The diagnostics of slight damage are extremely significant for providing the early warning damage information for in-service structures. This thesis presents the development of a systematic experimental method to identify structural damage by the experimental techniques. Three carbon fabric squared composite p1ates were used as the research objects. Two of them with light crack damage that can be classified as fiber breaking and matrix cracking were supposed to be identified through the dynamic experimental techniques. The tests of the frequency response functions (FRFs) of the investigated objects were conducted first to provide a general understanding of the dynamic properties of the material and the structures. Then the tests of the velocity fields at some specified frequencies are performed to acquire dynamic response data of the objects for the study purposes. A systematic method to process the experimental data has been developed first in this thesis. The best regressive mathematical models for the test velocity fields are built based on the linear polynomial regression procedures and statistical analysis. To perform damage identification, the correlation coefficient (CC) and spatial correlation coefficient (SCC) techniques based on the best-fitted models and the curvature models were used. Finally, the student t' statistical tests were applied to decide whether the two compared data sets are significantly different in statistical sense. / Master of Science

Using the principles of continuum mechanics, the general, incremental, geometrically nonlinear, equilibrium equations for the displacement-based finite element method are derived in both the Total Lagrangian and Updated Lagrangian form. The Updated Lagrangian equations are then specialized for the analysis of three-dimensional cable structures. Two node and three node isoparametric finite elements are derived and a program is developed which implements both elements. Comparisons between the two elements are made to determine their relative merits. In addition, four separate iterative solution techniques are investigated in the solution of the equilibrium equations: the Newton-Raphson method, the Modified Riks-Wempner method, an Orthogonal Trajectory Accession method, and a Dynamic Relaxation approach. An extension of this investigation is a comparison between the Riks-Wempner method and Orthogonal Trajectory Accession. / M.S.

This work discusses the effects of inherent variabilities on the damage identification problem. The goal of damage identification is to detect structural damage before it reaches a level which will detrimentally affect the structure’s performance. Inductive learning is one tool which has been proposed as an effective method to perform damage identification. There are many variabilities which are inherent in damage identification and can cause problems when attempting to detect damage. Temperature fluctuation and manufacturing variability are specifically addressed. Temperature is shown to be a cause-effect variability which has a measurable effect on the damage identification problem. The inductive learning method is altered to accommodate temperature and shown experimentally to be effective in identifying added mass damage at several locations on an aluminum plate. Manufacturing variability is shown to be a non-quantifiable variability. The inductive learning method is shown to be able to accommodate this variability through careful examination of statistical significances in dynamic response data. The method is experimentally shown to be effective in detecting hole damage in randomly selected aluminum plates from a manufactured batch. / Master of Science

A study was performed to verify the accuracy of the Intergraph MicasPlus structural analysis and design software and to evaluate its functionality. The components of MicasPlus to be considered in the study include linear static analysis and steel design and code checking in accordance with the AISC Allowable Stress Design specifications. The verification is based on a comparison of results obtained using the MicasPlus and GTSTRUDL programs for three dimensional steel structures. The factors to be considered in the comparison include support reactions, member forces, displacements, ratios of applied stresses to allowable stresses, and member sizes. The factors to be considered in the evaluation of the functionality of the program include the ease of modeling, amount of effort required to enter the structural models and loads, potential for user errors, quality of support and documentation, and format and quality of the final output. / Master of Science

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A study has been performed focusing on the calculation of sensitivities of displacements, velocities, accelerations, and stresses in linear, structural, transient response problems. One significant goal of the study was to develop and evaluate sensitivity calculation techniques suitable for large-order finite element analyses. Accordingly, approximation vectors such as vibration mode shapes are used to reduce the dimensionality of the finite element model. Much of the research focused on the accuracy of both response quantities and sensitivities as a function of number of vectors used. Two types of sensitivity calculation techniques were developed and evaluated. The first type of technique is an overall finite difference method where the analysis is repeated for perturbed designs. The second type of technique is termed semianalytical because it involves direct, analytical differentiation of the equations of motion with finite difference approximation of the coefficient matrices. To be computationally practical in large-order problems, the overall finite difference methods must use the approximation vectors from the original design in the analyses of the perturbed models. In several cases this fixed mode approach resulted in very poor approximations of the stress sensitivities. Almost all of the original modes were required for an accurate sensitivity and for small numbers of modes, the accuracy was extremely poor. To overcome this poor accuracy, two semi-analytical techniques were developed. The first technique accounts for the change in eigenvectors through approximate eigenvector derivatives. The second technique applies the mode acceleration method of transient analysis to the sensitivity calculations. Both result in accurate values of the stress sensitivities with a small number of modes. In both techniques the computational cost is much less than would result if the vibration modes were recalculated and then used in an overall finite difference method. / Ph. D.

This study presents analytical and experimental procedures and design tools for the control of flexible structure vibrations using a cantilevered adaptive truss. A specific six-actuator, octahedral-octahedral truss effects the control of different flexible structures. These structures could represent space structures or robotic manipulators or a variety of other flexible structures where unwanted structural vibration could reduce the performance of the system. Three of these structures; a slender beam, a single curved beam, and two curved beams are controlled both in simulation and with an experimental test article. The test article, comprised of the flexible structure, the adaptive truss, and the actuators shows excellent agreement between simulated and experimental responses to initial conditions in both open-loop and (LQR) closed-loop control. As a example of the ability of the truss to control the slender beam, a first mode simulated frequency of 3.11Hz (3.09Hz experimental) and damping ratio of 0.0044 (0.0044) are controlled to produce a 3.20Hz (3.14Hz) frequency and a damping ratio of 0.2876 (0.2746). This 6000% increase in damping without a significant change in the modal frequency shows the potential of the adaptive truss in vibration control. The agreement between simulated and experimental data shows the validity of the modeling and experimental procedures. From the information gained, conclusions are drawn about the uses of an adaptive truss in the control of flexible structure vibrations. / Ph. D.

A new technique that couples modal analysis and wavelet transforms for detection of multiple damage in structures is introduced. Structural damage may cause local changes in one or more of the following parameters; stiffness, mass and damping that affect the dynamic behavior of the structure. For example, a crack reduces the stiffness of the structure in a localized sense, and thus reducing its natural frequency, and causes changes in both modal damping and mode shapes. In this research, two different structures have been analyzed: beams and plates. First, uniform beams with a single damage (notch) of different sizes at different locations were considered. Then, a beam with multiple damages was analyzed. Secondly, a uniform plate with a single square damage was considered. For all structures considered, the responses were obtained experimentally (using experimental modal analysis) and numerically (using finite element analysis), then wavelet transform was used to detect and characterize these defects. Most vibration-based methods require knowledge of the undamaged state of the structure, which is unavailable in most cases. However, using wavelet transformation has the advantage of not requiring knowledge of the undamaged state. In addition, wavelet transform has the characteristics that make the visualization of the signal discontinuities clear. Here it has been found that some wavelets were able to detect the damage location for all cases. Also it was observed that the magnitude of the wavelet coefficient increased linearly with the increase in the amount of damage.