A simple, computationally efficient analysis approach is developed to predict the buckling of geodesically stiffened composite panels under in-plane loads. This procedure accounts for the discrete flexural contribution of each stiffener through the use of Lagrange multipliers in an energy method solution. An analysis is also implemented for the buckling of simply supported anisotropic rhombic plates. Examples are presented to verify results of the stability analyses and to demonstrate their convergence behavior. Analysis routines are coupled with a versatile numerical optimizer to create a package for the design of minimum-mass stiffened panels, subject to constraints on buckling of the panel assembly, local buckling of the stiffeners, and material strength failure. The design code is used to conduct a preliminary design study of structurally efficient stiffened aircraft wing rib panels. Design variables include thickness of the skin laminate, stiffener thickness, and stiffener height. Applied loads are uniaxial compression, pure shear, and combined compression-shear. Two different geodesically stiffened wing nib configurations with increasing numbers of stiffeners are considered. Results are presented in the form of structural efficiency curves and are compared with those for minimum-weight longitudinally stiffened panels and unstiffened flat plates. Trends in design parameters, including skin thickness and stiffener height, stiffener thickness, stiffener aspect ratio, stiffener load fraction, and stiffener mass fraction, are also examined for the geodesic panels under compression and shear. The effects of skin laminate geometry and anisotropy on the local buckling behavior of cross-stiffened geodesic panels are examined using the rhombic plate analysis. / Master of Science

A one-dimensional infiltration/cure model was developed to simulate fabrication of advanced textile composites by the resin film infusion process. The simulation model relates the applied temperature and pressure processing cycles, along with the experimentally measured compaction and permeability characteristics of the fabric preforms, to the temperature distribution, the resin degree of cure and viscosity, and the infiltration flow front position as a function of time. The model also predicts the final panel thickness, fiber volume fraction, and resin mass for full saturation as a function of compaction pressure. The infiltration model is based on D’arcy’s law for flow through porous media. Composite panels were fabricated using the RTM film infusion technique from knitted, knitted/stitched, and 2-D woven carbon preforms and Hercules 3501-6 resin. Prior to fabrication, the deflection and permeability of the preforms were measured as a function of compaction pressure. Measurements of the temperature distribution, the resin viscosity and degree of cure, and the infiltration flow front position were compared with the RTM simulation model results. The model predictions were within 12% of the experimental results. Fabric composites were fabricated at different compaction pressures and temperature cycles to determine the effects of the processing on the properties. The composites were C-scanned and micrographed to determine the quality of each panel. Composite panels fabricated using different temperature cycles to the same state of cure and similar compaction pressures were found to have similar compressive and shear properties. Advanced cure cycles, developed from the RTM simulation model, were utilized to reduce the total cure cycle times by a factor of 3 and the total infiltration times by a factor of 2. / Master of Science

Modern composites are non-homogeneous materials having high strength fibers embedded in a polymeric or metal matrix, and having directional properties. Shape Memory Alloys (SMAs) such as Nitinol can be embedded in composites for active control of structures. Since the micromechanics of composites in general and adaptive composites, in particular, are poorly understood, accurate values of material elastic properties and in-plane loads generated are seldom available. It is possible to determine these parameters if natural frequencies of the structure are available. The techniques used to determine these parameters from the modal response of a structure are generically called ‘System Identification’. This thesis reviews the various System Identification techniques applicable to vibrating structures. Three techniques are then adapted to an orthotropic laminate and implemented in FORTRAN. These techniques, being iterative in nature, require an initial estimate of the parameters to be identified as input. Because all the techniques discussed here are iterative, they are sensitive to the values of initial estimates. The robustness of these techniques in face of a scatter in the input data is tested using a randomized statistical analysis. The techniques are compared based on certain attributes and recommendations are made. / Master of Science

not available until scanned / Master of Science

In this study, a computational model for accurate analysis of composite laminates and laminates with including delaminated interfaces is developed. An accurate prediction of stress distributions, including interlaminar stresses, is obtained by using the Generalized Laminate Plate Theory of Reddy in which layer-wise linear approximation of the displacements through the thickness is used. Analytical, as well as finite-element solutions of the theory, are developed for bending and vibrations of laminated composite plates for the linear theory. Geometrical nonlinearity, including buckling and post-buckling are included and used to perform stress analysis of laminated plates. A general two-dimensional theory of laminated cylindrical shells is also developed in this study. Geometrical nonlinearity and transverse compressibility are included. Delaminations between layers of composite plates are modeled by jump discontinuity conditions at the interfaces. The theory includes multiple delaminations through the thickness. Geometric nonlinearity is included to capture layer buckling. The strain energy release rate distribution along the boundary of delaminations is computed by a novel algorithm. The computational models presented herein are accurate for global behavior and particularly appropriate for the study of local effects. / Ph. D.

As composites have been more widely accepted as structural materials, residual stresses in them have become a more serious issue. Various experimental methods have been developed and used to measure residual strains in different kinds and shapes of materials. The anisotropic and heterogeneous nature of composites and the complexity of residual stresses, however, pose limitations on current techniques. Those techniques lack the sensitivity or spatial resolution that is required for the measurement of local deformation of composites on a ply·by-ply basis, or give point-by-point and averaged information. Also they are incapable of resolving the complexity of combined effect of different residual stress components. I n order to measure residual strains more effectively, a new method of measuring then1 is required. Moire interferometry combined with the cut-and-sectioning method has been developed for effective measurement of residual strains in fiber-reinforced composites. This optical technique provided the capability of studying separately the effect of each component of residual stresses. It also allowed the determination of high-sensitivity full-field deformation information. This approach was applied to thick composite cylinders for measuring residual strains. The results showed a strong influence of curing procedure on residual stresses. Also, in order to determine residual strains on a within-the-ply basis, a new high-resolution data reduction procedure has been developed. This procedure enhanced the resolution of the existing data reduction technique without losing qualitative information. The combination of both aforementioned techniques provided an effective tool for measuring residual strains of composite materials. The technique is illustrated in an investigation of the effect of stacking sequence on residual strains in flat composite panels. / Ph. D.

Transient moisture conditions can accelerate the viscoelastic behavior of certain materials over that of constant moisture conditions. This is termed the mechano-sorptive phenomenon. The thrust of this research effort is to study the mechano-sorptive effects on the creep behavior of synthetic fibers and composite materials. This study consisted of two main parts: 1). a phenomenological investigation of the transient moisture effects in synthetic fibers and composite materials and 2). mechanistic studies of the observed phenomenon. The materials studied included Kevlar® fibers, Kevlar® fiber reinforced composites, Technora fibers, poly(methyl methacrylate) (PMMA) fibers, and Nylon 6,6 fibers. Unidirectional ( 0° ) Kevlar® 49/7714 epoxy coupons undergoing desorption exhibited an increase in tensile and bending creep deformations, a decrease in storage modulus, and an increase in the loss tangent (Tan δ) when compared to coupons maintained at a constant (saturated) moisture content. However, the transient moisture effects were not seen in composite coupons along the matrix direction. Experimental results showed that aramid fibers exhibited logarithmic creep behavior under tensile load. Even though different constant moisture conditions did not have appreciable effects on the creep behavior of aramid fibers, the creep process increased substantially under transient moisture conditions. The logarithmic creep rates and the mechano-sorptive effects increased with temperature. The creep activation energies of Kevlar® fibers are: 4.84 Kcal/mole for the cyclic moisture conditions and 1.04 Kcal/mole for the constant (saturated) moisture condition. Increases in stress may increase the logarithmic creep rates but may reduce the mechano-sorptive effect. In addition, the creep behavior under transient moisture conditions was nonlinearly dependent on stress. The fiber elastic compliance was observed to increase after creep deformation. Moreover, it was found that the fiber elastic compliance has correlation with the logarithmic creep rates. Aramid fibers contain hydrogen bonds between rod-like crystallites oriented at small angles relative to the fiber axis. These hydrogen bonds may be disrupted during a transient moisture process. The breakage of these hydrogen bonds may cause slippage of hydrogen bonded crystallites and result in accelerated crystallite rotations, thus causing increases in logarithmic creep rate. Analysis indicated that the obtained activation energy (4.84 Kcal/mole) and the reduction in fiber elastic compliance due to creep deformation support the proposed mechanisms. / Ph. D.

The results of a numerical study of interlaminar stresses within unsymmetrically laminated plates is presented. The focus of the study is upon the linear thermoelastic response of thin square laminated composite plates subjected to extensional, compressive, or thermal loading. Symmetric and unsymmetric 0/90, +45/-45, and 0/+45 laminate stacking sequences are examined to determine the effects of mismatch between adjacent layers in Poisson’s ratio, coefficient of mutual influence, and coefficients of thermal expansion. Since the out-of-plane (transverse) deflections of unsymmetric laminates are typically large, a geometrically nonlinear kinematic description is used to account for the large displacements and rotations. The geometrically nonlinear three-dimensional boundary value problems are formulated from nonlinear elasticity theory and approximate solutions are determined using the finite element method. A total Lagrangian, displacement-based, incremental finite element formulation is implemented using Newton’s method. Geometrically nonlinear global/local finite element analysis is used to obtain improved free edge stress predictions. For laminates subjected to external loading, the mismatch in material properties between adjacent layers causes interlaminar stresses to arise near the free edges. For unsymmetric laminates under external loading, the mismatch in material properties about the geometric midplane causes out-of-plane deflections. For the laminates and loading conditions considered, the results of this study show that the out-of- plane deflections of unsymmetric laminates reduce interlaminar shear stresses. In addition, the out-of-plane deflections reduce interlaminar normal stresses for some laminates and increase these stresses for others. For the two-layer unsymmetric laminates considered, the effect of out-of-plane deflections upon interlaminar normal stress was shown to be dependent upon the type of in-plane strain mismatch (i.e., normal and/or shear) caused by the dissimilar material properties. The results also show that as the out-of-plane deflections become large, the effects of geometric nonlinearity upon this stress-deformational response become important. These conclusions apply to extensional, compressive (prior to a change in mode shape), and thermal loading. The numerical results include interlaminar stresses for laminated plates which have buckled as a wide column under compressive loading. / Ph. D.

A postbuckling analysis procedure for simply-supported, symmetrically laminated, rectangular, generally orthotropic plates under uniaxial compression based on a Marguerre-type energy method was developed. The analysis assumes the out-of-plane displacement to be represented by using a truncated Fourier sine series. The unknown coefficients of the displacement function were obtained from a system of nonlinear algebraic equations by using the principle of minimum potential energy. The number of terms that are to be retained in the out-of-plane displacement function to obtain an accurate response was studied and identified for a wide range of generally orthotropic plates. In the postbuckling load range, plates are also allowed to change their buckled form. The magnitudes of the total potential energy for possible different deformed shapes of a plate were compared to determine the actual deformed shape. The effect of bending-twisting coupling terms on the postbuckling behavior of anisotropic laminates was also investigated. Several postbuckling problems for isotropic, orthotropic, and anisotropic plates were considered and the results obtained by the present approach were compared with available literature results and finite element solutions to demonstrate the present analysis procedure. The analysis procedure developed was, then, applied to minimum-weight design of laminated plates for postbuckling performance. Laminate failure load used in the postbuckling regime was calculated based on a maximum strain failure criterion. The failure criterion was demonstrated to predict the failure load reasonably well when compared with available test results. Weight comparison between the plates designed against buckling and the ones designed for the postbuckling strength was made to quantitatively evaluate the weight savings achieved for plates that are allowed to buckle. The design variables were assumed to be the layer thicknesses with specified fiber orientations and assumed to take only discrete values. A simple approach based on the penalty method was proposed to achieve the discrete-valued designs. In addition to the regular penalty terms for constraint violation, the proposed approach introduces penalty terms to reflect the requirement that the design variables take discrete values. A variable magnitude penalty term in the form of a sine function was implemented with the extended interior penalty method of the optimization package NEWSUMT-A. The proposed discrete optimization technique was applied to the classical truss and laminated composite plate design problems to demonstrate the performance of the procedure. / Ph. D.

Graphical simulation of crack growth using singular finite elements and interactive computer graphics is presented. The study consists of two main parts : (i) the formulation and application of an anisotropic singular element (ASE) for analyzing homogeneous anisotropic materials with cracks and, (ii) graphical simulation of crack growth in unidirectional composites. Lekhnitskii’s stress function method is used to formulate the traction-free crack boundary value problem with the stress function expressed in a Laurent series. The geometry of the element is arbitrary. The development of the stiffness matrix for general anisotropic materials is presented and it is shown how the singular element can be incorporated into a conventional displacement based finite element program. The anisotropic singular element (ASE) developed is implemented to analyze cracked anisotropic materials subjected to inplane loading. A 2-D, displacement based linite element code is used and center cracked on- and off-axis coupons under tensile loading are analyzed using the element developed. A general, interactive menu driven program is developed to track crack growth in composite materials. PHIGS (Programmers Hierarchical Interactive Graphics System) is used as the application program interface to integrate the finite element program with interactive graphics. Simulation studies are performed for center cracked on- and off-axis Iaminae using the normal stress ratio theory as the crack propagation criterion. The direction of crack propagation and values of the crack initiation stresses predicted are in reasonable agreement with the experimental values for the cases analyzed. / Ph. D.