Afin de limiter les impacts environnementaux, les industries du transport (automobile, aéronautique, etc.) cherchent à alléger leurs produits. Cette recherche de gain de masse peut conduire à l'utilisation des adhésifs dans la réalisation des structures assemblées. Dans les situations de crash, les joints de colle introduits peuvent jouer un rôle critique sur la tenue globale de l'assemblage. Par conséquent, la maîtrise de leur comportement mécanique devient un point clef de développement. Notamment en vue du « virtual testing », il est essentiel de disposer de lois de comportement fiables dans le but de fournir des outils de calcul prédictifs à l'ingénieur. Les travaux de thèse portent sur le développement d'une démarche expérimentale permettant de caractériser le comportement mécanique d'adhésifs sous sollicitations dynamiques combinées. Le développement d'une éprouvette Arcan et d'un dispositif de sollicitation dédiés à la caractérisation des adhésifs en dynamique constituent le coeur du travail de thèse. Après une phase de validation, le moyen d'essais développé a été utilisé afin de caractériser le comportement dynamique de trois adhésifs structuraux. Les résultats obtenus ont été comparés à des essais « structures » de type TAST. En parallèle de ces travaux expérimentaux, une ouverture sur la modélisation numérique des adhésifs est proposée. La stratégie de modélisation se base sur l'utilisation d'éléments cohésifs et d'une loi de comportement développée suivant la méthode de l'état local. L'identification des paramètres matériaux est réalisée par méthode directe et inverse à partir des résultats issus d'essais Arcan. La stratégie de modélisation développée a été transposée à des essais TAST et simple recouvrement. Les prédictions faites par les modèles sont proches des résultats expérimentaux validant ainsi l'approche numérique mise en oeuvre. / In order to limit environmental impacts, the transport industries (automotive, aeronautics, etc.) are seeking to reduce the weight of their products. This search for mass gain can lead to the use of adhesives in the manufacturing of assembled structures. In crash situations, introduced bonded joints can play a critical role in the overall strength of the assembly. Consequently, their mechanical behaviour becomes a key development point. Especially for virtual testing, it is essential to have reliable behaviour laws in order to provide predictive computing tools to the engineer. The thesis work focuses on the development of an experimental approach to characterize the mechanical behaviour of adhesives under combined dynamic loadings. The development of an Arcan specimen and a test rig dedicated to the dynamic characterization of adhesives is the core of the thesis work. After a validation step, the developed test mean was used to characterize the dynamic behaviour of three structural adhesives. The results obtained were compared with "structural" TAST tests. In parallel of this experimental work, a first study on the numerical modelling of adhesives is proposed. The modelling strategy relies on the use of cohesive elements and a behaviour law developed according to the local state method. The identification of material parameters is carried out by direct and inverse methods from the results of Arcan tests. The modelling strategy developed has been transposed to TAST and single lap joint tests. The predictions made by the models are close to the experimental results and therefore validate the implemented numerical approach.

L’assemblage des matériaux composites par collage présente des avantages incontestés par rapport à d’autres méthodes telles que le boulonnage ou le rivetage. Cependant, la principale difficulté que rencontrent les concepteurs est celle de la prévision du niveau et du mode de rupture de ces collages. Dans le cas des composites stratifiés, un des facteurs influents sur le comportement des joints collés est la séquence d'empilement, mais les travaux présentés dans la littérature ne séparent pas les effets globaux (modification des rigidités de membrane et de flexion) et les effets locaux (orientation des plis au contact de la colle). La présente étude s'intéresse à la caractérisation de ces effets dans le cas de joints de type simple recouvrement de stratifiés carbone/epoxy. Pour isoler les effets locaux, des séquences d'empilement spécifiques quasi isotropes quasi homogènes sont utilisées. A propriétés de raideur globale égales, des différences de résistance de plus de 30% sont observées selon les séquences considérées. Les essais réalisés avec un stratifié symétrique anisotrope en flexion montrent également que la raideur en flexion joue un rôle important dans le comportement des joints. Les modèles analytiques utilisés prédisent les effets globaux avec une bonne précision mais sont inappropriés lorsque des phénomènes locaux se produisent. Une approche par éléments finis permet de prendre en compte ces phénomènes, en modélisant explicitement les plis au contact de la colle et en rendant possible le décollement interlaminaire de ces plis à l'aide d'un modèle de zone cohésive. Cette modélisation est mise en œuvre pour réaliser une étude paramétrique de la géométrie du joint et pour produire une enveloppe de rupture en fonction de la direction de sollicitation. / Adhesive bonding of composite materials has undeniable advantages over other methods such as bolting or riveting. However, one of the difficulties encountered by designers is the prediction of the failure level and failure mode of these adhesively bonded assemblies. In the case of composite laminates, one of the factors acting on the bonded joint behaviour is the stacking sequence, but works presented in the literature do not separate global effects (membrane and bending stiffness modification) and local effect (ply orientation near the adhesive layer). This study deals with the characterization of such effects in the case of single lap joints of carbon/epoxy laminates. In order to isolate the local effects, specific quasi isotropic quasi homogeneous stacking sequences are used. When stiffness properties are maintained constant, strength variations of more than 30 % are observed. Tests performed with a symmetric laminate with bending anisotropy show that the bending stiffness plays also an important role in the joint behaviour. Closed form models are able to predict global effects with a good accuracy but are inappropriate when local effects occur. The use of finite element models can account for these phenomena, by explicitly modelling the laminate plies near the adhesive layer and introducing delamination between these plies with a cohesive zone model. This model is used to perform a parametric study of the joint geometry and to produce a failure envelope according to the orientation of the loading.

<p>In the heating, ventilation, air conditioning and refrigeration (HVAC&R) industry, bonding and joining play an important role in the manufacturing and assembly process, which is critical to the cost, safety, reliability, and design freedom of systems. The goal of this thesis is to understand and evaluate the usage of adhesive bonds in the manufacture of HVAC&R systems, specifically in regards to leakage/reliability characterization and stress analysis under loading.</p> <p>The bonding performance under static loading is first studied using a commercial epoxy adhesive product. In addition to the traditional surface preparation methods of mechanical and chemical etching, a novel laser-interference surface structuring preparation technique was utilized to improve bonding performance. Laser interference structuring uses a ND:YAG laser beam that is split into two beams that are re-directed to overlap on the same area of a copper alloy. A structuring pattern near the interference structuring limits is achieved due to the phase shift between the beams that is imparted as they are re-directed. Two different laser structuring methods were tested: spot-by-spot and laser raster. Different structuring parameters were varied including the laser spot size and pulses per spot (2, 4, 6, 8, 10, 12 pulses/spot) for the spot-by-spot method, and raster speed (2, 4, 6, 8, 10, 12 mm/s) for laser raster method. The microstructure morphology and surface profile after processing were characterized using the scanning electron microscopy (SEM) and profilometry for all surfaces. It was found that the laser-interference structuring removed the surface contaminants efficiently and formed dot- or net-shaped structures on the surface. This indicates that melting, vaporization, and solidification of the metal happened differently. Due to the much higher speed of the laser raster method, considering practical industrial applications, it is selected for additional investigation for shear strength improvement. The shear strength is measured by a single lap shear test which pulls apart adhesively bonded single lap joint specimen under shear loading using a mechanical tester.</p> <p>Based on the surface profiles, three different laser raster speeds of 2 mm/s, 6 mm/s and 12 mm/s were selected for the manufacture of single lap joint specimens for comparison with the traditional surface preparation methods. The shear lap strength and displacement at maximum load were obtained for the specimens. The laser raster at 6 mm/s increased these values by approximately 11.0% and 25.1%, respectively, while the 12 mm/s condition had an increase of 16.8% and 43.8%, compared with the baseline traditional surface preparation method. It is concluded that laser structuring can enhance the single lap shear joint bonding performance. Within the tested laser processing parameters, a higher laser raster speed results in a larger enhancement. </p> <p>In addition to the static loading test with epoxy adhesive, different adhesive formulations are investigated and developed by the collaborating adhesive manufacturer to determine their suitability for use under the temperature and pressure conditions in HVAC&R systems. Reliability, especially fatigue failure, is another major concern because the strength of the adhesive joints is sufficient for HVAC&R applications. Two primary types of fatigue may happen in practical applications: thermal fatigue and vibration fatigue. Two test facilities were designed and built to test the adhesive performance and understand the failure mechanisms. For the thermal fatigue testing, a novel pressure and temperature cyclic (PTC) test stand was designed to simulate the pressure and temperature changes that may occur in HVAC&R systems. The test stand was designed to switch between hot high-pressure gas and cold low-pressure gas by using a compressor with hot gas bypass setup. For the vibration testing, a standard industrial shaker was used to provide the required vibration at a given displacement and frequency with a specially designed fixture for the tested joints. In both tests, adhesive joints were tested in parallel with brazed joints, undergoing extreme thermal and vibration loading conditions. All the samples were leak-checked before and after the testing, which were found to be leak-free after the testing, indicating that they pass the required qualification test according to available standards. It is confirmed that adhesive joints can be a potential alternative when dealing with thermal and vibration fatigue in the common working conditions of HVAC&R systems.</p> <p>The qualification testing is specific to the required loading conditions, such as pressure and temperature variations, and limited to certain tube sizes. An analytical model is developed to allow for design and evaluation across various operating conditions. The model aims to predict the adhesive stress and strain fields of in tube-to-tube joints based on the geometric parameters, material properties, and the loading conditions. In particular, the model uniquely considers the influence of thermal expansion and contraction in the joint, which is necessary for the periodically changing temperatures in HVAC&R systems. It is numerically solved using Mathematica and validated against the published data in the literature. The exact same solutions are achieved using the reported data in the literature, under simplified conditions without any temperature change involved. The validated model is then used in parametric studies to investigate the influence of geometric sizes and temperature change. Several conclusions are made about the trend of stress changes as well as the maximum stress, which provide insight from a perspective of general design guidance. Adhesive bonding length should be selected such that the maximum stress is smaller than the allowed material strength for both normal and shear stress. Adhesive thickness has less impact in the parametric range considered and is nevertheless usually dictated by the manufacture recommendation in view of other practical considerations. In regard to the thermal stresses, it is found that in practical HVAC&R working environment, the temperature-induced thermal stress dominates the stress fields and leads to significant change in the stress distribution across the adhesive layer. If a temperature change is present, the combination of all possible loading and temperature change should be analyzed to find the most extreme loading condition. This work demonstrates the first stress and strain analysis of tube-to-tube adhesive joints considering the working conditions of HVAC&R applications involving temperature cycling. All of these results provide a detailed guidance for use of adhesive joints across different application or locations in HVAC&R systems. The model can be also used as a framework to evaluate and compare the performance of different adhesives, as long as the adhesive properties are available.</p> <p>Lastly, it is also essential to demonstrate the application of these joints in real HVAC&R systems. A proof-of-concept test was done to demonstrate that the use of adhesive joints in a real system would cause no change in operation or leakage. A commercial heat pump dryer system was used to perform the testing at the Ray W. Herrick Laboratories. Two adhesive joints were installed to replace the brazing joints at the compressor inlet and outlet, where the most extreme temperature and pressure conditions are present. Results show that the system operates without any change in performance and experience no obvious leakage after more than 50 hours of testing over 6 months. </p> <p>This work explores the feasibility and reliability of adhesive bonding of copper for HVAC&R applications. The bonding strength of adhesive was studied and tested with both traditional surface preparation and advanced laser-interference structuring technique. The results show that for the tested structural epoxy adhesive, the bonding strength is large enough considering the internal pressure in the tube and the laser structure technique can increase the shear strength. </p> <p>The long-term reliability with respect to thermal, stress and vibration fatigue are then experimentally investigated and the adhesive joints pass the qualifications tests required by the standard. Further modeling work for predicting the stress distribution in adhesively bonded joints is developed to understand the influence on geometric parameters and temperature change. The adhesive length can influence the stress distribution significantly and temperature-induced stress dominates the stress distribution under the HVAC&R loading conditions. Further material characterization is needed for crack propagation or detailed fatigue analysis, which is highly dependent on the adhesive formula, working environment and loading conditions, which can be performed with a more specific targeted application. The experimental and modeling work in this thesis provides a foundation for adhesives to be applied in HVAC&R applications and a framework to further develop, optimize, and utilize adhesive joining in HVAC&R applications. </p>

Friction Bit Joining (FBJ) is a new technology that can be used to join dissimilar materials together. This ability makes it a good candidate for creating lightweight structures for the automotive industry by combining lightweight materials such as aluminum to stronger materials like advanced high-strength steels. The automotive industry is putting significant effort into interest in reducing vehicle structure weight to increase fuel efficiency and reduce greenhouse gas emissions. Joining of dissimilar materials is a challenge they face in the light weighting the body of the vehicle. The purpose of the current research is to employ FBJ in the joining of a very challenging material combination: GA DP 1180 to AA 7085-T71. In accomplishing this purpose, the goal is to move FBJ toward a more production ready process by better understanding the effects of tooling, bit design, and process parameters on joint strength and reliability as they relate to load profiles captured during the joining process. It was found that the joint strength variation was influenced strongly by the hardness and the geometric consistency of the consumable bits. Bit hardness below 45 HRC led to joint strength that was less than the required specification (5kN in lap shear tension, and 1.5kN in cross-tension and T-peel). Variation in bit height and diameter also led to excessive scatter in joint strength values, where it was not possible to meet the standard for 10 consecutive specimens (for each of the three tests). Implementation of high-speed data acquisition (1000Hz) enabled the capture of load curve profiles generated during FBJ. Load curve profiles were correlated with destructive testing results to discover the impact of process parameter combinations. Analysis of load curve profiles led to improvements in parameter selections of spindle speeds (revolutions per minute) and spindle feed-rates (inches per minute). Process parameters of 5000 RPM and 15 IPM reduced variation in load-curve profiles and destructive testing. Satisfactory joint strength was achieved in lap shear tension, cross-tension, and T-peel testing configurations with values of 10.1 kN, 4.1 kN, and 1.8 kN, respectively. The presence of wet adhesive had little impact on joint performance. Finally, the analysis of a load-curve profiles resulted in a criterion that allowed for distinguishing "good" welds from "bad" ones, where a threshold load of 6kN, or higher, during the dwell phase of welding was required in order to meet joint strength standards.

Damages of metallic aircraft structures that occur during manufacturing, assembly and in service require local repair. Especially with current service-life extensions of ageing aircraft fleets, the importance of such repair methods is increasing. Typically, the repair of smaller damages on aluminium fuselage or wing skins is done by riveting a patch onto the flawed structure. However, the use of rivets reduces the strength of the structure and promotes fatigue. Joining the patch by adhesive bonding would not only offer more homogenous load distribution and weight savings, but even an increase of structural integrity. Metal adhesive bonding is commonly used in aeronautics, but requires elaborated surface treatments of the adherends, employing hazardous chemicals like chromates, due to the high durability demands. Furthermore, these treatments are usually tank processes that are not suitable for local repairs. Hence, there is a strong need for locally applicable surface preparation methods that allow safe and reliable adhesive bonding of primary aircraft structures. The aim of this thesis is to assess the – still emerging – method of atmospheric pressure plasma deposition of silicon (Si) containing compounds concerning its suitability as surface preparation for adhesive bonding of aluminium aerostructures. Atmospheric plasma deposition is not yet used in the aircraft industry, and the knowledge on functionality of this technology concerning bonding of aluminium parts is limited. Moreover, the durability requirements of the aircraft industry greatly exceed the standards in other industries. Hence, special attention is paid to a thorough analysis of the key characteristics of the deposited coupling films and their effectiveness in terms of adhesion promotion as well as joint durability under particularly hostile conditions. In order to do so, the altering mechanisms of the treated joints and the behaviour of the coupling films during accelerated ageing will be investigated in detail for the first time in this thesis. Furthermore, the influence of the aluminium surface pre-treatment (i.e. topography and oxide properties of the substrate) on the overall joint performance after coupling film deposition is thoroughly examined. Based on these findings, the surface preparation is optimised, and a process is developed to achieve maximal joint performance. As alternative local surface treatments prior to adhesive bonding, solution derived deposition of silane and sol-gel films have already been widely investigated and can be considered as reference, even though these techniques are rarely used in civil aeronautics. The knowledge on their effectiveness and capabilities in corrosive atmosphere is still very limited. Therefore, all analyses of degradation mechanisms are conducted for both plasma deposition and wet-chemical reference treatments to reveal the differences and communalities of the two Si-based coupling films. Physical and chemical analysis of the films, the oxides and the interfaces reveal differing, but interdependent failure mechanisms that are inhibited differently by the individual coupling films. Using the optimum deposition parameters, plasma films of only several nanometres in thickness significantly enlarge the corrosion resistance of bonded joints, reaching almost the level of anodising treatments with several micrometres thick oxides and strongly outperforming solution derived silane treatments. However, plasma film performance is found to be largely dependent on the precursor selection. With plasma deposition of 3-glycidoxypropyltrimethoxysilane, which has not been reported before, highest joint stability is achieved. Moreover, it is discovered that the properties of plasma and solution derived silane based films are complementary. It is shown that an optimised combined plasma and wet-chemical treatment process provides even superior resistance to bondline corrosion than state-of-the-art anodising techniques.

Este trabalho objetivou a caracterização mecânica e a análise de falha de dois tipos de juntas compósitas através de ensaios mecânicos, em que uma das juntas era composta por dois laminados de fibras de carbono recoberto em ambas as faces por tecido de fibra de vidro reforçando uma matriz termoplástica (PPS-C) unidos via soldagem por resistência elétrica, e a outra confeccionada com dois laminados de fibras de carbono reforçando uma matriz de resina epóxi (EPX-C) unidos via colagem por filme de resina epoxídica. Os dois tipos de juntas foram submetidos a impacto único transversal de 10 J, condicionamento higrotérmico, além de carregamento em fadiga compressiva no plano nas mais diversas combinações destes processos degradativos de suas propriedades mecânicas. Observou-se, que a junta termorrígida colada de EPX-C apresentou a maior resistência mecânica em flexão em quatro pontos (F4P) na condição original (como-manufaturada), assim como os maiores valores de resistência residual para as várias condições de degradação mecânica e higrotérmica a que foi submetida. Por sua vez, a junta termoplástica soldada de PPS-C exibiu, em termos percentuais, menores reduções dos valores de resistência à flexão sob as condições avaliadas, relativamente às perdas apresentadas pela junta EPX-C em idênticas circunstâncias. A análise macroscópica da superfície de fratura de ambas as juntas indicou que o modo de falha predominante da junta termorrígida foi interfacial enquanto que, para a junta PPS-C, o modo de falha predominante foi o intralaminar. Análises fratográficas através da microscopia eletrônica de varredura (MEV) evidenciaram para a junta termorrígida EPX-C, uma alta adesão entre fibra/matriz, porém uma relativamente fraca interação entre os aderentes (laminado) e o filme adesivo de colagem, enquanto que, para a junta termoplástica PPS-C, reduzidas interações fibra/matriz forem inferidas na camada externa de PPS-V do aderente assim como entre a malha metálica resistiva e os filmes puros de PPS que a revestiam. Em ensaios de resistência ao cisalhamento interlaminar (ILSS), os espécimes retirados da junta EPX-C na condição virgem evidenciaram uma colagem uniforme/homogênea, enquanto que, para a junta PPS-C, os espécimes de ensaio usinados a partir da junta virgem indicaram a ocorrência de efeitos de degradação térmica altamente localizada nas bordas soldadas. / This study aimed to mechanical characterization and failure analysis of two kinds of composite single-lap joints by mechanical tests, in which a single-lap joint was made of two adherents of carbon fibers coated on both sides with glass fiber fabric reinforcing a thermoplastic matrix (PPS-C) welded via resistance welding, and the another type made of two adherents of carbon fibers reinforcing an epoxy matrix (EPX-C) bonded by epoxy adhesive film. The two types of single-lap joints were subjected to single transverse impact of 10 J, hygrothermal conditioning, and compressive fatigue loading in the plane in various combinations of these degradative processes of mechanical properties. It was observed that the thermosetting bonded joint EPX-C showed the highest strength in four point-flexure test in the original condition (as-manufactured), as well as greater residual strength values for the various conditions of mechanical and hygrothermal degradation which was subjected. In turn, the welded thermoplastic joint PPS-C exhibited, in percentage terms, smaller reductions in flexural strength values under the tested conditions, in respect of losses showed by EPX-C in similar circumstances. Macroscopic analysis of the fracture surfaces from both joints indicated that the predominant failure mode was interfacial to thermosetting while for PPS-C joint, the predominant failure mode was intralaminar. Analysis of fracture surfaces by scanning electron microscopy (SEM) showed, for thermosetting joints EPX-C, a high adhesion between fiber/matrix, but a relatively weak interaction between adherents and the bonding adhesive film, whereas for thermoplastic joint PPS-C, reduced interactions fiber/matrix are inferred in the outer layers of PPS-V and between metal mesh and the pure PPS films that coated it. In the interlaminar shear strength tests (ILSS), the specimens removed from the EPX-C joint in the pristine condition showed a uniform/homogeneous bonding along the joint area, while for PPS-C joint, coupons extracted from pristine condition joint indicated the occurrence of degradation caused by thermal effects localized in the welded edges.

Este trabalho objetivou a caracterização mecânica e a análise de falha de dois tipos de juntas compósitas através de ensaios mecânicos, em que uma das juntas era composta por dois laminados de fibras de carbono recoberto em ambas as faces por tecido de fibra de vidro reforçando uma matriz termoplástica (PPS-C) unidos via soldagem por resistência elétrica, e a outra confeccionada com dois laminados de fibras de carbono reforçando uma matriz de resina epóxi (EPX-C) unidos via colagem por filme de resina epoxídica. Os dois tipos de juntas foram submetidos a impacto único transversal de 10 J, condicionamento higrotérmico, além de carregamento em fadiga compressiva no plano nas mais diversas combinações destes processos degradativos de suas propriedades mecânicas. Observou-se, que a junta termorrígida colada de EPX-C apresentou a maior resistência mecânica em flexão em quatro pontos (F4P) na condição original (como-manufaturada), assim como os maiores valores de resistência residual para as várias condições de degradação mecânica e higrotérmica a que foi submetida. Por sua vez, a junta termoplástica soldada de PPS-C exibiu, em termos percentuais, menores reduções dos valores de resistência à flexão sob as condições avaliadas, relativamente às perdas apresentadas pela junta EPX-C em idênticas circunstâncias. A análise macroscópica da superfície de fratura de ambas as juntas indicou que o modo de falha predominante da junta termorrígida foi interfacial enquanto que, para a junta PPS-C, o modo de falha predominante foi o intralaminar. Análises fratográficas através da microscopia eletrônica de varredura (MEV) evidenciaram para a junta termorrígida EPX-C, uma alta adesão entre fibra/matriz, porém uma relativamente fraca interação entre os aderentes (laminado) e o filme adesivo de colagem, enquanto que, para a junta termoplástica PPS-C, reduzidas interações fibra/matriz forem inferidas na camada externa de PPS-V do aderente assim como entre a malha metálica resistiva e os filmes puros de PPS que a revestiam. Em ensaios de resistência ao cisalhamento interlaminar (ILSS), os espécimes retirados da junta EPX-C na condição virgem evidenciaram uma colagem uniforme/homogênea, enquanto que, para a junta PPS-C, os espécimes de ensaio usinados a partir da junta virgem indicaram a ocorrência de efeitos de degradação térmica altamente localizada nas bordas soldadas. / This study aimed to mechanical characterization and failure analysis of two kinds of composite single-lap joints by mechanical tests, in which a single-lap joint was made of two adherents of carbon fibers coated on both sides with glass fiber fabric reinforcing a thermoplastic matrix (PPS-C) welded via resistance welding, and the another type made of two adherents of carbon fibers reinforcing an epoxy matrix (EPX-C) bonded by epoxy adhesive film. The two types of single-lap joints were subjected to single transverse impact of 10 J, hygrothermal conditioning, and compressive fatigue loading in the plane in various combinations of these degradative processes of mechanical properties. It was observed that the thermosetting bonded joint EPX-C showed the highest strength in four point-flexure test in the original condition (as-manufactured), as well as greater residual strength values for the various conditions of mechanical and hygrothermal degradation which was subjected. In turn, the welded thermoplastic joint PPS-C exhibited, in percentage terms, smaller reductions in flexural strength values under the tested conditions, in respect of losses showed by EPX-C in similar circumstances. Macroscopic analysis of the fracture surfaces from both joints indicated that the predominant failure mode was interfacial to thermosetting while for PPS-C joint, the predominant failure mode was intralaminar. Analysis of fracture surfaces by scanning electron microscopy (SEM) showed, for thermosetting joints EPX-C, a high adhesion between fiber/matrix, but a relatively weak interaction between adherents and the bonding adhesive film, whereas for thermoplastic joint PPS-C, reduced interactions fiber/matrix are inferred in the outer layers of PPS-V and between metal mesh and the pure PPS films that coated it. In the interlaminar shear strength tests (ILSS), the specimens removed from the EPX-C joint in the pristine condition showed a uniform/homogeneous bonding along the joint area, while for PPS-C joint, coupons extracted from pristine condition joint indicated the occurrence of degradation caused by thermal effects localized in the welded edges.

Advanced composite materials have enabled the conventional aircraft structures to reduce weight, improve fuel efficiency and offer superior mechanical properties. In the past, materials such as aluminum, steel or titanium have been used to manufacture aircraft structures for support of heavy loads. Within the last decade or so, demand for advanced composite materials have been emerging that offer significant advantages over the traditional metallic materials. Of particular interest in the recent years, there has been an upsurge in scientific significance in the usage of adhesively bonded composite joints (ABCJ’s). ABCJ’s negate the introduction of stress risers that are associated with riveting or other classical techniques. In today’s aircraft transportation market, there is a push to increase structural efficiency by promoting adhesive bonding to primary joining of aircraft structures. This research is focused on the issues associated with the durability and related failures in bonded composite joints that continue to be a critical hindrance to the universal acceptance of ABCJ’s. Of particular interest are the short term strength, contamination and long term durability of ABCJ’s. One of the factors that influence bond performance is contamination and in this study the influence of contamination on composite-adhesive bond quality was investigated through the development of a repeatable and scalable surface contamination procedure. Results showed an increase in the contaminant coverage area decreases the overall bond strength significantly. A direct correlation between the contaminant coverage area and the fracture toughness of the bonded joint was established. Another factor that influences bond performance during an aircraft’s service life is its long term strength upon exposure to harsh environmental conditions or when subjected to severe mechanical loading. A test procedure was successfully developed in order to evaluate durability of ABCJ’s comprising severe environmental conditioning, fatiguing in ambient air and a combination of both. The bonds produced were durable enough to sustain the tests cases mentioned above when conditioned for 8 weeks and did not experience any loss in strength. Specimens that were aged for 80 weeks showed a degradation of 10% in their fracture toughness when compared to their baseline datasets. The effect of various exposure times needs to be further evaluated to establish the relationship of durability that is associated with the fracture toughness of ABCJ’s.

No description available.

Radio-frequency microelectromechanical systems (RF~MEMS) are highly miniaturized devices intended to switch, modulate, filter or tune electrical signals from DC to microwave frequencies. The micromachining techniques used to fabricate these components are based on the standard clean-room manufacturing processes for high-volume integrated semiconductor circuits. RF~MEMS switches are characterized by their high isolation, low insertion loss, large bandwidth and by their unparalleled signal linearity. They are relatively simple to control, are very small and have almost zero power consumption. Despite these benefits, RF~MEMS switches are not yet seen in commercial products because of reliability issues, limits in signal power handling and questions in packaging and integration. Also, the actuation voltages are typically too high for electronics applications and require additional drive circuitry. This thesis presents a novel MEMS switch concept based on an S-shaped film actuator, which consists of a thin and flexible membrane rolling between a top and a bottom electrode. The special design makes it possible to have high RF isolation due to the large contact distance in the off-state, while maintaining low operation voltages due to the zipper-like movement of the electrostatic dual-actuator. The switch comprises two separately fabricated parts which allows simple integration even with RF circuits incompatible with certain MEMS fabrication processes. The two parts are assembled by chip or wafer bonding which results in an encapsulated, ready-to-dice package. The thesis discusses the concept of the switch and reports on the successful fabrication and evaluation of prototype devices. Furthermore, this thesis presents research results in wafer-level packaging of (RF) MEMS devices by full-wafer bonding with an adhesive intermediate layer, which is structured before bonding to create defined cavities for housing MEMS devices. This technique has the advantage of simple, robust and low temperature fabrication, and is highly tolerant to surface non-uniformities and particles in the bonding interface. It allows cavities with a height of up to many tens of micrometers to be created directly in the bonding interface. In contrast to conventional wafer-level packaging methods with individual chip-capping, the encapsulation is done using a single wafer-bonding step. The thesis investigates the process parameters for patterned adhesive wafer bonding with benzocyclobutene, describes the fabrication of glass lid packages based on this technique, and introduces a method to create through-wafer electrical interconnections in glass substrates by a two-step etch technique, involving powder-blasting and chemical etching. Also, it discusses a technique of improving the hermetic properties of adhesive bonded structures by additional passivation layers. Finally, it presents a method to substantially improve the bond strength of patterned adhesive bonding by using the solid/liquid phase combination of a patterned polymer layer with a contact-printed thin adhesive film. / QC 20100617