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

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 influence of thickness and hole radius on the fracture strengths of Narmco V 5245C-G40-600 graphite/bismaleimide laminates was studied. Tests were run on 8 ply, 40 ply, and 80 ply quasi-isotropic laminates of stacking sequence [0/ ± 45/90]. Both unnotched and notched laminates were tested. Unnotched strength was found to be inversely proportional to thickness. Notched strengths were compared to three different failure models based on the stress distribution around the hole. Damage development around the holes was studied using x-ray radiography. In general, the small holes created more damage than the large holes and the thin laminates were more susceptible to damage than the thick laminates. All notched specimens exhibited matrix cracking in the 90° plies around the hole and vertical splitting in the 0° plies at the edge of the hole. / 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

In an effort to obtain an improved mode III fracture toughness test suitable for a testing standard, mechanics analysis, experimental testing, and finite element analysis (FEA) have been conducted. Of particular concern are the merits of one-point and two-point edge crack torsion (ECT) test methods, the influence of specimen geometry that overhangs beyond load/support points, and the influence of crack length on the compliance and energy release rate. Shear stress distributions at the crack front are determined to examine the uniformity of mode III loading and mode II influence. The shear stress distributions in the one-point and two-point tests are virtually identical, indicating that either of the two tests could be used interchangeably. Based on the uniformity of the mode III shear stress distribution along the crack front, it was found that the ECT specimen should have minimum overhang. Longer crack lengths tend to produce nonuniform shear stress distributions. A modified two-point ECT test fixture was developed to allow testing of specimens with a range of dimensions. This development enabled experimental verification of the results from the FEA overhang series. The specimens with a minimum overhang produced consistant mode III toughness data. The most reliable way to reduce data is through the original compliance calibration method. A modified ECT specimen was developed with a staggered crack front to produce uniform mode III crack growth. Finite element analysis of the modified ECT specimen shows a uniform mode III stress distribution along the crack front with little mode II interaction. / by Grant Browning. / Thesis (M.S.C.S.)--Florida Atlantic University, 2009. / Includes bibliography. / Electronic reproduction. Boca Raton, Fla., 2009. Mode of access: World Wide Web.

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An experimental analysis of specimens taken from a thick, filament-wound composite material pressure vessel (cylinder) was performed by testing tensile coupons with various semi-elliptical surface notches. The strength of specimens with small notches was found to be notch insensitive. The strength of specimens with larger notches depended on the size of the notch. The fracture toughness of the laminate was found by applying a general fracture-toughness parameter approach. Using this value, several approaches were employed to predict failure loads. The accuracy of the approaches depended on the size of the notches. In general, the linear-elastic fracture mechanics method overpredicted the failure strength of specimens with intermediate sized notches, but predicted failure strength accurately for specimens with large notches. A strength of materials approach accurately predicted notched strength only for specimens with small notches. Notched strength was more accurately predicted for all notch sizes using an empirical approach, with the notch area used to predict failure instead of the notch depth, which was used in the linear-elastic fracture mechanics and strength of materials approaches. / M.S.

A mixed method of approximation based on Reissner's variational principle is developed for the linear analysis of interlaminar stresses in laminated composites, with special interest in laminates that contain terminated internal plies (dropped-ply laminates). Two models are derived, one for problems of generalized plane deformation and the other for the axisymmetric response of shells of revolution. A layerwise approach is taken in which the stress field is assumed with an explicit dependence on the thickness coordinate in each layer. The dependence of the stress field on the thickness coordinate is determined such that the three-dimensional equilibrium equations are satisfied by the approximation. The solution domain is reduced to one dimension by integration through the thickness. Continuity of tractions and displacements between layers is imposed. The governing two-point boundary value problem is composed of a system of both differential and algebraic equations (DAEs) and their associated boundary conditions. Careful evaluation of the system of DAEs was required to arrive at a form that allowed application of a one-step finite difference approximation. A two-stage Gauss implicit Runge-Kutta finite difference scheme was used for the solution because of its relatively high degree of accuracy. Patch tests of the two models revealed problems with solution accuracy for the axisymmetric model of a cylindrical shell loaded by internal pressure. Parametric studies of dropped-ply laminate characteristics and their influence on the interlaminar stresses were performed using the generalized plane deformation model Eccentricity of the middle surface of the laminate through the ply drop-off was found to have a minimal effect on the interlaminar stresses under longitudinal compression, transverse tension, and in-plane shear. A second study found the stiffness change across the ply termination to have a much greater influence on the interlaminar stresses. Correlations between the stiffness ratio of the thick to the thin sections of the laminates and the magnitude of a parameter based on a quadratic delamination criterion were found to be surprisingly good for longitudinal compression and in-plane shear loadings. For laminates with very stiff terminated plies loaded in longitudinal compression, inclusion of a short insert of softer composite material at the end of the dropped plies was found to significantly reduce the interlaminar stresses produced. / Ph. D.

Within the past several years, researchers have detected the presence of a third “phase” between the bulk fiber phase and bulk matrix phase in a polymeric matrix composite. This finite-thickness region — termed the interphase — possesses mechanical, physical, and chemical properties that are distinct from the fiber and matrix constituents. Thus, the interphase embodies the characteristics of the fiber/matrix bond, including the strength and stiffness of the bond. In essence, the interphase represents the composite system, since it defines the level of synergistic interaction that occurs between the load-carrying fibers and the binding matrix material. Recent interest in the interphase has spawned international conferences and a technical journal devoted to its study. Despite this spate of research, some very fundamental questions about the interphase have remained unanswered. One such question is: “What is best for the performance of a composite, a strong or weak or intermediate-strength interphase?” It is surprising that this question is even asked, since, until recently, it had been assumed that the stronger the fiber/matrix bond, the better the composite behavior. It is now known that this adage is far from true. Two formidable challenges await those who wish to correlate the strength of the interphase to the mechanical performance of polymeric matrix composite materials. First, one seeks to systematically alter the interphase in order to exploit this variable. In this study, fourteen material systems representing permutations of four carbon fibers, three matrix systems, percentages of fiber surface treatment, and three sizing conditions have been examined. Secondly, one needs to quantitatively characterize the properties of the resultant interphase in order to correlate the bond condition to the composite’s mechanical behavior. This investigation utilizes two techniques, the Continuous Ball Indentation Test and transverse flexure testing, as a means of interrogating the strength of the interphase. The influence of the interphase on the tensile and compressive strength and modulus of crossplied laminates possessing a center hole is investigated. Unnotched angle-ply ([±45]_ns) laminates are also tested in order to assess the role of the interphase in the strength of a “matrix-dominated” laminate. Fully-reversed (R =-1), axial fatigue of notched cross-plied laminates from each of the fourteen material systems 1s performed. During fatigue testing several data are monitored, including cycles to failure, dynamic modulus, and notch temperature. The tension-tension (R= 0.1) fatigue response of the unnotched angle-ply laminates is also studied. Results from X-ray radiography of fatigue-damaged specimens help to explain the relationship between the interphase and the initiation and propagation of life-critical damage mechanisms. Having observed the formative role played by the interphase in the performance of these laminates, an attempt is made to introduce variables representing the interphase into micromechanical models of composite behavior. / Ph. D.