[pt] O objetivo deste trabalho foi sintetizar pós submicrônicos de Y2W3O12 e Al2W3O12 visando estudar o efeito da incorporação destas cargas no coeficiente de expansão térmica (CET) e nas propriedades mecânicas de compósitos de polietileno de alta densidade (PEAD). As partículas de Y2W3O12 foram obtidas pelos métodos de coprecipitação por mistura de reagentes e coprecipitação reversa modificada, com o intuito de avaliar a influência destas rotas no grau de aglomeração dos pós preparados. Ambos os métodos permitiram obter pós submicrônicos de Y2W3O12 0. 0.7 um) com tamanhos de partículas primárias e graus de aglomeração semelhantes, conforme verificado por diversas técnicas de caracterização. As partículas de Al2W3O12 foram sintetizadas pelos métodos de coprecipitação por mistura de reagentes e coprecipitação normal, utilizando duas fontes de tungstênio e os precursores obtidos foram secos no micro-ondas. Os efeitos do pH da síntese (4.2 e 6.0) no estado de aglomeração dos pós sintetizados foram avaliados. Pós submicrônicos de Al2W3O12 0.22 0.87 um) foram produzidos utilizando estas rotas. A coprecipitação pela mistura de reagentes em pH=4.2 permitiram obter pós com aglomerados de menor tamanho (0.22 um). Os compósitos de PEAD foram fabricados por microextrusão-microinjeção, com partículas de Al2W3O12 e Y2W3O12 (0.70 um) em frações volumétricas na faixa de 0.001 0.011. Os compósitos PEAD/Y2W3O12 exibiram reduções no CET de 20 25 porcento quando comparados ao PEAD na faixa de 25 70 °C, enquanto os compósitos PEAD/Al2W3O12 mostraram reduções no CET de 7 8 porcento. O módulo de Young aumentou 11 32 porcento e 21 porcento para os compósitos reforçados com Y2W3O12 e Al2W3O12, respectivamente, enquanto a estabilidade térmica do PEAD foi preservada para todos os compósitos. / [en] The aim of this work was to synthesize Al2W3O12 and Y2W3O12 submicronic powders and study the effect of incorporation of these fillers on thermal expansion coefficient (CTE) and mechanical properties of high density polyethylene (HDPE) composites. Y2W3O12 was synthesized via co-precipitation by mixing reactants and modified reverse strike co-precipitation, to assess the influence of the method on agglomeration degree of powders. Both methods produced Y2W3O12 submicronic powders (0.60 0.70 um), with similar primary particle sizes and agglomeration degrees, as verified by different characterization techniques. Al2W3O12 was synthesized via co-precipitation by mixing reactants and normal strike method, using two distinct tungsten sources and the assynthesized precursors were microwave dryied. The effects of synthesis pH (4.2 and 6.0) on final agglomeration state of the as-synthesized powders were assessed. Al2W3O12 submicronic powders (0.22 0.87 um) were synthesized by these routes. Co-precipitation by mixing reactants at pH=4.2 led to Al2W3O12 powders with smaller agglomerate sizes (0.22 um). HDPE composites were manufactured by micro-compounding, using Al2W3O12 and Y2W3O12 powders with similar particle sizes (0.70 um), at volume fractions in the range of 0.001 0.011. HDPE/Y2W3O12 composites presented CTE reductions of 20 25 porcento compared to neat HDPE, in the temperature range of 25 70 Celsius degrees, while HDPE/Al2W3O12 composites displayed reductions on CTE of 7 8 porcent. Young s moduli of composites were increased 11 32 porcent nd 21 porcent when filled with Y2W3O12 and Al2W3O12, respectively, whilst thermal stability of HDPE was preserved for all composites.

No / The goal of this study was to investigate the fibre orientation distribution (FOD), and subsequent mechanical properties, of an injection moulded plate with two different number averaged fibre lengths, termed in this paper medium (1.35 mm) and long (2.40 mm). Fibre orientation measurements (FOD) were made using the 2D elliptical section method and an in-house developed image analyser. The samples were injected from a pin gate located at the centre and top of the plate. Expansion flow on the divergent flow front from this pin gate resulted in a core region with circumferential alignment, while through thickness shear resulted in the usual realignment of fibres in the flow direction either side of the core, termed the shell layers. Two interesting aspects were discovered from these measurements. First, and most importantly, the FOD was found to be independent of the two fibre lengths in this study, and so predominantly controlled by the mould shape and the interaction with the flow front. Second, the fibres in the core region were found to be much closer packed than those in the shell regions. The interaction between the flow front and the mould shape resulted in a range of FOD across the moulded plate, from equal in-plane orientation at the centre of the plate, to highly aligned at the plate edge. This gave a very useful set of samples from which to test out the well known modified rule of mixtures (MROM). Often the fibre orientation distribution cannot be measured directly, but indirectly using the modified rule of mixtures model in reverse. The samples from this moulding (at two different average fibre lengths) gave an excellent opportunity to validate this often used approach. Both the tensile modulus and strength (measured parallel to the injection direction) were found to show a strong correlation with the measured fibre orientation, with a significant increase in both measures between the centre and the edge of both plates. The increased length of the ‘long’ fibre plate was found to give only a small increase in tensile modulus but a much larger increase in tensile strength. The tensile modulus showed a linear dependence with the measured fourth order orientation tensor average, 〈cos4 θ〉, with respect to the injection direction of the plate, as predicted by the modified rule of mixtures. Excellent agreement was found between the measured modulus and the predictions from the modified rule of mixtures, based only on measured quantities (matrix modulus, fibre fraction and average fibre length) for both plates.

The purpose of this research is to investigate the influence of stitching architectures by using different stitching periodic patterns on the in-plane and out-of-plane mechanical properties. By using the inherent periodic architecture of these composites, their mechanical properties may be tailored for specific applications. Composite structures are extensively used in several industries such as aerospace, automotive, sports, and construction due to their many advantages, which include tailorable mechanical properties, high strength-to-weight ratios, and high specific stiffness. However, due to their low interlaminar tensile strength, composites are prone to delaminations, which can degrade the overall mechanical performance of the structure. Through-thickness stitching provides the third-direction reinforcement to enhance the interlaminar tensile and shear strengths. In this study, quasi-isotropic composite test articles were manufactured and stitched through-thickness using different chain stitch patterns. Full-field surface strain measurements were collected through the non-contact digital image correlation (DIC) technique. A design of experiments (DoE) approach was used to investigate the stitch parameters, such as stitch density (number of stitches per unit area), stitch angle (stitch seam orientation), and linear thread density (thread diameter), and their interactions on the in-plane and out-of-plane mechanical properties. Experimental results are then used to develop a statistically informed response surface model (RSM) to find optimal stitching parameters based on a maximum predicted tensile strength, tensile modulus and flexural strength.

Polymer matrix composites (PMC's) are now being used more and more extensively and over wider ranges of service conditions. Large changes in pressure, chemical environment or temperature influence the mechanical response of such composites. In the present effort, we focus on temperature, a parameter of primary interest in almost all engineering applications. In order to design composite structures without having to perform extensive experiments (virtual design), the necessity of establishing theoretical models that relate the macroscopic response of the structure to the microscopic properties of the constituents arises. In the first part of the present work, a new stiffness versus temperature model is established. The model is validated using data from the literature. The influence of the different polymer's properties (Molecular weight, crystallinity, and filler content) on the model are studied by performing experiments on different grades of four polymers PMMA, PEEK, PPS, and PB. This statistical model is proven to be applicable to very different polymers (elastomers, thermoplastics, crystalline, amorphous, cross-linked, linear, filled, unfilledâ ¦) over wide temperature ranges (from the glassy state to the flow region). The most attractive feature of the proposed model is the capability to enable a description of the polymer's mechanical behavior within and across the property transition regions. In order to validate the feasibility of using the model to predict the mechanical response of polymer matrix composites, the stiffness-temperature model is used in various micromechanical models (rule of mixtures, compression models for the life prediction of unidirectional PMC's in end-loaded bendingâ ¦). The model is also inserted in the MRLife prediction code to predict the remaining strength and life of unidirectional PMC's in fatigue bending. End-loaded fatigue experiments were performed. A good correlation between theoretical and experimental results is observed. Finally, the model is used in the Classical Lamination Theory; some laminates were found to exhibit stress reversals with temperature and behaved like thermally activated mechanical switches. / Ph. D.

Σύνθετα που ενσωματώνουν σιδηροηλεκτρικά και πιεζοηλεκτρικά νανοσωματίδια, ομοιογενώς διεσπαρμένα μέσα σε μήτρα άμορφου πολυμερούς, αντιπροσωπεύουν μια νέα κατηγορία υλικών. Τα νανοδιηλεκτρικά σύνθετα ανήκουν σε ένα νέο τύπο υλικών που παρασκευάζονται για βελτιωμένες επιδόσεις, σαν διηλεκτρικά και ηλεκτρικοί μονωτές. Ορισμένα κεραμικά υλικά μπορούν να επιλεγούν και να αναμιχθούν με πολυμερή για να επιτευχθεί συνέργια μεταξύ της υψηλής διηλεκτρικής αντοχής των πολυμερών και της υψηλή διηλεκτρικής σταθεράς των κεραμικών. Τα εν λόγω συστήματα μπορούν να χρησιμοποιηθούν σε πολλές εφαρμογές, όπως σε ολοκληρωμένους πυκνωτές αποσύζευξης, ακουστικούς αισθητήρες εκπομπής, επιταχυνσιόμετρα γωνιακής επιτάχυνσης και ελεγκτές ρεύματος διαρροής, καθώς και σε στρατιωτικούς εξοπλισμούς και εφαρμογές στις μεταφορές. Έχει διαπιστωθεί πως τα νανοσύνθετα παρουσιάζουν βελτιωμένες ιδιότητες, αλλά όχι πλήρως κατανοητές, συγκριτικά με τα μικροσύνθετα. Επιπλέον, μελέτες δείχνουν ενδιαφέρουσες συμπεριφορές, όταν πρόκειται για πολύ χαμηλές περιεκτικότητες σε νανοσωματίδια. Στην παρούσα μελέτη, νανοσύνθετα εποξειδικής ρητίνης και κεραμικών νανοσωματιδίων BaSrTiO3 (μεικτό οξείδιο τιτανικού στροντίου βαρίου, BST) (<100 nm), παρασκευάστηκαν με διαδικασία ανάμειξης σε ένα ευρύ φάσμα συγκεντρώσεων, με σκοπό να μελετηθεί η επίδραση της πολύ χαμηλής (ή πολύ υψηλής) περιεκτικότητας στα χαρακτηριστικά του συστήματος. Οι διηλεκτρικές ιδιότητες και τα φαινόμενα χαλάρωσης μελετήθηκαν με τη βοήθεια της διηλεκτρικής φασματοσκοπίας (BDS) στο εύρος θερμοκρασιών από 30 oC έως 160 oC και συχνοτήτων 10-1 Hz έως 107 Hz. Ο μορφολογικός χαρακτηρισμός έγινε μέσω της ηλεκτρονικής μικροσκοπίας σάρωσης (SEM) και διαπιστώθηκε πως η νανοδιασπορά των εγκλεισμάτων είναι επιτυχής. Ο δομικός χαρακτηρισμός, των δοκιμίων αλλά και των σωματιδίων BaSrTiO3, έγινε μέσω περίθλασης ακτίνων-Χ (XRD) και διαπιστώθηκε πως τα φάσματα έρχονται σε συμφωνία με τη σχετική βιβλιογραφία. Από την ανάλυση των αποτελεσμάτων της διηλεκτρικής φασματοσκοπίας καταγράφονται τρεις διηλεκτρικές χαλαρώσεις, που αποδίδονται με φθίνουσα σειρά των χρόνων χαλάρωσης, στη διεπιφανειακή πόλωση, την μετάπτωση από την υαλώδη στην ελαστομερική φάση της μήτρας (α-χαλάρωση) και σε επαναδιευθετήσεις πολικών πλευρικών ομάδων της κύριας πολυμερικής αλυσίδας (β-χαλάρωση). Η εξέταση της επίδρασης της περιεκτικότητας σε νανοεγκλέισματα, στη διηλεκτρική απόκριση των σύνθετων αποκαλύπτει μη-αναμενόμενες συμπεριφορές, σε χαμηλές (αλλά και υψηλές) περιεκτικότητες. Πιο συγκεκριμένα στις χαμηλές περιεκτικότητες εμφανίζονται φαινόμενα ακινητοποίησης των μακροαλυσίδων και επακόλουθα των διπόλων, με αποτέλεσμα τη μείωση της διαπερατότητας και την αύξηση της θερμοκρασίας υαλώδους μετάπτωσης του συστήματος. Αντίστοιχα σε υψηλές περιεκτικότητες υπάρχει εκ νέου αύξηση της θερμοκρασίας υαλώδους μετάπτωσης, λόγω περιορισμένης κινητικότητας των διπόλων. Συμπερασματικά, θεωρείται πως υπάρχουν τρεις ‘’ζώνες περιεκτικοτήτων’’ που προσδίδουν διαφορετικά χαρακτηριστικά στη διηλεκτρική συμπεριφορά του συστήματος, μέσω της ρύθμισης των αλληλεπιδράσεων ρητίνης-εγκλεισμάτων. / Ceramic–polymer composites incorporating ferroelectric and piezoelectric crystal nanoparticles, homogeneously dispersed within an amorphous polymer matrix represent a novel class of materials. Nanodielectric composites belong to a new type of engineering materials suitable for improved performance as dielectrics and electrical insulators. Certain ceramic materials can be selected to be blended with polymers providing synergy between the high breakdown strength of polymers and the high permittivity of ceramic materials. These type of material systems can be used in plenty applications such as integrated decoupling capacitors, acoustic emission sensors, angular acceleration accelerometers, smart skins and leakage current controllers, as well as in military equipment and transport applications. It has been found that nanocomposites exhibit enhanced properties, yet not fully understandable, comparably to microcomposites. Furthermore, literature demonstrates some interesting dielectric behaviors when filler’s concentration comes to very low nanoparticle loadings. In the present study, nanocomposites of epoxy resin and ceramic BaSrTiO3 (Barium-Strontium Titanate, BST) nanoparticles (<100 nm), were prepared with a mixing procedure in a wide range of nanofiller concentrations, aiming to investigate the impact of very low (or very high) loadings on the system’s properties. The dielectric properties and the related relaxation phenomena were studied by means of Broadband Dielectric Spectroscopy (BDS) in the temperature range from 30 oC to 160 oC and frequency range from 10-1 Hz to 107 Hz. Scanning electron microscopy (SEM) was employed in order to examine the morphology of the produced specimens. The dispersion of nanoinclusions can be considered as satisfactory. Structural characterization of the systems as well as of the BaSrTiO3 nanopowder was examined via x-ray diffraction (XRD). Obtained results are in accordance with literature. Three dielectric relaxation processes were detected form the analysis of the dielectric spectra. They are attributed, with descending order of relaxation time, to interfacial polarization, glass to rubber transition of the polymer matrix (α-relaxation), and re-arrangement of polar side groups of the main macromolecular chain (β-relaxation). The influence of the nanoparticles content upon the dielectric response of the composites, reveal unexpected behaviours at low and high filler loading. In particular, effects of immobilization/entanglement of macromolecular chains and subsequently of dipoles, resulting to a decrease of the permittivity values and enhancement of the glass to rubber transition temperature of the systems, were ascertained. At the opposite edge, at high filler loading, an increase of glass to rubber transition temperature was also found, due to the limited mobility of the chains and dipoles. Concluding, the existence of three “zones of filler content” is assumed, providing different characteristics in the dielectric response of the systems, because of the tunable polymer-inclusion interactions.

The thesis consists of four parts of which part 1 presents a brief overview of nanomaterials. Parts 2, 3 and 4 contain results of investigations of graphene, nanofilms of noble metal nanoparticles and ZnO nanostructures respectively. Investigations of graphene are described in Part 2 which consists of six chapters. In Chapter 2.1, changes in the electronic structure and properties of graphene induced by molecular charge-transfer have been discussed. Chapter 2.2 deals with the results of a study of the interaction of metal and metal oxide nanoparticles with graphene. Electrical and dielectric properties of graphene-polymer composites are presented in Chapter 2.3. Chapter 2.4 presents photo-thermal effects observed in laser-induced chemical transformations in graphene and other nanocarbons system. Chapter 2.5 describes the mechanical properties of polymer matrix composites reinforced by fewlayer graphene investigated by nano-indentation. The extraordinary synergy found in the mechanical properties of polymer matrix composites reinforced with two nanocarbons of different dimensionalities constitute the subject matter of Chapter 2.6. Investigations of noble metal nanoparticles have been described in Part 3. In Chapter 3.1, ferromagnetism exhibited by nanoparticles of noble metals is discussed in detail while Chapter 3.2 deals with surface-enhanced Raman scattering (SERS) of molecules adsorbed on nanocrystalline Au and Ag films formed at the organic–aqueous interface. Factors affecting laser-excited photoluminescence from ZnO nanostructures are examined in great detail in Part 4.

abstract: Polymer matrix composites (PMCs) are attractive structural materials due to their high stiffness to low weight ratio. However, unidirectional PMCs have low shear strength and failure can occur along kink bands that develop on compression due to plastic microbuckling that carry strains large enough to induce nonlinear matrix deformation. Reviewing the literature, a large fraction of the existing work is for uniaxial compression, and the effects of stress gradients, such as those present during bending, have not been as well explored, and these effects are bound to make difference in terms of kink band nucleation and growth. Furthermore, reports on experimental measurements of strain fields leading to and developing inside these bands in the presence of stress gradients are also scarce and need to be addressed to gain a full understanding of their behavior when UDCs are used under bending and other spatially complex stress states. In a light to bridge the aforementioned gaps, the primary focus of this work is to understand mechanisms for kink band evolution under an influence of stress-gradients induced during bending. Digital image correlation (DIC) is used to measure strains inside and around the kink bands during 3-point bending of samples with 0°/90° stacking made of Ultra-High Molecular Weight Polyethylene Fibers. Measurements indicate bands nucleate at the compression side and propagate into the sample carrying a mixture of large shear and normal strains (~33%), while also decreasing its bending stiffness. Failure was produced by a combination of plastic microbuckling and axial splitting. The microstructure of the kink bands was studied and used in a microstructurally explicit finite element model (FEM) to analyze stresses and strains at ply level in the samples during kink band evolution, using cohesive zone elements to represent the interfaces between plies. Cohesive element properties were deduced by a combination of delamination, fracture and three-point bending tests used to calibrate the FEMs. Modeling results show that the band morphology is sensitive to the shear and opening properties of the interfaces between the plies. / Dissertation/Thesis / Doctoral Dissertation Mechanical Engineering 2016

High performance polymer composites are materials with emphasis on specific chemical and mechanical properties due to their broad scope of applications. The main advantages are high strenght and toughness in comparison with their low weight and density. An increased adhesion at composite interfaces is important to ensure excellent composite properties. Bundles of glass fibers were coated by plasma-polymerised interlayers of tetravinylsilane (pp-TVS) of different thicknesses and at different effective powers. The prepared interlayers of pp-TVS were analyzed to evaluate chemical composition (RBS, FTIR, XPS) and mechanical properties (NI-AFM). Microindentation test and fiber-bundle pull-out test were used to determine the interfacial shear strenght.

<div>As the use of fiber-reinforced polymer composites grows in aerospace structures, there is an emerging need to implement damage tolerant approaches. The use of <i>in-situ</i> synchrotron X-ray tomography enables direct observations of progressive damage relative to the microstructural features, which is studied in a T650/5320 laminate composite with varying layup orientations (using 45^o and -45^o plies) in a compact tension specimen geometry. Specifically, the interactions of micromechanical damage mechanisms at the notch tip were analyzed through 3D image processing as the crack grew. First, monotonic tests were conducted where X-ray tomography was acquired incrementally between the unloaded state and maximum load. The analysis of the monotonic tension specimens showed intralaminar cracking was dominant during crack initiation, delamination became prevalent during the later stages of crack progression, and fiber breakage was, in general, largely related to intralaminar cracking. After the monotonic tension analysis, modifications were made to the specimen geometry and the loading assembly, and fatigue tests were conducted, also using <i>in-situ</i> synchrotron X-ray tomography. Specifically, tomography images were acquired after select intervals of cyclic loading to examine the crack growth behavior up to 5802 cycles. The analysis of the fatigue tests showed that intralaminar cracking was also dominant, while localized delamination allowed ply cross-over. A finite element analysis was conducted by comparing the crack profile at varying intervals of loading, and the change in stored energy per cycle, dU/dN, was calculated. The combined experimental and simulation analysis showed that when the per ply values of dU/dN were examined, the intralaminar cracking rate collapsed to one curve regardless of the ply orientation, where direct observations of fiber bridging were characterized and associated with a reduction in crack growth rate for the influenced ply. Overall, this work provides a physical understanding of the micromechanics facilitating intralaminar crack growth in composites, providing engineers the necessary assessments for slow crack growth approaches in structural composite materials.<br></div>

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