Increasing ecological and economic pressures, as well as increased technical requirements, have given rise to the need for research into alternative wear layers for hydrodynamic plain bearings. Previous investigations have tested and researched the bonded coating GL92. It has been possible to demonstrate that the bonded coating (plastic base) is a viable alternative to a conventional white metal alloy. However, this coating was not able to address all the issues experienced by industrial users, for example the behaviour of the system (changes to operating properties) during penetration of particles, or behaviour in a mixed friction area. The literature has not – or not sufficiently – answered the questions relating to damage patterns and damage tolerance. Therefore, regardless of the proven benefits, industrial use of this coating is currently not possible or is connected with significant risks. In the study, tests were carried out in the mixed friction area (start-stop testing, emergency testing without oil, insufficient lubrication tests) as well as in the hydrodynamic area. The comparative tests were performed on different wear layer materials. This paper demonstrates and discusses the results for both a conventional white metal alloy bearing and for the bearing with bonded coating (GL92). Depending on the operating point, the bearing systems exhibit significant differences at the maximum bearing load, lubricating film pressure distribution and maximum bearing temperature. The investigation has proven that GL92 is a technically useful alternative to conventional wear layers.

Compression tests were performed on damaged and undamaged rods and tubes made from unidirectional carbon/epoxy composite and lightweight core materials. Tested samples represent local members in an open, three-dimensional, composite lattice structure. Testing was performed in order to establish effective core materials to use in order to increase the buckling length of local IsoTruss® members while maintaining low weight. Members were formed from T700SC-12K-50C carbon fiber with UF6639-100 resin. Core materials consisted of 3/8-inch (0.953 cm) outside diameter Teflon® rods, Teflon® tubes, nylon rods, nylon tubes, Ertalyte® rods, and Duratron® rods. All 3/8-inch (0.953-cm) cores were each surrounded by 50 tows of carbon/epoxy prepreg. Control samples were also created with 50 carbon/epoxy prepreg tows. Half-inch (1.27 cm) outside diameter copper tubes were used as core materials for tubes consisting of 100 carbon/epoxy prepreg tows. Control samples to compare against samples with copper cores were also created with 100 tows of carbon/epoxy prepreg. Impact damage was inflicted using a cylindrical tup with 20 ft-lb impact energy.In undamaged specimens, nylon tube showed the highest structural efficiency. Nylon showed structural efficiencies much higher than other materials when comparing undamaged samples. In damaged specimens Ertalyte® rods showed the highest structural efficiency. Core stiffness appeared to control the level of absorbed impact energy with stiffer cores absorbing and dissipating more energy than softer equivalents during impact.

Compression strength after impact tests were conducted on unidirectional composite rods with sleeves. These elements represent local members of open three-dimensional composite lattice structures (e.g., based on isogrid or IsoTruss® technologies). The unidirectional cores composed of carbon, glass, or basalt fiber/epoxy composites were co-cured in aramid sleeves. Sleeve patterns included both bi-directional (unsymmetric) braids and unidirectional spiral wraps with sleeve coverage ranging from nominally half to full. The diameters were nominally 8 and 11 mm (5/16 and 7/16 in). The larger diameter had nominally twice the cross-sectional area, to quantify the effects of scaling. The specimens were long enough to encourage local buckling failure as expected in members of typical composite lattice structures. The unsupported lengths varied from 127 mm (5.0 in) to 160 mm (6.3 in). Specimens were radially impacted at mid-length with energy levels ranging from 0 to 20 J (0 to 14.8 ft-lbs) and tested in longitudinal compression to quantify the effects of local impact damage on the buckling strength. In undamaged specimens, sleeve type and sleeve coverage have no effect on the ultimate compression strength of carbon, glass, or basalt composites (7% or less standard deviation for each material). When impacted, the influence of sleeve type and sleeve coverage varies with the type of fiber in the unidirectional core. Sleeve type and coverage did not affect the compression strength after impact for fiberglass composites. On the other hand, both carbon and basalt composites exhibited improved performance with braided (vs. spiral) sleeves (up to 34% stronger) and full (vs. half) coverage (up to 38% stronger). The compression strength of carbon configurations decreases with increasing impact energy regardless of sleeve type or coverage. The higher flexibility of glass and basalt composites, however, allowed some configurations to maintain the same compression strength after impact as their undamaged counterparts, at lower impact energy levels. Doubling cross-sectional area of basalt composites significantly improves the stiffness and compression strength after impact, more than doubling the impact energy required to achieve the same compression strength.

The structural optimization of a cantilever aircraft wing with curvilinear spars and ribs and stiffeners is described. The design concept of reinforcing the wing structure using curvilinear stiffening members has been explored due to the development of novel manufacturing technologies like electron-beam-free-form-fabrication (EBF3). For the optimization of a complex wing, a common strategy is to divide the optimization procedure into two subsystems: the global wing optimization which optimizes the geometry of spars, ribs and wing skins; and the local panel optimization which optimizes the design variables of local panels bordered by spars and ribs. The stiffeners are placed on the local panels to increase the stiffness and buckling resistance. The panel thickness, size and shape of stiffeners are optimized to minimize the structural weight. The geometry of spars and ribs greatly influences the design of stiffened panels. During the local panel optimization, the stress information is taken from the global model as a displacement boundary condition on the panel edges using the so-called "Global-Local Approach". The aircraft design is characterized by multiple disciplines: structures, aeroelasticity and buckling. Particle swarm optimization is used in the integration of global/local optimization to optimize the SpaRibs. The interaction between the global wing optimization and the local panel optimization is usually computationally expensive. A parallel computing technology has been developed in Python programming to reduce the CPU time. The license cycle-check method and memory self-adjustment method are two approaches that have been applied in the parallel framework in order to optimize the use of the resources by reducing the license and memory limitations and making the code robust. The integrated global-local optimization approach has been applied to subsonic NASA common research model (CRM) wing, which proves the methodology's application scaling with medium fidelity FEM analysis. Both the global wing design variables and local panel design variables are optimized to minimize the wing weight at an acceptable computational cost. The structural weight of the wing has been, therefore, reduced by 40% and the parallel implementation allowed a reduction in the CPU time by 89%. The aforementioned Global-Local Approach is investigated and applied to a composite panel with crack at its center. Because of composite laminates' heterogeneity, an accurate analysis of these requires very high time and storage space. In the presence of structural discontinuities like cracks, delaminations, cutouts etc., the computational complexity increases significantly. A possible alternative to reduce the computational complexity is the global-local analysis which involves an approximate analysis of the whole structure followed by a detailed analysis of a significantly smaller region of interest. We investigate here the performance of the global-local scheme based on the finite element method by comparing it to the traditional finite element method. To do so, we conduct a 2D structural analysis of a composite square plate, with a thin rectangular notch at its center, subjected to a uniform transverse pressure, using the commercial software ABAQUS. We show that the presence of the thin notch affects only the local response of the structure and that the size of the affected area depends on the notch length. We investigate also the effect of the notch shape on the response of the structure. Stiffeners attached to composite panels may significantly increase the overall buckling load of the resultant stiffened structure. Buckling analysis of a composite panel with attached longitudinal stiffeners under compressive loads is performed using Ritz method with trigonometric functions. Results are then compared to those from ABAQUS FEA for different shell elements. The case of composite panel with one, two, and three stiffeners is investigated. The effect of the distance between the stiffeners on the buckling load is also studied. The variation of the buckling load and buckling modes with the stiffeners' height is investigated. It is shown that there is an optimum value of stiffeners' height beyond which the structural response of the stiffened panel is not improved and the buckling load does not increase. Furthermore, there exist different critical values of stiffener's height at which the buckling mode of the structure changes. Next, buckling analysis of a composite panel with two straight stiffeners and a crack at the center is performed. Finally, buckling analysis of a composite panel with curvilinear stiffeners and a crack at the center is also conducted. ABAQUS is used for these two examples and results show that panels with a larger crack have a reduced buckling load. It is shown also that the buckling load decreases slightly when using higher order 2D shell FEM elements. A damage tolerance framework, EBF3PanelOpt, has been developed to design and analyze curvilinearly stiffened panels. The framework is written with the scripting language PYTHON and it interacts with the commercial software MSC. Patran (for geometry and mesh creation), MSC. Nastran (for finite element analysis), and MSC. Marc (for damage tolerance analysis). The crack location is set to the location of the maximum value of the major principal stress while its orientation is set normal to the major principal axis direction. The effective stress intensity factor is calculated using the Virtual Crack Closure Technique and compared to the fracture toughness of the material in order to decide whether the crack will expand or not. The ratio of these two quantities is used as a constraint, along with the buckling factor, Kreisselmeier and Steinhauser criteria, and crippling factor. The EBF3PanelOpt framework is integrated within a two-step Particle Swarm Optimization in order to minimize the weight of the panel while satisfying the aforementioned constraints and using all the shape and thickness parameters as design variables. The result of the PSO is used then as an initial guess for the Gradient Based Optimization using only the thickness parameters as design variables. The GBO is applied using the commercial software VisualDOC. / Ph. D.

O presente trabalho, inicialmente, apresenta uma revisão a cerca de vários métodos de análise de fadiga e tolerância ao dano em materiais compósitos e, em particular, o conceito de elemento crítico e o método da resistência residual é estudado e aplicado à análise de materiais compósitos poliméricos, reforçados por fibras contínuas, sob cargas cíclicas. Casos de laminados com concentradores de tensão são também abordados, principalmente, devido ao crescente emprego de materiais compósitos. Diante do contexto apresentado, o presente trabalho buscou desenvolver uma ferramenta computacional, que fosse capaz de prever de forma consistente tanto a vida em fadiga de estruturas em material compósito como a resistência residual dessas estruturas sob a ação de carregamentos cíclicos. Dessa forma, um modelo baseado na resistência residual, que envolve integração numérica, é implementado em um programa computacional denominado SAFECom (Sistema de Análise de Fadiga de Estruturas em Compósito), o qual é aplicado a alguns estudos de casos. O SAFECom foi, então, avaliado, comparando resultados obtidos com dados da literatura. Verifica-se uma boa convergência entre os resultados nessa fase de avaliação. Por fim, são listadas possibilidades de aprimoramento do programa, que podem torná-lo aplicável a outros estudos de casos não abordados no presente trabalho, além da avaliação do mesmo através de ensaios experimentais. / This work firstly presents an overview about various methods of analysis of fatigue and damage tolerance of composite materials and, in particular, the concept of critical element and the method of residual strength are studied and applied to the analysis of polymer composites reinforced by continuous fibers under cyclic loading. Cases of laminates with stress concentrators are also addressed, mainly due to increasing use of composite materials. In this context, this work aimed to develop a computational tool that was able to consistently predict both the fatigue life of structures in composite material and the residual strength of these structures under the action of cyclic loads. Thus, a model based on residual strength, which involves numerical integration, is implemented in a computer program called SAFECom (System of Analysis of Fatigue for Composites), which is applied to some case studies. The SAFECom is evaluated by comparing results obtained from literature data. There is a good convergence between the results of this assessment. Finally, possibilities for improvement are listed on the program that can make it applicable to other case studies, which were not covered in this study, in addition to its evaluation by experimental tests.

The main objective of the thesis is to assess the damage resistance and damage tolerance of the non-conventional dispersed laminates and compare the response with the conventional ones. However, part of the effort is spent on understanding the delamination behavior in multidirectional laminates. In the first part of the thesis, the delamination in multidirectional laminates is studied. The objective is to design a proper stacking sequence, capable of avoiding intralaminar damage (crack jumping), to enable the fracture toughness characterization under pure mode I. The result of this study shows that the higher the crack arm bending stiffness, the lower the tendency to crack jumping. This phenomenon is also studied experimentally and the same conclusion is drawn. / El principal objectiu de la tesi és valorar la resistència al dany i la tolerància al dany dels laminats no-convencionals dispersos i comparar la seva resposta amb la dels laminats convencionals. No obstant, part de l'atenció es dedica a comprendre el comportament de la delaminació en laminats multidireccionals. En la primera part de la tesi, s'analitza la delaminació en laminats multidireccionals. L'objectiu és dissenyar una seqüència d'apilament apropiada per evitar el dany intralaminar (migració de la delaminació) i permetre la caracterització de la tenacitat a la fractura en model. Els resultats d'aquests estudi mostren que a major rigidesa a flexió dels braços de l'esquerda, menor és la tendència a la migració de l'esquerda. Aquest aspecte també s'ha analitzat experimentalment, obtenint les mateixes conclusions.

Les industriels du secteur aéronautique sont, de plus en plus, à la recherche de procédés de fabrication à forte valeur ajoutée sans modifier les paramètres d’infusabilité de la résine lorsque l’on change de tissu. Nous avons donc mis en œuvre le procédé d’infusion de résine liquide sur des composites carbone/époxyde de forte épaisseur (e>4 mm) en modifiant les cycles de polymérisation, les matériaux utilisés et les séquences d’empilement. Tous les tissus sont en carbone et la résine utilisée est la résine commerciale RTM6. Les structures aéronautiques sont sollicitées, en service, de différentes façons. Elles peuvent être accidentellement impactées par des engins de maintenance, des outils, de la grêle ou toute autre forme d’impact. Le problème pour les industriels est de pouvoir détecter l’endommagement créé et de comprendre les mécanismes mis en jeu lors de l’impact mais aussi leur évolution pendant un cyclage en fatigue. Nos travaux se sont donc inscrits dans cet objectif et différentes méthodes ont été mises en œuvre : détection de défauts d’impact et suivi en temps réel par thermographie infrarouge, détection de l’indentation résiduelle par numérisation par projection de franges. Parallèlement, le phénomène d’impact a aussi été traité par une étude statistique par plan d’expérience et une modélisation avancée a été créée avec l’utilisation de surfaces cohésives. / Aeronautical manufacturers are looking for the best manufacturing process giving high benefits. Moreover, it has to be implemented easily with change of woven fabrics. So, we realize thick carbon/epoxy composites (t>4 mm) by modifying cure cycles, woven fabrics and lay-up sequences. Concerning the constituent materials of the composites, woven fabrics are carbon made and the resin is the commercial product named RTM6. Aeronautical structures can be unfortunately stressed, in service, with different solicitations: holding, engine impact, falling objects impacts or other way. The main problem for industrials is to be able to detect the created damage, to understand the phenomena dealing with it and the damage evolution during fatigue cycles. By following this direction, different methods are developed: impact damage defects detection and evolution monitored with infrared thermography, impact residual depth measurement by fringes projection digitalization. At the same time, a statistical study made by design of experiments is completed. A numerical impact modeling is also developed by using cohesive surfaces.

O presente trabalho, inicialmente, apresenta uma revisão a cerca de vários métodos de análise de fadiga e tolerância ao dano em materiais compósitos e, em particular, o conceito de elemento crítico e o método da resistência residual é estudado e aplicado à análise de materiais compósitos poliméricos, reforçados por fibras contínuas, sob cargas cíclicas. Casos de laminados com concentradores de tensão são também abordados, principalmente, devido ao crescente emprego de materiais compósitos. Diante do contexto apresentado, o presente trabalho buscou desenvolver uma ferramenta computacional, que fosse capaz de prever de forma consistente tanto a vida em fadiga de estruturas em material compósito como a resistência residual dessas estruturas sob a ação de carregamentos cíclicos. Dessa forma, um modelo baseado na resistência residual, que envolve integração numérica, é implementado em um programa computacional denominado SAFECom (Sistema de Análise de Fadiga de Estruturas em Compósito), o qual é aplicado a alguns estudos de casos. O SAFECom foi, então, avaliado, comparando resultados obtidos com dados da literatura. Verifica-se uma boa convergência entre os resultados nessa fase de avaliação. Por fim, são listadas possibilidades de aprimoramento do programa, que podem torná-lo aplicável a outros estudos de casos não abordados no presente trabalho, além da avaliação do mesmo através de ensaios experimentais. / This work firstly presents an overview about various methods of analysis of fatigue and damage tolerance of composite materials and, in particular, the concept of critical element and the method of residual strength are studied and applied to the analysis of polymer composites reinforced by continuous fibers under cyclic loading. Cases of laminates with stress concentrators are also addressed, mainly due to increasing use of composite materials. In this context, this work aimed to develop a computational tool that was able to consistently predict both the fatigue life of structures in composite material and the residual strength of these structures under the action of cyclic loads. Thus, a model based on residual strength, which involves numerical integration, is implemented in a computer program called SAFECom (System of Analysis of Fatigue for Composites), which is applied to some case studies. The SAFECom is evaluated by comparing results obtained from literature data. There is a good convergence between the results of this assessment. Finally, possibilities for improvement are listed on the program that can make it applicable to other case studies, which were not covered in this study, in addition to its evaluation by experimental tests.

Die Arbeit ruht auf zwei Säulen: Die eine besteht in der Aufbereitung, Erprobung und konsequenten Anwendung von Methoden der Skaleninvarianzanalyse, die andere in einem breiten Fundus an experimentellen Daten für aushärtbare Aluminiumknetlegierungen in der Form dünner Bleche, die unter gleichartigen, streng kontrollierten Bedingungen gewonnen worden sind. Als methodische Weiterentwicklungen sind die Fundierung des Umgangs mit der algebraischen Korrelation zwischen Vorfaktor und Exponent einer beliebigen Potenzgleichung, die Übertragung des Ansatzes der finiten Skaleninvarianz auf die Ermüdungsrißausbreitung sowie die Kombination der Idee eines geschwindigkeitsbestimmenden Schrittes mit der Dimensionsanalyse der umgebungsabhängigen Ermüdungsrißausbreitung bis hin zur Kartierung der geschwindigkeitsbestimmenden Schritte zu nennen. Auf experimenteller Seite wurde eine Datensammlung mit gemessenen Streubändern für die Ermüdungsrißausbreitung und das Verfestigungsverhalten von 39 Orientierungen bzw. Auslagerungszuständen von Aluminiumlegierungen aufgebaut. Diese Sammlung wird durch ausgewählte Messungen der Ermüdungsrißausbreitung im schwellenwertnahen Bereich, Restfestigkeitsversuche, Rißschließmessungen, Rauheitsmessungen an Bruchflächen, frequenzabhängige Messungen zum Umgebungseinfluß sowie Untersuchungen an drei Stählen und einer Magnesiumlegierung sinnvoll ergänzt. Auf der Basis der Meßdaten und der Analysemethoden wurde der Werkstoffeinfluß auf die Ermüdungsrißausbreitung in dünnen Blechen aus Aluminiumknetlegierungen bei Belastung mit konstanter Amplitude im Gültigkeitsbereich der linear-elastischen Bruchmechanik untersucht. Dabei wurden folgende Größen als wesentliche Einflußfaktoren identifiziert: - für die Gruppenzugehörigkeit: der Kohärenz- und Ordnungsgrad der festigkeitsbestimmenden Ausscheidungen und die resultierende Gleitverteilung, - für den gemeinsamen Vorfaktor der Legierungen der Gruppe 1: die elastischen Eigenschaften und das Spannungsverhältnis (Translation der Paris-Geraden), - für die Exponenten der Legierungen der Gruppe 1: 0,2%-Dehngrenze, athermischer Verfestigungsparameter, Probendicke und Kc-Wert als dimensionsloses Potenzprodukt (Rotation der Paris-Geraden), - für die Legierungen der Gruppe 2: das Ausmaß der Rißablenkung und eine bleibende Mode-II-Komponente der Rißöffnungsverschiebung, - für den Umgebungseinfluß der Legierung 6013 T6: Frequenz und Schwingbreite des Spannungsintensitätsfaktors. Die Diskussion umfaßt den wertenden Vergleich der experimentellen Ergebnisse mit Befunden und Modellen aus der Literatur, Erklärungsansätze für die Ursachen der Einflußnahme der wesentlichen Parameter sowie einen Modellansatz für die Legierungen der Gruppe 1 auf der Basis einer Mischungsregel. Dabei hatte sich erwiesen, daß keines der aus der Literatur bekannten Modelle alle Befunde richtig wiedergibt. Einige der ausgearbeiteten Erklärungsansätze bedürfen der zukünftigen Vertiefung. / The work is based upon two essentials: the first one is the preparation and application of techniques of scale invariance analysis, the second one consists in a database of experimental results for heat-treatable thin-sheet wrought aluminium alloys obtained under uniform conditions. Progress with respect to methodology was achieved regarding, first, the algebraic correlation between sets of coefficients and exponents of any power law, second, the transfer of the concept of finite scale invariance to the phenomenon of fatigue crack growth (FCG), and third, the combination of the ideas of a rate-controlling step and dimensional analysis of environmental-assisted FCG including the mapping of rate-controlling steps. In the experimental part, a database containing both measured scatterbands of FCG and strengthening characteristics for several orientations and aging conditions of aluminium alloys amounting to a total of 39 different material conditions was established. This database was supplemented with results of selected measurements of near-threshold FCG rates, residual strength, crack closure, roughness of fatigue cracks, and frequency-dependent environmental-assisted FCG as well as investigations of three plain-carbon steels and a magnesium alloy. Based on these prerequisites, the influence of the material on the FCG behaviour of thin-sheet wrought aluminium alloys under constant-amplitude loading was investigated within the limits of validity of linear-elastic fracture mechanics. The following influence factors were identified to be essential: The assignment of alloys to one out of two groups is mainly determined by the degrees of coherency and order of the strength-controlling precipitates and the resulting type of slip distribution. The normalized-Paris-law coefficient for the first group is mainly dependent on the modulus of elasticity and the stress ratio. The Paris-law exponents for the first group are dominated by a dimensionless power monomial of the 0.2% proof stress, the athermal strengthening coefficient, sheet thickeness and the critical stress intensity factor. The retardation of the FCG rates of alloys of the second group relative to the first group is mainly determined by the amount of crack deflection and by a residual mode-II component of crack opening displacement. Finally, the environment-assisted FCG for aluminium alloy 6013 T6 reveals a coupled dependence on loading frequency and cyclic stress intensity factor. The discussion covers the evaluation of the results in relation to observations and models from the literature, the explanation of the modes of operation of the major influence factors and a model based on a mixing rule for the alloys of the first group. It turned out that there is not any model that reflects all of the observations simultaneously. Some of the ideas presented require to be worked out in more detail.

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