Current Federal Aviation Regulations require costly and time consuming crashworthiness testing to certify aircraft. These tests are only capable of a limited assessment of progressive damage and all crash configurations and scenarios cannot be physically evaluated. Advancements in technology have led to accurate and effective developments in numerical modeling that have the possibility of replacing these rigorous physical experiments. Through finite element analysis, an in-depth investigation of an aircraft equipped with a fabricated composite undercarriage was evaluated during water ditching. The severe impact of aircraft ditching is dynamic and nonlinear in nature; the goal of this work to develop a methodology that not only captures the structural response of the aircraft, but also the fluidic behavior of the water. Fundamental studies were first conducted on a well-researched fluid-solid interaction problem, the water entry of a wedge. Typical modeling strategies did not capture the desired detail of the event. An advanced meshing scheme combining meshed and meshless Lagrangian techniques was developed and multiple wedge angles were tested and compared to analytic and qualitative results. The meshing technique proved valid, as the difficult to model phenomena of splashing was captured and the maximum impact force was within five percent of analytical calculations for the 20° and 30° deadrise wedge. Physical small scale aircraft ditching experiments were then performed with an innovative testing platform capable of producing varied aircraft approach configurations. The model was outfitted with an instrumented composite undercarriage to record data throughout the impact while a high-speed camera recorded the event. Numerical simulations of the model aircraft were then compared to experimental results with a strong correlation. This methodology was then ultimately tested on a deformable model of a fuselage section of a full-size aircraft. / Master of Science

The problem of transverse cracks in T300/5208 graphite-epoxy cross-ply laminates was studied using generalized plane strain finite element analysis. The degradation of elastic modulus, Poisson's ratio, shear modulus, and coefficient of thermal expansion was predicted and then compared with analytical models and experimental data. The elastic modulus was predicted to exhibit relatively small degradation as a function of crack density as compared to the Poisson's ratio, shear modulus, and coefficient of thermal expansion which were predicted to have large degradations. The resulting state of stress was also studied for several crack spacings. Interlaminar stresses were predicted to form as a result of the free surface of the crack. The crack tip also caused all the nonzero stresses to exhibit singular behavior at the crack tip. / Master of Science

The objective of this study was to investigate the out-of-plane tensile strength of graphite/epoxy laminates as a function of porosity. An experimental test program was designed to apply tension to the faces of circular graphite/epoxy specimens in a direction perpendicular to the laminate mid-plane. The specimens were removed from the webs of angle sections fabricated by Lockheed Georgia Company using (AS4/1806 and AS4/3501-6 graphite/epoxy material systems with a stacking sequence of (±45/90₂/ ±45/0₂)_S or (±45/0₂/ +̅ 45/90₂)_S. The specimen porosities were the result of four distinct processing methods: a baseline hand lay-up, low pressure cure-cycle, a solvent wipe of pre-preg to remove resin, and the addition of water between pre-pregs. The experimental results have shown a significant reduction in the out-of-plane tensile strength as a function of increasing void content. The volume fraction of pores, pore geometry, size, and orientation were determined for a representative number of specimens by metallography and optical analysis methods. This data was combined with the out-of-plane tensile data and used in the theoretical model, prepared by Brown et al, to predict the out-of-plane strength as a function of porosity. The predicted strength values compared very well with the experimental data when the pores were found to be uniformly distributed throughout the laminate. / Master of Science

Dynamic instability in a laminated composite plate is studied using the finite element technique. The governing equations are derived based on the first order shear deformation theory with a linear strain-displacement relationship. The regions of instability for the resulting set of coupled Mathieu equations are obtained using a method of simultaneous diagonalization. Boundary frequencies generated using a first subdeterminant approximation to the infinite determinant are compared with those obtained by using the more accurate second subdeterminant as well as with frequencies from an analytical solution. These values are verified by checking the nature of responses near the boundaries between stability and instability. Results are presented for plates with different laminations, boundary conditions, thicknesses, number of layers, etc. Some unstable regions for a damped plate are also shown. Results from the first order plate theory are compared with those from a higher order shear deformation theory. / Master of Science

The effects of time and stress level were investigated in cross-ply laminates to gain more understanding on the damage events in composites. Analytical predictions of the effect of stress level were performed for the case of linear elastic materials. The predictions were based on energy methods and linear elastic fracture mechanics. Damage was simulated with a Monte Carlo numerical scheme. The predicted results corresponded well with experimental data in the literature. Experimental testing was performed on cross-ply laminates to gain a better understanding of the effect of time and rate on matrix cracking. The tests were performed on Kevlar/epoxy and graphite/epoxy [0/90₃]₈ laminates. The results indicate that the stress levels required for matrix cracking are a function of how fast the specimens were loaded. Also, significant time dependent damage was observed in cross-ply laminates which were subjected to sustained loads. / Master of Science

Two dimensional and quasi-three dimensional, linear elastic finite element models for the prediction of crack growth characteristics, including crack growth direction, in laminated composite materials are presented. Mixed-mode crack growth in isotropic materials, unidirectional and laminated composites is considered. The modified crack closure method is used to predict the applied load level for crack extension and two new failure theories, modifications of the point stress and the Hashin failure criteria, are proposed to predict the direction of crack extension in composites. Comparisons are made with the Tsai-Wu failure criterion and the Sih strain energy density criterion as well as with experimental results. It is shown that the modified versions of point stress and Hashin criteria compare well with experiment. / Master of Science

Adaptive composites utilizing embedded nitinol fibers have the unique ability to change their material properties, induce large internal distributed forces in a structure, and can modify the stress and strain distribution within a structure in a controlled manner. In this study, nitinol fibers are embedded in graphite-epoxy and are used as distributed actuators to actively tune the dynamic response of clamped-clamped beams. The natural frequencies of clamped-clamped nitinol composite beams are shown, experimentally. to increase linearly as a function of temperature. Beams with nitinol volume fractions of 5% 10%, and 15% can increase their first natural frequency by factors of 1.7, 2.5, and 3.0 respectively. Classical lamination theory is used to formulate a mathematical model of the dynamic response which includes the adaptive properties of the embedded nitinol fibers as a function of temperature, as well as the thermal aspects of the matrix material. Experimental characterization of nitinol for use as constrained thermosets is performed and the results are used in the mathematical model. The mathematical model is used to calculate the natural frequencies of clamped-clamped nitinol composite beams and the results are compared to experimental results. It is clear that adaptive composites represent a new concept in active control of structural responses and may act as a catalyst for future developments in both material and structures technology. Demonstrating, experimentally and computationally, the ability to alter the dynamic response using unique adaptive qualities will hopefully inspire new material/structural interaction paradigms. / Master of Science

A novel optical fiber sensor for the measurement of strain and temperature in graphite-epoxy composite materials using differential interferometry is described. The sensor uses two single-mode optical fiber waveguides imbedded within the composite during prepreg ply lay-up. Strain and temperature changes arc obtained as a motion of an optical interference pattern. Values arc calculated for the strain and temperature dependence of the fringe motion. The results of measurement which attempt to duplicate modeled loading conditions are reported and compared with analytical results. Analytical and experimental extensions of the technique to the measurement of the differential temperature in graphite-epoxy composite specimens during cure cycle processing are also considered. / M.S.

The objectives of this study were to characterize the effects of the space environment on the compressive behavior of T300/934 graphite/epoxy composite material and on the tensile properties of the neat (unfilled) epoxy matrix material. Both materials were tested in the baseline state and after exposure to electron radiation (total dose of 10,000 Mrads of 1 MeVelectrons at a dose rate of 50 Mrads/hr). Irradiation was conducted under vacuum and simulates 30 year, "worst case", exposure in geosynchronous earth orbit. A compressive test method was developed to characterize thin (8-ply) unidirectional coupons. Compression tests were conducted at cryogenic (-250°F; -157°C), room, and elevated (250°F; 121°C) temperatures. Elastic and strength properties were obtained in the principal material directions (E₁, E₂, v₁₂, v₂₁, X_c, Y_c). Tensile specimens of the neat Fiberite 934 epoxy resin were fabricated and tests were conducted at room and elevated (250°F; 121°C) temperatures. Elastic and strength properties (E, ν, δ_ult) were obtained. Irradiation and temperature were found to have a significant effect on composite and neat resin properties. Properties tended to improve at cryogenic temperature and degrade at elevated temperature. Irradiation degraded properties at all temperatures, with the degradation being most severe at elevated temperature. / M.S.

Further development of the modern aeronautical and aerospace industries will require increased use of composite materials. It is predicted that the influence of composite materials on the aerospace industry will be revolutionary. Composite materials have many good qualities; however, due to the heterogeneity of their thermal and mechanical properties, they are particularly susceptible to the influences of their thermal environment during manufacture and in certain applications. Therefore, it is essential to be able to perform thorough thermoelasticity analyses of these kinds of materials before they can be considered for certain applications where they will be exposed to high temperatures or steep temperature gradients or thermal shock. Up to now, no results for the transient three-dimensional analysis of the thermoelastic response of fiber-reinforced composite materials, which include the mechanical coupling during a thermal shock, have been reported in the open literature; yet there are a number of practical applications where it might be important to know the thermal-mechanical coupling effects. This thesis serves as a first step toward developing a comprehensive model for the transient thermoelastic behavior of fiber-reinforced composite materials which includes full coupling between the thermal and mechanical processes. / M.S.