de réciprocité. Les développements théoriques réalisés dans cette thèse ont abouti à la mise en place d’une formulation générale permettant la prise en compte d’un nombre quelconque de défauts fins, d’orientations et des géométries différentes, pouvant être situés dans des couches différentes de la pièce. Par la suite, cette méthode innovante a été implémentée au sein de la plateforme de simulation CIVA, développée au CEA LIST, et a été validée expérimentalement à plusieurs reprises. Une extension de cette méthode a également permis la mise en place d’une approche la couplant à une modélisation volumique standard pour la simulation de configurations complexes comme le contrôle de fissures au voisinage d’un alésage. Ce travail, qui a fait l’objet d’une diffusion internationale affirmée, a permis de lever avec succès un certain nombre de difficultés théoriques et pratiques liées à la modélisation du CND CF de défauts fins. / Non Destructive Testing (NDT) with Eddy Current (EC) techniques are is widely employed in several industrial sectors for cracks detection. Numerical simulation tools are largely used in order to design sensors, understand the signals collected during the measurements process and to provide a support in expertise. This work has been accomplished inside CEA LIST in collaboration with L2S-Supélec. It is also a part of the CIVAMONT 2012 project, with the active participation of MEANDER laboratory members from University of Western Macedonia (Greece) and Technological Educational Institute of Western Macedonia (Greece). The main goal of our work has consisted in to developing a semi-analytical modeling approach, devoted to Eddy Current Testing (ECT) of multiple narrow cracks in planar multilayered structures. From the numerical point of view, simulation of multiple narrow cracks problems is a difficult task for classical methods, like for example the Volume Integral Method (VIM) or the Finite Element Method (FEM). The main issues reside in geometrical characteristics of narrow crack themselves. Indeed, a narrow crack presents a small opening as well as complex profile and a complex shape, with possible electrical contacts inside it. All these features increase enormously, with classical methods, the difficulty to simulate in rapid and/or precise way problems involving narrow cracks. We have tackled the narrow crack issue by developing a Boundary Element Method (BEM) dedicated to ECT signal modeling, starting from an approach presented in literature. Then, we have extended its capability to more realistic and challenging cases, such as the ECT of multilayered structures affected by complex narrow cracks. The principle of this method is to introduce additional assumptions, leading to the description of the crack perturbation as the effect of a dipole distribution, oriented toward the crack opening. Numerically speaking, such a description makes it possible to largely reduce, compared to the VIM, the number of unknowns that one needs to properly solve the problem. A particular attention has been devoted to the analytical formulation, in order to achieve generality, accuracy and efficiency. A precise derivation of the spectral-domain Dyadic Green Function (DGF) associated to our problem has first been developed. In this work, analytical expressions of the spectral-domain DGF have been obtained via the Discrete Complex Image Method (DCIM). Then, an accurate approximation of the spectral-domain DGF has been achieved via the Generalized Pencil of Function (GPOF) method. Therefore, the closed-form of the spectral-domain DGF, expressed under the form of Sommerfeld Integrals (SIs), has been calculated analytically. Finally, the integral equation(s) associate to the electromagnetic problem is solved by applying the Method of Moments (MoM).Validations with respect to experiments and commercial simulation software have been performed to test the model. A large set of configurations have been chosen in order to address realistic configurations involving multiple narrow cracks embedded in different layers of a given multilayered structure. The model proposed has shown its promising performance in terms of computational time compared with the VIM and the FEM. Moreover, a very good agreement with respect to the experimental data has always been observed. In the last and very recent part of our work, a coupled approach between BEM and VIM has been studied and developed in order to address, in a efficient way, problems where narrow cracks appear in the vicinity of with volumetric flaws (for example the simulation of fastener sites inspections). Comparisons with experimental measures have shown that the coupled approach is capable to achieve, overall, better results than the VIM and saves a lot of computational time.

For a PV plant it is of fundamental importance that the operation of the PV modules is free from faults or at least that the faults can be detected early, to ensure efficient electricity production. Some defects such as cracks can be seen in visible light while microcracks and damage to the silicon material can only be seen through special lighting. This study focuses on the most common defects in photovoltaic (PV) systems. Compare the infrared (IR) technology with the new ultraviolet (UV) fluorescence image technique for PV characterization, based on their accuracy and uncertainty factors under an experimental field investigation. In this study, first a literature study was conducted to the most common defects in PV system and their impact on electricity generation. Then a simulation model of a PV system was created in PVsyst and exported to Microsoft Excel which was used to evaluate how different defects at different stages of the PV cell's life cycle impact electricity generation, performance parameters and economic exchange. Furthermore, experiments with UV and IR was implemented at a PV system located in Dalarna and some PV modules at MDH. It was conducted that occurrence of snail tracks, delamination and hot spots in combination with bypass failures and non-functioning cell will affect the economic profitability in the long run and the payback time will increase since their impacts on electricity generation and performance parameters are huge. The worst case is when PV modules are affected by the fault in bypass diode and non-functioning cell which result to a payback time longer than the module's lifetime and huge amount electricity losses in different bypass diodes configurations. Since UV and IR are two different methods that are performed in two different ways, different errors occurred during the measurements. The biggest external factor was the weather that determined if the experiment could be implemented. The IR method gave decent results and was quicker to use, but the UV method highlighted some defect which could not be seen with the IR technology.

La détection des fissures fermées constitue un verrou important pour les méthodes conventionnelles de CND. En revanche, les méthodes basées sur le principe du contact acoustique non-linéaire (CAN) représentent un outil potentiel capable de détecter et éventuellement de caractériser les fissures fermées. Dans ce travail de thèse, le CAN et la génération du second harmonique sont au centre de l’attention dans le but d’analyser les mécanismes mis en jeu en vue de la détection et la caractérisation de fissures fermées. Notre approche repose sur une analyse numérique de l’interaction entre une interface de contact et une onde longitudinale et une analyse expérimentale du CAN sur une éprouvette contenant une fissure de fatigue.Le CAN est modélisé par une loi de contact unilatéral avec (ou sans) frottement de Coulomb, en considérant l’interaction non-linéaire entre une onde longitudinale et une interface de contact, d’abord dans un milieu unidimensionnel en utilisant la méthode des différences finies, puis dans un milieu bidimensionnel par la méthode des Eléments Finis. Cette analyse a permis d’expliquer la génération des harmoniques supérieurs et de mettre en évidence les principaux paramètres gouvernant le CAN dans un cas unidimensionnel (onde plane/interface de contact plane), et aussi dans un cas bidimensionnel plus complexe considérant une onde non-plane et des morphologies d’interfaces complexes. Afin de valider les résultats numériques et d’apprécier l’applicabilité de la méthode dans un cas réel, des mesures expérimentales du second harmonique dans le cas d’une fissure de fatigue réelle sont effectuées. Les résultats de cette étude sont en accord avec les tendances numériques obtenues.Enfin, dans le but d’exploiter au maximum la méthode de génération d’harmonique, nous proposons également une stratégie numérique/expérimentale de reconstruction des sources acoustiques, basée sur une méthode d’holographie acoustique, permettant ainsi d’accéder à la distribution des sources de génération du second harmonique au niveau de la fissure. Ces résultats sont prometteurs en vue de la caractérisation des fissures fermées. / The detection of closed cracks constitutes an important obstacle for conventional nondestructiftechnics (NDT). However, nonlinear methods based on contact acoustic nonlinearity (CAN)represent a potential tool for the detection of these defects. In this work, a detailed analysis of thecontact acoustic nonlinearity is proposed in view of the characterization of closed crack.Our approach is based on an association between a numerical analysis of the nonlinear interactionbetween a longitudinal wave and a contact interface, as well as an experimental study of the CAN inthe case of a real fatigue crack.Numerically, the CAN is modeled by using a unilateral contact law with (or without) Coulombfriction. The nonlinear interaction between a longitudinal wave and a contact interface is considered,first in a one-dimensional medium by using the Finite Differences method, and then in a twodimensionalmedium by using the Finite Elements method. This analysis explains the generation ofsuper-harmonics and defines the main physical parameters controlling the CAN in a one-dimensionalcase (plane wave / plane contact interface) and also in a more complex two-dimensional caseconsidering a non-planar wave and complex interface morphologies. In order to validate the numericalresults and to test the applicability of the method in a real case, experimental measurements of thesecond harmonic in the case of a real fatigue crack are realized. The results of this study are inagreement with the numerical tendencies obtained.Finally, in order to exploit well the harmonic generation method, a numerical / experimentalstrategy based on acoustic holography principal for the reconstruction of acoustic sources is proposed.This strategy gives access to the distribution of second harmonic sources at the crack. The results ofthis study are promising in view of the characterization of closed cracks.

Le développement de systèmes mécaniques, toujours plus performants et légers, demande l’optimisation continue des matériaux utilisés ainsi que leurs traitements. Dans le domaine des transmissions de puissance, la nitruration gazeuse apporte une hausse significative des durées de vie en fatigue des pièces fortement sollicitées. Alors que le rôle des gradients de propriétés mécaniques générés sur la résistance en fatigue est reconnu, l’influence de cette microstructure nitrurée sur les mécanismes de plasticité ou de microplasticité menant à de l’endommagement puis à l’initiation d’une fissure reste relativement méconnue. Ce travail de thèse concerne exclusivement la nitruration de l’acier de nuance 33CrMoV12-9, particulièrement utilisée dans les roulements et engrenages des industries aéronautiques et navales. Les propriétés mécaniques liées aux phénomènes menant à l’initiation de fissures en fatigue à grand nombre de cycles ont, tout particulièrement, été étudiées. Les propriétés mécaniques, comme le module d’élasticité, la dureté, la limite d’élasticité ou encore les contraintes résiduelles sont déterminées à plusieurs échelles, de celles de la microstructure à celles plus macroscopiques. Des observations in-situ ou post-mortem des couches nitrurées sollicitées jusqu’à l’endommagement ayant pour but d’estimer l’influence de la microstructure sur les hétérogénéités de déformations, les phénomènes de microplasticité et les mécanismes d’endommagement correspondants n’ont pas permis d’identifier clairement des mécanismes d’initiations de fissures. Pour ces essais, une éprouvette nitrurée à cœur représentative de la couche nitrurée a été développée. Cet outil expérimental a permis d’effectuer des essais simples de sollicitations des microstructures et ainsi déterminer des caractéristiques élastiques, d’écrouissage et de tenue en fatigue. Enfin, il a été démontré que les propriétés mesurées comme le module d’élasticité dépendent de l’échelle d’observation, des moyens et des méthodes utilisés. / Design of high-performance and lighter mechanical systems needs a perpetual work on materials used. In the power transmission field, gaseous nitriding thermochemical treatment enhances significantly the life-cycle of the parts which are higher stressed on the surface. While the contribution of the mechanical property gradients due to the treatment on the fatigue life is well-known, the microstructure effects on the (micro-)plasticity mechanisms remains undetermined. This work concerns exclusively the nitrided 33CrMoV12-9 graded steel, used for gears and bearings in the naval and aeronautical industry. It aims particularly at defining mechanical properties linked to crack initiation mechanisms during high-cycle fatigue. Properties such as Young modulus, hardness, yield stress and fatigue limits are looked at several scales, from microstructural ones to hardened case one. In-situ and post-mortem investigation on loaded and damaged nitrided layers leads to estimating the microstructural influence on local deformations, microplasticity and damage but no particular mechanisms were found. Special fully-nitrided parts were designed to perform the experiments. This tool helps to carry out simple loadings and measure the strain-hardening curve and fatigue limits. Finally, it as been attested that measured properties depend on scales, means and methodology.

Os aços de alta resistência e baixa liga (ARBL) são amplamente empregados nas indústrias automotiva, petrolífera e naval por apresentarem boas propriedades mecânicas e boa soldabilidade. A seleção do processo de soldagem utilizado para a fabricação de componentes depende de vários fatores, entre eles a qualidade do cordão de solda e a capacidade de produção, ambas obtidas ao se utilizar a soldagem a laser, processo que vem sendo cada vez mais utilizado nas indústrias. As propriedades dos cordões de solda são afetadas pelo processo de soldagem e pelos parâmetros do processo, e o conhecimento do desempenho mecânico do cordão de solda é essencial para garantir a confiabilidade da união soldada. A caracterização mecânica de cordões de solda é complexa devido aos diversos fatores que podem afetá-lo, como as alterações metalúrgicas, diferentes propriedades mecânicas nas regiões do cordão de solda, presença de descontinuidades, tensão residual e complexo estado de tensão devido à geometria da junta. O presente trabalho obteve uniões entre chapas de aço ARBL pelo processo a laser em duas condições diferentes: com alto aporte térmico e baixo aporte térmico. A utilização de diferentes aportes térmicos gerou cordões de solda com microestruturas distintas, e avaliou-se, deste modo, o comportamento mecânico associando-o à microestrutura gerada. Para a avaliação da microestrutura utilizou-se da microscopia óptica e eletrônica de varredura, e para a caracterização mecânica empregou-se a aferição da dureza, ensaio de tração, ensaio de tenacidade à fratura, avaliada pelo CTOD, e ensaio de propagação da trinca por fadiga. Devido às alterações metalúrgicas causadas pelos diferentes aportes térmicos verificou-se diferente comportamento mecânico entre os cordões de solda em função da microestrutura obtida. / High-strength low-alloy steels (HSLA) are widely used in the automotive and oil industries due to their good mechanical properties and good weldability. The selection of the welding process used to manufacture components depends on several factors, including the quality of the weld bead and the production capacity, both obtained when using laser welding, a process, which is being increasingly used in the industries. The weld beads properties are affected by the welding process and by its parameters, and the knowledge of the weld bead mechanical performance is essential to ensure the reliability of the welded joint. The weld beads mechanical characterization is complex due to various factors that may affect it, such as metallurgical changes, different mechanical properties in the regions of the weld bead, presence of discontinuities, residual stress and complex state of tension due to the geometry of the joint. The present work obtained the joining of HSLA steel sheets by laser process in two different conditions, with high heat input and low heat input. The use of different heat inputs produced weld beads with different microstructures; therefore, the welded joints were evaluated according to their mechanical behavior, relating it with the microstructural generated. For the microstructure evaluation, optical microscopy and scanning electron microscopy were used, and for the mechanical characterization hardness determination, tensile test, fracture toughness test (evaluated by CTOD) and fatigue crack propagation were performed. Due to the metallurgical changes caused by the different heat inputs, a different mechanical behavior was observed between the weld beads as a function of the microstructure obtained.