Abstract Biodegradable starch-glycerol based films were obtained. The influence of lipid compounds (palmitic, stearic and oleic acid), other polymers (hydroxypropylmethylcellulose and sodium caseinate) and bioactive compounds (¿-tocoferol, D-limonene and orange essential oil) on film properties (oxygen and water vapour barrier, optical, mechanical, nano- and microstructural). Furthermore the effect of storage time on films¿ properties was also considered. Fatty acids addition did not improve the water vapour ability of films except for non-stored saturated fatty acids containing films. X-ray diffraction results showed that cristallinity of films increased with storage time, thus increasing the stiffness and decreasing the gloss of films. Furthermore, crystallinity affected the water sorption capacity of films as function of relative humidity and temperature. Glass transition temperature of starch films varied with saturated fatty acids addition. However, oleic acid did not affect this parameter. The presence of fatty acids promoted the formation of V-type structures, thus indicatin the formation of amylose-lipid complexes that inhibited the developmet of other crystalline structures. The effect of the incorporation of other biopolymers to improve the functionality of starch films was also studied. Hydroxypropylmethylcellulose (HPMC) addition inhibited starch retrogradation. However, obtained films were more permeable, specially in case of oxygen. HPMC addition produced phase separation as it was observed by scanning electron microscopy. On the contrary, sodium caseinate incorporation (NaCas) allowed to obtain homogeneous films and less permeable to oxygen. Obtained films showed less mechanical resistance in comparison with pure starch films but a greater flexibility without increasing the water vapour permeability. Rearrangement of polymers chains during storage reduced the mechanical resistance, the extensibility and the gloss of composite films. Regarding the obtained results, the film including a starch:protein ratio of 50:50 was choosen as the film with the most adequate properties. Composite film (starch:Nacas ratio = 50:50) was studied as a matrix for the incorporation o active compounds (¿-tocopherol, D-limonene and orange essential oil). The effect of ¿-tocopherol addition was compared with the incorporation of oleic acid and their mixture. Lipids addition promoted phase separation between starch and NaCas due to the different interactions between each polymer and the lipids. Furthermore, oleic acid addition increased significantly the oxygen permeability whereas ¿-tocopherol greatly improved the antioxidant capacity of films without affecting the oxygen permeability. D-limonene and orange essential oil incorporation was carried out by forming rapeseed and soy nanoliposomes, which acted as carriers of bioactive components. Nanoliposomes incorporation was performed directly in starch-NaCas dispersions without any homogenization, to avoid nanoliposomes damages. Bioactive compounds addition did not confer antimicrobial capacity to the films (except for soy-orange oil nanoliposomes containing film) probably due to the high stability of nanoliposomes and the low antibacterial activity of D-limonene and orange essential oil. / Se han desarrollado y caracterizado films biodegradables a base de almidón de maíz y glicerol como plastificante, evaluando al mismo tiempo el efecto de la adición de componentes lipídicos (ácido palmítico, esteárico y oleico), otros polímeros (hidroxipropilmetilcelulosa y caseinato de sodio) y compuestos bioactivos (¿-tocoferol, aceite esencial de naranja y D-limoneno) sobre las propiedades de los films (propiedades barrera al vapor de agua y al oxígeno, ópticas, mecánicas, micro y nanoestructurales). Asimismo se evaluó la influencia del tiempo de almacenamiento en las propiedades de los films. La adición de ácidos grasos no mejoró notablemente la permeabilidad al vapor de agua excepto en el caso de los films con ácidos grasos saturados y solo en films no almacenados. Los resultados de difracción de rayos X mostraron que la cristalinidad aumentó con el tiempo de almacenamiento, incrementándose la rigidez, y disminuyendo el brillo de los films. Del mismo modo, la cristalinidad afectó a la capacidad de sorción de agua de los films en función de la humedad relativa y la temperatura. La temperatura de transición vítrea de los films de almidón se vio afectada por la adición de ácidos grasos saturados pero no por la adición de ácido oleico. La presencia de dichos componentes promovió la formación de estructuras cristalinas tipo V, indicando la formación de complejos entre los lípidos y las cadenas de amilosa e inhibiendo la formación de otros tipos de formas cristalinas. Se analizó también el efecto de la incorporación de otros biopolímeros en la posible mejora de la funcionalidad de los films de almidón. En las mezclas con hidroxipropilmetilcelulosa (HPMC), se inhibió la retrogradación del almidón en los films composite, pero se observó un efecto negativo en las propiedades barrera de los mismos, que fueron más permeables, principalmente al oxígeno. La adición de HPMC produjo separación de fases en los films (observada por microscopía electrónica de barrido). Por el contrario, la incorporación de caseinato de sodio (NaCas) permitió formar films homogéneos y menos permeables al oxígeno. Los films presentaron una resistencia mecánica algo menor que los films de almidón puro pero una mayor flexibilidad sin incrementar los valores de permeabilidad al vapor de agua. La reorganización de las cadenas de los polímeros con el tiempo de almacenamiento provocó la disminución de la resistencia mecánica, la deformabilidad y el brillo de los films composite. Atendiendo a los efectos observados, se eligió como formulación más adecuada el film composite formado por almidón y NaCas con un ratio de polímeros del 50:50. El film composite de almidón y NaCas (50:50) se estudió como matriz para la incorporación de compuestos bioactivos como son el ¿-tocoferol y el aceite esencial de naranja o su principal componente, el D-limoneno. El efecto de la adición de ¿-tocoferol se comparó con la influencia de la adición de ácido oleico y también con la adición de ambos compuestos. La adición de lípidos provocó una separación de fases entre el almidón y el NaCas debido a la diferente interacción entre cada polímero y los lípidos. Asimismo la adición de ácido oleico incrementó significativamente la permeabilidad al oxígeno, al contrario que el ¿-tocoferol, que además impartió a los films una elevada capacidad antioxidante. La incorporación de aceite esencial de naranja y D-limoneno se realizó utilizando nanoliposomas de lecitina de soja y lecitina de colza que encapsularon los compuestos activos. La incorporación de nanoliposomas en los films se realizó directamente en las dispersiones acuosas sin posterior homogeneización para evitar su ruptura. La adición de los compuestos bioactivos en forma de nanoliposomas no confirió capacidad antimicrobiana a los films, salvo en el caso de los nanoliposomas de lecitina de soja con aceite esencial, debido probablemente a la dificultad de los compuestos encapsulados para difundir en el film por la gran estabilidad de los liposomas y a la baja actividad antilisteria del D-limoneno y el aceite esencial de naranja. / Jiménez Marco, A. (2013). Propiedades de films de almidón de maíz. Influencia de la incorporación de lípidos, biopolímeros y compuestos bioactivos [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/28214 / TESIS / Premios Extraordinarios de tesis doctorales

The goal of the present dissertation is to develop and apply novel and systematic data-driven optimization approaches that can efficiently optimize Additive Manufacturing (AM) systems with respect to targeted properties of final parts. The proposed approaches are capable of achieving sets of process parameters that result in the satisfactory level of part quality in an accelerated manner. First, an Accelerated Process Optimization (APO) methodology is developed to optimize an individual scalar property of parts. The APO leverages data from similar—but non-identical—prior studies to accelerate sequential experimentation for optimizing the AM system in the current study. Using Bayesian updating, the APO characterizes and updates the difference between prior and current experimental studies. The APO accounts for the differences in experimental conditions and utilizes prior data to facilitate the optimization procedure in the current study. The efficiency and robustness of the APO is tested against an extensive simulation studies and a real-world case study for optimizing relative density of stainless steel parts fabricated by a Selective Laser Melting (SLM) system. Then, we extend the idea behind the APO in order to handle multi-objective process optimization problems in which some of the characteristics of the AMabricated parts are uncorrelated. The proposed Multi-objective Process Optimization (m-APO) breaks down the master multi-objective optimization problem into a series of convex combinations of single-objective sub-problems. The m-APO maps and scales experimental data from previous sub-problems to guide remaining sub-problems that improve the solutions while reducing the number of experiments required. The robustness and efficiency of the m-APO is verified by conducting a series of challenging simulation studies and a real-world case study to minimize geometric inaccuracy of parts fabricated by a Fused Filament Fabrication () system. At the end, we apply the proposed m-APO to maximize the mechanical properties of AMabricated parts that show conflicting behavior in the optimal window, namely relative density and elongation-toailure. Numerical studies show that the m-APO can achieve the best trade-off among conflicting mechanical properties while significantly reducing the number of experimental runs compared with existing methods.

This paper presents an innovative, eco-friendly and sustainable tape manufacturing technology that transforms waste carbon and polyamide fibers into a new class of fibrous structure with unidirectional fiber orientation, termed “unidirectional tapes structure” for the fabrication of high performance composites. This novel technology imparts homogeneity, uniformity, orientation and thermal stability in unidirectional tapes structure that resemble conventional prepreg material. Unidirectional configuration of the tapes structure brings a revolution towards development of cost efficient carbon fiber composites for load bearing structural applications. This paper introduces the concept of tape manufacturing technology and highlights the modifications, optimization, and technological developments carried out to develop unidirectional tapes. The structural parameters that play a significant role in the properties of the high performance composite, such as fiber length, fiber orientation, fiber damage, and uniformity, were assessed during tape manufacturing. The results reveal composites fabricated from unidirectional tape structures with optimum parameters deliver tensile strength and modulus of 1370 ± 22 MPa and 85 ± 4 GPa, respectively.

Made available in DSpace on 2017-07-21T20:43:45Z (GMT). No. of bitstreams: 1 Luis Otavio Ribas Lima.pdf: 8124964 bytes, checksum: d16bc8c64f9e15d57f770b1b271d6b3b (MD5) Previous issue date: 2014-06-18 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Aluminum alloys have been the primary material of choice for the aircraft due to their properties such as low density, high mechanical and corrosion resistance. Commercial aircraft apply aluminum alloys for the fuselage, wings and supporting structure due to the extensive knowledge in design and production of aluminum components, and most importantly, aluminum alloys continue to be developed, keeping it highly competitive. A great development happen with the heat-treatable alloys, which allow improvement of the mechanical properties. Among this alloys stand out the Al-Zn-Mg-Cu series, known for high strength, toughness and corrosion resistance. The improvement of those alloys occurs by the precipitation of nanometric particles MgZn2, called η phase. This study’s aim was to promote an interrupted heat treatment T6I4-65 in an AA7050 aluminum alloy, with fatigue crack growth resistance as priority. Interrupted heat treatments’ goal is optimizing the consumption of solute atoms during process of nucleation and growth of precipitates as a finely dispersion. The T6I4-65 condition obtained was analyzed by differential scanning calorimetry, DSC, optical and electronic microscopy, mechanical tests as hardness, tensile and fatigue crack growth. The T6I4-65 treatment results in a microstructure with a fine dispersion of precipitated phase η’, about 75% smaller than those resulting from in current use, T7451. This microstructure resulted in a reduction of up to 24% in fatigue crack growth rate compared to that resulting from T7451 treatment, keeping the ductility of 17% of area reduction and yield strength higher than 400MPa. / Ligas de alumínio são o principal material de uso na indústria aeronáutica devido a suas propriedades como baixa densidade, alta resistência mecânica e a corrosão. Aeronaves comerciais utilizam ligas de alumínio em sua fuselagem, asas e na estrutura de suporte devido ao extenso conhecimento no projeto e produção de componentes em alumínio, e mais importante, as ligas de alumínio continuam a serem desenvolvidas, mantendo-se altamente competitivas. Ocorreu um grande avanço com o desenvolvimento das ligas tratáveis termicamente, que permitiram a otimização das propriedades mecânicas. Entre estas ligas tratáveis, destaca se a família Al-Zn-Mg-Cu, conhecidas pela alta resistência mecânica, tenacidade e resistência a corrosão. O aperfeiçoamento destas ligas ocorre pela precipitação de partículas nanométricas de MgZn2, conhecida como fase η. O objetivo deste trabalho foi obter um tratamento térmico interrompido T6I4-65 em uma liga de alumínio AA7050 com prioridade ao aumento de resistência à propagação de trinca por fadiga. Tratamentos interrompidos tem por objetivo otimizar o consumo de átomos de soluto durante os processos de nucleação e crescimento dos precipitados endurecedores na liga na forma de dispersão finamente dispersa. A condição T6I4-65 obtida foi analisada por meio de calorimetria diferencial de varredura, DSC, microscopia ótica e eletrônica de varredura e transmissão, ensaios mecânicos dureza, tração e propagação de trinca por fadiga. Este tratamento resultou em uma microestrutura com uma dispersão de finos precipitados de fase η’, cerca de 75% menores que os resultantes de tratamentos de uso corrente, T7451. Esta microestrutura promoveu a redução de até 24% na taxa de propagação de trinca por fadiga em comparação à resultante do tratamento T7451, mantendo grande ductilidade, até 17% de redução de área e limite de escoamento superior a 400MPa.

In recent years, due to growing environmental awareness, considerable attention has been given to the development and production of natural fibre reinforced polymer (both thermoset and thermoplastic) composites. The main objective of this study was to reinforce epoxy and polylactic acid (PLA) with hemp fibre to produce improved composites by optimising the fibre treatment methods, composite processing methods, and fibre/matrix interfacial bonding. An investigation was conducted to obtain a suitable fibre alkali treatment method to: (i) remove non-cellulosic fibre components such as lignin (sensitive to ultra violet (UV) radiation) and hemicelluloses (sensitive to moisture) to improve long term composites stability (ii) roughen fibre surface to obtain mechanical interlocking with matrices (iii)expose cellulose hydroxyl groups to obtain hydrogen and covalent bonding with matrices (iv) separate the fibres from their fibre bundles to make the fibre surface available for bonding with matrices (v) retain tensile strength by keeping fibre damage to a minimum level and (vi) increase crystalline cellulose by better packing of cellulose chains to enhance the thermal stability of the fibres. An empirical model was developed for fibre tensile strength (TS) obtained with different treatment conditions (different sodium hydroxide (NaOH) and sodium sulphite (Na2SO3) concentrations, treatment temperatures, and digestion times) by a partial factorial design. Upon analysis of the alkali fibre treatments by single fibre tensile testing (SFTT), scanning electron microscopy (SEM), zeta potential measurements, differential thermal analysis/thermogravimetric analysis (DTA/TGA), wide angle X-ray diffraction (WAXRD), lignin analysis and Fourier transform infrared (FTIR) spectroscopy, a treatment consisting of 5 wt% NaOH and 2 wt% Na2SO3 concentrations, with a treatment temperature of 120oC and a digestion time of 60 minutes, was found to give the best combination of the required properties. This alkali treatment produced fibres with an average TS and Young's modulus (YM) of 463 MPa and 33 GPa respectively. The fibres obtained with the optimised alkali treatment were further treated with acetic anhydride and phenyltrimethoxy silane. However, acetylated and silane treated fibres were not found to give overall performance improvement. Cure kinetics of the neat epoxy (NE) and 40 wt% untreated fibre/epoxy (UTFE) composites were studied and it was found that the addition of fibres into epoxy resin increased the reaction rate and decreased the curing time. An increase in the nucleophilic activity of the amine groups in the presence of fibres is believed to have increased the reaction rate of the fibre/epoxy resin system and hence reduced the activation energies compared to NE. The highest interfacial shear strength (IFSS) value for alkali treated fibre/epoxy (ATFE) samples was 5.2 MPa which was larger than the highest value of 2.7 MPa for UTFE samples supporting that there was a stronger interface between alkali treated fibre and epoxy resin. The best fibre/epoxy bonding was found for an epoxy to curing agent ratio of 1:1 (E1C1) followed by epoxy to curing agent ratios of 1:1.2 (E1C1.2), 1: 0.8 (E1C0.8), and finally for 1:0.6 (E1C0.6). Long and short fibre reinforced epoxy composites were produced with various processing conditions using vacuum bag and compression moulding. A 65 wt% untreated long fibre/epoxy (UTLFE) composite produced by compression moulding at 70oC with a TS of 165 MPa, YM of 17 GPa, flexural strength of 180 MPa, flexural modulus of 10.1 GPa, impact energy (IE) of 14.5 kJ/m2, and fracture toughness (KIc) of 5 MPa.m1/2 was found to be the best in contrast to the trend of increased IFSS for ATFE samples. This is considered to be due to stress concentration as a result of increased fibre/fibre contact with the increased fibre content in the ATFE composites compared to the UTFE composites. Hygrothermal ageing of 65 wt% untreated and alkali treated long and short fibre/epoxy composites (produced by curing at 70oC) showed that long fibre/epoxy composites were more resistant than short fibre/epoxy composites and ATFE composites were more resistant than UTFE composites towards hygrothermal ageing environments as revealed from diffusion coefficients and tensile, flexural, impact, fracture toughness, SEM, TGA, and WAXRD test results. Accelerated ageing of 65 wt% UTLFE and alkali treated long fibre/epoxy (ATLFE) composites (produced by curing at 70oC) showed that ATLFE composites were more resistant than UTLFE composites towards hygrothermal ageing environments as revealed from tensile, flexural, impact, KIc, SEM, TGA, WAXRD, FTIR test results. IFSS obtained with untreated fibre/PLA (UFPLA) and alkali treated fibre/PLA (ATPLA) samples showed that ATPLA samples had greater IFSS than that of UFPLA samples. The increase in the formation of hydrogen bonding and mechanical interlocking of the alkali treated fibres with PLA could be responsible for the increased IFSS for ATPLA system compared to UFPLA system. Long and short fibre reinforced PLA composites were also produced with various processing conditions using compression moulding. A 32 wt% alkali treated long fibre PLA composite produced by film stacking with a TS of 83 MPa, YM of 11 GPa, flexural strength of 143 MPa, flexural modulus of 6.5 GPa, IE of 9 kJ/m2, and KIc of 3 MPa.m1/2 was found to be the best. This could be due to the better bonding of the alkali treated fibres with PLA. The mechanical properties of this composite have been found to be the best compared to the available literature. Hygrothermal and accelerated ageing of 32 wt% untreated and alkali treated long fibre/PLA composites ATPLA composites were more resistant than UFPLA composites towards hygrothermal and accelerated ageing environments as revealed from diffusion coefficients and tensile, flexural, impact, KIc, SEM, differential scanning calorimetry (DSC), WAXRD, and FTIR results. Increased potential hydrogen bond formation and mechanical interlocking of the alkali treated fibres with PLA could be responsible for the increased resistance of the ATPLA composites. Based on the present study, it can be said that the performance of natural fibre composites largely depend on fibre properties (e.g. length and orientation), matrix properties (e.g. cure kinetics and crystallinity), fibre treatment and processing methods, and composite processing methods.

The influence of seven additives on the crystallization ability of polylactide (PLA), melt flow index (MVR) and mechanical tensile properties was studied. Pressed plates with a thickness of 0.8 mm were tested. Selected additives added in amounts of 0.5 and 1.0% were as follows: talc, sodium benzoate, mixtures of organic salts with amorphous SiO2 and zinc stearate, metal salt, phosphate salt, and potassium salt of 5-dimethylsulfoisophthalate (LAK-301 - nucleating agent developed for PLA). Non-isothermal crystallization measurements were performed at different cooling rates (0.3; 0.5; 0.7; 1.0 and 1.5 ° C). All nucleation agents increased the MVR of PLA except talc; the largest increase (9-fold and 24-fold) was the addition of metal salt. The additives did not fundamentally change the mechanical properties. All samples were rather brittle (the most brittle with LAK-301), the modulus of elasticity was around 1.2 GPa for all samples, the strength of PLA was increased the most by the addition of 1% talc (by 12%) and the elongation at break was increased by organic salt with SiO2. All samples with nucleating agents content of 1% were amorphous (crystalline content did not exceed 2%). Thus, the addition of reagents did not support the crystallization process during rapid cooling, even in the case of LAK-301. However, LAK-301 was acting as an excellent nucleating agent at slow cooling rates (1.5 °C / min and below). The nucleation activity of the additives decreased in the following order: LAK-301, organic salt with zinc stearate, talc, organic salt modified with amorphous SiO2 and phosphate salt. Samples with sodium benzoate and metal salt were crystallizing on cooling in several steps and it was not possible to use the method of Dobrev and Gutzow to evaluate the nucleation activity.

[ES] Los procesos convencionales de transformación de materiales requieren afrontar nuevos retos que se presentan en la actual sociedad industrial como es la propia sostenibilidad medioambiental. De la misma forma, los productos fabricados en el futuro deberán cumplir ciertos requisitos medioambientales, como la reciclabilidad de la materia prima utilizada. Dentro de este contexto la fabricación de productos mediante técnicas tridimensionales como la fabricación aditiva, permite utilizar únicamente el material necesario que se requiere para un producto completo. Dichas técnicas de fabricación son las solicitadas por el sector aeronáutico, entre otros, que requiere de unos valores de calidad muy exigentes. Dentro de estos ensayos, el estudio del comportamiento del material ante crecimiento de grieta es de gran importancia. Mediante este tipo de fabricación se obtiene un producto en estado casi final a través de la adición de capas de alrededor de 100 micras, que da como resultado una orientación de grano metalúrgico preferente y diferente a la misma aleación fabricada por forja convencional. Los fenómenos ocurridos durante la fabricación pueden dar lugar a defectos como grietas o porosidades que disminuyen las capacidades resistentes, por lo que un estudio para predecir la vida del componente es importante. Dentro de los procesos de fabricación aditiva nos encontramos con la fabricación por haz de electrones, que consigue calidades de material casi con porosidad nula, por lo que empresas del sector aeronáutico o médico consideran esta técnica como de gran fiabilidad. El trabajo desarrollado en esta tesis se basa en el estudio de aleaciones de titanio fabricadas mediante fabricación aditiva por haz de electrones. En concreto, el estudio se centra en el comportamiento a tenacidad a la fractura para relacionarlo con las características microestructurales más relevantes. Los análisis llevados a cabo consideran diversas orientaciones que tienen lugar en la bandeja de fabricación, realizándose ensayos mecánicos tanto estáticos como dinámicos. Una segunda parte de la tesis se basa en el modelado mediante elementos finitos extendido, XFEM, que se desarrolla como alternativa a los métodos tradicionales de mallado. En el XFEM una aproximación de elementos finitos se construye de forma que sea capaz de representar funciones de enriquecimiento dentro de los elementos mediante grados de libertad adicionales. Un punto crítico en el proceso de cálculo mediante elementos finitos es el proceso de mallado. La precisión obtenida en la aproximación depende del tamaño de los elementos de la malla. Por tanto, el cálculo con precisión en puntos importantes como la zona cercana a grieta exige una malla con un tamaño de elemento muy pequeño. Con la técnica XFEM se alcanza una mayor precisión mediante un proceso de enriquecimiento de extremo de grieta. Los resultados que ofrece la herramienta XFEM se comparan con los obtenidos experimentalmente con componentes fabricados mediante impresión 3D. Esta comparativa se lleva a cabo sobre diversas geometrías con la presencia de agujeros, de tal forma que se ha podido predecir el crecimiento de grieta que tiene lugar en materiales por impresión 3D. De la misma forma, se llevan a cabo comparativas de piezas con geometría compleja, para validar el modelo desarrollado. / [CA] Els processos convencionals de transformació de materials requereixen afrontar nous reptes que es presenten en l'actual societat industrial com és la pròpia sostenibilitat mediambiental. De la mateixa forma, els productes fabricats en el futur hauran de complir certs requisits mediambientals, com el reciclatge de la matèria primera. Dins d'aquest context, la fabricació de productes mitjançant tecnologia 3D com la fabricació additiva, permet usar només el material necessari que es requereix per a un producte complet. Aquestes tècniques de fabricació són les sol·licitades pel sector aeronàutic que requereix d'uns valors de qualitat molt exigents. Dins d'aquests assajos, l'estudi del comportament del material a través de creixement de clivella és vital. Mitjançant aquesta mena de fabricació s'obté un producte en estat quasi final a través de l'addició de capes d'alçària al voltant de 100 micres, que dona com a resultat una orientació de gra metal·lúrgic preferent i diferent al mateix però fabricat convencionalment. Els fenòmens ocorreguts durant la fabricació poden donar lloc a defectes com a clivelles o porositats que poden disminuir les capacitats resistents, per la qual cosa un estudi per a predir la vida del material és important. Dins dels processos de fabricació additiva ens trobem amb la fabricació per feix d'electrons la qual aconsegueix qualitats de material quasi amb porositat nul·la, per la qual cosa empreses del sector aeronàutic i mèdic han conclòs a aquesta tècnica com la més fiable. El treball desenvolupat en aquesta tesi es basa en l'estudi d'aliatges de titani fabricades mitjançant fabricació additiva per feix d'electrons, principalment el seu comportament a la tenacitat a la fractura per a relacionar-ho amb les característiques microestructurals més rellevants. Les anàlisis dutes a terme se centren en diverses orientacions que tenen lloc en la plataforma de fabricació, realitzant-se assajos mecànics tant estàtics com dinàmics. Una segona part de la tesi es basa en el modelatge mitjançant elements finits estesos, XFEM, que es desenvolupa com a alternativa als mètodes lliures de malla. En el XFEM una aproximació d'elements finits es construeix de manera que siga capaç de representar funcions (enriquiment) dins dels elements. Un punt crític en el procés de càlcul en qualsevol mètode que usa una malla és el procés d'emmallat. La precisió obtinguda en l'aproximació depén de la grandària dels elements de malla. Per tant, el càlcul amb precisió en punts importants, com la zona pròxima a clivella, exigeix l'ús d'una malla amb una grandària d'element molt xicoteta. Amb la tècnica XFEM aconseguim aqueixa precisió mitjançant un procés de enriquiment. / [EN] Conventional material transformation processes require facing new challenges that arise in today's industrial society, such as environmental sustainability. Similarly, products manufactured in the future must meet certain environmental requirements, such as the recyclability of the raw material used. Within this context, the manufacture of products using 3D technology such as additive manufacturing, allows using only the necessary material that is required for a complete product. These manufacturing techniques are requested by the aeronautical sector, which requires very demanding quality values. Within these tests, the study of the behavior of the material through crack growth is of great importance. By means of this manufacturing technology, a product is obtained in an almost final state through the addition of layers of about 100 microns, which results in a preferential metallurgical grain orientation and different from the same alloy manufactured by conventional methods. The phenomena occurring during manufacturing can lead to defects such as cracks or porosities that can reduce the strength capabilities, so a study to predict the life of the component is important. Within the additive manufacturing processes we find the electron beam manufacturing which achieves material qualities with almost zero porosity. As a consequence, companies in the aeronautical or medical sector have concluded this technique as very reliable. The work developed in this thesis is based on the study of titanium alloys manufactured by electron beam additive manufacturing. More precisely, the work is focused on the fracture toughness behavior in order to relate it to the most relevant microstructural characteristics. The analyses carried out consider different orientations and positions that take place in the fabrication tray, performing both static and dynamic mechanical tests. A second part of the thesis is based on the application of the extended finite element method, XFEM, which is developed as an alternative to conventional finite element method. In XFEM a finite element approximation is constructed in such a way that it is able to represent functions within the elements. A critical point in the calculation process in the finite element method is the meshing process. The accuracy obtained in the approximation depends on the size of the elements of the mesh. Therefore, accurate computation at important points such as the near-crack zone requires the use of a mesh with a very small element size. With the XFEM technique, we achieve this accuracy by means of an enrichment process. The results provided by the XFEM tool are compared with those obtained experimentally with respect to components manufactured by 3D printing. This comparison is carried out on different geometries with the presence of holes, in such a way that it has been possible to predict the crack growth that takes place in 3D printed materials. In the same way, comparisons of parts with complex geometry are carried out to validate the developed model. / Niñerola González, R. (2022). Estudio del comportamiento por crecimiento de grieta de aleaciones fabricadas por adición mediante haz de electrones [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/183818 / TESIS

The rapid growth of plastic waste from food packaging around the world demands renewable substitutes, such as natural fibers and biocomposites. Wood fibers are natural fibers extracted from trees and are commonly used in packaging. In order for renewable alternatives to compete against plastics and other non-renewable materials, a better understanding of the mechanical properties of single fibers at the micro-scale are necessary. A great deal of previous research into the mechanical properties of single wood fibers has focused on their tensile behavior, however, little work has been published about their compressive behavior. It is difficult to measure the compressive strength of single fibers directly due to fiber buckling. The purpose of this study is to investigate how post-buckling of single wood pulp fibers affects the mechanical properties of fibers in tension. Two alternative hypotheses were tested through experiments in The Odqvist Laboratory for Experimental Mechanics at KTH. The major part of the thesis process has been invested in developing components called grippers, and testing methods for the Single Fiber Testing System, in order to be able to perform the experiments. The existing grippers were tested and alternative grippers were developed, as well as an alternative testing method without grippers, called the Paper frame method (PFM). PFM was used in the final experimental work to test the hypotheses. The main finding from this study is that there is not enough evidence to suggest that the tensile strength or tensile stiffness of single wood fibers are significantly reduced by post-buckling damage. This finding is mostly relevant in the research and development of fibrous material with larger distances between individual fibers, such as low-density fiber network materials. The main findings from the single fiber testing methods development were that the existing grippers cannot prevent fiber slippage. Furthermore, the alternative gripper 22A with its arc design generates higher grip force than previous grippers but lacks surface friction in the contact region in order to prevent fiber slippage. PFM has an experimental success rate of over 80 % for trained users and easy usage for the operator. The testing equipment Single Fiber Testing System displays several systematic errors occurring in the post-processing process of tests with cyclic loads. / Den snabba tillväxten av plastavfall från livsmedelsförpackningar runt om i världen kräver förnybara alternativ, såsom förpackningar gjorda av naturfibrer och biokompositer. Träfibrer är naturliga fibrer som utvinns från trä och används ofta i förpackningar. För att dessa förnybara alternativ ska kunna konkurrera mot plast och andra icke-förnybara material krävs en bättre förståelse av de mekaniska egenskaperna hos enskilda fibrer på mikronivå. Det finns en omfattande forskning om de mekaniska egenskaperna i drag hos enskilda träfibrer. Däremot existerar det lite publicerad forskning om träfibrers kompressionsegenskaper. Kompressionsegenskaperna är svåra att mäta direkt på grund av fiberknäckning. Syftet med denna studie är att undersöka hur skadan som uppstår efter knäckning av enskilda träfibrer påverkar de mekaniska egenskaperna hos fibrer i drag. Två alternativa hypoteser testades genom experiment i Odqvistlaboratoriet för experimentell mekanik vid KTH. Huvuddelen av examensarbetet har investerats i att utveckla grepparmar och testmetoder för testmaskinen Single Fiber Testing System, för att kunna utföra experiment. De befintliga grepparmarna testades och nya grepparmar utvecklades, och även en alternativ testmetod utan grepparmar som kallas Paper frame method (PFM) utvecklades. PFM användes i det sista experimentella arbetet för att pröva hypoteserna. Huvudslutsatsen från denna studie är att det inte finns tillräckligt med bevis för att stödja hypotesen att enskilda träfibrers draghållfasthet eller dragstyvhet reduceras av skada som uppstår efter knäckning. Detta resultat är mest relevant för forskning och utveckling av fibernätverks material med större avstånd mellan fibrerna, såsom fibermaterial med låg densitet. Huvudslutsatserna från utvecklingen av testmetoder var att de befintliga grepparmarna inte kunde förhindra fiberglidning. Den alternativa grepparmen 22A med sin bågkonstruktion genererade högre greppkraft än tidigare grepparmar men saknar rätt beläggning i kontaktområdet för att förhindra glidning av fiber. PFM har en hög test framgångsgrad med över 80 % för erfarna användare och den är enkel att arbeta med. Testmaskinen Single Fiber Testing System visar flera systematiska fel som blir märkbar under dataanalys av tester med cykliska belastningar.

Additive Manufacturing (AM) is an innovative manufacturing process which offers near-net shape fabrication of complex components, directly from CAD models, without dies or substantial machining, resulting in a reduction in lead-time, waste, and cost. For example, the buy-to-fly ratio for a titanium component machined from forged billet is typically 10-20:1 compared to 5-7:1 when manufactured by AM. However, the production rates for most AM processes are relatively slow and AM is consequently largely of interest to the aerospace, automotive and biomedical industries. In addition, the solidification conditions in AM with the Ti alloy commonly lead to undesirable coarse columnar primary β grain structures in components. The present research is focused on developing a fundamental understanding of the influence of the processing conditions on microstructure and texture evolution and their resulting effect on the mechanical properties during additive manufacturing with a Ti6Al4V alloy, using three different techniques, namely; 1) Selective laser melting (SLM) process, 2) Electron beam selective melting (EBSM) process and, 3) Wire arc additive manufacturing (WAAM) process. The most important finding in this work was that all the AM processes produced columnar β-grain structures which grow by epitaxial re-growth up through each melted layer. By thermal modelling using TS4D (Thermal Simulation in 4 Dimensions), it has been shown that the melt pool size increased and the cooling rate decreased from SLM to EBSM and to the WAAM process. The prior β grain size also increased with melt pool size from a finer size in the SLM to a moderate size in EBSM and to huge grains in WAAM that can be seen by eye. However, despite the large difference in power density between the processes, they all had similar G/R (thermal gradient/growth rate) ratios, which were predicted to lie in the columnar growth region in the solidification diagram. The EBSM process showed a pronounced local heterogeneity in the microstructure in local transition areas, when there was a change in geometry; for e.g. change in wall thickness, thin to thick capping section, cross-over’s, V-transitions, etc. By reconstruction of the high temperature β microstructure, it has been shown that all the AM platforms showed primary columnar β grains with a <001>β.

<p>Co and Ni are two major elements in high temperature structural alloys that include superalloys for turbine engines and hard metals for cutting tools. The recent development of complex concentrated alloys (CCAs), loosely defined as alloys without a single principal element (e.g. CoNiFeMn), offers additional opportunities in designing new alloys through extensive composition and structure modifications. Within CCAs and Co- and Ni-based superalloys, solid-solution strengthening and stacking fault energy engineering are two of the most important strengthening mechanisms. While studied for decades, the potency and quantitative materials properties of these mechanisms remain elusive. </p> <p><br></p> <p>Solid-solution strengthening originates from stress field interactions between dislocations and solute of various species in the alloy. These stress fields can be engineered by composition modification in CCAs, and therefore a wide range of alloys with promising mechanical strength may be designed. This thesis initially reports on experimental and computational validation of newly developed theories for solid-solution strengthening in 3d transition metal (MnFeCoNi) alloys. The strengthening effects of Al, Ti, V, Cr, Cu and Mo as alloying elements are quantified by coupling the Labusch-type strengthening model and experimental measurements. With large atomic misfits with the base alloy, Al, Ti, Mo, and Cr present strong strengthening effects comparable to other Cantor alloys. </p> <p> </p> <p>Stacking fault energy engineering can enable novel deformation mechanisms and exceptional strength in face-centered cubic (FCC) materials such as austenitic TRIP/TWIP steels and CoNi-based superalloys exhibiting local phase transformation strengthening via Suzuki segregation. We employed first-principles calculations to investigate the Suzuki segregation and stacking fault energy of the FCC Co-Ni binary alloys at finite temperatures and concentrations. We quantitatively predicted the Co segregation in the innermost plane of the intrinsic stacking fault (ISF). We further quantified the decrease of stacking fault energy due to segregation.  </p> <p><br></p> <p>We further investigated the driving force of segregation and the origin of the segregation behaviors of 3d, 4d and 5d elements in the Co- and Ni-alloys. Using first-principles calculations, we calculated the ground-state solute-ISF interaction energies and revealed the trends across the periodic table. We discussed the relationships between the interaction energies and the local lattice distortions, charge density redistribution, density of states and local magnetization of the solutes. </p> <p><br></p> <p>Finally, this thesis reports on new methodologies to accelerate first-principles calculations utilizing active learning techniques, such as Bayesian optimization, to efficiently search for the ground-state energy line of the system with limited computational resources. Based on the expected improvement method, new acquisition strategies were developed and will be compared and presented. </p>