Several search methods for unconstrained minimization in the minimum weight design of a stiffened cylindrical shell are investigated. The cylindrical shell, under study, is stiffened with bar shaped stiffeners in the longitudinal and circumferential directions, and capable of being loaded both axially and radially with up to ten separate load conditions. For this investigation, it was necessary to alter an unconstrained minimization program for use on the IBM 360 computer by employing double precision techniques. The computer runs obtained on this system are compared with those provided by the prior work of William Morrow and Lucien Schmit. The results show, as great as, a 7% reduction in the final weight. Either slight alterations in the initial stiffener and cylinder dimensions, or the conversion of the program to double precision is responsible for the change. Regardless of which, the sensitivity or inexactness of the minimization process is apparent.

This dissertation evaluates higher-order modal methods for predicting thermal and structural response. More accurate methods or ones which can significantly reduce the size of complex, transient thermal and structural problems are desirable for analysis and are required for synthesis of real structures subjected to thermal and mechanical loading. A unified method is presented for deriving successively higher-order modal solutions related to previously developed, lower-order methods such as the mode-displacement and mode-acceleration methods. A new method, called the force derivative method, is used to obtain higher-order modal solutions for both uncoupled (proportionally-damped) structural problems as well as thermal problems and coupled (non-proportionally damped) structural problems. The new method is called the force-derivative method because, analogous to the mode-acceleration method, it produces a term that depends on the forcing function and additional terms that depend on the time derivatives of the forcing function. The accuracy and convergence history of various modal methods are compared for several example problems, both structural and thermal. The example problems include the case of proportional damping for: a cantilevered beam subjected to a quintic time varying tip load and a unit step tip load and a muItispan beam subjected to both uniform and discrete quintic time-varying loads. Examples of non-proportional damping include a simple two-degree-of-freedom spring-mass system with discrete viscous dampers subjected to a sinusoidally varying load and a multispan beam with discrete viscous dampers subjected to a uniform, quintic time varying load. The last example studied is a transient thermal problem of a rod subjected to a linearly-varying, tip heat load. / Ph. D.

Traditionally, in reliability design, the random variables acting on a system are assumed independent. This assumption is usually poor because in most real life problems the variables are correlated. The available information, most of the time, is limited to the first and second moments. Very few methods can handle correlation between the variables when the joint probability density function is unknown. There are no reports that provide information of the accuracy of these methods. This work presents an evaluative study of reliability under incomplete information, comparing three existing methods for calculating the probability of failure: The method presented by Ang and Tang which assumes the correlation between the variables to be invariant; Kiureghian and Liu/s method which accounts for the change in correlation and; Rackwitz's method under the assumption of independence. We have also developed a new algorithm to generate random samples of correlated random variables when the marginal distributions and correlation coefficients of these variables are specified. These samples can be used in Monte Carlo simulation which is a tool for comparison of the three methods described above. This Monte Carlo simulation approach is based on the assumption of normal joint probability density function as considered by Kiureghian and Liu. To examine if this approach is biased towards Kiureghian and Liu, a second Monte Carlo simulation approach with no assumption about the joint probability density function is developed and compared with the first one. Both methods that account for correlation show a clear advantage over the traditional approach of assuming that the variables are independent. Moreover, Kiureghian and Liu's approach proved to be more accurate in most cases than Ang and Tang's method. In this study, it is also shown that there is an error in calculating the safety index for correlated variables when either one of the methods in study is implemented, because the joint probability density function of the random variables is neglected. / Master of Science

Glycerol thermal processing (GTP) is studied as a novel biomass pretreatment method in this research with the purposes to facilitate biopolymer fractionation and biomass saccharification. This approach is performed by treating sweet gum particles on polymer processing equipment at high temperatures and short times in the presence of anhydrous glycerol. Nine severity conditions are studied to assess the impact of time and temperature during the processing on biopolymer structure and conversion. The GTP pretreatment results in the disruption of cell wall networks by increasing the removal of side-chain sugars and lignin-carbohydrate linkages based on severity conditions. After pretreatment, 41% of the lignin and 68% of the xylan is recovered in a dry powdered form by subsequent extractions without additional catalysts, leaving a relatively pure cellulose fraction, 84% glucan, as found in chemical pulps. Lignin structural analysis indicated GTP processing resulted in extensive degradation of B-aryl ether bonds through the C-y elimination, followed by abundant phenolic hydroxyl liberation. At the same time, condensation occurred in the GTP lignin, providing relatively high molecular weight, near to that of the enzymatic mild acidolysis lignin. Better thermal stability was observed for this GTP lignin. In addition to lignin, xylan was successfully isolated as another polymer stream after GTP pretreatment. The recovered water insoluble xylan (WIX) was predominant alkali soluble fraction with a maximum purity of 84% and comparable molecular weight to xylan isolated from non-pretreated fibers. Additionally, the narrow molecular weight distribution of recovered WIX, was arisen from the pre-extraction of low molecular weight water-soluble xylan. Additionally, a 20-fold increase of the ultimate enzymatic saccharification for GTP pretreated biomass was observed even with significant amounts of lignin and xylan remaining on the non-extracted fiber. The shear and heat processing caused a disintegrated cell wall structure with formation of biomass debris and release of cellulose fibrils, enhancing surface area and most likely porosity. These structural changes were responsible for the improved biomass digestibility. Additionally, no significant inhibitory compounds for saccharification are produced during GTP processing, even at high temperatures. While lignin extraction did not promote improvement in hydrolysis rates, further xylan extraction greatly increases the initial enzymatic hydrolysis rate and final level of saccharification. The serial of studies fully demonstrate glycerol thermal processing as a novel pretreatment method to enhance biomass saccharification for biofuel production, as well as facilitate biopolymer fractionation. Moreover, the study shows the impact of thermally introduced structural changes to wood biopolymers when heated in anhydrous environments in the presence of hydrogen bonding solvent. / Ph. D.

A user friendly preprocessor was developed and documented for the plane and space frame and space truss structural analysis programs that are based on the matrix displacements method. This preprocessor is comprised of three programs. The main program in the preprocessor is to allow the user to create error free input data files. This program also allows modifications of existing input data files. The two other programs are the library manager and the graphics presentation. The library manager is used to manage the libraries of the element and material properties. The graphics presentation is used to display a plane structure on the graphics display. In Chapter 2, the development of a user friendly preprocessor is discussed. After a short review of the extension of the analysis program from plane frame to space frame in Chapter 3, the preprocessor and its supporting programs are described in detail in the user manual in Chapter 4. Possible extensions to the preprocessor are discussed in Chapter 5. The appendix contains examples of input data files for these structural analysis programs. / Master of Science

When structural analysis is performed via iterative solution technique it is possible to integrate the analysis and design iterations in an integrated analysis and design procedure. The present work seeks to apply an integrated analysis and design approach due to Rizk to the problem of hole shape optimization in thick plates. The plates are modeled by three dimensional eight noded elements. An element by element (EBE) preconditioned conjugate gradient (PCG) method is used for the structural analysis, because this method is well suited for poorly banded three dimensional problems. The plates were optimized so as to minimize the stress concentration near the hole measured by the ratio of the Von Mises stress to the applied boundary stress. The analysis program was validated by comparison to a commercial finite-element program as well as photoelastically obtained stress concentrations. Similarly, the optimization procedure was checked against plates optimized by a photoelastic technique. Good agreement was observed. The integrated analysis and design approach tested here is based on partially converged solutions of the EBE-PCG iterative process. A study of the effect of the number of iterations on analysis and derivative accuracy was performed. Based on this analysis a choice was made for the number of iterations to be used in the integrated analysis and design procedure. It was found that the cost of the design could be significantly reduced with only minimal effects on the final shape and stress concentration factor. / Master of Science

In conclusion, it can be said that the method of approach (whether by the minimum complementary energy or column analogy) is very straightforward, and the final set of equations can be developed very quickly. However, the final simultaneous solution presents a very difficult part of obtaining the final answers. / Master of Science

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

The HECS wing was developed by NASA Langley Research Center and has a nonplanar, hyper-elliptically swept leading and trailing edge as well as spanwise camber. For this wing, the leading and trailing edges are swept back according to a hyper-elliptical equation. The span of the wing is also defined with hyper-elliptical anhedral giving it nonplanar spanwise camber. A single-degree-of-freedom mechanism is developed to provide a means for the wing to continuously change shape from its nonplanar to planar configuration. The mechanism uses a repeating quaternary-binary link configuration to translate motion from one segment to the next. A synthesis of the mechanism is performed, such that with one input to the first segment of the chain, the other wing segments move into their desired positions. Linear aerodynamic theory is applied to the HECS wing configuration at certain morphed positions in order to predict the aerodynamic loads. This work performs a linear static analysis of the mechanism at different morphed positions. A finite element representation of the mechanism as a structure is developed. Using the predicted aerodynamic loads, a structural analysis is performed. The analysis investigates different materials and cross sections of the members to determine a need for redesign due to failure from buckling and bending stress. From the analysis of the mechanism, a design is finalized which lightens the structure as well as increases the strength. These results are beneficial for the next phase of model development of the mechanism. / 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.