Advanced composite materials are widely used for many structural applications in the aerospace/aircraft industries today. Joining of composite structures using adhesive bonding offers several advantages over traditional fastening methods. However, this technique is not yet employed for fastening the primary structures of aircrafts or space vehicles. There are several reasons for this: There are not any reliable non-destructive evaluation (NDE) methods that can quantify the strength of the bonds, and there are no certifications of quality assurance for inspecting the bond quality. Therefore, there is a significant need for an effective, reliable, easy to use NDE method for the analysis of composite adhesive joints. This research aimed to investigate an adhesively bonded composite-aluminum joints of variable bond strength using digital image correlation (DIC). There are many future possibilities in continuing this research work. As the application of composite materials and adhesive bond are increasing rapidly, the reliability of the composite structures using adhesive bond should quantified. Hence a lot of similar research using various adhesive bonds and materials can be conducted for characterizing the behavior of adhesive bond. The results obtained from this research will set the foundation for the development of ultrasonic DIC as a nondestructive approach for the evaluation of adhesive bond line.

It was the objective of this research to develop high temperature and high performance polyimides that also display (a) thermal stability; (b) crystallinity in the initial material and ability to crystallize from the melt; (c) fast crystallization kinetics and (d) melt processability. This unique combination of properties is presently unavailable in any other polyimide. In this regard, the present work investigates the crystallization, morphology and thermal stability of two novel semicrystalline polyimides based on the same diamine, 1,3-bis (4-aminophenoxy) benzene (TPER), but two different dianhydrides, 3,3',4',4'-biphenyltetracarboxylic dianhydride (BPDA) and 3,3',4,4'-benzophenonetetracarboxylic dianhydride (BTDA). Phthalic anhydride was used as an endcapper to improve the thermal stability of the polyimides. The BPDA based polyimide was also tested extensively as a structural adhesive using Ti-6Al-4V coupons. Additionally, these polyimides are based on monomers, that are presently commercially available. The bulk thermal stability of the polyimides was first evaluated using dynamic and isothermal thermogravimetric experiments. DSC was utilized to test the ability of the polyimides to crystallize from the melt after exposures to varying melt times and temperatures. Exceptional thermal stability was demonstrated by BPDA based polyimide with no change in the melting behavior after 40 min at 430°C or 30 min at 440°C. The semicrystalline morphology of the material was studied using hot stage polarized optical microscopy (OM) and atomic force microscopy. The spherulitic growth rates were determined as a function of crystallization temperature after quenching from various melt times and temperatures. The effect of crystallization temperature, previous melt time and melt temperature on the morphology was considered. The spherulitic growth rates increased with increasing undercooling in the temperature range studied (nucleation controlled), while the growth rate at a specific crystallization temperature decreased on increasing the previous melt time and temperature. The melting behavior was studied after different crystallization times and temperatures and also as a function of different heating rates. Crystallization kinetics was followed both isothermally and non-isothermally using DSC and OM. Avrami analysis was performed for TPER-BPDA and the obtained results were correlated with microscopic observations. Melt viscosity measurements were carried out as a function of melt temperature, melt time and frequency. The adhesive investigations for TPER-BTDA utilized lap-shear test, wedge test and double cantilever beam tests. The durability of the adhesive and the fracture surface was studied after exposure to various solvents and after high aging and testing temperatures. The polyimide demonstrated very high average room temperature lap-shear strengths (8400 psi or 59 MPa), excellent solvent resistance and durability of strengths at high aging and testing temperatures. / Ph. D.

[pt] Os materiais compósitos foram desenvolvidos para substituir o uso de ligas metálicas em aplicações de alto desempenho, se tornando materiais muito utilizados devido a uma ampla combinação de propriedades mecânicas. O setor de oleodutos, com o passar dos anos, tem enfrentado problemas nas tubulações devido ao envelhecimento e deterioração causados, principalmente, pela corrosão. Para solucionar esses problemas, normalmente, são utilizados reparos convencionais feitos com materiais metálicos tendo a finalidade de substituir a parte danificada. Porém, esse processo é muito trabalhoso e o setor de oleodutos tem visto como alternativa o uso de materiais compósitos poliméricos reforçados com fibra como reparo. As principais vantagens em relação aos reparos convencionais são a alta relação resistência-peso, a baixa concentração de tensão e a fácil aplicação. Entretanto, muitos estudos já mostraram que quando expostos a ambientes agressivos como temperatura, umidade e radiação ultravioleta, os materiais compósitos podem se deteriorar, ou seja, perdem suas propriedades mecânicas. Com isso, é importante analisar o efeito desses ambientes com o intuito de prever o comportamento mecânico, a vida útil e as possíveis falhas do reparo de juntas coladas. Com a finalidade de entender o comportamento mecânico, a adesão e a vida útil da junta adesiva, o objetivo desse trabalho foi analisar o envelhecimento de reparos em uma atmosfera salina. Foram utilizadas juntas Double-Lap Shear – DLS de compósito polimérico reforçado com fibra de vidro sobre um substrato de aço. As juntas foram fabricadas com quatro tipos tratamentos superficiais diferentes: I) Sem Silano / lixamento manual; II) Sem Silano / máquina Monti; III) Com Silano / lixamento manual; IV) Com Silano / máquina Monti. Os reparos de juntas coladas foram analisados em relação aos efeitos de temperatura (35 graus C, 55 graus C e 70 graus C) e da exposição a ambientes de névoa salina por um tempo de envelhecimento de até 6830h. O efeito do envelhecimento e a variação das propriedades mecânicas foram avaliados pelo ensaio destrutivo DLS e foi utilizado o ensaio não destrutivo de ultrassom para detectar o tamanho e localização dos defeitos. Como resultado, observou-se que quanto maior a temperatura de envelhecimento mais rápida foi a degradação das propriedades mecânicas e que as amostras que possuem silano, mantiveram maior resistência ao longo do tempo de envelhecimento. Também foi observado que o modo de falha de junta mudou com o passar do tempo e da temperatura tornando a falha adesiva a mais comum. Em relação ao ensaio de ultrassom, foi observado que a técnica de ultrassom foi eficiente para localizar e dimensionar o tamanho dos defeitos. Porém, não foi possível fazer uma comparação do aumento da área de defeitos com a variação das propriedades mecânicas. / [en] Composite materials have been developed to replace the use of metal alloys in high-performance applications, becoming widely used materials due to their broad combination of mechanical properties. Over the years, the pipeline sector has faced problems in pipelines due to aging and deterioration caused mainly by corrosion. To solve these problems, conventional repairs made with metallic materials are usually used to replace the damaged part. However, this process is very laborious, and the pipeline sector has seen the use of fiber-reinforced polymer composite materials as an alternative for repair. The main advantages compared to conventional repairs are high strength-to-weight ratio, low stress concentration, and easy application. However, many studies have shown that when exposed to aggressive environments such as temperature, humidity, and ultraviolet radiation, composite materials may deteriorate, meaning they lose their mechanical properties. Therefore, it is important to analyze the effect of these environments to predict the mechanical behavior, service life, and possible failures of adhesive joint repairs. In order to understand the mechanical behavior, adhesion, and service life of the adhesive joint, the aim of this work was to analyze the aging of repairs in a saline atmosphere. Double-Lap Shear (DLS) joints made of fiberglass-reinforced polymer composite on a steel substrate were used. The joints were made with four different surface treatments: I) Without Silane / manual sanding; II) Without Silane / Monti machine; III) With Silane / manual sanding; IV) With Silane / Monti machine. The bonded joint repairs were analyzed for the effects of temperature (35 degrees C, 55 degrees C, and 70 degrees C) and exposure to salt spray environments for an aging time of up to 6830h. The effect of aging and variation of mechanical properties were evaluated by destructive DLS testing, and non-destructive ultrasonic testing was used to detect the size and location of defects. As a result, it was observed that the higher the aging temperature, the faster the degradation of mechanical properties, and that samples with silane maintained greater resistance over the aging time. It was also observed that the joint failure mode changed over time and temperature, making adhesive failure more common. Regarding ultrasonic testing, it was observed that the technique was effective in locating and sizing defects. However, it was not possible to compare the increase in defect area with the change of the mechanical properties.

In this thesis, a new modeling methodology applicable to adhesively bonded joints for crash simulations is presented. Using this approach, adhesive joints can be modeled without using minute solid elements thus reducing the size of the model. Moreover, coarse mesh can be used for substrates in the overlap region of a joint. Both of these improvements together yield significant reduction in simulation run times in crash analysis when compared to solid element representation of adhesive. The modeling can also capture effects of strain rate for a given ambient temperature. In order to develop the efficient modeling procedure mentioned above, experimental, analytical and numerical studies have been carried out. Mechanical behaviors of adhesively bonded joints are studied with the help of double lap shear (DLS) coupon tests conducted at different extension rates and temperatures. The joint specimens are made from dual-phase (DP) steel coupons bonded with epoxy resin. Tests are also carried out to ascertain the behaviors of these component materials at different extension rates and temperatures. A new semi-analytical solution procedure is developed considering material nonlinearity to predict mechanical behaviors of adhesively bonded DLS joints. The joint behaviors using the semi-analytical approach are predicted separately using the Von Mises and exponent Drucker-Prager yield criteria. The predicted force versus extension curves using semi-analytical solution are compared with test results. It is also hypothesized here that, the semi-analytical solution procedure can be used as a base to develop efficient modeling procedures of adhesively bonded joints in FEA. In finite element analysis, both adhesive and substrates are modeled as elastic-plastic materials. It is shown that the shell-solid model of the DLS joint, in which substrates are modeled using shell elements and adhesive is modeled using solid elements, can accurately predict the mechanical behavior of the joint. Both exponent Drucker-Prager and Von Mises material models in ABAQUS are used to calculate force versus extension curves. Numerical and experimental forces versus extension curves are compared. A new methodology for efficient modeling of adhesively bonded joints in LS-DYNA using equivalent material properties in the joint overlap region is proposed. Various models using this methodology are assessed by comparing their results with shell-solid model and test results. Finally, it is also shown that strain rate effects can be included in the efficient modeling approach.

Orientador: Marcos Valério Ribeiro / Resumo: A utilização de materiais compósitos em componentes do setor aeronáutico vem crescendo muito nos últimos anos. Isso se deve ao fato destes materiais apresentarem boas propriedades mecânicas, aliadas a sua baixa massa específica. Dentre estes, os laminados metal fibra, são uma classe de materiais que vem ganhando destaque. Contudo seu emprego na maioria dos casos, requer a confecção de furos, algo que é muito complexo e pode causar danos ao material. Portanto, o objetivo desta pesquisa foi estudar o processo de furação em cheio de um laminado de metal fibra de alumínio 2024-T3/epóxi/fibra de vidro apoiado no alumínio 7075, aplicando diferentes parâmetros de usinagem para avaliar a influência na qualidade dos furos e o desgaste das ferramentas, correlacionando com a potência consumida, vibração, força de avanço, variações dimensionais nos furos e alteração das propriedades mecânicas do material. Para tal, o material foi processado via moldagem por compressão a quente e caracterizado por meio dos ensaios de cisalhamento interlaminar (ILSS), cisalhamento por compressão (CST) e Lap shear. Posteriormente, foram realizados processos de furação utilizando 4000, 6000 e 8000 rpm, bem como avanços de 0,05; 0,1 e 0,2 mm/rot. Após a análise dos resultados, pode-se concluir que os parâmetros influenciaram nos dados de potência, vibração e força de avanço, bem como houve variação significativa nos diâmetros obtidos, sendo que o melhor resultado ocorreu para combinação de 6000 rpm com 0,05mm... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: The use of composite materials in aeronautical components has been increased in the last years. This is due to these material presente good mechanical properties, allied with low specific mass. Among them, the fiber metal laminated (FML) are a class of materials that has been gaining prominence. However, its use in most cases requires the drilling of holes, which is very complex and can cause damage to the material. Therefore, the objective of this research was to study the conventional drilling process in aeronautical aluminum structures reinforced with fiber metal laminates. Applying different machining parameters, to evaluate the delamination and the wear tools, correlating with the power consumed, vibration, advance force, dimensional variations in the holes and variation of the mechanical properties on material. For this, the material was processed by hot compression molding and characterized by interlaminar shear stress (ILSS), compression shear test (CST) and Lap-shear. Afterwards, drilling processes were carried out using 4000, 6000 and 8000 rpm, as well as an advance of 0.05; 0.1 and 0.2 mm / rot. After the analysis of the results, it was possible to conclude that the parameters influenced the data of power, vibration and force of advance, as well as there was significant variation in the diameters obtained, being the best result occurred for a combination of 6000 rpm with 0,05mm / rot. It can also be stated that there is a tendency of shear strength of the Lap Shear... (Complete abstract click electronic access below) / Mestre

A cold atmospheric pressure plasma source was investigated as an alternative pre-treatment for carbon fibre reinforced epoxy substrates prior to bonding. For reference, common surface pre-treatments were also investigated (peel ply, manual abrasion, and grit blasting). In the aerospace industry, the peel ply, is usually added to one side of the composite surface during manufacture and peeled off prior to bonding. Peel ply can be used independently or in combination with other techniques. The strength of the bonded joints of the different pre-treatments was assessed through tensile lap shear tests. It was found that combining peel ply with plasma increased the joint strength by 10% whereas manual abrasion or grit blasting after peel ply improved the strength of the joints by 15% and 20% respectively. The effect of pre-treating the composite substrate side without peel ply (bag side) was also investigated. The strength of the joints produced without any pre-treatment was increased by 99% for manual abrasion, 134% for grit blasting and by 146% for plasma. Comparing both surfaces of the composite substrates, it was found that using peel ply improved the performance of the joints by 91%. In order to understand better the effects of the different pre-treatments, surface characterisation of the substrates (surface roughness, surface free energy, and analysis of chemical changes) was also conducted. The effect of roughness did little to affect the strength values (for both surfaces of the composite). The adhesive used in this research was very good at wetting the surface, regardless of the roughness. However, when the adhesive was able to wet the surface, the relationship between bond strength and surface free energy was unclear. Plasma was shown to increase levels of oxygen at the surface and reduce/eliminate the concentration of fluorine at the surface on the bag side of the composite.

Carbon nanotubes (CNTs) were dispersed into polyvinyl acetate (PVAc ) adhesive cured by radio frequency (RF) to investigate their loading effects on PVAc adhesive curing time and lap shear bond strength performance. Main factors are CNT loading (0.08, 0.25 and 0.41%), RF curing time (15, 30, 45, 60 seconds), and clamping pressure (100, 160, and 240 psi). Experimental results indicated that CNT loading had significant effects on PVAc curing time and lap shear bond strength. Specifically, single lap shear joints bonded with 0.41% CNT loading adhesive had significantly higher lap shear bond strengths than one with PVAc only (0% CNT loading). The curing time of PVAc dispersed with a 41% CNT loading (15 seconds) was 30 seconds shorter than the one with a 0% CNT loading (45 seconds).

Reducing the weight of the truck vehicle conveys more cargo to be carried by thetrailer. This has a significant impact on the efficiency of the transport lowering both the total cost of cargo moved and the total carbon dioxide emitted. Half of the body in-white weight of a truck is comprised out of panels made out of thin mild forming steel which cannot be made thinner to reduce weight due to the lowered stiffness it would entail. Sandwich materials have a high stiffness to weight ratio and would for the same panel thickness as regular forming steel have a comparable bending stiffness but lowered weight. This master thesis is intended to be a preliminary study for Scania CV AB on sandwich materials and its potential use as lightweight panels in their trucks. With the intention of investigating whether a commercial sandwich material is capable of filling the role as outer panels of a truck, comparative tests regarding significant matters such as forming and painting was made on identically manufactured demonstrators comparing a sandwich material and a regular forming steel material. The tests identified weaknesses in the current manufacturing process for parts of a sandwich material. Such limitations are problems with painting and joining due to isolated cover sheets, forming problems revealing sink marks likely due to different spring back of the material and hemming flaws due to inadequately optimized hemming technique and anisotropy. Now that more knowledge of sandwich materials has been gained, counter measures for these findings can be made in order to take another step towards lowering the weight of the truck and a more efficient way of transporting goods. / Genom att minska vikten på lastbilen frigörs mer last att bäras av släpvagnen. Detta har en betydande inverkan på effektiviteten hos transporten som sänker både den totala kostnaden för transporterad last och de totala koldioxidutsläppen. Hälften aven lastbils rena karossvikt består av paneler gjorda av tunt mjukt formningsstål vilke tinte kan bli tunnare för att minska vikten på grund av den sänkta styvheten som detskulle medföra. Sandwichmaterial har en hög styvhet till viktförhållande och skulle församma paneltjocklek som vanligt formningsstål ha en jämförbar böjstyvhet men sänkt vikt. Denna uppsats är avsedd att vara en preliminär studie för Scania CV AB om sandwichmaterial och dess potentiella användning av lättvitkspaneler i lastbilar.Med avsikt att undersöka huruvida ett kommersiellt sandwichmaterial kan fylla rollen som lastbilens ytterpaneler utfördes jämförande tester med avseende på signifikanta frågor såsom formning och målning på identiskt tillverkade demonstratorer som jämförde ett sandwichmaterial och ett vanligt formningsstål. Testerna identifierade svagheter med materialet samt hur processen behöver anpassas för att kunna använda sandwichmaterialet i rådande tillverkningsprocess. Identifierade problem var bland annat problem med målning och sammanfogning på grund av isolerade ytterskickt i sandwichmaterialet, problem med formning som gav upphov till limdragningar som troligen beror på materialets olika återfjädring samt falsningsfel på grund av otillräckligt optimerad falsteknik och anisotropi. Nu när mer kunskap om sandwichmaterial erhållits kan motåtgärder för de funna resultaten undersökas för att ta ytterligare ett steg mot att sänka lastbilens vikt och därmed få ett effektivare transportmedel.

Composite structures are commonly joined using adhesive or mechanical joints, withmechanical joints being preferred when components need to be removable for maintenancepurposes. However, the presence of mechanical joints introduces a discontinuity in theload path, which can serve as an initiation point for failure and needs to be taken intoaccount in the design of the joint. Additionally, delaminations may occur around thefastener hole during the manufacturing and assembly processes, further impacting thestrength of the laminate under compressive loading. While some studies have assessedthe residual strength of open-hole specimens, limited information exists regarding theresidual bearing strength in delaminated composite joints. This study aims to assessthe significance of delaminations of varying sizes on the bearing strength of single-bolt,single-lap shear joints under static loading using numerical analysis methods. The effectsof countersinking and bolt size are also examined. Stress and progressive failure analysisare utilized to evaluate different parameters and account for the nonlinear behavior of thematerials. The study reveals that the presence of delamination leads to degradation ofthe bearing strength of approximately five percent when bolt pretension is applied and15 percent in the absence of pretension. Countersinking increases maximum and averagestresses on the cylindrical section of the hole, while a larger bolt size enhances bearingstrength by reducing bolt bending in single-lap shear joints. / Kompositstrukturer sammanfogas vanligtvis med hjälp av lim eller mekaniska fogar,där mekaniska fogar är att föredra när komponenter måste kunna demonteras förunderhållsändamål. Mekaniska fogar introducerar dock en diskontinuitet i lastvägen, somkan fungera som en startpunkt för strukturella fel och måste beaktas vid utformningenav fogen. Dessutom kan delaminering uppstå runt fästelementhålet under tillverkningsochmonteringsprocesserna, vilket ytterligare påverkar laminatets tryckhållfasthet. Någrastudier har utvärderat resthållfastheten hos prov med öppna hål, men det finns begränsadinformation om den resterande tryckhållfastheten vid hålkanten hos delamineradekompositfogar. Denna studie syftar till att bedöma påverkan av delamineringar avvarierande storlek på tryckhållfastheten vid hålkanten hos enskärsförband med ettfästelement under statisk belastning med hjälp av numeriska analysmetoder. Ävenpåverkan av försänkning och bultstorlek undersöks. Analyser av spänning och progressivskada används för att utvärdera olika parametrar och ta materialens olinjära beteendetill hänsyn. Studien ställer fast att förekomsten av delaminering leder till en försämringav tryckhållfastheten vid hålkanten med cirka fem procent när fästelementet är underförspänning och 15 procent utan förspänning. Försänkning av fästelementets huvud ökarmaximala och genomsnittliga spänningar på den cylindriska delen av hålet, medan enstörre bultstorlek höjer tryckhållfastheten vid hålkanten genom att minska bultböjningen ienskärsförband.

Many applications require adhesives with high strength to withstand the exhaustive loads encountered in regular operation. In aerospace applications, advanced adhesives are needed to bond metals, ceramics, and composites under shear loading. The lap shear test is the experiment of choice for evaluating shear strength capabilities of adhesives. Specifically during single-lap shear testing, two overlapping rectangular tabs bonded by a thin adhesive layer are subject to tension. Shear is imposed as a result. Debonding occurs when the shear strength of the adhesive is surpassed by the load applied by the testing mechanism. This research develops a finite element model (FEM) and material model which allows mechanicians to accurately simulate bonded joints under mechanical loads. Data acquired from physical tests was utilized to correlate the finite element simulations. Lap shear testing has been conducted on various adhesives, specifically SA1-30-MOD, SA10-100, and SA10-05, single base methacrylate adhesives. The adhesives were tested on aluminum, stainless steel, and cold rolled steel adherends. The finite element model simulates what is observed during a physical single-lap shear test consisting of every combination of the mentioned materials. To accomplish this, a three-dimensional model was created and the cohesive zone approach was used to simulate debonding of the tabs from the adhesive. The thicknesses of the metallic tabs and the adhesive layer were recorded and incorporated into the model in order to achieve an accurate solution. From the data, force output and displacement of the tabs are utilized to create curves which were compared to the actual data. Stress and strain were then computed and plotted to verify the validity of the simulations. The modeling and constant determination approach developed here will continue to be used for newly-developed adhesives.