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.

This thesis demonstrates the use of fiber optic methods for nondestructive testing of composite materials and aluminum specimens using the acousto-ultrasound approach. A noncontact method using a hybrid interferometer is devised for measuring absolute surface acoustic wave (SAW) amplitudes. The J1..J4 spectrum analysis technique is used for calibrating the piezoelectric transducer cylinder (PZT) and JO/J2 spectrum analysis technique is used for demodulating the SAW signal from the interferometer. An extrinsic Fabry-Perot interferometric (EFPI) sensor is utilized for sensing acoustic emission, measuring speed and attenuation in aluminum and composite specimens. A broadband preamplifier is designed for amplifying signals from the EFPI sensor. Theoretical and practical minimum detectable air gap change of an EFPI sensor are calculated for the system. The directional sensitivity of the EFPI sensor to SAW is studied. / Master of Science

Three test methods were employed to characterize the response of unidirectional Boron/Aluminum metal matrix composite material under monotonic and cyclic loading conditions, namely: Iosipescu Shear, Off-Axis Tension and Compression. The characterization of the elastic and plastic response includes the elastic material properties, yielding and subsequent hardening of the unidirectional composite under different stress ratios in the material principal coordinate system. The elastic response is compared with the prediction of the transformation theory, based on the far field stress ōₓₓ, the Pagano-Halpin Model, and finite element analysis. Yield loci were generated for different stress ratios and were compared for the three different test methods, taking into account residual stresses and specimen geometry. The yield locus for in-plane shear was compared with the prediction of an analytical micromechanical model. The influence of the scatter in the experimental data on the predicted yield surface was also analyzed. Likewise the experimental material strength in tension and compression was compared with the Maximum Stress and the Tsai-Wu failure criterion. / M.S.

Vlasov’s one-dimensional structural theory for thin-walled open section bars was originally developed and used for metallic elements. The theory was recently extended to laminated bars fabricated from advanced composite materials. The purpose of this research is to provide a study and assessment of the extended theory. The focus is on flexural and torsional-flexural buckling of thin-walled, open section, laminated composite columns. Buckling loads are computed from the theory using a linear bifurcation analysis, and are compared to available experimental data. Also, a geometrically nonlinear beam column analysis by the finite element method is developed from the theory. Results from the nonlinear compression response analysis are compared to limited available test data. The merits of the theory and its implementation are discussed. / Master of Science / incomplete_metadata

This investigation was aimed at understanding the global and local strain and deflection responses of stiffened skins. Global deformations of the stiffened skins, under load, produce high local stresses in the interface region between the stiffener and skin. Test panels were designed to study the stiffener and skin interactions using parameters typical of stiffened skins for aircraft fuselages. A total of six panels were tested. Two skin laminates, both 0.04 in. thick, and three stiffener configurations were studied. The panels, having clamped edge boundary conditions, were subjected to pressure loads of up to 14.5-14.8 psi. Out-of-plane deflections and longitudinal and transverse strains were measured in several locations. The deflection responses showed a strongly nonlinear behavior at pressure loads of less than 5 psi. In addition relatively severe gradients of both longitudinal and transverse strains were measured in the interface region of the stiffener and skin. Finite element models incorporating geometric nonlinearities were made of four of the panels. Results of these models substantiated the overall findings of the experimental measurements. / Master of Science / incomplete_metadata

A method for reinforcing the free edges of a symmetric 11-ply graphite-epoxy laminate by adding a one-layer Kevlar-epoxy edge cap is studied. Generalized plane strain finite element analysis is used to predict that interlaminar stresses are reduced when an edge cap is added to the laminate. Different edge cap designs are evaluated. A three-dimensional composite failure criterion and finite element analysis are used in a progressive laminate failure analysis to predict the failure load of the reinforced luminate. The results of an experimental program are presented. Cappad laminates are shown to be on average 130% to 140%. stronger than uncapped laminates when subjected to static tensile or tension-tension fatigue loading. In addition, the coefficient of variation of the static tensile failure load decreases from 24% to 8% with the addition of edge caps. The predicted failure load which is calculated with the finite element results is 10%. lower than the actual failure load. For both the capped and the uncapped laminates, actual failure loads are much lower than those predicted using classical lamination theory stresses and a 2-D failure criterion. Possible applications of the free edge reinforcement concept are given. Suggestions for future research are made. / M.S.

A change in laminate thickness due to terminating internal plies acts as a stress riser for both intralamina and interlaminar stresses. This laminate configuration is referred to as a ply drop. The linear elastic, three-dimensional stress distributions in the vicinity of a ply drop are determined for a graphite-epoxy laminate subject to axial tension and compression by a finite element analysis. It is shown that the interlaminar stresses have a maximum magnitude at the ply drop-off, and decrease proceeding away from the drop-off. Two modes of failure initiation are analyzed. In the pure resin regions surrounding the dropped plies, the maximum stress criterion is assumed to govern failure. The Tsai-Wu criterion is used for intralamina failure prediction. The influence of two laminate lay-ups and a variety of ply drop geometries on the response and failure are presented. / M.S.

M.S.

M.S.