In the American Institute of Steel Construction (AISC) LRFD specification, the shear lag coefficient for plate members is dependent on only the ratio of the welding length to the plate width. To determine how this ratio influences the shear lag coefficient and whether there are other factors that should be considered, a finite element investigation was conducted. Experimental data from a previous study conducted at Virginia Tech was used for comparison with the analytical model. Upon completion of the verification study, several parameters were investigated to determine their influence on the shear lag coefficient. These included the effects of weld length, eccentric load and imperfection created by the welding process. Shear lag coefficients were determined from the finite element analyses and compared with the AISC specification values. / Master of Science

University of Macau / Faculty of Science and Technology / Department of Civil and Environmental Engineering

Natural frequencies of the double and triple-walled carbon nanotubes are determined exactly and approximately for both types. Approximate solutions are found by using Bubnov-Galerkin and Petrov-Galerkin methods. For the first time explicit expressions are obtained for the natural frequencies of double and triple-walled carbon nanotubes for different combinations of boundary conditions. Comparison of the results with recent studies shows that the above methods constitute quick and effective alternative techniques to exact solution for studying the vibration properties of carbon nanotubes. The natural frequencies of the clamped-clamped double-walled carbon nanotubes are obtained; exact solution is provided and compared with the solution reported in the literature. In contrast to earlier investigation, an analytical criterion is derived to establish the behavior of the roots of the characteristic equation. Approximate Bubnov-Galerkin solution is also obtained to compare natural frequencies at the lower end of the spectrum. Simplified version of the Bresse-Timoshenko theory that incorporates the shear deformation and the rotary inertia is proposed for free vibration study of double-walled carbon nanotubes. It is demonstrated that the suggested set yields extremely accurate results for the lower spectrum of double-walled carbon nanotube. The natural frequencies of double-walled carbon nanotubes based on simplified versions of Donnell shell theory are also obtained. The buckling behavior of the double-walled carbon nanotubes under various boundary conditions is studied. First, the case of the simply supported double-walled carbon nanotubes at both ends is considered which is amenable to exact solution. / Then, approximate methods of Bubnov-Galerkin and Petrov-Galerkin are utilized to check the efficacy of these approximations for the simply supported double-walled carbon nanotubes. Once the extreme accuracy is demonstrated for simply supported conditions, the approximate techniques are applied to two other cases of the boundary conditions, namely to clamped-clamped and simply supported-clamped double-walled carbon nanotubes. For the first time in the literature approximate expression for the buckling loads are reported for these boundary conditions. The dynamic deflection of a single-walled carbon nanotube under impact loading is analyzed by following a recently study reported on the energy absorption capacity of carbon nanotubes under ballistic impact. / by Demetris Pentaras. / Thesis (Ph.D.)--Florida Atlantic University, 2009. / Includes bibliography. / Electronic reproduction. Boca Raton, Fla., 2009. Mode of access: World Wide Web.

The primary goal of this thesis is to evaluate vibration control of an all-clamped plate having an unknown disturbance. The vibration control is implemented using a piezoelectric actuator placed at an optimal location. The first part of this thesis considers a robust actuator placement with respect to varying spatial distributions of disturbances. The treatment here, is different from performance-based LQR approaches, since it is based on minimizing the effect of the disturbance distributions. The second part of this thesis addresses a more general case where the plate is under an unknown disturbance. An unknown disturbance is also characterized by the case where the disturbance signal moves randomly over the entire spatial domain. An optimal switching controller algorithm is developed, based on LQR performance, which switches between piezoelectric actuators employed for the vibration control of the plate. A single actuator is selected from the various actuator locations during each time interval, which leads to performance enhancement.

Thesis. 1976. Sc.D.--Massachusetts Institute of Technology. Dept. of Mechanical Engineering. / Microfiche copy available in Archives and Engineering. / Vita. / Includes bibliographical references. / by Nathan Clay Martin II. / Sc.D.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Aeronautics and Astronautics, 1981. / Microfiche copy available in Archives and Barker. / Includes bibliographical references. / by Steven James Hollowell. / M.S.

No description available.

Design of commercial aircraft structure, composed of composite material, requires the prediction of failure loads given large scale damage. In particular, a fuselage of graphite/epoxy lamination was analyzed for damage tolerance given a standard large crack that severed both skin and internal structure. Upon loading, a zone of damage is known to develop in front of a crack-tip in composite laminates; and, its material behavior within the damage zone is characterized as strain softening. This investigation sought to develop a computational model that simulates progressive damage growth and predicts failure of complex laminated shell structures subject to combined tensile and flexural load conditions. This was accomplished by assuming a macroscopic definition of orthotropic damage that is allowed to vary linearly through the shell thickness. It was further proposed that nonlocal plate strain and curvature act to force damage growth according to a set of uniaxial criteria. Damage induced strain softening is exhibited by degradation of laminate stiffness. An expression for the damage reduced laminated plate stiffness was derived which assumed the familiar laminated plate [AM] stiffness matrix format. The model was implemented in a finite element shell program for simulation of fracture and evaluation of damage tolerance. Laminates were characterized for damage resistance according to material parameters defining nonlocal strain and the damage growth criteria. These parameters were selected using an inverse method to correlate simulation with uniaxial strength and fracture test results. A novel combined tension-plus-flexure fracture test was developed to facilitate this effort. Analysis was performed on a section of pressurized composite fuselage containing a large crack. Good agreement was found between calculations and test results. / Graduation date: 1998

Thesis (M.Eng.)--Memorial University of Newfoundland, 2005. / Bibliography: leaves 116-121.

Thesis (Ph.D.)--Memorial University of Newfoundland, 2001. / Bibliography: leaves 166-174.