The strength of materials solution for a new bonded cantilever beam test specimen to determine adhesive shear properties is reviewed and discussed. A parametric analysis for the adhesive shear stress and for the end deflection reveals the specimen dimensions required for reliable bonded adhesive shear properties determination. Recommendations are provided for conducting reproducible tests. A pure and quasi-uniform shear test for stiff adhesives is proposed. Analytical solutions are compared with Finite Element solutions from VISTA and NOVA for the stresses in the adhesive. It appears that the assumption of pure shear is nearly valid even for very stiff and/or very thick adhesives. In order to increase the end point deformations for stiff adhesives, a modified specimen is proposed. Three-dimensional effects through the thickness of the adhesive layer are studied with the program ABAQUS. Experiments were performed using the two methods derived from theory and good correlation between theory and experiment were obtained with some restrictions. For both methods, experimental results underlined the need for defining proper specimen geometry prior to testing. Simple numerical codes are proposed to facilitate this purpose. / Master of Science

The large deformation response of composite beams subjected to a dynamic axial load was studied. The beams were loaded with a moderate amount of eccentricity to promote bending. The study was primarily experimental but some finite element results were obtained. Both the deformation and the failure of the beams were of interest. The static response of the beams was also studied in order to determine the difference between the static and dynamic failure. Twelve different laminate types were tested. The beams tested were 23 in. by 2 in. and generally 30 plies thick. The beams were loaded dynamically with a gravity-driven impactor traveling at 19.6 ft./sec. and quasi-static tests were done on identical beams in a displacement controlled manner. For laminates of practical interest, the failure modes under static and dynamic loadings were identical. Failure in most of the laminate types occurred in a single event involving 40% to 50% of the plies. However, failure in laminates with 30° or 15° off axis plies occurred in several events. All laminates exhibited bimodular properties. The compressive flexural moduli in some laminates was measured to be 1/2 the tensile flexural modulus. No simple relationship could be found among the measured ultimate failure strains of the different laminate types. Using empirically determined flexural properties, a finite element analysis was reasonably accurate in predicting the static and dynamic deformation response. / Master of Science

A mathematically one-dimensional model was developed to predict the static response of composite sandwich beams subjected to loads induced by piezoelectric (PZT) actuators. The model was derived using Reddy's (1984) displacement field for a laminated plate which consists of cubic variation of the in-plane displacement through the thickness. In this model, beam deformations include extension, bending, transverse shear, St. Venant torsion, and torsion due to warping of the cross section out of its plane. The PZT actuators can be configurated to induce a bimoment, resulting in twist cf the beam through the warping of the cross section. Hence directionally attached PZT (DAP) actuator elements, which cause twist by inducing tensile and compressive strains at 45° to the longitudinal axis of the beams, are not necessary to actuate twist. For an aluminum beam example, it is shown that the PZT bimomet control produced about 2.7 times more twist than the conventional DAP control. / Master of Science

periments and predictions was found to be excellent around the net-section region. Although predictions for other strains were not as good, they were within the range of experimental data. Distribution of contact stresses between the pins and the hole edges was also studied. Numerical analysis suggests that the prevalent assumption of radial cosine distribution of contact stress between the pin and hole edge is in substantial error. It can also be concluded that the strength of the joint is for the most part, independent of its width, though for narrower specimens, the holes were quite highly loaded. / Master of Science

This study investigates the effects of typical construction details on the strength of steel deck reinforced concrete composite slabs. Past research on composite slabs has been centered primarily around single span, single panel width slabs with unrestrained ends. The test specimens in this study are more representative of actual slab construction. The effects of multiple spans, multiple panels, end restraint from pour stops, and deck anchorage from shear studs and welds are investigated. The results of this experimental study are analyzed using methods given in the Steel Deck Institute Composite Deck Design Handbook. The models were found to conservatively predict the strength of the composite slabs. Recommended modifications to the calculation methods are given. / Master of Science

The results of sixty-five double-sided pushout tests utilizing the Elco grade 8, 5/16 in. diameter, standoff screw functioning as a mechanical shear connector are presented. In all tests, the base material, through which the screw was fastened, was fabricated out of back to back angle to simulate the top chord of an open web steel joist. Varied test parameters include: steel deck profile, base angle thickness, screw embedment depth, slab thickness, and slab width. The objectives of the testing were (1) to provide an understanding of the strength of the standoff screw in various test geometries and (2) to provide an understanding of the slip characteristics, or ductility, of the connector. A review of literature presents several applicable existing welded stud strength models based on post-test observations of failure mechanisms. Modeled failure mechanisms include: concrete splitting, concrete pullout, rib shear, and stud shear. After applying the existing models to the pushout test data, the following conclusions are drawn: (1) the concrete splitting model developed by Oehlers (1989) can be used to predict the strength of the standoff screw in flat slab geometries and (2) no existing model adequately predicts the strength of the standoff screw in geometries utilizing profiled steel deck. An equation, based on a rederivation of a wedged shaped shear-cone pullout model (Lloyd & Wright 1990), is presented which predicts the strength of the standoff screw in geometries with steel deck with acceptable accuracy. / 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

Within this work a study of the vibrations of laminated composite cantilevers exhibiting structural uncertainties and damage is accomplished. The study is performed within both the Classical Lamination (CLT) and the First-order Transverse Shear Deformation Theories (FSDT). Upon comparing the natural frequencies and mode shapes obtained by both theories, the effects of transverse shear deformation will be emphasized. Other nonclassical effects as e.g. the bending-twist coupling and the warping restraint on the cantilevered structure are also considered. As passive techniques of vibration control, structural tailoring and optimization are implemented. To deal with structural uncertainties, a probabilistic discretization technique for the governing system is developed. Statistical properties of natural frequencies are obtained by means of a second-moment method and a first-order perturbation technique. Structural tailoring is reconsidered to reduce the sensitivity of the dynamic behavior to parameter uncertainties. Next, the damage effect on the structure is considered in the design process. As a result, the problem of the robustness of structures in the presence of damage is addressed. This work also deals with the active feedback control of cantilevered structural systems. An efficient control technique for continuous structures, namely modal control, is adopted and the control gain is obtained by an optimal control law. The comparison of controlled and uncontrolled dynamic responses is made between two models based on CLT and FSDT with emphasis on the influence played by transverse shear deformation and warping restraint. / Ph. D.

Damage states in laminated composites were studied by considering the model problem of a laminated beam subjected to three-point bending. A combination of experimental and theoretical research techniques was used to correlate the experimental results with the analytical stress distributions. The analytical solution procedure was based on the stress formulation approach of the mathematical theory of elasticity. The solution procedure is capable of calculating the ply-level stresses and beam displacements for any laminated beam of finite length using the generalized plane deformation or plane stress state assumption. The beam lamination can be any arbitrary combination of monoclinic, orthotropic, transversely-isotropic, and isotropic layers. Prior to conducting the experimental phase of the study, the results from preliminary analyses were examined. Significant effects in the ply-level stress distributions were seen depending on the fiber orientation, aspect ratio, and whether or not a grouped or interspersed stacking sequence was used. The experimental investigation was conducted to determine the different damage modes in laminated three-point bend specimens. The test matrix consisted of three-point bend specimens of 0° unidirectional, cross-ply, and quasi-isotropic stacking sequences. The dependence of the damage initiation loads and ultimate failure loads were studied, and their relation to damage susceptibility and damage tolerance of the beam configuration was discussed. Damage modes were identified by visual inspection of the damaged specimens using an optical microscope. The four fundamental damage mechanisms identified were delaminations, matrix cracking, fiber breakage, and crushing. The correlation study between the experimental results and the analytical results was performed for the midspan deflection, indentation, damage modes, and damage susceptibility. The correlation was primarily based on the distributions of the in-plane component of shear stress, t_xz. The exceptions were for the case of a very small aspect ratio (less than 1.0) where the crushing mode of damage was predicted based on the maximum contact pressure, and for very large aspect ratios (greater than 12.0) where a maximum tensile bending stress criterion was used for predicting the damage initiation loads. / Ph. D.

Scale model testing can improve the cost-effectiveness of composite structures by reducing the reliance on full size component testing. Use of scale models requires the relationship be known between the responses of the small scale model and full size component. This relationship may be predicted by dimensional analysis or through mechanics formulations. The presence of physical constraints may prevent the complete reproduction of all responses in small scale models. Scaling relationships may not be available at the level necessary to predict all scaled responses. Investigations of the scalability of composite structures are needed in order to evaluate the reliability of small scale model predictions of the responses of full size components. The scaling of the responses of graphite-epoxy laminated composite Z-section stiffeners subjected to uniaxial, compressive loading has been evaluated. The response regimes investigated are prebuckling, initial local buckling, postbuckling and crippling. A mechanistic approach to scaling has been used, in which the scalability of the responses has been judged relative to governing mechanics models. A linked-plate analytical model has been obtained which predicts the buckling loads, and from which two nondimensional load parameters have been obtained. The finite element method has been used for prediction of the buckling and postbuckling responses. The analytical and numerical analyses were used to define an experimental program involving fifty-two specimens of seventeen basic geometrical configurations and three stacking sequences. The buckling, postbuckling and crippling responses were largely determined by the flange-to-web width ratio and both the absolute and relative values of the bending stiffnesses. Buckling loads increased with decreasing flange width and the laminate orthotropy ratio, and increasing flange-to-web corner radii and laminate thickness. The postbuckling load range was the greatest for specimens having wider flanges, but the failure stresses were greatest among the narrower specimens. The crippling mechanisms included flange free edge delamination at both nodal and anti-nodal axial positions, material crushing in the flange-to-web corner at nodal axial positions, and ply splitting in the flange-to-web corner at anti-nodal axial locations. The constraint of the potted end supports of the experimental specimens was not scaled. The effect of displacements within the end supports was manifested by lower prebuckling axial stiffnesses than predicted based on the gage length properties alone. This phenomenon required a post-test adjustment to the data in order to permit comparisons of the experimental and finite element predictions of the response of the gage length on an equivalent basis. Once corrected, the prebuckling stiffnesses were generally observed to have scaled. One of the nondimensional load parameters normalized the buckling loads for specimens of various web widths only. The second parameter normalized the buckling loads for all of the geometric and material variables contained in the model. This parameter also normalized the postbuckling loads, and is, therefore, a general nondimensional parameter for the buckling and postbuckling responses of the Z-section stiffeners. No scale effects were observed in the buckling response. The quality of the postbuckling load predictions degraded with the width of the postbuckling load range. It was not determined whether genuine scale effects were present in the postbuckling response or whether the observed error was a result of inadequate modelling of structural and material nonlinearities or other effects such as damage development in the specimens. Good correlation between experimental and finite element predictions of the out-of-plane displacements and load-axis strains has been demonstrated. Predicted local material strain development has been related to the structural deformation characteristics. Consideration of individual strain values, however, could not predict which of several competing failure modes would determine the actual crippling response. Neither could the strain data provide any quantitative prediction of the crippling loads. Thus, the determination of strength scale effects is hindered by the complex structural-material interaction and the lack of a mechanics-based interactive failure model. / Ph. D.