In order for future space transportation vehicles to be considered economically viable, the extensive use of lightweight materials is critical. For spacecraft with liquid fueled rocket engines, one area identified as a potential source for significant weight reduction is the replacement of traditional metallic cryogenic fuel tanks with newer designs based on polymer matrix composites. For long-term applications such as those dictated by manned, reusable launch vehicles, an efficient cryo-tank design must ensure a safe and reliable operating environment. To execute this design, extensive experimental data must be collected on the lifetime durability of PMC's subjected to realistic thermal and mechanical environments. However, since polymer matrix composites (PMC's) have seen limited use as structural materials in the extreme environment of cryogenic tanks, the available literature provides few sources of experimental data on the strength, stiffness, and durability of PMC's operating at cryogenic temperatures. It is recognized that a broad spectrum of factors influence the mechanical properties of PMC's including material selection, composite fabrication and handling, aging or preconditioning, specimen preparation, laminate ply lay-up, and test procedures. It is the intent of this thesis to investigate and report performance of PMC's in cryogenic environments by providing analysis of results from experimental data developed from a series of thermal/mechanical tests. The selected test conditions represented a range of exposure times, loads and temperatures similar to those experienced during the lifetime of a cryogenic, hydrogen fuel tank. Fundamental, lamina-level material properties along with properties of typical design laminates were measured, analyzed, and correlated against test environments. Material stiffness, strength, and damage, will be given as a function of both cryogenic test temperatures and pre-test cryogenic aging conditions. This study focused on test temperature, preconditioning methods, and laminate configuration as the primary test variables. The material used in the study, (IM7/PETI-5), is an advanced carbon fiber, thermoplastic polyimide composite. / Master of Science

This dissertation contains the results from an experimental study, numerical, and analytical study conducted on quasi-isotropic carbon fiber laminates containing a center hole (notch) subjected to constant amplitude tension-tension fatigue loading in order to investigate the effect of fatigue damage development on the residual properties. Quasi-static tests were initially performed on notched composites using digital image correlation (DIC) to determine the strain profiles at selected transverse sections of the outer ply of the laminates and the static strength of the laminates. Subsequently, tension-tension fatigue tests were carried out at 70%, 75% and 80% of the nominal static failure load. A finite element model was developed using Abaqus and Digimat in which Digimat was used to implement the damage evolution model via a user-defined material subroutine. Damage initiation has been assessed using Hashin's failure criteria and the Matzenmiller model was adopted for damage evolution. A second finite element model was developed using Abaqus and Autodesk Simulation Composite Analysis (ASCA) in which ASCA was used to implement the user-material subroutine. The subroutine includes a failure initiation criterion based upon multi-continuum theory (MCT) and an energy-based damage evolution law. Numerical and experimental strain results were presented and compared for different section lines on the outer surface of the laminate at different loading stages. Additionally, the experimentally measured notched composite strength was compared with the predictions from the finite element solutions. These results are used as baseline for subsequent comparison with strain profiles obtained using DIC for specimens fatigued at different stress levels and fatigue lifetime fractions. The results showed a significant effect of fatigue damage development on strain redistribution even at early stages of fatigue. The results also showed the capability of DIC technique to identify damage initiation and its location. Furthermore, X-ray computed tomography (CT) was used to examine the sequence of damage development throughout the fatigue lifetime and to connect the observed damage mechanisms with the occurred change of strain profiles. Experimental vibrational modal analysis tests were also conducted to assess the effect of fatigue damage on the residual frequency responses (RFRs). Vibrational measurements were initially performed on pristine notched composites. The results are used as baseline for subsequent comparison with strain profiles obtained with DIC. Finite element models based on the classical plate theory (Kirchhoff) and the shear deformable theory (Mindlin) within the framework of equivalent single-layer and layer-wise concepts as well as the three-dimensional theory of elasticity are developed to predict the natural frequencies of non-fatigued specimen. These models are implemented using the finite element software, Abaqus, to determine the natural frequencies and the corresponding mode shapes. In addition, an analytical model based on Kirchhoff plate theory is developed. Using this approach, an equivalent bending-torsion beam model for cantilever laminated plates is extracted taking into account the reduction in local stiffness and mass induced by the center hole. Numerical and analytical natural frequency values are then compared with those obtained through experimental vibrational tests, and the accuracy of each finite element (FE) and analytical model type is assessed. It is shown that the natural frequencies obtained using the analytical and FE models are within 8% of the experimentally determined values. / Ph. D.

Made available in DSpace on 2014-12-17T14:57:56Z (GMT). No. of bitstreams: 1 SergioRLT_TESE_Capa_ate_pag58.pdf: 1954386 bytes, checksum: d88b3653eaeaeec9f2c0ce6f5549f9ef (MD5) Previous issue date: 2014-02-07 / Coordena??o de Aperfei?oamento de Pessoal de N?vel Superior / The growing demand in the use of composite materials necessitates a better understanding of its behavior related to many conditions of loading and service, as well as under several ways of connections involved in mechanisms of structural projects. Within these project conditions are highlighted the presence of geometrical discontinuities in the area of cross and longitudinal sections of structural elements and environmental conditions of work like UV radiation, moisture, heat, leading to a decrease in final mechanical response of the material. In this sense, this thesis aims to develop studies detailed (experimental and semi-empirical models) the effects caused by the presence of geometric discontinuity, more specifically, a central hole in the longitudinal section (with reduced cross section) and the influence of accelerated environmental aging on the mechanical properties and fracture mechanism of FGRP composite laminates under the action of uniaxial tensile loads. Studies on morphological behavior and structural degradation of composite laminates are performed by macroscopic and microscopic analysis of affected surfaces, in addition to evaluation by the Measurement technique for mass variation (TMVM). The accelerated environmental aging conditions are simulated by aging chamber. To study the simultaneous influence of aging/geometric discontinuity in the mechanical properties of composite laminates, a semiempirical model is proposed and called IE/FCPM Model. For the stress concentration due to the central hole, an analisys by failures criteria were performed by Average-Stress Criterion (ASC) and Point-Stress Criterion (PSC). Two polymeric composite laminates, manufactured industrially were studied: the first is only reinforced by short mats of fiberglass-E (LM) and the second where the reinforced by glass fiber/E comes in the form of bidirectional fabric (LT). In the conception configurations of laminates the anisotropy is crucial to the final mechanical response of the same. Finally, a comparative study of all parameters was performed for a better understanding of the results. How conclusive study, the characteristics of the final fracture of the laminate under all conditions that they were subjected, were analyzed. These analyzes were made at the macroscopic level (scanner) microscope (optical and scanning electron). At the end of the analyzes, it was observed that the degradation process occurs similarly for each composite researched, however, the LM composite compared to composite LT (configurations LT 0/90? and LT ?45?) proved to be more susceptible to loss of mechanical properties in both regarding with the central hole as well to accelerated environmental aging / A crescente demanda na utiliza??o dos materiais comp?sitos torna imprescind?vel uma melhor compreens?o do seu comportamento frente ?s mais diversas condi??es de carga e servi?o, bem como sob as variadas formas de conex?es envolvendo mecanismos nos projetos estruturais. Dentro dessas condi??es de projeto destaque se faz para a presen?a de descontinuidades geom?tricas na ?rea de se??es transversal e ou longitudinal dos elementos estruturais, assim como, condi??es ambientais de servi?o como a radia??o UV, a umidade, o calor, que levam a um decr?scimo na resposta mec?nica final do material. Neste sentido, essa tese tem como objetivo o desenvolvimento de estudos (experimental e modelos semi-emp?ricos) detalhados dos efeitos causados pela presen?a de descontinuidade geom?trica, mais especificamente um furo conc?ntrico na se??o longitudinal (com redu??o da se??o transversal), assim como, da influ?ncia do envelhecimento ambiental acelerado nas propriedades mec?nicas e mecanismo de fratura de laminados comp?sitos de PRFV sob a a??o de cargas uniaxiais de tra??o. Estudos referentes ao comportamento morfol?gico e de degrada??o estrutural dos laminados comp?sitos s?o realizados, atrav?s de an?lises macrosc?picas e microsc?picas das superf?cies afetadas, al?m da avalia??o mediante a T?cnica de Media??o de Varia??o de Massa (TMVM). As condi??es de envelhecimento ambiental acelerado ser?o simuladas atrav?s da c?mara de envelhecimento. Para avalia??o da influ?ncia simult?nea do envelhecimento/descontinuidade geom?trica nas propriedades mec?nicas dos laminados comp?sitos, um modelo semi-empirico ? proposto e denominado de Modelo IE/FCPM. Para o caso da presen?a da concentra??o de tens?o devido ao furo conc?ntrico, a mesma ? avaliada a partir do uso dos crit?rios de falhas da Tens?o M?dia (Average-Stress Criterion) e da Tens?o Pontual (Point-Stress Criterion). Dois laminados comp?sitos polim?ricos, confeccionados industrialmente, foram estudados: o primeiro ? unicamente refor?ado com mantas de fibras curtas de vidro/E (LM) e o segundo onde o refor?o de fibras de vidro/E vem na forma de tecido bidirecional (LT). Na concep??o das configura??es dos laminados a presen?a da anisotropia ? fundamental para a resposta mec?nica final dos mesmos. Por fim, um estudo comparativo entre todos os par?metros foi realizado para uma melhor compreens?o dos resultados. Como estudo conclusivo, as caracter?sticas da fratura final dos laminados sob todas as condi??es que os mesmos foram submetidos, foram analisadas. Essas an?lises foram feitas em n?veis macrosc?pico (scanner) microsc?pico (?ptico e eletr?nico de varredura). Ao t?rmino das an?lises foi observado que o processo de degrada??o se d? de forma semelhante para cada comp?sito pesquisado, no entanto, o laminado comp?sito LM quando comparado ?s configura??es LT 0/90? e LT ?45? do laminado comp?sito LT, se mostrou mais suscept?vel a perda de propriedades mec?nicas tanto em rela??o ? presen?a do furo conc?ntrico como em rela??o ao envelhecimento ambiental acelerado

Cette thèse a pour objet l’étude des automorphismes des groupes d’Artin à angles droits. Etant donné un graphe simple fini G, le groupe d’Artin à angles droits GG associé à G est le groupe défini par la présentation dont les générateurs sont les sommets de G, et dont les relateurs sont les commutateurs [v,w], où {v,w} est une paire de sommets adjacents. Le premier chapitre est conçu comme une introduction générale à la théorie des groupes d’Artin à angles droits et de leurs automorphismes. Dans un deuxième chapitre, on démontre que tout sous-groupe sous-normal d’indice une puissance de p d’un groupe d’Artin à angles droits est résiduellement p-séparable. Comme application de ce résultat, on montre que tout groupe d’Artin à angles droits est résiduellement séparable dans la classe des groupes nilpotents sans torsion. Une autre application de ce résultat est que le groupe des automorphismes extérieurs d’un groupe d’Artin à angles droits est virtuellement résiduellement p-fini. On montre également que le groupe de Torelli d’un groupe d’Artin à angles droits est résiduellement nilpotent sans torsion, et, par suite, résiduellement p-fini et bi-ordonnable. Dans un troisième chapitre, on établit une présentation du sous-groupe Conj(GG) deAut(GG) formé des automorphismes qui envoient chaque générateur sur un conjugué de lui-même / The purpose of this thesis is to study the automorphisms of right-angled Artin groups. Given a finite simplicial graph G, the right-angled Artin group GG associated to G is the group defined by the presentation whose generators are the vertices of G, and whose relators are commuta-tors of pairs of adjacent vertices. The first chapter is intended as a general introduction to the theory of right-angled Artin groups and their automor-phisms. In a second chapter, we prove that every subnormal subgroup ofp-power index in a right-angled Artin group is conjugacy p-separable. As an application, we prove that every right-angled Artin group is conjugacy separable in the class of torsion-free nilpotent groups. As another applica-tion, we prove that the outer automorphism group of a right-angled Artin group is virtually residually p-finite. We also prove that the Torelli group ofa right-angled Artin group is residually torsion-free nilpotent, hence residu-ally p-finite and bi-orderable. In a third chapter, we give a presentation of the subgroup Conj(GG) of Aut(GG) consisting of the automorphisms thats end each generator to a conjugate of itself