No description available.

The goal of this investigation was to develop a two-dimensional model which describes the winding and curing processes of filament wound composite structures. The model was developed in two parts. The first part is the cure model which relates the cure temperature, applied at the boundaries of the composite, to the thermal, chemical, and physical processes occurring in the case during cure. For a specified cure cycle, the cure model can be used to calculate the temperature distribution, the degree of cure of the resin, and the resin viscosity inside the composite case. The second part is the layer tension loss and compaction model which relates the winding process variables (i.e., winding pattern, mandrel geometry, initial winding tension, and the properties of the fiber and resin system) to the instantaneous position and tension of the fibers in each layer of the case. A finite element computer code "FWCURE" was developed to obtain a numerical solution to the model. Verification of the cure sub-model was accomplished by measuring the temperature distributions in a 5.75 inch diameter graphite - epoxy test bottle and a 4 inch diameter graphite - epoxy tube during cure. The data were compared with the temperature distributions calculated using FWCURE. Differences between the measured and calculated temperatures was no more than 10 °C for both the test bottle and the cylindrical tube. A parametric study was performed by using FWCURE computer code. Results of the simulation illustrate the information that can be generated by the models and the importance of different processing and material parameters on the fabrication process. / Ph. D.

A comprehensive fabrication stress model was developed to compute fiber stresses in axisymmetric filament wound composite structures at any stage of the fabrication process, a prerequisite for the evaluation of the performance of the composite structures from fabrication process variables. The stress model uses an isoparametric axisymmetric finite element formulation and a double-layered composite element to model the mechanical behavior of the composite material in any cure state. An incremental finite element formulation was used to model the winding and mandrel removal stages. A thermo-mechanical formulation was used to model the curing stage. Also, all major physical phenomena occurring in the fabrication stages which significantly affect the fiber stresses are taken into account: instantaneous tension loss in winding, tension loss due to multiple circuit winding, tension loss due to fiber motion through the uncured resin, material cure transition, and fiber stiffness degradation in a compressive strain state. Two case studies were selected to evaluate and to illustrate the use of the fabrication stress model: the space shuttle booster Joint overwrap and a filament wound composite bottle. The analysis results of the overwrap case study show excellent agreement with experimental hoop strain data. The fabrication stresses from the analysis indicate that the overwrap should experience no strength degradation due to adverse fabrication stresses and strains. Very favorable residual stress results were also predicted by the model for the overwrap. The analysis results of the bottle case study, while having no experimental data to compare with, show very reasonable behaviors, which can be readily explained by a qualitative consideration of the actual winding problem. The stress and strain results from the case study show that the bottle would experience strength degradation when a sand/PVA mandrel is used, but it would retain maximum strength when a steel mandrel is used. / Ph. D.

Within this work a study of the vibrations of laminated composite cantilevers exhibiting structural uncertainties and damage is accomplished. The study is performed within both the Classical Lamination (CLT) and the First-order Transverse Shear Deformation Theories (FSDT). Upon comparing the natural frequencies and mode shapes obtained by both theories, the effects of transverse shear deformation will be emphasized. Other nonclassical effects as e.g. the bending-twist coupling and the warping restraint on the cantilevered structure are also considered. As passive techniques of vibration control, structural tailoring and optimization are implemented. To deal with structural uncertainties, a probabilistic discretization technique for the governing system is developed. Statistical properties of natural frequencies are obtained by means of a second-moment method and a first-order perturbation technique. Structural tailoring is reconsidered to reduce the sensitivity of the dynamic behavior to parameter uncertainties. Next, the damage effect on the structure is considered in the design process. As a result, the problem of the robustness of structures in the presence of damage is addressed. This work also deals with the active feedback control of cantilevered structural systems. An efficient control technique for continuous structures, namely modal control, is adopted and the control gain is obtained by an optimal control law. The comparison of controlled and uncontrolled dynamic responses is made between two models based on CLT and FSDT with emphasis on the influence played by transverse shear deformation and warping restraint. / Ph. D.

Scale model testing can improve the cost-effectiveness of composite structures by reducing the reliance on full size component testing. Use of scale models requires the relationship be known between the responses of the small scale model and full size component. This relationship may be predicted by dimensional analysis or through mechanics formulations. The presence of physical constraints may prevent the complete reproduction of all responses in small scale models. Scaling relationships may not be available at the level necessary to predict all scaled responses. Investigations of the scalability of composite structures are needed in order to evaluate the reliability of small scale model predictions of the responses of full size components. The scaling of the responses of graphite-epoxy laminated composite Z-section stiffeners subjected to uniaxial, compressive loading has been evaluated. The response regimes investigated are prebuckling, initial local buckling, postbuckling and crippling. A mechanistic approach to scaling has been used, in which the scalability of the responses has been judged relative to governing mechanics models. A linked-plate analytical model has been obtained which predicts the buckling loads, and from which two nondimensional load parameters have been obtained. The finite element method has been used for prediction of the buckling and postbuckling responses. The analytical and numerical analyses were used to define an experimental program involving fifty-two specimens of seventeen basic geometrical configurations and three stacking sequences. The buckling, postbuckling and crippling responses were largely determined by the flange-to-web width ratio and both the absolute and relative values of the bending stiffnesses. Buckling loads increased with decreasing flange width and the laminate orthotropy ratio, and increasing flange-to-web corner radii and laminate thickness. The postbuckling load range was the greatest for specimens having wider flanges, but the failure stresses were greatest among the narrower specimens. The crippling mechanisms included flange free edge delamination at both nodal and anti-nodal axial positions, material crushing in the flange-to-web corner at nodal axial positions, and ply splitting in the flange-to-web corner at anti-nodal axial locations. The constraint of the potted end supports of the experimental specimens was not scaled. The effect of displacements within the end supports was manifested by lower prebuckling axial stiffnesses than predicted based on the gage length properties alone. This phenomenon required a post-test adjustment to the data in order to permit comparisons of the experimental and finite element predictions of the response of the gage length on an equivalent basis. Once corrected, the prebuckling stiffnesses were generally observed to have scaled. One of the nondimensional load parameters normalized the buckling loads for specimens of various web widths only. The second parameter normalized the buckling loads for all of the geometric and material variables contained in the model. This parameter also normalized the postbuckling loads, and is, therefore, a general nondimensional parameter for the buckling and postbuckling responses of the Z-section stiffeners. No scale effects were observed in the buckling response. The quality of the postbuckling load predictions degraded with the width of the postbuckling load range. It was not determined whether genuine scale effects were present in the postbuckling response or whether the observed error was a result of inadequate modelling of structural and material nonlinearities or other effects such as damage development in the specimens. Good correlation between experimental and finite element predictions of the out-of-plane displacements and load-axis strains has been demonstrated. Predicted local material strain development has been related to the structural deformation characteristics. Consideration of individual strain values, however, could not predict which of several competing failure modes would determine the actual crippling response. Neither could the strain data provide any quantitative prediction of the crippling loads. Thus, the determination of strength scale effects is hindered by the complex structural-material interaction and the lack of a mechanics-based interactive failure model. / Ph. D.

No description available.

In this study two one-dimensional finite element formulations based on higher-order displacement models have been developed. Both theories account for geometric nonlinearities, a parabolic shear strain distribution through the thickness, and satisfy the shear stress free boundary conditions at the upper and lower surfaces of the beam. The theories also account for the bend-stretch, shear-stretch, and bend-twist couplings inherent to generally laminated composite beams. Further, a coupling between the shear deformation and the twisting is introduced. The lateral strains are assumed nonzero and retained in the formulation. The first model termed SVHSDT also accounts for the continuity of the interlaminar shear stresses at the layer interfaces, while keeping the number of degrees of freedom independent of the number of layers. This theory though is restricted to the analysis of symmetrically laminated cross-ply beams. The formulation has been applied to the linear static and free vibration analysis. The second model termed RHSDT is valid for generally laminated beams. This model has been applied to the nonlinear static and transient analysis of generally laminated beams, free vibration analysis, and impact analysis. The effect of axial stresses on the nonlinear transient response has also been investigated using this theory. For generally laminated beams the lateral strains and the shear-twist coupling were found to have a significant effect on the vibrations frequencies. Also, as expected, initial stresses, boundary conditions and the lamination scheme were found to have a significant effect on the nonlinear responses. / Master of Science

A comprehensive study was performed to overcome the design issues related to Ocean Current Turbine (OCT) blades. Statistical ocean current models were developed in terms of the probability density function, the vertical profile of mean velocity, and the power spectral density. The models accounted for randomness in ocean currents, tidal effect, and ocean depth. The proposed models gave a good prediction of the velocity variations at the Florida Straits of the Gulf Stream. A novel procedure was developed to couple Fluid-Structure Interaction (FSI) with blade element momentum theory. The FSI effect was included by considering changes in inflow velocity, lift and drag coefficients of blade elements. Geometric non-linearity was also considered to account for large deflection. The proposed FSI analysis predicted a power loss of 3.1 % due to large deflection of the OCT blade. The method contributed to saving extensive computational cost and time compared to a CFD-based FSI analysis. The random ocean current loadings were calculated by considering the ocean current turbulence, the wake flow behind the support structure, and the velocity shear. The random ocean current loadings had large probability of high stress ratio. Fatigue tests of GFRP coupons and composite sandwich panels under such random loading were performed. Fatigue life increased by a power function for GFRP coupons and by a linearlog function for composite sandwich panels as the mean velocity decreased. To accurately predict the fatigue life, a new fatigue model based on the stiffness degradation was proposed. Fatigue life of GFRP coupons was predicted using the proposed model, and a comparison was made with experimental results. As a summary, a set of new design procedures for OCT blades has been introduced and verified with various case studies of experimental turbines. / Includes bibliography. / Dissertation (Ph.D.)--Florida Atlantic University, 2017. / FAU Electronic Theses and Dissertations Collection