Thesis (Ph. D.)--School of Electrical and Computer Engineering, Georgia Institute of Technology, 2004. Directed by Mark G. Allen. / Vita. Includes bibliographical references (leaves 155-166).

The present study involves the analysis and design optimization of thin and thick laminated composite structures using symbolic computation. The fibre angle and wall thickness of balanced and unbalanced thin composite pressure vessels are optimized subject to a strength criterion in order to maximise internal pressure or minimise weight, and the effects of axial and torsional forces on the optimum design are investigated. Special purpose symbolic computation routines are developed in the C programming language for the transformation of coordinate axes, failure analysis and the calculation of design sensitivities. In the study of thin-walled laminated structures, the analytical expression for the thickness of a laminate under in-plane loading and its sensitivity with respect to the fibre orientation are determined in terms of the fibre orientation using symbolic computation. In the design optimization of thin composite pressure vessels, the computational efficiency of the optimization algorithm is improved via symbolic computation. A new higher-order theory which includes the effects of transverse shear and normal deformation is developed for the analysis of laminated composite plates and shells with transversely isotropic layers. The Mathematica symbolic computation package is employed for obtaining analytical and numerical results on the basis of the higher-order theory. It is observed that these numerical results are in excellent agreement with exact three-dimensional elasticity solutions. The computational efficiency of optimization algorithms is important and therefore special purpose symbolic computation routines are developed in the C programming language for the design optimization of thick laminated structures based on the higher-order theory. Three optimal design problems for thick laminated sandwich plates are considered, namely, the minimum weight, minimum deflection and minimum stress design. In the minimum weight problem, the core thickness and the fibre content of the surface layers are optimally determined by using equations of micromechanics to express the elastic constants. In the minimum deflection problem, the thicknesses of the surface layers are chosen as the design variables. In the minimum stress problem, the relative thicknesses of the layers are computed such that the maximum normal stress will be minimized. It is shown that this design analysis cannot be performed using a classical or shear-deformable theory for the thick panels under consideration due to the substantial effect of normal deformation on the design variables. / Thesis (Ph.D.)-University of Natal, Durban, 1994.

The resin film infusion process or RFI is a vacuum assisted moulding method for producing high quality fibre reinforced components. The goals of this research have been to investigate this new process, with the aim of determining how the process could be used by the South African composites industry. This included factors such as suitable materials systems, and optimum process parameters. The RFI process is a new composite moulding method designed to allow fibre reinforced products to be manufactured with the ease of pre-preg materials while still allowing any dry reinforcement material to be used. The high pressures required for traditional manufacturing methods such as autoclaves, matched dies and R TM can be avoided while still having very accurate control over the fibre / resin ratio. Moreover, the RFI process is a "dry" process and hence avoids many of the environmental and health concerns associated with wet lay-up and vacuum bag techniques. Furthermore the simple lay-up process requires less skill than a wet lay-up and vacuum bag method. Through a combination of mathematical modelling and physical testing, a material system has been identified. The primary process parameters were identified and a strenuous regime of testing was performed to find optimum values of these parameters. These results were finally feed back into the development of the mathematical model. / Thesis (Ph.D.)-University of Natal, Durban, 1999.

The present study involves the analysis and design optimisation of composite structures using analytical and numerical methods. Five different problems are considered. The first problem considers the design of laminated plates subject to non-uniform temperature distributions. The plates are optimised for maximum buckling temperature using the fibre angle as the optimising variable. The method of solution involves the finite element method based on Mindlin theory for thin laminated plates and shells, and numerical optimisation. A computational approach is developed which involves successive stages of solution for temperature distribution, buckling temperature and optimal fibre angle. Three different temperature loadings are considered and various combinations of simply supported and clamped boundary conditions are studied. The effect of plate aspect ratio on the optimal fibre angle and the maximum buckling temperature is investigated. The influence of bending-twisting coupling on the optimum design is studied by considering plates with increasing number of layers. The second problem concerns the optimal design of composite pressure vessels. Finite element solutions are presented for the design of hemispherically and flat capped symmetrically laminated pressure vessels subjected to external pressure. The effect of vessel length, radius and wall thickness, as well as bending-twisting coupling and hybridisation on the optimal ply angle and buckling pressure are numerically studied. Comparisons of the optimal fibre angles and maximum buckling pressures for various vessel geometries are made with those for hybrid pressure vessels. In the third problem, the multiobjective design of a symmetrically laminated shell is obtained with the objectives defined as the maximisation of the axial and torsional buckling loads. The ply angle is taken as the optimising variable and the performance index is formulated as the weighted sum of individual objectives in order to obtain Pareto optimal solutions of the design problem. Single objective design results are obtained and compared with the multiobjective design. The effect of weighting factors on the optimal design is investigated. Results are given illustrating the dependence of the optimal fibre angle and performance index on the cylinder length, radius and wall thickness. In the fourth problem, the optimal layup with least weight or cost for a symmetrically laminated plate subject to a buckling load is determined using a hybrid composite construction. A hybrid construction provides further tailoring capabilities and can meet the weight, cost and strength constraints while a non-hybrid construction may fail to satisfy the design requirements. The objective of the optimisation is to minimise either the weight or cost of the plate using the ply angles, layer thicknesses and material combinations as design variables. As the optimisation problem contains a large number of continuous (ply angles and thicknesses) and discrete (material combinations) design variables, a sequential solution procedure is devised in which the optimal variables are computed in different stages. The proposed design method is illustrated using graphite, kevlar and glass epoxy combinations and the efficiency of the hybrid designs over the non-hybrid ones are computed. Finally, the minimum deflection and weight designs of laminated composite plates are given in the fifth and last problem. The finite element method is used in conjunction with optimisation routines in order to obtain the optimal designs, as was the procedure in the first problem. Various boundary conditions are considered and results are given for varying aspect ratios and for different loading types. / Thesis (Ph.D.)-University of Natal, Durban, 1996.

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The goal of this academic project was to study the effects of different variables on the damage progression around a central hole in carbon fiber composite coupon specimens. The tracked variables included the type of layup, stress ratio, stress levels, and damage mechanisms observed in each specimen. In-situ x-ray of the individual laminates recorded the extent of damage, mostly longitudinal splitting, as a function of the cycle count. The following lay-ups were included in the experiment: [45/90/-45/02/45/02/-45/0]s, [±5/65/(±5)2/-65/±5]s, and [±5/65/(±5)2/-65/5/65]s. More specifically, the objective of this study was to determine the stress levels at which detectable damage started to develop. The researchers chose to apply 50,000 cycles at each stress level and once damage was detected, the stress level was typically raised by 34.5 MPa (5 KSI), and then cycled another 50,000 cycles until damage exceeding 1.27 cm (0.50") in length was observed. Once the damage exceeded 1.27 cm (0.50"), cycling was continued to 1,000,000 cycles. Upon completion of the fatigue cycling, each specimen's residual strength was determined. The damage length versus stress level was plotted as a way to compare damage onset stresses and growth as a function of lay-up and stress ratio.

Low velocity impact behaviour of bi-layered laminates of acrylic and polycarbonate was investigated using a combination of drop tower impact experiments and explicit finite element analysis in LS-DYNA. Material characterisation tests were conducted in tension and in compression to obtain material properties for input to the material model in the numerical analysis. Quasistatic plate bending tests were conducted at different loading rates to compare the quasistatic response of the materials to the impact behaviour. Impact tests on circular plates of monolithic acrylic and polycarbonate were carried out using an instrumented drop weight impact tester. The impact force histories were recorded and a multiparameter approach was used to determine critical energy. Acrylic exhibited radial cracking, spalling and pene- tration while polycarbonate underwent large deformation and failed by dishing and plugging. The damage caused by impact in the bilayered laminate included partial or full delamination at the interface and radial cracks in the acrylic layer. The low velocity impact responses were simulated using 8-noded solid elements in LS- DYNA. A node-splitting technique based on maximum tensile stress failure criterion and an erosion approach based on maximum principal stress criteria was used to model the failure of acrylic. A material model that takes into account the asym- metric behaviour in tension and compression was investigated. The delamination between the acrylic and polycarbonate plate was modelled by a tiebreak contact with a shear strength based failure. The results of the finite element simulations are in good agreement with the experimental data.

Thesis (Ph.D.)--McMaster University, 2005. / Supervisor: P.S. Nicholson. Includes bibliographical references (leaves 135-157).