Dissertations / Theses on the topic 'Multiscale mechanical characterization'

Oggigiorno, la crescente consapevolezza sociale e politica per le questioni ambientali si sta orientando verso lo sviluppo di tecnologie a basso consumo ed emissioni. In questo contesto, tecnologie come le miscele bituminose a freddo possono rappresentare una valida alternativa ai tradizionali conglomerati bituminosi a caldo, per le pavimentazioni stradali. Inoltre, quando vengono utilizzati materiali provenienti dal riciclaggio di pavimentazioni stradali ammalorate, il consumo di aggregati vergini può essere considerevolmente ridotto. In passato, l'uso di miscele bituminose a freddo ha riscosso limitato successo a causa dei problemi legati al tempo necessario per il completo sviluppo di resistenza e la suscettibilità all’acqua nei primi mesi di vita. Il presente dottorato di ricerca è volto a valutare scientificamente i vantaggi/svantaggi dell’adozione di miscele bituminose a freddo. Oltre alle tradizionali indagini di laboratorio, è stata adottata una metodologia originale basata sulla caratterizzazione multiscala del materiale, sia dal punto di vista fisico che reologico. Infatti, la miscela bituminosa a freddo può considerarsi un materiale evolutivo poiché il suo stato fisico evolve nel tempo a causa della continua perdita di umidità. In questo contesto, la caratterizzazione delle miscele bituminose a freddo deve essere sviluppata su scale temporali differenti durante la vita in servizio del materiale, e a diversi livelli di indagine (scala dimensionale). I risultati raccolti hanno mostrato una correlazione ottimale tra i diversi livelli di indagine; a dimostrazione del fatto che il metodo di ricerca adottato può ritenersi scientificamente valido e inoltre, nessun elemento scoraggia l'uso delle miscele bituminose a freddo come strati di supporto per la sovrastruttura stradale. Ad ogni modo, i materiali impiegati devono essere adeguatamente progettati in termini di assortimento granulometrico, contenuto d’acqua e leganti (tipologia e dosaggio).
Nowadays, the growing social and political awareness about environmental issues is moving towards the development of low-energy and low-emission technologies. In this context, technologies as cold mixtures may represent a valid alternative to traditional hot mix asphalt for road pavements. Moreover, when materials obtained from the recycling of old pavements are adopted, the consumption of virgin aggregate can be significantly reduced. In the past, the use of cold mixture for structural layers has attracted relatively little attention largely because of problems related to the time required for full strength to be achieved after paving and its susceptibility to early life damage by rainfall. The PhD research aimed at scientifically evaluating advantages and disadvantages of cold mixtures. Besides the traditional laboratory investigations, an original research methodology based on the multiscale characterization of the material, from both physical and rheological point of view. In fact, cold mixture can be considered as an evolutive material because its physical state evolves over time according to moisture loss. In this context, the characterization of cold mixture should be developed at different time during its in-service life (time-scale) and at different level of investigation (size-scale). Optimum correlation was found between results collected from different levels of investigation (size and time-scales); hence demonstrating the scientific validity of the adopted research approach. Based on the overall findings, no elements discourage the use of cold mixtures as support layers for pavement structure. Therefore, materials should be properly designed in terms of aggregate blend, water content and binding agents (type and dosage).

The vocal folds are two membranous lips of soft tissue located within the human larynx. During phonation, they undergo self-sustained oscillations. Common voice disorders are believed to result from excessive mechanical stresses within the vocal folds' mucosal layer. The overarching goal of the present study was to better understand the relationship between mechanical loading and vocal fold tissue response. The bulk mechanical properties of the vocal folds were initially investigated using uniaxial traction testing and shear rheometry. These methods were used to quantify the material parameters of porcine vocal folds. A linear transversely-isotropic model was used to relate stresses and strains. The assumption of incompressibility was used to reduce the number of independent parameters.The effects of water loss induced by an osmotic pressure potential on vocal fold tissue's viscoelastic properties were investigated. Uniaxial traction testing was used to impose slow-rate, cyclical extensions of porcine vocal folds while a hypertonic solution was used to absorb interstitial fluid from the tissue. The elastic modulus and the loss factor were then determined for normal and dehydrated tissues. A non-linear eight-chain hyperelastic model was used to relate the stress and the stretch of the bi-phasic tissue. Significant changes in mass were observed as a result of the traction tests. The mechanics of the vocal folds were investigated under linear poroelasticity, where interstitial fluids are assumed to be freely moving within the extracellular matrix proteins. The one-dimensional consolidation problem was used to model the contact between tissue and a spherical indenter. Atomic force microscopy based indentation data, obtained from creep and dynamic oscillation testing, were used to calibrate the model. Nanoscale viscoelastic characteristics of porcine lamina propria were characterized from the force response to frequency-dependent displacement oscillations, with an amplitude of 30-50 nm. Nonlinear laser scanning microscopy was used to visualize the morphology of extracellular matrix proteins within human and animal vocal folds. A custom-built multimodal nonlinear laser scanning microscope was used to scan fibrous proteins in human and porcine vocal folds. Collagen and elastin were imaged using second-harmonic generation and two-photon fluorescence, respectively. An experimental protocol was introduced to characterize the geometrical properties of the collagen fibrils. Nonlinear laser scanning microscopy was then used to investigate the remodeling of scarred rat vocal folds. The volume fraction of collagen was found to be 12% greater in scarred vocal fold tissue 12 month after injury. Atomic force microscopy images suggest a rope-shaped structure of collagen fibrils in the vocal folds. A hyperelastic theory was developed for collagen-reinforced soft tissues. The relevant formulation was derived for finite element simulations. The model captured the role of helical hierarchies of the collagen fibrils in the nonlinear response of vocal folds under load.
Les cordes vocales sont des membranes de tissus mous situées à l'intérieur du larynx. Pendant la phonation, elles sont soumises à des oscillations auto-entretenues. Certains troubles de la voix répandus sont connus pour résulter de contraintes mécaniques excessives au sein de la muqueuse des cordes vocales. Les propriétés viscoélastiques des cordes vocales pathologiques diffèrent de celles dont les tissus sont sains. L'objectif global de cette étude est de mieux comprendre la relation entre le chargement mécanique et la réponse des tissus des cordes vocales. Les propriétés mécaniques de compression des cordes vocales ont tout d'abord été étudiées à l'aide d'essai de traction et d'un rhéomètre à cisaillement. Ces méthodes ont servi à quantifier les paramètres mécaniques de cordes vocales porcines. Un modèle linéaire, isotrope transverse a été utilisé pour la relation entre les contraintes et les déformations. La condition d'incompressibilité a permis de réduire le nombre de paramètres indépendants. Les effets de déshydratation, induite par le potentiel de pression osmotique, sur les propriétés des tissus des cordes vocales ont été étudiés. Des essais de traction uniaxiaux ont servi pour imposer des extensions cycliques à faible vitesse sur des cordes vocales porcines pendant qu'une solution hypertonique absorbait le fluide interstitiel des tissus. Le module élastique et le facteur de perte ont été calculés pour des tissus normaux et déshydratés. Un modèle d'hyperélasticité non linéaire à huit chaînes a servi pour décrire la relation entre les contraintes et les déformations du tissu biphasique. Des variations de masse significatives ont été observées à la suite des essais de traction. La mécanique des cordes vocales a été étudiée à l'aide de conditions de poroélasticité linéaire. Les fluides interstitiels sont supposés libres de mouvement au sein des matrices extra-cellulaires des protéines. Le problème de consolidation à une dimension a servi à la modélisation du contact entre les tissus mous et une indentation sphérique. Les données d'entrée du modèle étaient obtenues par la microscopie à force atomique basée sur des données d'indentation utilisant des signaux de rampe ou d'oscillations dynamiques. Des caractéristiques viscoélastiques furent mises en valeur à partir de la réponse des cordes vocales aux oscillations, dont le déplacement était contrôlé en fréquence, avec une amplitude de 30 à 50 nm. La microscopie optique non linéaire a permis la visualisation de la morphologie des matrices extra-cellulaires des protéines au sein de cordes vocales humaines et porcines. Un microscope non linéaire multimodale a été conçu pour scanner les protéines fibreuses de cordes vocales humaines et porcines. Le collagène et l'élastine ont été imagés respectivement à l'aide de la génération de second harmonique et de la fluorescence sous excitation à deux photons. L'introduction d'un protocole expérimental a servi à caractériser les propriétés géométriques des fibres de collagène. Cette méthode d'imagerie a ensuite été utilisée pour étudier le remodelage de cordes vocales de rats cicatrisées. Ceci a permis de montrer que la fraction volumique de collagène était 12% plus importante dans les tissus de cordes vocales cicatrisées 12 mois après la blessure.Les images du microscope à force atomique suggèrent que des fibres de collagène avec une structure de corde sont présentes dans les cordes vocales. Une théorie hyperélastique a été développée pour des tissus mous supportés par le collagène, ainsi que la formulation adaptée pour les calculs éléments finis. Le modèle capture le rôle de la structure hélicoïdale des fibres de collagène d'après la réponse non linéaire des cordes vocales soumises à un chargement.

The present dissertation is an in-depth investigation from the macro to atomic scale of industrial wear-resistant CVD hard coatings deposited on cemented carbides for cutting tool applications. Micro-compression tests at the micro-scale and contact damage induced by means of millimetric spherical indentation were deployed to study deformation mechanisms of two systems consisting of a defined cemented carbide substrate coated with two different films: Ti(C,N) and Zr(C,N). The latter system exhibited a superior tool life in comparison to the conventional Ti(C,N) one. Several characterization techniques were used: confocal microscopy, scanning electron microscopy, focused ion beam, electron back scattered diffraction, X-ray synchrotron and atom probe tomography. It was found that remnant structural integrity related to the absence of an extensive cracking network for Zr(C,N) - in the as deposited state - is one of the main reasons that could explain better performance in interrupted cutting. Adapted coefficient of thermal expansion toward the substrate, plastic deformation and better cohesive strength at the grain boundaries (which renders more toughness) are factors that contribute not only to this preserved structural integrity but also to the extended tool life during in-service interrupted cutting.
En esta tesis doctoral se presenta una investigación extensa y detallada, desde la escala macroscópica hasta la atómica, de recubrimientos industriales - duros y resistentes al desgaste - depositados por CVD sobre carburos cementados para su aplicación como herramientas de corte. El estudio se realizó en dos sistemas recubiertos empleando diferentes capas cerámicas - Ti(C,N) y Zr(C,N) - pero sin variar el carburo cementado empleado como sustrato. Los mecanismos de deformación de ambos sistemas se evaluaron mediante ensayos de micro-compresión de pilares, así como de indentación esférica (con bolas de radios milimétricos), estos últimos buscando inducir daño de forma controlada a nivel superficial y subsuperficial. El sistema recubierto con la capa de Zr(C,N) exhibió una vida útil superior al más convencional - Ti(C,N). El estudio incluyó la implementación de varias técnicas de caracterización: microscopía confocal, microscopía electrónica de barrido, haz de iones focalizados, difracción de electrones retrodispersados, sincrotrón de rayos X, y tomografía con sonda atómica. Se encontró que la elevada integridad estructural remanente relacionada con la ausencia de fisuración interconectada en el caso de Zr(C,N) – justo después de ser depositado – es alguna de las principales razones para explicar el mayor rendimiento de este sistema recubierto en operaciones de mecanizado que involucran corte interrumpido. La adecuación del coeficiente de expansión térmica, relativo al que exhibe el sustrato, la capacidad de absorber deformación plástica, y la relevante resistencia cohesiva en los bordes de granos (lo que proporciona una mayor tenacidad) son factores que contribuyen no sólo a preservar la integridad estructural, sinó también a prolongar la vida útil de la herramienta durante condiciones de servicio que conlleven corte interrumpido.
Die vorliegende Dissertation ist eine eingehende Untersuchung vom makrobis zu der atomaren Skala von industrieller verschleißfester CVD-Hartschichten auf Hartmetallschneidwerkzeugen abgeschieden. Mikrodruckversuche und Kontaktschädigung ausgelöst durch millimetergenaue Kugel Eindruck wurden eingesetzt, um Verformungsmechanismen von zwei Systemen, bestehend aus einem definierten Hartmetallsubstrat, das mit zwei verschiedenen Schichten beschichtet ist: Ti(C,N) und Zr(C,N). Letzteres System zeigt eine höhere Standzeit als das herkömmliche Ti(C,N). Es wurden eine Vielzahl von Charakterisierungstechniken eingesetzt: Konfokale Mikroskopie, Rasterelektronenmikroskopie, fokussierter Ionenstrahl, Elektronenrückstreubeugung, Synchrotron und Atomsonden- Tomographie. Es wurde festgestellt, dass die erhaltene strukturelle Integrität in Bezug auf das Fehlen eines ausgedehnten Rissnetzwerks für Zr(C,N) - im abgeschiedenen Zustand - einer der Hauptgründe ist, der die bessere Leistung beim unterbrochenen Schnitt Verfahren erklären könnte. Angepasste Wärmeausdehnungskoeffizienten entgegen das Substrat, plastische Verformung und bessere Korngrenzen-Kohäsion (was zu mehr Zähigkeit führt) sind Faktoren, die nicht nur zu dieser erhaltenen strukturellen Integrität beitragen, sondern auch zu einer verlängerten Standzeit beim Fräsen im Einsatz.

For many decades, fiber reinforced polymers (FRPs) have been extensively utilized in load-bearing structures. Their formability and superior in-plane mechanical properties have made them a viable replacement for conventional structural materials.  A major drawback to FRPs is their weak interlaminar properties (e.g., interlaminar fracture toughness). The need for lightweight multifunctional structures has become vital for many applications and hence alleviating the out-of-plane mechanical (i.e., quasi-static, vibration, and impact) and electrical properties of FRPs while retaining minimal weight is the subject of many ongoing studies. The primary objective of this dissertation is to investigate the fundamental processes for developing hybrid, multifunctional composites based on surface grown carbon nanotubes (CNTs) on carbon fibers\' yarns. This study embraces the development of a novel low temperature synthesis technique to grow CNTs on virtually any substrate. The developed method graphitic structures by design (GSD) offers the opportunity to place CNTs in advantageous areas of the composite (e.g., at the ply interface) where conventional fiber architectures are inadequate. The relatively low temperature of the GSD (i.e. 550 C) suppresses the undesired damage to the substrate fibers. GSD carries the advantage of growing uniform and almost aligned CNTs at pre-designated locations and thus eliminates the agglomeration and dispersion problems associated with incorporating CNTs in polymeric composites. The temperature regime utilized in GSD is less than those utilized by other synthesis techniques such as catalytic chemical vapor deposition (CCVD) where growing CNTs requires temperature not less than 700 C.
It is of great importance to comprehend the reasons for and against using the methods involving mixing of the CNTs directly with the polymer matrix, to either fabricate nanocomposites or three-phase FRPs. Hence, chapter 2 is devoted to the characterization of CNTs-epoxy nanocomposites at different thermo-mechanical environments via the nanoindentation technique. Improvements in hardness and stiffness of the CNTs-reinforced epoxy are reported. Long duration (45 mins) nanocreep tests were conducted to study the viscoelastic behavior of the CNT-nanocomposites. Finally, the energy absorption of these nanocomposites is measured via novel nanoimpact testing module.
Chapter 3 elucidates a study on the fabrication and characterization of a three phase CNT-epoxy system reinforced with woven carbon fibers. Tensile test, high velocity impact (~100 ms-1), and dynamic mechanical analysis (DMA) were employed to examine the response of the hybrid composite and compare it with the reference CFRP with no CNTs. Quasi-static shear punch tests (QSSPTs) were also performed to determine the toughening and damage mechanisms of both the CNTs-modified and the reference CFRP composites during transverse impact loading.
The synthesis of CNTs at 550 C via GSD is the focus of chapter 4. The GSD technique was adjusted to grow Palladium-catalyzed carbon filaments over carbon fibers. However, these filaments were revealed to be amorphous (turbostratic) carbon.  Plasma sputtering was utilized to sputter nickel nano-films on the surface of the substrate carbon fibers. These films were later fragmented into nano-sized nickel islands from which CNTs were grown utilizing the GSD technique.  The structure and morphology of the CNTs are evaluated and compared to CNTs grown via catalytic chemical vapor deposition (CCVD) over the same carbon fibers.
Chapter 5 embodies the mechanical characterization of composites based on carbon fibers with various surface treatments including, but not limited to, surface grown CNTs. Fibers with and without sizing were subjected to different treatments such as  heat treatment similar to those encountered during the GSD process, growing CNTs on fabrics via GSD and CCVD techniques, sputtering of the fibers with a thin thermal shield film of SiO2 prior to CNT growth, selective growth of CNTs following checkerboard patterns, etc.
The effects of the various surface treatments (at the ply interfaces) on the on-axis and off-axis tensile properties of the corresponding composites are discussed in this chapter. In addition, the DMA and impact resistance of the hybrid CNT-CFRP composites are measured and compared to the values obtained for the reference CFRP samples. While the GSD grown CNTs accounted for only 0.05 wt% of the composites, the results of this chapter contrasts the advantages of the GSD technique over other methods that incorporate CNTs into a CFRP (i.e. direct growth via CCVD and mixing of CNTs with the matrix).
Understanding the behavior of the thin CFRPs under impact loadings and the ability to model their response under ballistic impact is essential for designing CFRP structures.  A precise simulation of impact phenomenon should account for progressive damage and strain rate dependent behavior of the CFRPs. In chapter 6, a novel procedure to calibrate the state-of-the-art MAT162 material model of the LS-DYNA finite element simulation package is proposed. Quasi-static tensile, compression, through thickness tension, and in-plane Isopescu shear tests along with quasi-static shear punch tests (QSSPTs) employing flat cylindrical and spherical punches were performed on the composite samples to find 28 input parameters of MAT162. Finally, the capability of this material model to simulate a transverse ballistic impact of a spherical impactor with the thin 5-layers CFRP is demonstrated.
It is hypothesized that the high electrical conductivities of CNTs will span the multifunctionality of the hybrid composites by facilitating electromagnetic interference (EMI) shielding. Chapter 6 is devoted to characterizing the electrical properties of hybrid CNT-fiberglass FRPs modified via GSD method. Using a slightly modified version of the GSD, denser and longer CNTs were grown on fiberglass fabrics.  The EMI shielding performance of the composites based on these fabrics was shown to be superior to that for reference composites based on fiberglass and epoxy. To better apprehend the effect of the surface grown CNTs on the electrical properties of the resulting composites, the electrical resistivities of the hybrid and the reference composites were measured along different directions and some interesting results are highlighted herein.
The work outlined in this dissertation will enable significant advancement in protection methods against different hazards including impact, vibrations and EMI events.

Ph. D.

This thesis aims to investigate electrospun structures by means their production process and morpho-mechanical characterization. Considering the results obtained, the electrospun devices developed, will be useful for tendon and ligament tissue applications.

Nature develops several materials with remarkable functional properties composed of comparatively simple base substances. Biological materials are often composites, which optime the conformation to their function. On the other hand, synthetic materials are designed a priori, structuring them according to the performance to be achieved. 3D printing manufacturing is the most direct method for specific component production and earmarks the sample with material and geometry designed ad-hoc for a defined purpose, starting from a biomimetic approach to functional structures. The technique has the advantage of being quick, accurate, and with a limited waste of materials. The sample printing occurs through the deposition of material layer by layer. Furthermore, the material is often a composite, which matches the characteristics of components with different geometry and properties, achieving better mechanical and physical performances. This thesis analyses the mechanics of natural and custom-made composites: the spider body and the manufacturing of metallic and non-metallic graphene composites. The spider body is investigated in different sections of the exoskeleton and specifically the fangs. The study involves the mechanical characterization of the single components by the nanoindentation technique, with a special focus on the hardness and Young's modulus. The experimental results were mapped, purposing to present an accurate comparison of the mechanical properties of the spider body. The different stiffness of components is due to the tuning of the same basic material (the cuticle, i.e. mainly composed of chitin) for achieving different mechanical functions, which have improved the animal adaptation to specific evolutive requirements. The synthetic composites, suitable for 3D printing fabrication, are metallic and non-metallic matrices combined with carbon-based fillers. Non-metallic graphene composites are multiscale compounds. Specifically, the material is a blend of acrylonitrile-butadiene-styrene (ABS) matrix and different percentages of micro-carbon fibers (MCF). In the second step, nanoscale filler of carbon nanotubes (CNT) or graphene nanoplatelets (GNP) are added to the base mixture. The production process of composite materials followed a specific protocol for the optimal procedure and the machine parameters, as also foreseen in the literature. This method allowed the control over the percentages of the different materials to be adopted and ensured a homogeneous distribution of fillers in the plastic matrix. Multiscale compounds provide the basic materials for the extrusion of fused filaments, suitable for 3D printing of the samples. The composites were tested in the configuration of compression moulded sheets, as reference tests, and also in the corresponding 3D printed specimens. The addition of the micro-filler inside the ABS matrix caused a notable increment in stiffness and a slight increase in strength, with a significant reduction in deformation at the break. Concurrently, the addition of nanofillers was very effective in improving electrical conductivity compared to pure ABS and micro-composites, even at the lowest filler content. Composites with GNP as a nano-filler had a good impact on the stiffness of the materials, while the electrical conductivity of the composites is favoured by the presence of CNTs. Moreover, the extrusion of the filament and the print of fused filament fabrication led to the creation of voids within the structure, causing a significant loss of mechanical properties and a slight improvement in the electrical conductivity of the multiscale moulded composites. The final aim of this work is the identification of 3D-printed multiscale composites capable of the best matching of mechanical and electrical properties among the different compounds proposed. Since structures with metallic matrix and high mechanical performances are suitable for aerospace and automotive industry applications, metallic graphene composites are studied in the additive manufacturing sector. A comprehensive study of the mechanical and electrical properties of an innovative copper-graphene oxide composite (Cu-GO) was developed in collaboration with Fondazione E. Amaldi, in Rome. An extensive survey campaign on the working conditions was developed, leading to the definition of an optimal protocol of printing parameters for obtaining the samples with the highest density. The composite powders were prepared following two different routes to disperse the nanofiller into Cu matrix and, afterward, were processed by selective laser melting (SLM) technique. Analyses of the morphology, macroscopic and microscopic structure, and degree of oxidation of the printed samples were performed. Samples prepared followed the mechanical mixing procedure showed a better response to the 3D printing process in all tests. The mechanical characterization has instead provided a clear increase in the resistance of the material prepared with the ultrasonicated bath method, despite the greater porosity of specimens. The interesting comparison obtained between samples from different routes highlights the influence of powder preparation and working conditions on the printing results. We hope that the research could be useful to investigate in detail the potential applications suitable for composites in different technological fields and stimulate further comparative analysis.

L'agriculture 4.0 se développe actuellement rapidement en termes de recherche, de développement et d'applications commerciales. L'objectif de l'agriculture 4.0 est d'utiliser la technologie pour améliorer tous les domaines de l'agriculture. L'agriculture 4.0 est tellement vaste que si l'on veut y contribuer, il faut choisir un domaine spécifique. Le domaine choisi pour l'étude de ce doctorat est la production de fibres de lin. Les fibres de lin sont des fibres naturellement solides qui peuvent être extraites des tiges de lin. Les tiges de lin ont évolué pour avoir des fibres robustes d'un diamètre de l'ordre du micromètre qui courent le long de l'extérieur de la tige et sont maintenues en place dans le tissu externe de la tige. Une fois extraites et isolées, les fibres de lin ont de nombreuses applications, allant des textiles aux matériaux composites. Afin de faciliter l'extraction mécanique des fibres de lin de leurs tiges mères, les tiges subissent un processus connu sous le nom de « rouissage ». Le rouissage entraîne la décomposition du tissu externe (appelé lamelle moyenne) entre les fibres. Une forme courante de rouissage est connue sous le nom de « rouissage de rosée ». Dans le rouissage de la rosée, des processus naturels tels que les bactéries et les champignons produisent des enzymes qui décomposent la lamelle centrale et séparent progressivement les grappes de fibres et les fibres des grappes. La durée du rouissage dépend fortement des conditions météorologiques. Un rouissage insuffisant entraîne une extraction difficile des fibres dans l'usine, tandis qu'un rouissage excessif peut compromettre la qualité des fibres. On sait depuis longtemps qu'il existe un point de rouissage optimal - même les anciens le savaient. Certains agriculteurs artisans qualifiés sont capables de juger ce point par une combinaison de manipulation manuelle des tiges, d'observation des dommages causés aux tissus externes par cette manœuvre, et aussi d'observation de la couleur et de l'odeur des tiges au cours de ce test très habile, mais artisanal. Il est clair que l'artisan effectue des tests de laboratoire rudimentaires littéralement « sur le terrain ». Il semblerait donc logique d'essayer de quantifier ces tests et de voir si un outil fiable peut être mis au point pour aider l'artisan. Et c'est exactement ce que d'autres ont tenté de faire. L'introduction de la thèse donne des exemples de tentatives de fabrication d'outils de rouissage optimal dans les années 1980 et suivantes. Inspirés par ces premiers travaux, les travaux de cette thèse tentent une caractérisation mécanique multi-échelle complète des tiges et des fibres de lin pendant un cycle de rouissage (été 2022) et, de manière quelque peu ambitieuse, réalisée en temps réel - à notre connaissance pour la première fois. La caractérisation mécanique comprend des essais mécaniques macroscopiques (flexion, écrasement et torsion de la tige), ainsi que des essais mécaniques microscopiques inédits sur des fibres de lin individuelles à l'aide de nouvelles méthodes inspirées des MEMS. En outre, les propriétés mécaniques nanoscopiques de la paroi cellulaire primaire des fibres de lin en cours de rouissage ont été caractérisées à l'aide de l'AFM par nanoindentation. Au fur et à mesure que le travail expérimental, l'analyse via la modélisation analytique et l'interprétation descendent en échelle, de la macro au nano en passant par le micro, nous en apprenons un peu plus sur la manière dont le rouissage affecte les tiges, leurs propriétés et leurs fibres. En plus de l'apprentissage, un résultat très positif du doctorat est que l'on est capable de suggérer un mécanisme de dommage induit mécaniquement dans les tiges, qui pourrait être la base d'un outil. On peut cependant noter que la nature multiparamétrique incontrôlable du sujet, par exemple le temps, signifie que plusieurs études seraient nécessaires pour confirmer sans aucun doute les observations d'un seul cycle de rouissage
Agriculture 4.0, also known under several aliases such as ‘digital agriculture', ‘smart farming', and ‘e-farming' is currently developing rapidly in terms of research, development, and commercial applications. As with Agriculture 1.0, 2.0, and 3.0, the objective of Agriculture 4.0 is the use of technology to improve all areas of agriculture. In Agriculture 4.0 it is the application of microelectronics and microtechnologies. Unlike before, these technologies bring things such as the internet-of-things, big data, telecommunications, novel sensing, rapid feedback, data analysis, connectivity, artificial intelligence etc. In principle, all these areas should result in a massive modernization of farming in terms of organisation, yield, efficiency, and quality of produce. However, Agriculture 4.0 is so vast that if one is to contribute to it, even in a minor way, one has to choose a specific area to contribute. The area chosen for the study in this PhD was flax fibre production. Flax fibres are naturally strong fibres which can be extracted from flax stems. The flax stems have evolved to have robust micrometre-diameter fibres running the length of the outside of the stem, and held in place in the external tissue of the stem. Once extracted and isolated, flax fibres have numerous applications ranging from textiles to composite materials. In order to facilitate the mechanical extraction of flax fibres from their parent stems, the stems undergo a process known as ‘retting'. Retting leads to the breakdown of the external tissue between the fibres. A common form of retting is known as ‘dew retting'. In dew retting, natural processes such as bacteria and fungi result in enzymes which break down the middle lamella and gradually separate fibre bunches and fibres from bunches. The length of dew retting depends heavily on the weather. Too little retting results in difficult fibre extraction in the factory, too much retting can result in a compromise in fibre quality. It has long been known that there is an optimum retting point-even the ancients knew this. Certain skilled artisan farmers are able to judge this point via a combination of manual manipulation of the stems, observation of damage caused to the external tissue via this manoeuvre, and also observing the colour and the smell of the stems during this very skilled, but artisanal, testing. It is clear that the artisan is performing rudimentary laboratory tests quite literally ‘in-the-field'. It would seem logical therefore to try to quantify such tests and see if a reliable tool can be made to help the artisan. And indeed, this is exactly what others have attempted. The introduction of the PhD gives examples of attempts to make optimal-retting tools in the 1980s and after. Inspired by this early work, the work of this PhD attempts a full multiscale mechanical characterization of flax stems and fibres during a retting cycle (summer 2022) and, somewhat ambitiously, performed in real time-to our knowledge for the first time. The mechanical characterization involved macroscopic mechanical tests (bending, crushing, and twisting), as well as novel microscopic mechanical testing of single flax fibres using novel methods inspired by MEMS. In addition, the nanoscopic mechanical properties of the primary cell wall of retting flax fibres was characterised using nanoindentation AFM. As the experimental work, analysis via analytical modelling, and interpretation descends in scale from macro, through micro, to nano, we learn a little more of how the retting affects the stems, their properties, and their fibres. In addition to simply learning, a very positive outcome of the PhD is that one is able to suggest a mechanically-induced damage mechanism in stems which could be the basis for a tool. One can note however, that the uncontrollable multiparameter nature of the subject, e.g. the weather, means that several studies would be needed to confirm beyond doubt observations from a single retting cycle

Asphalt mastics act as a binding phase in asphalt mixtures and their rheological properties strongly affect the performance of asphalt mixtures with respect to virtually all damage modes. In order to measure mastics properties, relevant for field performance, testing should be performed at size-scales representative for the morphology and material inhomogeneity of asphalt mixtures. This thesis aims to contribute to solving these important issues by developing new experimental and modelling tools for the multi-scale characterization of asphalt mastics. An instrumented indentation test for viscoelastic characterization of asphalt mastics is proposed as a new alternative to existing techniques. A methodology for spherical indentation testing of bituminous materials is developed allowing measuring their viscoelastic properties at arbitrary non-decreasing loading. The potential of indentation tests for multi-scale measurements of viscoelastic properties of binder-aggregate composites is investigated for the special case of asphalt mortar, composed of mastic and aggregates smaller than 2.36 mm. The effect of the test parameters on the measured apparent shear relaxation modulus of asphalt mortar is evaluated. Experimental and modelling results indicate that the measurement scale in the indentation tests can be controlled efficiently by testing with different indenter-specimen contact areas. Accordingly, indentation tests may be used for reliable viscoelastic characterization of binder-aggregate composites on macro-scale as well as on the mastic phase level. It may thus potentially provide a relatively simple tool for measuring viscoelastic properties of mastics in situ in asphalt mixtures.  In order to establish a quantitative link between material design parameters of mastics and its rheology, a new finite element (FE) micromechanical modelling approach has been developed. It allows predicting the viscoelastic properties of bitumen-filler mastic from its volumetric, mechanical and geometrical design parameters. The influence of modelling parameters on the model’s accuracy is evaluated and optimal parameter combinations are identified. The model is validated with the measurements performed on several mastics and for a range of volumetric concentration of filler. It is shown that the proposed model can capture the measured viscoelastic behaviour of mastics for the examined range of loading, temperature and material parameters. Accordingly, it may be a useful tool for optimizing mastics material design for the target viscoelastic properties.
Asfaltmastix fungerar som bindemedel i asfaltsblandningar och blandningens uppträdande vad gäller i stort sett alla skadetyper är starkt beroende av asfaltmastixens reologiska egenskaper. Att förstå de mekanismer och parametrar som beskriver asfaltmastixens reologi är därför nödvändigt för att försäkra sig om ett tillräckligt bra beteende hos asfaltsblandningar. Dessutom, för att kunna mäta mastix egenskaper, relevanta för materialets uppträdande i fält, bör provning genomföras för längdskalor som är relevanta för blandningens morfologi. Inhomogeniteter hos materialet måste också beaktas. Denna avhandling strävar mot att lösa dessa viktiga problem genom att utveckla experimentella verktyg och modelleringsverktyg för flerskalekarakterisering av mastix.Instrumenterad intryckningsmetodik, för viskoelastisk karakterisering av mastix, beskrivs i avhandlingen som ett alternativ till andra provningsmetoder. En ny metod, som utgår ifrån sfärisk intryckningsprovning av asfaltmastix, har tagits fram med avsikten att mäta viskoelastiska storheter vid godtycklig men ökande last. I avhandlingen undersöks nyttan med att använda intryckningsprov för flerskalekarakterisering av bindemedel/partikel kompositer, speciellt för fallet asfaltsbruk. Provparametrarnas inverkan på den uppmätta relaxationsmodulen utvärderas. De framtagna resultaten visar att mätskalan vid intryckningsproven kan kontrolleras effektivt genom att styra kontaktytans storlek vid experimenten. Sammantaget visas i avhandlingen att intryckningsprov är ett trovärdigt alternativ för viskoelastisk karakterisering av de aktuella kompositmaterialen, på både makronivå och komponentnivå. Metoden har alltså potential att vara ett relativt enkelt alternativ för att på plats mäta materialegenskaper hos asfaltsmaterial/asfaltsblandningar.För att fastställa en kvantitativ länk mellan materialets modellparametrar och dess reologi så har ett nytt mikromekaniskt angreppssätt, baserat på finit elementmetodik, utvecklats. Avsikten är då att beräkna de viskoelastiska egenskaperna hos asfaltmastix utgående från de av problemet givna parametrarna. Modellparametrarnas inverkan på noggrannheten utvärderas och optimala parameterkombinationer identifieras. Modellen valideras med hjälp av experiment på olika asfaltsmaterial och den visar sig kunna fånga det uppmätta viskoelastiska beteendet för det aktuella intervallet av olika undersökta parametrar. Följaktligen kan det vara ett användbart verktyg för att optimera framtagningen av asfaltsmaterial utgående från de riktmärken för det viskoelastiska beteendet som sätts upp.

QC 20200506

L’objectif de cette thèse est d’établir le scénario multi-échelles de déformation du PEEK lorsqu’il est sollicité en traction uniaxiale. Préalablement à la mise en oeuvre d’échantillons de deux grades commerciaux de PEEK, les propriétés thermiques et mécaniques de ces matériaux ont été caractérisées. La température d’oubli thermodynamique ainsi que la sensibilité aux vitesses de refroidissement ont été établies. Des éprouvettes de traction ont été obtenues à partir de plaques thermocompressées, procédé choisi pour obtenir des morphologies les plus isotropes possibles. Les propriétés mécaniques en traction ont ensuite été caractérisées au-dessus et au-dessous de la transition vitreuse de la phase amorphe (Tg). Grâce à un dispositif expérimental fabriqué sur mesure, des essais de traction à deux températures distinctes au-dessous et au-dessus de Tg ont été suivis par diffusion des rayons X aux petits (SAXS) et grands angles (WAXS) pour caractériser les déformations à l’échelle des empilements lamellaires et à l’échelle de la maille cristalline. Simultanément, le champ de déformation a été mesurée par corrélation d’images (DIC) afin de comparer la déformation macroscopique et microscopique. Pour les deux températures, les lamelles tendent à s’orienter perpendiculairement à la direction de traction (TD). Ce mécanisme d’orientation local (que nous appelons « modèle de réseau de chaînes ») est induit par la transmission des contraintes par les chaînes amorphes reliant les lamelles cristallines adjacentes. Au-dessus de Tg, l’allongement local est plus faible que l’allongement macroscopique dans les lamelles perpendiculaire à TD, ce qui implique que les lamelles inclinées doivent être cisaillées. L’évolution de la distribution d’orientation des lamelles appuie ce résultat. Une morphologie fortement orientée est finalement obtenue quelle que soit la température. Cependant, le profil d’endommagement est différent. En-dessous de Tg, le profil de diffusion centrale indique l’existence de petites entités (lamelles ou crystallites) orientées aléatoirement. A hautes température, le matériau est fibrillaire et présente des cavités
The aim of this PhD work is accessing the microscopic deformation mechanisms of bulk poly-ether-ether-ketone (PEEK) under tensile stretching. Beforehand, the thermal and mechanical properties of two commercial grades of PEEK were characterized. Tensile specimens were then compression-molded to obtain morphologies as isotropic as possible and characterized below and above the glass transition temperature. Deformations at the scales of lamellar stacks and of the crystalline unit cell have been characterized by small and wide-angle X-ray scattering (SAXS and WAXS) performed in-situ during tensile tests. Simultaneously, the strain field within the samples was followed by digital image correlation (DIC) in order to compare microscopic and macroscopic strains. At both temperatures, lamellae tend to orient perpendicular to the tensile direction (TD). This orientation mechanism (which we denote as ‘Chain Network model’) is driven by the amorphous chains which transmit the stress between adjacent lamellae. The tensile strain in lamellar stacks perpendicular to TD is lower than the macroscopic tensile strain, which must be compensated by increased shear in inclined stacks. Some differences of behavior have been observed depending on the test temperature, especially at high deformation. A highly oriented morphology is ultimately obtained in all cases. However, the central scattering profiles changes with testing temperatures. Below Tg, the presence of small entities randomly oriented is indicated. Above Tg, the material is fibrillar and contains cavities

Ces dernières années, les fibres naturelles de chanvre affichent un grand potentiel d’application dans les secteurs de l’aérospatiale et de l’automobile en raison de leur recyclabilité, renouvelabilité et biodégradabilité. Associés à une matrice polymère, les tissés à base de fibres végétales permettent l’élaboration des pièces structurelles bio-sourcées de hautes performances mécaniques et thermiques avec une bonne aptitude à la mise en forme. Une maîtrise de la faisabilité de la chaîne d’éco-conception, caractérisation et fabrication est nécessaire pour une meilleure optimisation des ressources. À l’échelle mésoscopique les propriétés des tissus de fibres de chanvre secs (tissage simple et tissage en sergé) sont étudiées. Des stratifiés agro-composites fibres de chanvre/matrice polypropylène sont élaborés par thermocompression. Les propriétés mécaniques des stratifiés à différentes températures en traction uni-axiale, cisaillement et traction bi-axiale ont été analysées. Enfin des mises en œuvre de pièces automobiles sont proposées par rétro-conception basée sur la numérisation 3D de la coque du rétroviseur de voiture, la reconstruction de la géométrie; la conception des moules, la mise en forme de la pièce par thermocompression et l’analyse des propriétés mécaniques et de la faisabilité du process. Le comportement des pièces de structure sandwich nids d'abeilles en fibres de chanvre élaborées par thermocompression et par impression 3D est étudié en compression et flexion quatre points
In recent years, natural hemp fibres have great potential application in the aerospace and automotive due to their recyclability, renewability and biodegradability. Associated with a polymer matrix, hemp woven fabrics allow the eco-friendly manufacturing of bio-sourced composite parts with high mechanical and thermal properties. A multiscale analysis is essential for the better understanding of the mechanical and thermoforming behaviour of hemp fabrics based composite. In this study, the tensile behaviour of hemp fibre yarns were studied (microscopic scale) and the tensile, shear and biaxial tests were performed to characterize the behaviour of plain and twill dry hemp fibre woven fabrics (mesoscopic scale). Hemp fibre woven fabrics/polypropylene composite laminates were manufactured by thermo-compression process and the mechanical properties of the laminates specimens were analysed at different temperature and for different fibre volume fraction by uniaxial tensile, shear and biaxial test (macroscopic scale). Complex hemp woven fabric composite structures (truncated cone and automotive rear side mirror) were also thermoformed in order to analyse the feasibility of the process. Hemp fibre based honeycomb sandwich structures were manufactured by thermo-compression and 3D printing process. The mechanical behaviour of specimens in compression and four-point bending were analysed experimentally and numerically using Finite Element software

It is perhaps paradoxical that many material properties arise from the absence of material rather than the presence of it. For example, the strength, stiffness, and toughness of a concrete are related to its pore structure. Furthermore, the volume, size distribution, and interconnectivity of porosity is important for understanding permeability, diffusivity, and capillary action occurring in concrete, which are necessary for predicting service lives in aggressive environments. This research advances the state-of-the-art of multiscale characterization of cement-based materials, and uses this characterization information to model the material behavior under competing durability concerns. In the first part of this research, a novel method is proposed to characterize the entrained air void system. In the second and third parts of this research, microstructural characterization is used in tandem with mechanical models to investigate the behavior of cementitious materials when exposed to rapid rates of loading and to cyclic freezing and thawing. First, a novel analytical technique is presented which reconstructs the 3D entrained air void distribution in hardened concrete using 2D image analysis. This method proposes a new spacing factor, which is believed to be more sensitive to microstructural changes than the current spacing factor commonly utilized in practiced, and specified in ASTM C457, as a measure of concrete's ability to resist to damage under cyclic freeze/thaw loading. This has the potential to improve economy by improving the quality of petrographic assessment and reducing the need for more expensive and time-consuming freeze/thaw tests, while also promoting the durability of concrete. Second, quantitative measurements of the sizes, shapes, and spatial arrangements of flaws which are through to drive failure at strain rates above 100/s were obtained in order to model mortar subjected to high strain-rate loading (i.e., extremes in load rate). A micromechanics model was used to study the ways in which flaw geometry and flaw interaction govern damage. A key finding suggests that dynamic strength may be multimodal, with larger flaws shifting the dynamic strength upwards into the highest strength failure mode. Third, a robust theoretical approach, based upon poroelasticity, is presented to further validate the utility of the novel spacing factor proposed this research. The model is truly multiscale, using in its formulation pore size data ranging from the nanoscale to the micro-scale, entrained air data from the micro-scale to the millimeter scale, and infers a representative volume element on the centimeter scale. The results provide an underlying physical basis for the performance of the novel spacing factor. Furthermore, the framework could be used as a forensic tool, or as a tool to optimize the entrained air void system against freeze/thaw damage.

The physiology of walled cells is dramatically different from that of human cells, but the biomechanics of walled cells are far less studied. Most bacterial, fungal, and plant cells have a strong cell wall (CW), which allows them to withstand large hydrostatic pressures in the cytoplasm, called turgor. Turgor pressure conflates the mechanics of subcellular components and complicates the characterization of the cell. In this dissertation, new models are introduced and explored for single cells to investigate the multiscale mechanics of plant and bacterial cells using micro- and nano-indentation experiments.

A multi-scale biomechanical assay is used to study the mechanical properties of plant cells. The plant CW is typically around 5% of the width of the entire cell, and is thought to carry most of the mechanical load. Large-scale indentations using a micro-indentation system probe the behavior of the overall cell structure, and atomic-force microscopy (AFM) nano-scale indentations are used to isolate the CW response. To determine the effect of external osmotic pressure, indentations are performed on cells in different osmotic conditions: hypotonic, isotonic, and hypertonic. The cell is idealized as two springs acting in series, one to represent the CW and one to represent the cytoplasm. The model uses the experimentally determined initial stiffnesses as input to the model to determine the relative stiffness contributions of the CW and the cytoplasm.

The first type of walled cells investigated is the xylem vessel element of Arabidopsis thaliana. The xylem is responsible for transporting water through the stem of any vascular plant (more commonly known as a land plant), and hence it must maintain structural integrity against high internal pressures while transporting water from the roots to the leaves. For extra structural support, xylem vessel elements develop secondary cell walls (SCWs), which are known to be a key component for mediating mechanical strength and stiffness in vascular plants. The structure and biomechanics of cultured plant cells are investigated during the cellular developmental stages associated with SCW formation using the multi-scale biomechanical assay described above. To determine the effect of morphological changes during differentiation, micro- and nano-indentations are performed on cells in different observed stages of the differentiation process.Prior to triggering differentiation, cells in hypotonic pressure conditions are significantly stiffer than cells in isotonic or hypertonic conditions, highlighting the dominant role of turgor pressure. Plasmolyzed cells with a SCW reach similar levels of stiffness as cells with maximum turgor pressure. Analysis using the two-spring model shows that the stiffness of the primary CW in all of these conditions is lower than the stiffness of the fully-formed SCW. These results provide the first experimental characterization of the mechanics of SCW formation at the single-cell level in plant cells.

Next, the mechanical response of individual Nicotiana tabacum cells from a suspension culture is studied using the same multi-scale biomechanical assay. The role played by the microtubules (MTs) and actin filaments (AFs) is determined through the use of drug treatments which selectively remove MTs and AFs. A generative statistical model is added to the two-spring model to quantify the stiffnesses of the CW, cytoplasm, turgor pressure, MTs, and AFs. Analysis of the initial stiffness and energy dissipation calculated from micro-indentation experiments indicates that the MTs and AFs contribute significantly to the mechanical response of a cell under compression. Micro- and nano-indentation tests confirm that turgor pressure is the most significant contributor to the stiffness response of turgid cells in compression. Finally, the results reveal that turgor pressure exerts stress on the CW, which leads to a measurable stiffening of the CW.

The studies described above focused on developing a discrete model to describe the mechanics of a cell in indentation experiments. However, the most common type of model used to evaluate the mechanics of a cell are continuum models. Continuum models are also necessary to decouple the material properties of subcellular components from their structure. In the final section, AFM indentations are simulated on a gram-negative bacterium, Escherichia coli, and a sensitivity study and inverse analysis are performed to solve for the CW elastic modulus and turgor pressure simultaneously. Sensitivity study results reveal that uncertainty in turgor pressure and CW elasticity indeed contribute the most to variability in force spectra from AFM measurements. The parameter space of possible values for CW elastic modulus and turgor pressure is discretized using triangular elements. "Simulated experiments" are tested throughout the parameter space, and correlations between the CW elastic modulus and turgor pressure, which depend on the type of objective function, are investigated. Two unique objective functions are tested in the inverse analysis, and a third objective function, which is a weighted sum of the first two, is found to reduce errors in estimated CW elastic modulus and turgor pressure by 20% and 11%, respectively. The use of this type of inverse analysis has the potential to elucidate the material properties of CWs using a single indentation measurement and reliably decouple these properties from the high turgor pressures inside walled cells.

abstract: Damage detection in heterogeneous material systems is a complex problem and requires an in-depth understanding of the material characteristics and response under varying load and environmental conditions. A significant amount of research has been conducted in this field to enhance the fidelity of damage assessment methodologies, using a wide range of sensors and detection techniques, for both metallic materials and composites. However, detecting damage at the microscale is not possible with commercially available sensors. A probable way to approach this problem is through accurate and efficient multiscale modeling techniques, which are capable of tracking damage initiation at the microscale and propagation across the length scales. The output from these models will provide an improved understanding of damage initiation; the knowledge can be used in conjunction with information from physical sensors to improve the size of detectable damage. In this research, effort has been dedicated to develop multiscale modeling approaches and associated damage criteria for the estimation of damage evolution across the relevant length scales. Important issues such as length and time scales, anisotropy and variability in material properties at the microscale, and response under mechanical and thermal loading are addressed. Two different material systems have been studied: metallic material and a novel stress-sensitive epoxy polymer. For metallic material (Al 2024-T351), the methodology initiates at the microscale where extensive material characterization is conducted to capture the microstructural variability. A statistical volume element (SVE) model is constructed to represent the material properties. Geometric and crystallographic features including grain orientation, misorientation, size, shape, principal axis direction and aspect ratio are captured. This SVE model provides a computationally efficient alternative to traditional techniques using representative volume element (RVE) models while maintaining statistical accuracy. A physics based multiscale damage criterion is developed to simulate the fatigue crack initiation. The crack growth rate and probable directions are estimated simultaneously. Mechanically sensitive materials that exhibit specific chemical reactions upon external loading are currently being investigated for self-sensing applications. The "smart" polymer modeled in this research consists of epoxy resin, hardener, and a stress-sensitive material called mechanophore The mechanophore activation is based on covalent bond-breaking induced by external stimuli; this feature can be used for material-level damage detections. In this work Tris-(Cinnamoyl oxymethyl)-Ethane (TCE) is used as the cyclobutane-based mechanophore (stress-sensitive) material in the polymer matrix. The TCE embedded polymers have shown promising results in early damage detection through mechanically induced fluorescence. A spring-bead based network model, which bridges nanoscale information to higher length scales, has been developed to model this material system. The material is partitioned into discrete mass beads which are linked using linear springs at the microscale. A series of MD simulations were performed to define the spring stiffness in the statistical network model. By integrating multiple spring-bead models a network model has been developed to represent the material properties at the mesoscale. The model captures the statistical distribution of crosslinking degree of the polymer to represent the heterogeneous material properties at the microscale. The developed multiscale methodology is computationally efficient and provides a possible means to bridge multiple length scales (from 10 nm in MD simulation to 10 mm in FE model) without significant loss of accuracy. Parametric studies have been conducted to investigate the influence of the crosslinking degree on the material behavior. The developed methodology has been used to evaluate damage evolution in the self-sensing polymer.
Dissertation/Thesis
Doctoral Dissertation Mechanical Engineering 2014

abstract: There are many applications for polymer matrix composite materials in a variety of different industries, but designing and modeling with these materials remains a challenge due to the intricate architecture and damage modes. Multiscale modeling techniques of composite structures subjected to complex loadings are needed in order to address the scale-dependent behavior and failure. The rate dependency and nonlinearity of polymer matrix composite materials further complicates the modeling. Additionally, variability in the material constituents plays an important role in the material behavior and damage. The systematic consideration of uncertainties is as important as having the appropriate structural model, especially during model validation where the total error between physical observation and model prediction must be characterized. It is necessary to quantify the effects of uncertainties at every length scale in order to fully understand their impact on the structural response. Material variability may include variations in fiber volume fraction, fiber dimensions, fiber waviness, pure resin pockets, and void distributions. Therefore, a stochastic modeling framework with scale dependent constitutive laws and an appropriate failure theory is required to simulate the behavior and failure of polymer matrix composite structures subjected to complex loadings. Additionally, the variations in environmental conditions for aerospace applications and the effect of these conditions on the polymer matrix composite material need to be considered. The research presented in this dissertation provides the framework for stochastic multiscale modeling of composites and the characterization data needed to determine the effect of different environmental conditions on the material properties. The developed models extend sectional micromechanics techniques by incorporating 3D progressive damage theories and multiscale failure criteria. The mechanical testing of composites under various environmental conditions demonstrates the degrading effect these conditions have on the elastic and failure properties of the material. The methodologies presented in this research represent substantial progress toward understanding the failure and effect of variability for complex polymer matrix composites.
Dissertation/Thesis
Doctoral Dissertation Mechanical Engineering 2016

abstract: A hybrid molecular dynamics (MD) simulation framework is developed to emulate mechanochemical reaction of mechanophores in epoxy-based nanocomposites. Two different force fields, a classical force field and a bond order based force field are hybridized to mimic the experimental processes from specimen preparation to mechanical loading test. Ultra-violet photodimerization for mechanophore synthesis and epoxy curing for thermoset polymer generation are successfully simulated by developing a numerical covalent bond generation method using the classical force field within the framework. Mechanical loading tests to activate mechanophores are also virtually conducted by deforming the volume of a simulation unit cell. The unit cell deformation leads to covalent bond elongation and subsequent bond breakage, which is captured using the bond order based force field. The outcome of the virtual loading test is used for local work analysis, which enables a quantitative study of mechanophore activation. Through the local work analysis, the onset and evolution of mechanophore activation indicating damage initiation and propagation are estimated; ultimately, the mechanophore sensitivity to external stress is evaluated. The virtual loading tests also provide accurate estimations of mechanical properties such as elastic, shear, bulk modulus, yield strain/strength, and Poisson’s ratio of the system. Experimental studies are performed in conjunction with the simulation work to validate the hybrid MD simulation framework. Less than 2% error in estimations of glass transition temperature (Tg) is observed with experimentally measured Tgs by use of differential scanning calorimetry. Virtual loading tests successfully reproduce the stress-strain curve capturing the effect of mechanophore inclusion on mechanical properties of epoxy polymer; comparable changes in Young’s modulus and yield strength are observed in experiments and simulations. Early damage signal detection, which is identified in experiments by observing increased intensity before the yield strain, is captured in simulations by showing that the critical strain representing the onset of the mechanophore activation occurs before the estimated yield strain. It is anticipated that the experimentally validated hybrid MD framework presented in this dissertation will provide a low-cost alternative to additional experiments that are required for optimizing material design parameters to improve damage sensing capability and mechanical properties. In addition to the study of mechanochemical reaction analysis, an atomistic model of interphase in carbon fiber reinforced composites is developed. Physical entanglement between semi-crystalline carbon fiber surface and polymer matrix is captured by introducing voids in multiple graphene layers, which allow polymer matrix to intertwine with graphene layers. The hybrid MD framework is used with some modifications to estimate interphase properties that include the effect of the physical entanglement. The results are compared with existing carbon fiber surface models that assume that carbon fiber has a crystalline structure and hence are unable to capture the physical entanglement. Results indicate that the current model shows larger stress gradients across the material interphase. These large stress gradients increase the viscoplasticity and damage effects at the interphase. The results are important for improved prediction of the nonlinear response and damage evolution in composite materials.
Dissertation/Thesis
Doctoral Dissertation Mechanical Engineering 2017