No description available.

An experimental study was carried out on a single-ply low-slope roof using a series of different underlayment layers to test and understand distributional characteristics of negative air pressure throughout the roof assembly, when subjected to suction pressure corresponding to different wind speeds. Various underlayments were tested at the Research & Demonstration Facility at Virginia Polytechnic Institute and State University to develop design guidelines for the installation of recently developed omni-directional roof vent on low slope membrane roofing systems. The newly designed and patented low-pressure roof vent works on Bernoulli and Venturi air flow principals and helps prevent uplift and detachment of the roofing membrane during storm or high wind conditions by creating a negative pressure zone underneath the roof membrane. The objective of this research is to further develop the Pressure Equalizing Vent System by testing the wind vent in combination with different underlayments and to determine the appropriate tributary area of each vent under dynamic wind conditions. The determined area of influence of each wind vent therefore serves as a guideline for appropriate spacing of the vents on the roof. It will also suggest the wind speed at which the pressure is lowered and its ability to weather storm events. The pressure sensors permanently installed beneath the membrane will continuously monitor the pressures in and around the wind vent. An additional objective of the research project is to understand the distributional characteristics of negative pressure in various layers of roof assembly in combination with varying underlayment membranes under different suctionals pressure to further develop the Pressure Equalizing Vent System. The proposal seeks to develop a cost effective roof assembly method, which can withstand the effect of extreme wind through improved understanding of air movement through different layers of roof assembly. The study explores the effect of various underlayments on spatial and distributional character of pressure field on the single ply flat roof deck in combination with omni-directional roof vent system. / Master of Science

<p>  </p> <p>Thermal shrinkage in advanced composite manufacturing causes residual stress in a cylindrical anisotropic segment. The residual stress later induces a spring-in angle when  the temperature change is negative. The superposition method in the finite element method (FEM) by ABAQUS©  proves that only the residual stress in the circumferential direction controls the spring-in angle and induces the radial residual stress. To predict the angle change, the residual stress is firstly determined by using the closed-loop geometry in FEM and then implemented into the cylindrical cross-ply symmetric laminate segment. Consequently, the geometry creates the spring-in angle under the traction-free surface. The angle change is in good agreement with the Radford equation and is found to depend on the coefficient of thermal expansion (CTE) in the circumferential and radial directions rather than other material properties and geometry dimensions. </p> <p>The study found a new limitation of the Radford equation, in that it is accurate when the part is anisotropic symmetric laminate, but not when it is unsymmetric. The accuracy of the Radford equation is further explored with the double curve geometry. Using the superposition method, the circumferential residual stress along the major curve is found to have an influence on the angle change not only of the major curve, but also of the minor curve. The negative temperature change produces the spring-in angle on the major curve, and both spring-in and -off angles on the minor curve, which rely on the radius ratio. In addition, the spring-in angle on the major curve is coincident with the Radford equation. In sum, knowing the spring-in angle is very helpful in designing a tool in advanced composite manufacturing, and the superposition method and the Radford equation are applicable to predict the spring-in angle.</p>

No description available.

This paper presents a semianalytical method for the prediction of interlaminar stresses and displacements near the free edges and ply cracks in general angle-ply laminates subjected to biaxial extensions and/or in plane shear deformation. The method is based on a state space representation of the three-dimensional equations of elasticity. Numerical solutions are obtained by using layer refinement in the through thickness direction and Fourier series expansion in the other directions. By this approach, an angle-ply laminate may be composed of an arbitrary number of monoclinic layers and each layer may have different material property and thickness. This method guarantees continuous fields of all interlaminar stresses across interfaces between material layers. Numerical results are compared with those obtained from other methods. It is found that the theory provides a satisfactory approximation to the stress singularities near the free edges and ply cracks. Numerical solutions for antisymmetric laminates under extension and general laminates under shearing are new in the literature and can be used as benchmarks for validating new models.

The quality of continuous fiber reinforced polymer matrix composite parts and structures depends strongly on the friction during the composite forming process. The two major types of friction that cause deformations during this process are ply-ply friction and tool-ply friction. One of the challenges in the composite forming process is the occurrence of wrinkling and shape distortion of the fabric caused by the surface differences between the forming tool and surface of the laminate. Frictional measurements of composites can vary widely depending on processing parameters, measurement technique, and instruments used. In this study, a commercial rheometer was used to evaluate tool-ply friction of unidirectional carbon fiber epoxy prepreg at various contact pressures, temperatures and sliding velocities. Viscoelastic properties such as the complex viscosity (η*), storage modulus (G'), loss modulus (G"), and loss factor (tan δ) were used to determine the critical transition events (such as gelation) during cure. An understanding of changes in viscoelastic properties as a function of time, temperature, and cure provides insight for establishing a suitable processing range for compression forming of prepreg systems. Surface imaging results were coupled with rheological results to qualitatively examine the effects of processing parameters on prepreg distortions. Changes in gap height over the measurement interval qualitatively describe the changes in contact area and contact mechanisms between the tool-ply surfaces. The results indicate that friction behavior of the prepreg system is a contribution of adhesive and frictional forces, where increase in viscosity, reduction in gap height, and cure of the sample correlate to higher friction values. / Master of Science / The quality of composite parts and structures depends strongly on the friction present during the composite forming process. One of the major challenges in the forming process is the occurrence of wrinkling and shape distortions of the fabric caused by the surface differences between the forming tool and material. The presence of these defects can compromise the final material property and lead to failure when in use. Frictional measurements of composites can vary widely depending on processing parameters, measurement technique, and instruments used. The extent of interaction between the tool and surface of the material depends on the tooling height, and by extension, contact area, which cannot easily be monitored with traditional test designs. A commercial rheometer was used in this study to evaluate tool-ply friction of unidirectional carbon fiber epoxy prepreg at various contact pressures, temperatures, and sliding velocities. Gap height and torque were monitored to provide information on the frictional dependence of processing parameters. In addition, surface-imaging results were coupled with rheological results to examine the relationship between friction and fiber distortions. The understanding of changes in material property with respect to the tooling process is the key to optimizing the composite forming process.

La tenue résiduelle à l'impact basse vitesse est un critère dimensionnant des structures composites minces pour l'aéronautique. La majorité des travaux réalisés jusqu'ici porte sur l'analyse du comportement en zone courante. Ce travail élargit le domaine d'étude aux zones de variation d'épaisseur. L'objectif est d'étudier la tolérance aux dommages d'impact d'un stratifié comportant une zone de reprise de plis (ZRP) dans le cadre d'un dialogue essai/ calcul, en menant en parallèle une campagne d'essais expérimentaux et l'adaptation d'un modèle numérique de l'endommagement des composites.Le volet expérimental étudie successivement l'impact, la compression et la fatigue à R=-l après impact (CAI et FAI) d'éprouvettes comportant une ZRP. L'analyse des essais d'impact a permis d'identifier la résistance à l'impact et le mécanisme d'endommagement des éprouvettes. Ensuite, un outillage d'essai adapté à la géométrie des éprouvettes a été conçu pour les essais de CAI et de FAI. Les essais de compression montrent une tenue résiduelle statique après impact similaire à celle des éprouvettes sans perte de plis. Les essais de fatigue menés à 60 070 de la tenue résiduelle statique montrent une propagation des délaminages d'impact (en dessous du BVID) qui mène à la rupture des éprouvettes pour un nombre de cycles relativement faible. Alors que la tolérance aux dommages d'impact des ZRP sous chargement statique est comparable à celle des zones courantes lisses, on constate une forte vulnérabilité de ces zones sous chargement de fatigue.Le volet numérique a permis de tester l'approche DPM (Discrete Ply Model), développé lors de travaux précédents, sur une configuration particulière. Un maillage spécifique a été réalisé pour tenir compte des discontinuités de la ZRP. Au niveau de la loi matériau, une formulation unifiée de la rupture de fibres en traction/compression a été implémentée. Les résultats de la simulation d'impact sont en bonne corrélation avec les données expérimentales. Le modèle est capable de prédire la réponse globale de l'éprouvette ainsi que l'étendu des dommages internes. La modélisation de la CAI a permis de confirmer les mécanismes de rupture identifiés lors des essais. Ces résultats numériques sont remarquables puisqu' aucune modélisation locale particulière n'a été faite pour les arrêts de plis. L'approche DPM s'est révélée suffisamment robuste et bien adaptée à la modélisation de l'endommagement des stratifiés unidirectionnels. / The residual strength after low velocity impact is a sizing criterion of thin composite structures in aeronautics. The majority of work on the subject is focused on the analysis of plain laminates. This study expands the field of interest to tapered area. The objective is to study the impact damage tolerance of a laminate with ply drop-off using an experiment/ modelling dialogue: on one hand carrying experimental test campaign and on other hand adapting numerical modelling of composite damage. The experimental part successively examines the impact, compression and fatigue R=-l after impact (CAI and FAI) of specimens with ply drop-off. Analysis of the impact results has enabled the identification of impact resistance and damage mechanism. Then, a testing tool for CAI and FAI was specially designed to suit the geometry of the specimens. Compression tests show a static residual strength after impact similar to plain laminates. Fatigue tests carried at 60% of CAI strength show a propagation of impact delamination (below B VID). Failure of specimens occurs after a relatively small number of cycles. While the impact damage tolerance of tapered laminates is comparable to plain laminates under static loading, high vulnerability is observed under fatigue loading. The numerical part allowed to test the Discrete Ply Model (DPM), developed in previous works, on a particular configuration. A specific meshing was realized to account for the discontinuities in the ply drop-off area. Regarding the material law, a unified formulation of the fiber breakage in tension/ compression is implemented. Impact simulation results correlated well with the experimental data. The model is able to predict the overall response of the specimen and internal damage. The modeling of the CAI enabled to confirm the failure mechanisms identified during tests. These numerical results are remarkable since no particular local modeling has been realized for the ply drop-off area. DPM approach has proven robust enough and well suited to damage modeling of unidirectional laminates.

O pneu é o único vínculo entre o veículo e o solo, é ele que transmite toda a potência e carga, e garante a dirigibilidade e condução do automóvel. A estrutura resistente de um pneu é um dos pontos de maior importância para o rendimento, tipo de aplicação e segurança. E conhecê-la é condição primária para o projeto. Pneus convencionais, via de regra, apresentam uma estrutura par de lonas cruzadas (cross-ply), dispostas em ângulos opostos, menores que 90º. Este trabalho visa avaliar as influências de uma estrutura ímpar de lonas cruzadas, em pneus convencionais. Objetiva-se com isso uma redução na matéria prima e uma otimização no tempo de processo. As influências da estrutura ímpar foram verificadas através de uma análise de elementos finitos, examinando o andamento das tensões internas na carcaça do pneu e observando a geometria da região de contato pneu/solo. Verificou-se também a variação da uniformidade utilizando-se do ensaio SAE J332 em uma máquina Akron FD90. A utilização de uma estrutura ímpar, em pneus de lonas cruzadas, acarreta numa deformação na região de contato pneu/solo, devido ao desbalanceamento de tensões nos fios da carcaça, um aumento das componentes de ply-steer e uma variação de força lateral nas componentes dinâmicas avaliadas. A utilização de uma estrutura ímpar deve ser cuidadosamente selecionada dependendo da velocidade, severidade e condições de utilização. / The tire is the only bond between the vehicle and the ground, is it that transmits all the power and load, and guarantees the driven and conduction of the automobile. The resistant structure of a tire is one of the most important factors for the efficiency, type of application and security. Knowing these parameters is the primary condition to design a tire. Conventional tires, usually have a pair structure, made of crossed plies (cross-ply) in opposite angles lesser than 90º. The present work aim to evaluate the influence of an odd cross-ply structure, in conventional tires, looking forward to a material reduction and also an optimization on time process. The influence of an odd structure was evaluated through a finite element analysis, examining the cord stress at the tire carcass and the tire/ground contact region (foot-print). The variation of the uniformity was also verified through a SAE332 test did on Akron FD90 machine. It was observed that the use of an odd structure in cross-ply tires cause a tire/ground contact region deformation, because of the unbalance internal cord stress (at the carcass), and an increase of uniformities components (ply-steer and variation of lateral force). The use of an odd structure must be carefully selected, depending on the speed, severity and condition of use.

The objective of this thesis is to investigate the progressive failure behavior of laminated fiber reinforced composite shell structures under different loading conditions. The laminates are assumed to be orthotropic and the first order shear deformation theory is applied. Three-node layered flat-shell elements are used in the analysis. To verify the numerical results obtained, experimental and analytical results found in literature are compared with the outputs of the study, and the comparison is found to have shown good agreement with the previous work. Rectangular graphite/epoxy composite laminates under transverse loading are analyzed through several boundary conditions and stacking sequences. Maximum stress criteria, Hashin&rsquo / s criteria and Tsai Wu criteria are employed to detect the failure and progressive failure methodology is be implemented according to instantaneous degradation approach. First ply failure, final failure loads, corresponding deformations and failure patterns are presented and compared.

Eine nachträgliche Verfestigung von nähgewirkten multiaxialen Gelegen führt zu einer verbesserten Ausnutzung der Verstärkungsfadeneigenschaften. Zu diesem Zweck wurden Lösungsansätze für eine entsprechende Verfestigungseinrichtung gesucht und bewertet. Drei Anlagenvarianten wurden durch die Kombination verschiedener Verfahren (Strahlungswärme, die Kombination aus Wärme und Druck sowie Walzenbeschichtung) und Bindemittel (Thermoplaste in verschiedenen Aufmachungen, Beschichtungsmassen) entworfen. Sie bieten auf die Einsatzzwecke Produktion, Laborbetrieb und geringster Aufwand abgestimmte Eigenschaften. / The additional stabilization of open grid warp knits provides a better exploitation of the reinforcing yarns. To realize such an additional stabilization, various possible methods have been examined and assessed. Three different types of stabilization installations have been developed by combining the most promising technologies (infrared radiation, combination of heat and pressure, roll coater) and binding agents (thermoplastics, liquid agents). These installations offer special fea-tures for different needs: production, laboratory and least expense.