H υποβάθμιση της δομικής ακεραιότητας των αεροπορικών κατασκευών με το χρόνο και η αποτίμηση της αποτελεί ένα σημαντικό τεχνολογικό πρόβλημα της αεροναυπηγικής. Το πρόβλημα αυτό σε συνδυασμό με το ότι εξακολουθούν να χρησιμοποιούνται αεροσκάφη, τα οποία έχουν ξεπεράσει τον αρχικά σχεδιασμένο χρόνο της επιχειρησιακής τους λειτουργίας, οδήγησε τη διεθνή επιστημονική κοινότητα και την αεροπορική βιομηχανία να ασχοληθεί με το πρόβλημα του “γηράσκοντος” αεροσκάφους (aging aircraft). Το πρόβλημα της γήρανσης της δομής ενός αεροσκάφους οφείλεται κυρίως στην εμφάνιση ευρείας έκτασης βλάβης κόπωσης (wide spread fatigue damage) σε διάφορες περιοχές της κατασκευής, η οποία αυξάνει την πιθανότητα εμφάνισης καταστροφικών αστοχιών. Οι ηλωτές συνδέσεις των ελασμάτων που συνθέτουν την άτρακτο ή τις πτέρυγες ενός αεροσκάφους αποτελούν περιοχές συγκέντρωσης τάσεων και εστίες διάβρωσης με συνέπεια την εμφάνιση και εξέλιξη μικρών ρωγμών σε πολλαπλά σημεία της δομής. Η κατάσταση της ύπαρξης πολλών αλληλεπιδρουσών ρωγμών στο ίδιο δομικό στοιχείο ονομάζεται πολλαπλή βλάβη (Multiple Site Damage – MSD) και οδηγεί σε σημαντική μείωση της εναπομένουσας αντοχής του δομικού στοιχείου και σε υποβάθμιση της δομικής του ακεραιότητας. Η μελέτη του προβλήματος της πολλαπλής βλάβης και η αντιμετώπισή του αποτελεί σημαντικό πεδίο έρευνας, καθώς περιλαμβάνει πλήθος επιμέρους φαινομένων, τα οποία αλληλεπιδρούν μεταξύ τους. Τέτοια φαινόμενα είναι η έναρξη, η διάδοση και η συνένωση πολλαπλών ρωγμών, η τελική αστοχία δομικού στοιχείου σε συνθήκες πολλαπλής βλάβης, καθώς και η επίδραση περιβαλλοντικών παραγόντων όπως η διάβρωση στη συμπεριφορά των υλικών. Σκοπός της παρούσας εργασίας είναι η ανάπτυξη μιας ολοκληρωμένης μεθοδολογίας για την εκτίμηση της εμφάνισης και εξέλιξης της πολλαπλής βλάβης κόπωσης σε αεροπορικά δομικά στοιχεία, έτσι ώστε να είναι εφικτή η εκτίμηση της δομικής τους ακεραιότητας σε οποιαδήποτε στιγμή της διάρκειας λειτουργίας τους. Τα τεχνολογικά προβλήματα, που αντιμετωπίζονται στην παρούσα εργασία με ανάπτυξη πρωτότυπων μεθοδολογιών, περιλαμβάνουν: • Ανάλυση τάσεων σε πολλαπλά ρηγματωμένες κατασκευές και υπολογισμό συντελεστών έντασης τάσεων στις ρωγμές • Πρόβλεψη έναρξης και διάδοσης ρωγμών • Πρόβλεψη συνένωσης ρωγμών και εναπομένουσας αντοχής Για την ανάλυση τάσεων και τον υπολογισμό των συντελεστών έντασης τάσεων σε πολλαπλά ρηγματωμένα δομικά στοιχεία με ή χωρίς ενισχύσεις (stiffeners) αναπτύσσονται στη βάση της τεχνικής της υπο-μοντελοποίησης της μεθόδου των πεπερασμένων στοιχείων (ΠΣ) κατάλληλα υπερ-στοιχεία (super-elements) για τη μοντελοποίηση των κρίσιμων τμημάτων της κατασκευής. Η μεθοδολογία υπερ-στοιχείων επιλέγεται στην παρούσα εργασία, με σκοπό τη σημαντική μείωση του υπολογιστικού κόστους σε σχέση με τις κλασικές μεθόδους των πεπερασμένων στοιχείων, έτσι ώστε να καταστεί πρακτικά δυνατή η επίλυση του πολύπλοκου αυτού προβλήματος. Το πρόβλημα της δημιουργίας νέων ρωγμών λόγω κόπωσης σε κάποιο δομικό στοιχείο παρουσιάζει έντονα πιθανοθεωρητικό χαρακτήρα. Για την αντιμετώπισή του αναπτύσσεται μεθοδολογία σύμφωνα με την οποία η πιθανότητα να εμφανιστεί μια ρωγμή σε κάποιο σημείο μετά από ορισμένο αριθμό κύκλων φόρτισης εξαρτάται τόσο από την τάση που αναπτύσσεται στο σημείο αυτό όσο και από την πιθανότητα ύπαρξης τοπικών ατελειών του υλικού στο υπό εξέταση σημείο. Για την εφαρμογή της μεθοδολογίας χρησιμοποιούνται στατιστικά δεδομένα που προέρχονται από πειράματα κόπωσης απλών δοκιμίων. Για την εκτίμηση της εναπομένουσας αντοχής ενός πολλαπλά ρηγματωμένου δομικού στοιχείου αναπτύσσεται ένα ενεργειακό κριτήριο συνένωσης ρωγμών. Το κριτήριο βασίζεται της μεταβολής της ενέργειας παραμόρφωσης που λαμβάνει χώρα κατά τη διάρκεια της αστοχίας του υλικού που συνδέει δύο παρακείμενες ρωγμές. Τέλος, τα μοντέλα έναρξης, διάδοσης και συνένωσης ρωγμών συνδυάζονται σε μια ολοκληρωμένη μεθοδολογία, η οποία μπορεί να χρησιμοποιηθεί για την πρόβλεψη της εξέλιξης της πολλαπλής βλάβης κόπωσης. Η αποτελεσματικότητα της μεθόδου επαληθεύεται με τη βοήθεια πειραμάτων κόπωσης πολλαπλά ρηγματωμένων δοκιμίων. / Structural integrity degradation of aeronautical structures due to fatigue, as well as its assessment, represents a major problem of aeronautics. This problem, combined to the fact that numerous aircrafts that have already exceeded their initial design operational life are still in service, led the international scientific community and the aeronautical industry to the in-depth study of aging aircraft. The main problem of an aging aircraft structure is widespread fatigue damage at various areas of the structure, that increase the probability of catastrophic failures. Riveted joints of fuselage or wing panels are areas of high stress concentration and corrosion, therefore, sites susceptible to multiple cracks development. The state of multiple interacting cracks existence at the same structural element is called Multiple Site Damage (MSD) and leads to residual significant strength reduction and structural integrity degradation of the structure. Study and handling of multiple site damage problem is an important research field, as it involves a number of different interacting phenomena, such as crack initiation, propagation and link-up and final structural failure under MSD conditions, as well as the effect of environmental factors like corrosion on the material behaviour. The aim of the present work is the development of an integrated methodology for the prediction of MSD evolution at aeronautical structural elements, in order to make feasible the assessment of their structural integrity throughout their service life. The technological issues that are faced in the frame of the present work by the development of innovative methodologies are: • Stress analysis and stress intensity factors calculation of structures under MSD condition. • Crack initiation and propagation prediction. • Prediction of crack link-up and residual strength. For the stress analysis and stress intensity factor calculation of un-stiffened of stiffened structural elements suitable super-elements are developed for the simulation of crucial structural elements. The super-element methodology is based on finite element sub-structuring technique and, in the present work, is chosen in order to obtain a significant reduction of computation cost with compared to the classical finite element method, so that handling of such complex problems becomes feasible. Fatigue crack initiation at a structural element is a problem exhibiting a strongly stochastic character. A crack initiation prediction methodology is developed according to which the probability of crack initiation at a site of the structure depends on the local stress state combined to a stochastic (random) variation of the fatigue strength at every site susceptible to crack initiation. For the methodology application statistical data from simple fatigue tests are used. For the crack link-up and the residual strength estimation of a multiple cracked structural element an energy-based link-up criterion is developed. The proposed criterion is based on the strain energy variation during the failure of the ligament of two adjacent cracks. Finally, the initiation, propagation and link-up models are combined and implemented in an integrated methodology that can be used for the assessment of multiple site damage evolution. The effectiveness of the methodology is validated using suitable fatigue tests of multiple-site damaged panels.

Made available in DSpace on 2014-10-09T12:43:11Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T13:56:42Z (GMT). No. of bitstreams: 1 06170.pdf: 3620725 bytes, checksum: 20c306e1c39ebb9a11fa8090fb30a119 (MD5) / Tese (Doutoramento) / IPEN/T / Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP

The thesis presents the development and validation of novel computational technology for modelling and analysis of damage and failure in quasi-brittle materials. The technology is demonstrated mostly on concrete, which is the most widely used quasi-brittle material exhibiting non-linear behaviour. Original algorithms and procedures for generating two-dimensional (2D) and three-dimensional (3D) heterogeneous material samples are developed, in which the mesoscale features of concrete, such as shape, size, volume fraction and spatial distribution of inclusions and pores/voids are randomised. Firstly, zero-thickness cohesive interface elements with softening traction-separation relations are pre-inserted within solid element meshes to simulate complex crack initiation and propagation. Monte Carlo simulations (MCS) of 2D and 3D uniaxial tension tests are carried out to investigate the effects of key mesoscale features on the fracture patterns and load-carrying capacities. Size effect in 2D concrete is then investigated by finite element analyses of meso-structural models of specimens with increasing sizes. Secondly, a 3D meso-structural damage-plasticity model for damage and failure analysis of concrete is developed and applied in tension and compression. A new scheme for identifying interfacial transition zones (ITZs) in concrete is presented, whereby ITZs are modelled by very thin layers of solid finite elements with damage-plasticity constitutive relations. Finally, a new coupled method named non-matching scaled boundary finite element-finite element coupled method is proposed to simulate crack propagation problems based on the linear elastic fracture mechanics. It combines the advantage of the scaled boundary finite element method in modelling crack propagation and also preserves the flexibility of the finite element method in re-meshing. The efficiency and effectiveness of the developed computational technology is demonstrated by simulations of crack initiation and propagation problems.

[pt] Durante o tempo em serviço, estruturas e componentes de equipamentos podem falhar e causar acidentes. Isto geralmente conduz a danos materiais podendo, ainda, causar perda de vidas humanas e danos ambientais. Frequentemente, os eventos danosos são originários de falhas de materiais e de fabricação, sobrecargas não previstas ou outros problemas oriundos do tempo de operação ou deficiência de manutenção de um componente. Neste estudo, com base numa análise de propagação de trinca superficial submetida a carregamento, se avaliou a integridade de um flange da conexão de topo de uma linha flexível, sob condições criogênicas de operação. Quando em serviço, a falha deste conector pode ocasionar interrupções na produção, dano a equipamentos e meio ambiente, bem como fatalidades em plataformas offshore de petróleo. As linhas (dutos) flexíveis utilizadas para injetar gás CO2 nos reservatórios de petróleo no país trabalham submetidas a elevadas pressões internas (da ordem de 10.000 psi), o que aumenta o risco de falha do componente. Além disto, a selagem da conexão flangeada pode falhar e provocar um vazamento de fluido gasoso, proporcionando uma queda acentuada da temperatura local e potencializando a fratura frágil do componente. O cenário proposto para o estudo foi relacionado a possível falha na vedação do anel BX, localizado entre a conexão da linha flexível e a plataforma de petróleo. A metodologia foi dividida em duas etapas: a primeira corresponde à análise global linear elástica do conector na ausência de trincas e baseada no método de elementos finitos (MEF). Na segunda etapa da análise, se introduziu uma trinca no conector na região com os maiores níveis de tensão e onde é possível realizar ensaio por líquido penetrante. Assim, o estudo se concentrou na conexão flangeada do conector, adotando como um aço com 9 por cento de níquel em sua composição química, liga ferrosa de larga aplicação em tanques de armazenagem de GNL. Se buscou avaliar a resistência à fratura do material através da associação de condições severas de carregamento, baixas temperaturas e tamanho de trinca em flanges de dutos flexíveis, conforme critério severizado da norma API 6A. Desta maneira, se espera contribuir para que a indústria offshore de petróleo tenha mais uma opção de material para ser adotado na fabricação de conectores de topo das linhas flexíveis que trabalhem em condição criogênica. / [en] During the time in service, structures and equipment components can fail and cause accidents. This usually leads to damage and may also cause loss of human lives and environmental damages. Often, harmful events are originated from materials and manufacturing faults, unforeseen overloads or other problems arising from the operation time or component maintenance deficiencies. In this study, based on the analysis of surface crack propagation under loading, was assessed the integrity of a top connecting flange of a flexible line operation under cryogenic conditions. When in service, the failure of this connector may cause production disruptions, damages to the equipment and the environment, as well as human fatalities on offshore oil platforms. In Brazil, flexibles lines (ducts) used to inject CO2 gas in oil reservoirs work subjected to high internal pressures (of the order of 10,000 psi), which increases the risk of a component failure. Moreover, the damaged flange may cause leakage of gaseous fluid resulting in very low local temperatures and resulting in brittle fracture enhancing of the component. The scenario proposed in this study is related to a possible seal BX ring failure which is located between the flexible line connection and the oil platform. The methodology was divided into two stages: the first corresponds to the linear elastic global analysis of the connector in the absence of cracks and based on the finite element method (FEM). In the second stage of analysis, it introduced a crack on the connector in the region with the highest stress levels and where it is possible to test for penetrant. Thus, the study focused on the flange connection connector, adopting as a steel with 9 per cent nickel in its chemical composition, ferrous alloy wide application in LNG storage tanks. He sought to evaluate the fracture resistance of the material through the combination of severe loading conditions, low temperatures and 9 cracks in flanges of flexible ducts as severe criteria standard API 6A standard. Thus, it is expected to contribute to the offshore oil industry has one more material option to be adopted in the manufacture of top connectors of the flexible lines working in cryogenic condition.

Le collage structural est une technique d’assemblage de plus en plus demandée aujourd’hui dans beaucoup de domaines comme l’automobile, l’aéronautique, l’aérospatial et dans d’autres domaines comme la construction, le sport et les loisirs. Cette technique très avantageuse, permet l’assemblage de matériaux semblables ou différents à l’aide d’un adhésif, la réduction importante du poids et la répartition uniforme des charges sur l’assemblage. Malgré ses avantages, le collage souffre encore de quelques inconvénients liés à l’existence de défauts dans les joints de colle. Parmi eux, il existe des défauts qui sont situés à l’interface colle/substrat comme un « kissing bond » ou un mauvais état de surface, qui restent indétectables ou difficilement détectables utilisant les techniques de contrôle non destructives. Donc, afin de prendre en compte l’existence des défauts d’adhésion dans les assemblages collés lors de la phase de conception, il est nécessaire de fournir un modèle analytique capable de prédire la propagation de fissure. Dans cette thèse, un modèle analytique qui prédit la propagation de fissure et qui évalue la résistance effective d’un assemblage collé contenant des défauts d’adhésion a été développé. Un défaut a généralement une géométrie complexe, et une étude générique est difficilement réalisable ce qui nous amène à considérer des géométries de défauts idéales. Le modèle a été vérifié par des expériences réalisées sur des éprouvettes DCB. Des simulations numériques utilisant la méthode de zone cohésive ont été réalisées également pour décrire plus complètement le processus de décohésion et simuler les essais expérimentaux. La dernière partie de ce travail a été dédiée à l’étude de la fissuration des éprouvettes en alliage de titane. Profitant de la collaboration avec Safran et Alphanov, les substrats ont subi un traitement de surface laser en laissant des zones non traitées. Le but de cette partie était de vérifier le modèle analytique proposé avec des configurations plus complexes. / Structural adhesive bonding has known an increasing use in many fields like aeronautics, aerospace and automotive and other fields like construction and sports. This very advantageous technique allows the assembly of similar or different materials using an adhesive, the significant reduction in weight and a uniform distribution of loads on the assembly. Despite its advantages, the bonding still suffers from some disadvantages related to the existence of defects in the bonded joints. Among them, there are defects that are located at the interface glue / substrate as "kissing bond" or poor surface due to bad surface treatment, which remain undetectable or hardly detectable using non-destructive control techniques. Therefore, in order to take into account the existence of adhesion defects in bonded assemblies during the design phase, it is necessary to provide an analytical model capable of predicting crack propagation and estimate the criticality of a defect. In this thesis, an analytical model that predicts crack propagation and evaluates the effective strength of a bonded assembly containing adhesion defects has been developed. A defect usually has a complex geometry, and a generic study is difficult to achieve, which leads us to consider ideal defect geometries. The model was verified by experiments performed on DCB specimens. Numerical simulations using the cohesive zone method were also performed to more fully describe the decohesion process and to simulate the experimental tests. The last part of this work was devoted to the study of titanium alloy assembly containing patterns. Taking advantage of the collaboration with Safran and Alphanov, the substrates underwent a laser surface treatment leaving untreated areas. The purpose of this part was to check the proposed analytical model with more complex configurations.

Submitted by Cátia Araújo (catia.araujo@unipampa.edu.br) on 2017-01-25T12:54:53Z No. of bitstreams: 2 license_rdf: 1232 bytes, checksum: 66e71c371cc565284e70f40736c94386 (MD5) Modelagem matemática de propagação de trinca em uma haste em queda livre chocando-se transversalmente contra um apoio rígido.PDF: 2441445 bytes, checksum: a03a1299e0ddd70217e9c31431f4fc61 (MD5) / Approved for entry into archive by Cátia Araújo (catia.araujo@unipampa.edu.br) on 2017-01-25T12:55:20Z (GMT) No. of bitstreams: 2 license_rdf: 1232 bytes, checksum: 66e71c371cc565284e70f40736c94386 (MD5) Modelagem matemática de propagação de trinca em uma haste em queda livre chocando-se transversalmente contra um apoio rígido.PDF: 2441445 bytes, checksum: a03a1299e0ddd70217e9c31431f4fc61 (MD5) / Made available in DSpace on 2017-01-25T12:55:20Z (GMT). No. of bitstreams: 2 license_rdf: 1232 bytes, checksum: 66e71c371cc565284e70f40736c94386 (MD5) Modelagem matemática de propagação de trinca em uma haste em queda livre chocando-se transversalmente contra um apoio rígido.PDF: 2441445 bytes, checksum: a03a1299e0ddd70217e9c31431f4fc61 (MD5) Previous issue date: 2014-04-11 / A modelagem da propagação de trinca é sempre um assunto importante para a segurança de componentes estruturais. Contudo, este assunto é muito complexo, devido à propagação de trinca sofrer influência de vários fatores, tais como: propriedades mecânicas dos materiais, tipo de carregamento e plastificação na região da trinca etc. Quando o componente estrutural é submetido a uma carga de impacto, a situação se complica muito, devido ao tempo da ação de impacto ser extremamente curto. Inicialmente este trabalho desenvolveu uma solução para o carregamento dinâmico de uma barra em queda livre chocando-se transversalmente contra um apoio rígido. Através desta solução podemos obter os dados como: deslocamento, velocidade, aceleração, momento fletor e força cortante em qualquer instante ao longo do seu comprimento. Pela análise do resultado numérico obtido, denotamos que a solução em forma de série pode ser trancada no quinto item, pois os termos subsequentes não tem influência relevante no resultado. Podemos constatar com auxílio dos gráficos, que a tensão máxima na superfície da barra não ocorre na seção da trinca, até certo tempo ocorrido. Em seguida o presente trabalho relacionou a tensão de escoamento e a tensão última do material, com a dureza. Através do método de Palmeri e Cicirello, foi modelada a propagação de trinca, na seção da barra localizada sobre o apoio. Os resultados obtidos mostram que a maior contribuição à força motriz na propagação de trinca é da energia cinética; a maior contribuição à resistência a propagação de trinca é dada pelo trabalho feito pela mola elástica de rotação, representando a trinca. Quando a força motriz é menor que a resistência, a trinca cessa sua propagação. Porém o resultado revelou algumas necessidades para melhorar a modelagem, especificando: a maneira de obter o coeficiente de flexibilidade da haste contendo trinca, e o tratamento dos dados sobre a aceleração instantânea. / Modeling crack propagation is always an important issue for the safety of structural components. This matter is very complex, due to crack propagation be influenced by several factors such as: mechanical properties of materials, type of loading, lamination in the crack region etc. When the structural component is subjected to an impact load, the situation is further complicated, for the impact of action time is extremely short. Initially this work developed a solution for the dynamic loading of a free falling bar crashing transverselly against a rigid support. Through this solution data as displacement, velocity, acceleration, bending moment and shear force at any point along its length can be obtained. By analyzing the numerical result, we denote the solution in serial form can be locked in the fifth term, for subsequent terms has no relevant influence on the result. We can see with the help of graphics, the maximum stress on the surface of the bar does not occur in the crack section until certain time has passed. Then this work has related yield stress and ultimate strength of the material with hardness. Through Palmeri and Cicirello’s method, crack propagation was modeled, in the bar section located over the support. The results show that the major contribution to the driving force in the crack propagation is the kinetic energy; the largest contribution to the crack propagation resistance is given by the work done by the spring elastic rotation, which represents the crack. When the driving force is less than the resistance crack propagation ceases. Still the results showed the need to improve the modeling, specifying: a way to get the stem flexibility coefficient containing the crack, and the processing of data concerning the instantaneous acceleration.

Submitted by Cátia Araújo (catia.araujo@unipampa.edu.br) on 2017-01-26T10:58:30Z No. of bitstreams: 2 license_rdf: 1232 bytes, checksum: 66e71c371cc565284e70f40736c94386 (MD5) Análise por método de diferenças finitas da propagação dinâmica de trinca em uma haste em queda livre chocando-se transversalmente contra um apoio rígido.pdf: 2272346 bytes, checksum: 880f3013b8a3711317e2a11e114edfaf (MD5) / Approved for entry into archive by Cátia Araújo (catia.araujo@unipampa.edu.br) on 2017-01-26T10:58:58Z (GMT) No. of bitstreams: 2 license_rdf: 1232 bytes, checksum: 66e71c371cc565284e70f40736c94386 (MD5) Análise por método de diferenças finitas da propagação dinâmica de trinca em uma haste em queda livre chocando-se transversalmente contra um apoio rígido.pdf: 2272346 bytes, checksum: 880f3013b8a3711317e2a11e114edfaf (MD5) / Made available in DSpace on 2017-01-26T10:58:59Z (GMT). No. of bitstreams: 2 license_rdf: 1232 bytes, checksum: 66e71c371cc565284e70f40736c94386 (MD5) Análise por método de diferenças finitas da propagação dinâmica de trinca em uma haste em queda livre chocando-se transversalmente contra um apoio rígido.pdf: 2272346 bytes, checksum: 880f3013b8a3711317e2a11e114edfaf (MD5) Previous issue date: 2015-06-19 / Atualmente o Brasil é um dos maiores produtores de minério de ferro do mundo. Após a extração ocorre a trituração através de equipamentos de grande porte. O Moinho de Barras é um dos equipamentos mais utilizados, essas máquinas apresentam elevado custo de manutenção devido a constante quebra das barras internas de moagem. Para reduzir esses custos é necessário realizar análise matemática e de mecânica da fratura na barra. O objetivo desse trabalho é analisar a propagação de trinca em uma barra no interior do tambor do moinho de barras, para isso, é considerado que a barra cai em queda livre sobre uma pedra (apoio rígido) posicionada exatamente no centro. A análise foi realizada a partir da solução dinâmica da barra em queda livre chocando-se transversalmente em um apoio rígido pelo método de diferenças finitas (MDF). O algoritmo do MDF foi aferido inicialmente por comparação entre os resultados de simulação realizada no ANSYS LS-DYNA e os resultados analíticos para caso de sem trinca, onde ficou comprovada a eficiência do MDF. A resolução da propagação da trinca consiste na determinação de quando ocorre o início da trinca, ou seja, quando a tensão superficial supera a tensão de ruptura do material a trinca inicia, e na verificação das condições para a propagação, isto é, após a iniciação da trinca utiliza-se o balanço de energia de Griffith em cada instante de tempo para verificar se a trinca continua propagando. Ao analisar os resultados do trabalho conclui-se que: a barra pode quebrar em mais de um pedaço; em materiais com dureza elevada não ocorre o início da trinca; a tenacidade à fratura influência diretamente na velocidade e no comprimento propagado da trinca; o coeficiente de encruamento do material é eficiente para interromper a propagação da trinca e reduzir a velocidade de propagação. O presente trabalho poderá ser considerado como base para analisar propagação de trinca em uma barra com múltiplas trincas e perfil de dureza variável. / Brazil is one of major producers of iron ore in the world. After the ore is explored, the first process of the production is to triturate iron ore. A rods mill is most common grinding equipment. These machines are of high maintenance costs due to constant failure in the milling bars caused by the chock when the bar drops down and impacts on the ores. To reduce such costs, mathematical and mechanical analysis for the bar failure is necessary. The aim of this study is to analyze the crack propagation in a bar, subjected to the shock inside the rod mill barrel. It is considered that the bar falls down freely on a rock (hard support) positioned exactly at the center. The dynamic analysis was performed by the finite difference method (FDM). At first the algorithm based on the FDM was compared with the results performed in ANSYS LS-DYNA and with analytical solution without crack propagation. It was verified FDM efficiency. The numerical analysis is to check when crack onset occurs and if there are conditions for the crack propagation, in other words, when the stress exceeds the ultimate strength of the material initiating crack, thereafter, using the energy balance Griffith at each instant of time to check the conditions of crack continue propagation. By analyzing the results we conclude that: the bar can break into several pieces; in materials with elevated hardness may not onset crack; the fracture toughness influences the speed and length of the crack propagation; the work hardening coefficient of the material is efficient to arrest crack propagation and decrease the speed of crack propagation. This work has laid the foundation for analyzing multi-cracks propagation in a bar with a profile of variable hardness.

Made available in DSpace on 2014-10-09T12:50:12Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T13:58:42Z (GMT). No. of bitstreams: 1 10555.pdf: 13498632 bytes, checksum: a2f985eebcd01db42fa692b0aad0df6d (MD5) / Tese (Doutoramento) / IPEN/T / Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP

A modelagem adequada da propagação de trincas por fadiga em fuselagens pressurizadas é complexa devido a uma série de fatores, tais como: o efeito geometricamente não-linear conhecido como bulging, o revestimento das fuselagens ser bastante delgado e a presença de reforçadores que alteram a distribuição de tensão na fuselagem. Uma metodologia para simular computacionalmente a propagação de trincas por fadiga em estruturas de casca pressurizadas baseada no método dos elementos finitos e em conceitos da Mecânica do Fraturamento Linear Elástica é apresentada. Como resultado dessa simulação, obtêm-se a trajetória de propagação da trinca e o número de ciclos para ocorrer a propagação. Um estudo paramétrico é realizado para avaliar a influência de diversos parâmetros, tais como: espessura do revestimento, nível de pressurização, comprimento de trinca, área da seção transversal e distância entre reforçadores, entre outros, no fator de bulging em estruturas de cascas pressurizadas com ou sem reforçadores. A metodologia apresentada é validada por meio da simulação de propagação de trinca em uma fuselagem de um Boeing 737, sendo que os resultados obtidos nessa simulação são comparados com resultados experimentais e com os obtidos por POTYONDY (1993). A influência do tamanho do incremento de trinca e do número de nós por elemento (4 ou 9 nós) nos resultados obtidos é investigada. / The modeling of fatigue crack growth in pressurized fuselages is very complex due to many factors, such as the nonlinear stiffening effect referred to as bulging, the fuselage skin being very thin and the presence of stiffeners which modifies the stress distribution on the fuselage. A methodology for the computational modeling of the fatigue crack growth in pressurized shell structures, based on the finite element method and concepts of Linear Elastic Fracture Mechanics, is presented. As a result of this numerical simulation, the crack trajectory and the fatigue propagation life are obtained. A parametric study is performed to investigate the influence of several parameters, such as skin thickness, pressurization level, crack length, structural area of the stiffeners, distance between stiffeners, on the bulging effect for unstiffened and stiffened pressurized shells. The methodology is validated by means of a fatigue crack propagation simulation in a Boeing 737 airplane fuselage. The results of this simulation are compared with experimental results and those obtained by POTYONDY (1993). Additionally, the influence of the crack increment and the number of nodes per element (8 or 9 nodes) on the simulation results are investigated.

O Método dos Elementos Finitos Generalizados (MEFG) tem se mostrado uma ferramenta bastante eficaz para a obtenção de soluções dos problemas da Mecânica da Fratura. O MEFG/MEFX permite que trincas estejam imersas na malha de elementos, o que contribui para a redução do custo computacional. Particularmente, ao se tratar da modelagem do processo de propagação, a combinação do método geométrico denominado level-set com o MEFG/MEFX traz a vantagem de permitir descrever mais claramente o posicionamento e o caminho de propagação da trinca. A previsão das soluções dos problemas de fratura obtidas com o MEFG/MEFX o indicam como ferramenta importante para a simulação de processos que envolvem carregamento repetido e previsão de resposta em fadiga. Nesse contexto, a vida em fadiga é um dos principais fatores determinantes na análise de integridade estrutural. Esta pesquisa tem por objetivo avaliar, mediante simulações numéricas, a representatividade e as dificuldades da aplicação do MEFG/MEFX, combinado ao método level-set, para a estimativa de vida em fadiga de elementos estruturais aeronáuticos. Consideram-se abordagens estática e cinemática, de um modo geral e em particular para as análises em fadiga. A abordagem estática requer precisão na determinação dos fatores de intensidade de tensão. A abordagem cinemática inclui a propagação da trinca. Os exemplos considerados consistem em problemas planos e tridimensionais. As ferramentas empregadas são códigos computacionais para o MEFG/MEFX: MXFEM e ABAQUS. Conclui-se que o MEFG/MEFX pode se constituir em instrumento de grande interesse para a indústria aeronáutica, ao permitir análises de integridade estrutural que viabilizam a definição de um plano de inspeção personalizado para os operadores. Além disso, o uso do MEFG/MEFX no campo de definição e projeto de reparos estruturais também é algo promissor, dadas suas características e vantagens demonstradas neste trabalho. / The Generalized Finite Element Method (GFEM) has proved to be a very effective tool for obtaining solutions to the problems of Fracture Mechanics. The GFEM/XFEM allows cracks to be immersed in a finite the element mesh, which contributes to the reduction of the computational cost. Particularly, regarding modeling the propagation process, the combination of the geometric method called \'level-set\' with the GFEM/XFEM has the advantage of being able to describe more clearly the positioning and propagation path of the crack. The prediction of solutions of the fracture problems obtained with the GFEM/XFEM indicate it as an important tool for the simulation of processes involving repeated loading and prediction of fatigue response. In this context, life in fatigue is one of the main determining factors in the structural integrity analysis. The aim of this research was to evaluate the representativeness and difficulties of the application of the GFEM/XFEM, combined with the \'level-set\' method, to estimate fatigue life of aeronautical structural elements using numerical simulations. Static and kinematic approaches were considered, and in particular for fatigue analysis. The static approach requires precision in the determination of the stress intensity factors. The kinematic approach includes the propagation of the crack. The considered examples consist of plane and three-dimensional problems. The tools used are computational codes for the GFEM/XFEM: MXFEM and ABAQUS. As a conclusion, the GFEM/XFEM can be an instrument of great interest for the aeronautical industry, allowing structural integrity analyzes that allow the definition of a personalized inspection plan for the operators. In addition, the use of GFEM/XFEM in the field of definition and design of structural repairs is also promising, given its characteristics and advantages demonstrated in this work.