Este trabalho trata das falhas que podem ocorrer em uniões e juntas de materiais compósitos unidas mecanicamente por parafusos. O compósito de fibra de carbono (tecido) embutido em resina epoxy foi estudado neste trabalho devido ao amplo uso em estruturas de vários segmentos da indústria. O trabalho apresenta vários critérios de falha, demonstrando as vantagens e desvantagens de cada um para materiais compósitos. A fabricação dos corpos de provas e os ensaios necessários para obtenção de parâmetros e validação de estruturas são descritos. A resistência da junta mostrou-se muito menor do que a da estrutura de compósito, demonstrando a importância de estudos assim. Criou-se um modelo numérico utilizando critérios de falhas como o critério de Hashin e o de máxima tensão. Os resultados da simulação de elementos finitos tiveram uma boa relação com os ensaios experimentais e o modelo foi então validado e considerado representativo. / This work shows failures that can occur in composite mechanically fastened joints. The composite carbon fiber embedded in epoxy resin, used in this study, was chosen due to it\'s wide use in structures of any segment of the industry. Many failure criteria, showing the advantages and disadvantages for each, regarding composite structures are presented in this work. Test specimens\' manufacturing is described along with required tests for parameter definition and structures validation. The countersunk fastened joint strength is much lower than the composite structure itself, demonstrating the necessity of studies like this. A numerical model using criteria like Hashin and maximum stress was created. The finite elements\' simulation results had a close response to the experimental results and the model was validated and considered representative.

Si l'emploi des matériaux composites dans l'aéronautique est déjà effectif sur des éléments de structures principales et de grande taille, leur généralisation aux structures secondaires bute sur leur positionnement en termes de coûts et performances face aux métaux. Il s'agit dans ce travail de contribuer à la mise en place d'une filière française de pièces composites hautes performances à bas coûts en s'appuyant sur un procédé de moulage en grande série, à savoir le thermoformage à haute pression. Ainsi, il a été question dans un premier temps d'optimiser ce procédé vis-à-vis des principales matières rencontrées dans les structures aéronautiques. Ensuite, les stratifiés moulés ont été caractérisés et les effets des conditions environnementales sévères (humidité, température, impact) sur leur comportement mécanique étudiés. Par ailleurs, réduire les coûts des pièces signifie également réduire les coefficients de sécurité qui restent très élevés pour les pièces composites. Cela passe par une meilleure prédictibilité de la rupture des matériaux et du comportement mécanique au-delà du linéaire. Sur le carbone/PEEK satin de 5 pris comme matériau d'illustration, les phénomènes non linéaires (viscoplasticité) ainsi que les mécanismes d'endommagement et de rupture ont été étudiés. Un accent particulier a été mis sur le délaminage et un critère permettant de prédire son amorçage a été proposé. La possibilité de faire des modèles éléments finis des pièces directement à l'échelle mésoscopique (du pli) a été également explorée et laisse entrevoir des pistes prometteuses pour des dimensionnements plus sûrs et donc moins conservatifs. / If the use of composite materials is already effective on elements of main structures and large size parts, their generalization to secondary parts is not effective due to their cost and their performances compared to metals. The framework of this thesis is to contribute to the establishment of a French chain of high performance composite parts at low cost. Thus, it was initially question of optimizing the process vis-à-vis the main composite materials used in the aerospace structures. Then, the molded laminates were characterized and the effects of severe conditions (humidity, temperature, impact) on their mechanical behavior were studied. Also, reduce the cost of parts also means reducing the safety factors which remain very high. This requires a better prediction of the failure and the mechanical behavior beyond the linear. Taking the five harness satin weave carbon/PEEK material as example, non-linear phenomena (viscoplasticity), damage mechanisms and failure criteria were studied, with particular emphasis on the delamination. The possibility to perform finite element analysis of the parts directly at the mesoscopic scale (ply-scale) was also explored and suggests promising expectations for a less conservative sizing of composite structures.

[pt] Eixos de virabrequim estão sujeitos a carregamentos multiaxiais quando em serviço. Por se tratar de um estado complexo de tensões, os modelos aplicados na determinação da vida em fadiga de tais componentes devem permitir, também, uma abordagem multiaxial, mais avançados do que aqueles adotados para carregamentos uniaxiais. O objetivo deste trabalho foi prever a falha em serviço de eixos virabrequins de unidades geradoras Diesel ou gás de plantas termoelétricas. Falhas recentemente ocorridas em eixos virabrequins no parque termoelétrico nacional justificam o presente estudo, para garantir eficiência e segurança nas operações industriais. Com base no método de elementos finitos, foram calculadas as tensões atuantes em um eixo virabrequim de unidade geradora a gás, com 10 mancais, duas bielas por mancal e fabricado com aço estrutural DIN 34CrNiMo6. Em sequência, adotando-se estas tensões atuantes calculadas, foram aplicados diversos critérios de fadiga para prever sua falha. Para tal, adotaram-se os modelos de Papadopoulos, Findley, Matake, McDiarmid, Carpinteri e Spagnoli, Liu e Mahadevan, Mises, Sines e Crossland, todos de fadiga de alto ciclo, baseados no plano crítico ou na tensão de von Mises. Propriedades de resistência à fadiga do material foram retiradas da literatura. Os resultados obtidos indicaram que o componente é seguro quando avaliado usando tais critérios. / [en] Crankshaft axles are subject to multiaxial loading when in service. Because the resulting state of stresses is complex, models applied to determining the fatigue life of such components employ a multiaxial approach as well, more advanced than those adopted for uniaxial loads. The objective of this work is to predict the failure in service of crankshafts of diesel or gas generating units of thermoelectric plants. Crankshafts reported recent failures in the national thermoelectric power plant justifies the present study, to guarantee efficiency and safety in such industrial operations. Based on the finite element method, the resulting stresses on a DIN 34CrNiMo6 structural steel gas generating unit crankshaft, with 10 bearings, two connecting rods per bearing were calculated. Using these finite element calculated stresses, several fatigue criteria were applied to predict this cranckshaft structural failure. Models due to Papadopoulos, Findley, Matake, McDiarmid, Carpinteri and Spagnoli, Liu and Mahadevan, Mises, Sines and Crossland, all of high cycle fatigue based on the critical plane or von Mises strain, were adopted. Material fatigue properties used in the analyses were compiled from specidized literature. Obtained results indicated that the component considered is safe regarding fatigue loadings, as evaluated using such criteria.

V současnosti jedna z metod spojování zejména tlustých a vysoce zatížených kompozitních komponent je šroubový spoj, který je možné rozebrat pro případ opravy na rozdíl od lepeného spoje. Kompozitní konstrukce se tradičně dimenzují tak, aby během provozu nedošlo k porušení první vrstvy laminátu, nicméně důležité je taky poznat chování laminátu po porušení první vrstvy. Pro strukturální analýzu nejenom spojů, ale také dalších komponent se používá metoda konečných prvků a protože moderní nelineání řešiče jsou schopné modelovat chování laminátu po porušení první vrstvy, tato schopnost jednoho z nich byla využita v této práci při zkoumaní chování sklolaminátu spojeného s hliníkovou částí šrouby. Konečno-prvkové modely dvou spojů kovové a kompozitní části konstrukce schopné popsat progresivní porušování laminátu byly postaveny s využitím tří různých poruchových kritérií – kritéria maximálního napětí, kritéria Hill a kritéria Tsai-Wu. Problém byl řešen s využitím řešiče Nastran. Křivky síla-posuv, tuhost-posuv a hodnoty zatížení při hraničním posuvu byly porov-nány s výsledky experimentů. Jelikož faktor zbytkové tuhosti ovlivňuje výsledky ana-lýzy progresivního porušování, byly provedeny citlivostní studie zkoumajíci vliv faktoru na přesnost a stabilitu výpočtu. Shoda výpočtu s experimentem v případe prvního šroubového spoje je méně uspokojivá, nicméně shoda v případě druhého spoje, který má zesilující tenkou ocelovou destičku na spodní straně, je podstatně lepší. Vý-borná shoda je zejména při použití interaktivních kritérií Hill a Tsai-Wu.

Birch plywood has favorable mechanical properties that could be used in new types of connections for timber structures, and thus enable a substitution of the current system with steel plates. Such new connections could result in significant advantages in terms of environmental impact and economy as well as ease of prefabrication and mountability. However, there is a lack of data concerning some of the mechanical properties of birch plywood that would be necessary in order to perform a safe design. In particular, there is a lack of reliable data and understanding of the mechanical properties of birch plywood in directions other than along and perpendicular to the face grain. The aim of this thesis is to gain new knowledge about this anisotropy and to study the variation of the in-plane mechanical properties of birch plywood at different loading angles to the face grain, including effects of size and moisture changes. The goal is that this knowledge will serve as input for the design of birch plywood connections under various loading conditions in timber structures. Specifically, birch plywood specimens were laboratory tested in in-plane tension, compression, shear and bending. The results show that birch plywood possesses the highest tensile, compressive and bending strength and elastic modulus parallel to the face grain and the lowest ones at 45° to the face grain. The opposite findings were noticed for the shear strength and the shear modulus. Moreover, a size effect on the in-plane bending strength property was observed at 0° (parallel) and 90° (perpendicular) to the face grain but not at other angles, which is attributed to the different failure mechanisms. In addition, the increase of moisture leads to the decreased bending strength and elastic modulus in the hygroscopic range. Validated by the experimental work, both analytical and numerical models to predict the mechanical performance of birch plywood under different load conditions and various moisture contents are proposed. / Björkplywood har gynnsamma mekaniska egenskaper som skulle kunna användas i nya typer av knutpunkter (förband) för träkonstruktioner, och därmed möjliggöra en substituering av dagens system med stålplåtar. Sådana nya knutpunkter kan innebära avsevärda fördelar när det gäller miljöpåverkan och ekonomi samt en enkel prefabricering och monterbarhet. Det saknas dock data när det gäller vissa mekaniska egenskaper hos björkplywood som är nödvändiga för att kunna utföra en säker design. I synnerhet saknas tillförlitliga data och förståelse kring de mekaniska egenskaperna hos björkplywood i andra riktningar än längs och tvärs dess ytfanér. Syftet med denna avhandling är att ta fram ny kunskap om denna anisotropi och studera variationen av de mekaniska egenskaperna hos björkplywood i dess plan vid olika belastningsvinklar mot ytfanérets fiberriktning, inklusive effekter av storlek och fuktförändringar. Målet är att denna kunskap ska fungera som input för design av björkplywood-förband under olika belastningsförhållanden i träkonstruktioner. Specifikt testades prover av björkplywood i dess plan i drag, tryck, skjuvning och böjning. Resultaten visar att björkplywood har den högsta drag-, tryck- och böjhållfastheten och elasticitetsmodulen parallellt med ytfanérets fiberriktning och den lägsta vid 45° mot densamma. Det motsatta gäller för skjuvhållfasthet och -modul. Dessutom observerades en storlekseffekt avseende böjhållfastheten vid 0° (parallell) och 90° (vinkelrätt) mot ytfanérets fiberriktning men inte vid andra vinklar, vilket förklaras genom de olika brottmekanismerna. Dessutom leder ökningen av fukt till minskad böjhållfasthet och elasticitetsmodul i det hygroskopiska området. Validerat av det experimentella arbetet föreslås både analytiska och numeriska modeller för att förutsäga den mekaniska prestandan hos björkplywood under olika belastningsförhållanden och olika fuktkvoter. / <p>QC 20221117</p>

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Κίνητρο της παρούσας διατριβής αποτέλεσε η αποτίμηση της δομικής ακεραιότητας κελυφοειδών κατασκευών από σύνθετα υλικά που παρουσιάζουν ανοχή στη βλάβη, με τη χρήση ακουστικών τεχνικών μη καταστροφικού ελέγχου. Στόχος ήταν η πειραματική και θεωρητική μελέτη επίδρασης της αστοχίας, που αναπτύσσεται σε μια πολύστρωτη μετά από φόρτιση, σε μετρήσιμα χαρακτηριστικά της κυματικής διάδοσης. Χωρίζεται σε δύο τμήματα, στη μοντελοποίηση της βλάβης και στη μελέτη επίδρασης αυτής στην κυματική διάδοση. Η έρευνα εστιάστηκε σε μορφές αστοχίας που συναντώνται σε πολύστρωτες υπό επίπεδη εντατική κατάσταση και συσσωρεύεται κατά το πάχος τους στη διάρκεια φόρτισης. Για την προσομοίωση της δημιουργήθηκαν διαφορετικά μηχανικά μοντέλα. Έμφαση δόθηκε στην προσέγγιση της συμπεριφοράς του υλικού υπό μονότονη στατική φόρτιση. Γι’ αυτό αναπτύχθηκε ένα φαινομενολογικό πρότυπο προοδευτικής αστοχίας για gl/ep πολύστρωτες. Η δομή του στηρίχθηκε σε τέσσερις πυλώνες. Πρώτον στην πειραματική διαδικασία χαρακτηρισμού μηχανικών ιδιοτήτων της μονοαξονικής στρώσης, ως το βασικό δομικό υλικό μιας πολύστρωτης. Ο ενδελεχής χαρακτηρισμός του υλικού σπάνια συναντάται σε τέτοια έκταση. Δεύτερον από τις δοκιμές προέκυψαν οι καταστατικές εξισώσεις της στρώσης. Η προσέγγιση της ανισότροπης μη – γραμμικότητας του υλικού έγινε με βηματική, γραμμική ανά βήμα, τασική ανάλυση στο επίπεδο της στρώσης χρησιμοποιώντας εφαπτομενική ελαστικότητα. Ο τρίτος πυλώνας αφορά στον προσδιορισμό έναρξης αστοχίας. Υιοθετήθηκαν κριτήρια ευρείας αποδοχής στο σχεδιασμό με σύνθετα υλικά, όπως π.χ. του Puck, των Shokrieh και Lessard κ.α., προτείνοντας και έναν νέο συνδυασμό τους. Τέλος, στρατηγικές υποβάθμισης των μηχανικών ιδιοτήτων της στρώσης προσομοίωσαν το αποτέλεσμα της συσσώρευσης αστοχίας μετά την έναρξή της. Το πρότυπο προοδευτικής αστοχίας ενσωματώθηκε σε στοιχείο κελύφους εμπορικού κώδικα πεπερασμένων στοιχείων. Ακολούθησε αξιολόγηση του, συγκρίνοντας τα αριθμητικά αποτελέσματα με πλειάδα μονοαξονικών και πρωτότυπων διαξονικών πειραμάτων. Η διαδικασία αυτή οδήγησε αφενός στην σημαντική για τον σχεδιασμό παρατήρηση εξάρτησης του μέτρου διάτμησης από το υπάρχον επίπεδο εντατικό πεδίο και αφετέρου στην εξέλιξη του προτύπου ώστε παρά τον περιορισμό των καταστατικών εξισώσεων που το διέπουν να μπορεί να προσομοιώσει τη διαστρωματική αποκόλληση. Έχοντας αναπτύξει τα εργαλεία περιγραφής της βλάβης, η διατριβή ολοκληρώνεται με τη μελέτη δομικής ακεραιότητας, χρησιμοποιώντας τη μη – καταστροφική τεχνική των ακουστό - υπέρηχων. Παρουσιάζεται το πειραματικό και θεωρητικό υπόβαθρο της διάδοσης τασικών κυμάτων σε κελύφη. Πρότυπα πολύστρωτων που υπέστησαν αριθμητική βλάβη υποβλήθηκαν σε αριθμητικές μη – καταστροφικές δοκιμές, καταλήγοντας σε συμπεράσματα όπως π.χ. τη μείωση της φασικής ταχύτητας με τη συσσώρευση βλάβης. / The motivation for the present research was the integrity estimation of shell – like structures made of damage tolerant composite materials, using acoustic non destructive testing techniques. An experimental and theoretical study was held aiming to investigate the influence of the damage, accumulated in a loaded laminate, in measurable wave propagation characteristics. The thesis is separated in two major parts. One described with detail the damage simulation model and the other the damage effects on the wave propagation and the wave mechanics. The study was focused on damage modes developed in composite laminates under in – plane complex stress fields due to several loading conditions and various mechanical models were developed for simulation purposes. Emphasis was given in the description of the material performance under monotonic static loading. Thus, a phenomenological progressive damage model for gl/ep multiaxial laminates was developed. This was structured based on four pillars. Primarily, as the laminate basic building block, the unidirectional layer was mechanically characterized. Such an extended experimental procedure can hardly be found. Secondly, the test results defined the ply constitutive equation laws. The highly anisotropic material non – linearity was approximated with piece – wise linear incremental layer by layer stress analysis using tangential elasticity. The third pillar regarded the damage initiation conditions. Thus, well defined criteria widely accepted in composite design were implemented i.e. Puck, Shokrieh and Lessard, etc. Finally post failure strategies were deployed, simulating material mechanical properties degradation emerging during damage accumulation. The progressive damage model was incorporated in a shell element of a commercial finite element code. An extended validation procedure took place comparing numerical results with several uniaxial and innovative biaxial test data. During this procedure the G12 shear modulus dependence on the developed plane stress field was thoroughly studied, resulting in recommendations for the designer and the selection of the appropriate modulus value. Additionally, the material model was further enhanced, taking into account incompatible failures with its constitutive equations e.g. delamination. Having developed several tools that described damage existence or accumulation, this dissertation was finished with the structural integrity study, using the acousto – ultrasonics non destructive testing technique. The experimental and theoretical background for stress wave propagation in waveguides was presented. Numerically damaged material models were additionally inspected with numerical non – destructive tests, resulting in specific conclusions for damage effect on measurable wave propagation characteristics, e.g. phase velocity reduction with damage growth.