The objective of this study was to investigate the bending strength and failure characteristics of AS4/3501-6 and AS4/1806 graphite/epoxy angles sections as a function of processing induced defects and porosity. The angle sections were removed from 30-inch long angles fabricated at Lockheed Georgia Company with two quasi-isotropic stacking sequences, (± 45/90₂/ ∓ 45/0₂), and (± 45/90₂ ∓ 45/0₂)₃. Various degrees of porosity were introduced into the angles using four processing techniques: a standard lay-up, a solvent wipe during lay-up, moisture introduction between plies during lay-up, and a low pressure cure cycle. Two 2.5-inch wide angle sections, each with a 1.5-inch short leg and a 3.0-inch long leg, were bonded together along their long leg to form a T-shaped specimen. Bending of the T-specimen was introduced by pressing up on the underside of the flanges while holding the base of the specimen fixed. The experimental results have shown a significant effect of the processing induced defects on the failure load and bending stiffness for AS4/3501-6 specimens, but not for AS4/1806 specimens. An anisotropic analysis of the angle curved section was performed using Lekhnitskii's stress function approach. Stress and strain fields were studied and two failure criteria (Dual maximum stress and Tsai-Wu) were investigated in order to predict T-specimen failure load and failure mode. Reasonable correlation between prediction and experiments was found for the AS4/3501-6 (± 45/90₂/ ∓ 45/0₂)₃ T-specimens, but both failure criteria were found to be too conservative in predicting failure for the AS4/3501-6 (± 45/O₂/ ∓ 45/90₂)₃, T-specimens. The predicted failure modes were in good agreement with the experimental observations for both Iaminates. / Master of Science

"lnterlayering" - the incorporation of low-modulus film adhesive between the plies of composite Iaminates - has proved to be a successful technique for reducing debilitating out-of-pIane stresses. This work seeks to determine the effect interlayering has on a composite Iaminate's in-plane performance. Two Iaminate systems, an unnotched, 16-ply, quasi-isotropic, AS4/C985 and a centernotched, 32-ply, quasi-isotropic, AS4/C1808, were furnished in an interlayered and baseline (non-interlayered) configuration. The interlayers, 0.0005 in. each in thickness, appeared between each ply in every Iaminate tested. Both configurations of these two material systems were subjected to a regimen of in-plane loading tests. These tests included monotonic tension and compression, fully-reversed (R=-1), tension-compression fatigue cycling, and long-term tensile loading. A new test method, called the Incremental Strain Test (IST), was developed in an attempt to isolate and distinguish the long-term, tensile response of the interlayered and baseline Iaminates. This technique and its utility are described herein. The interlayered Iaminates exhibited superior performance during monotonic and IST loading. Distinctly higher ultimate loads and strains were achieved by the interlayered laminates. The notched fatigue performance of the interlayered Iaminates was sub-standard in comparison to the baseline results at the load level tested. The residual tensile strength of the fatigued interlayered Iaminates fell sharply at an early fraction of the laminates’ total life. The presence of the interlayers did not degrade the laminates’ IST performance. Several non-destructive techniques were used to monitor the damage mechanisms. These results, when combined with the experimental findings, helped explicate the disparity found between the interlayered and baseline Iaminate response. / Master of Science

Several damage modes, including fiber breakage, delamination, and transverse cracking, have been observed to contribute to the mechanical degradation of fiber-reinforced composite laminates. In this investigation, the effect of transverse cracking on laminate stiffness was studied. Four. glass-epoxy laminates ([0,90₃]_s, [90₃,0]_s, [0,90]_s, and [0,±45]_s) were evaluated. Two experimental test sequences were performed. In the first test sequence, longitudinal stiffness was measured at various stages of damage development. Damage development was monitored via edge replication. In the second test sequence, four laminate stiffnesses (E_xx, v_xy, G_xy, and D_yy) were measured in the undamaged and near-saturation damage states. Two analytical models were evaluated. A one dimensional shear lag model was used to predict longitudinal stiffness as a function of crack density for the [0,90₃]_s and 90₃,0]_s laminates. Correlation between theory and experiment was good. A modified laminate analysis was used to predict four laminate stiffnesses (E_xx, v_xy, G_xy, and D_yy). Except for the [0,±45]_s case, a laminate in which significant amounts of damage - s other than transverse cracking were observed, agreement between pre- · dieted and observed stiffness changes was good. / 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

A higher-order shear deformation theory is used to analyze laminated anisotropic composite plates for deflections, stresses, natural frequencies, and buckling loads. The theory accounts for parabolic distribution of the transverse shear stresses, satisfies the stress-free boundary conditions on the top and bottom planes of the plate, and, as a result, no shear correction coefficients are required. Even though the displacements vary cubically through the thickness, the theory has the same number of dependent unknowns as that of the first-order shear deformation theory of Whitney and Pagano. Exact solutions for cross-ply and anti-symmetric angle-ply laminated plates with all edges simply-supported are presented. A finite element model is also developed to solve the partial differential equations of the theory. The finite element model is validated by comparing the finite element results with the exact solutions. When compared to the classical plate theory and the first-order shear deformation theory, the present theory, in general, predicts deflections, stresses, natural frequencies, and buckling loads closer to those predicted by the three dimensional elasticity theory. / 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

This paper discusses an investigation into thermal buckling and post-buckling of symmetrically laminated composite plates. In this study, thermal buckling is investigated for laminates under two different simple support conditions, fixed and sliding. These laminates are subjected to the conditions of a uniform temperature change and a linearly varying temperature change along the length of the plate. Postbuckling in the presence of a uniform temperature change and nonlinear response to imperfections in the form of a thermal gradient through the thickness of the plate and a lack of initial flatness are also studied. The buckling response is studied using variational methods, specifically the Trefftz criterion. Postbuckling and responses to imperfections are studied using nonlinear equilibrium conditions. A Rayleigh-Ritz formulation is used to obtain numerical results from the formulations for the prebuckling response, the buckling response, and the post-buckling and imperfection responses. The analyses are applied to graphite-reinforced materials with (± 45/0₂)_s and (± 45/0/90)_s lamination sequences. Numerical results are obtained for these laminates and also for the case of these laminates being rotated 30° inplane. For the first laminate, for example, such a rotation results in a (+75/ — 15/30₂)_s. stacking sequence. Such skewing of the principal material directions may be encountered when using fiber-reinforced materials in a structurally tailored design. In addition, the influence on thermal buckling of a lack of ideal boundary conditions in the form of boundary compliance and thermal expansion, which would occur in any real set-up, are investigated. / Master of Science

not available until scanned / Master of Science