A hierarchical, 2-D, displacement-based, global/local finite element model is developed to permit an accurate, efficient analysis of localized 3-D effects in laminated composite plates. The model is developed using hierarchical, multiple assumed displacement fields at two different levels: (1) at the element level, and (2) at the mesh level. First, by superimposing a hierarchy of assumed displacement fields within the same finite element domain, a new variable kinematic, finite element is developed. The displacement field hierarchy contains both a conventional 2-D plate expansion and a full layerwise expansion. Depending on the accuracy desired, the variable kinematic element can use various terms from the composite displacement field, thus creating a hierarchy of different elements having a wide range of kinematic complexity and representing a number of different mathematical models. Since the resulting model is hierarchic, these different element types can easily be connected together in the same computational domain to permit simultaneous multiple model analysis. Despite the obvious utility of variable kinematic finite elements, a multiple model analysis based solely on the use of these elements has a significant restriction: all subregions of the computational domain must maintain in-plane mesh compatibility along subregion boundaries. This restriction necessitates the use of 2-D transition zones. In an effort to avoid the problems associated with 2-D transition zones, hierarchical, multiple assumed displacement fields are used at the mesh level in a finite element mesh superposition scheme. In this application of the finite element mesh superposition technique. the variable kinematic elements are used to form the independent, local, overlay meshes that can be superimposed on a pre-existing mesh of conventional 2-D plate elements. Due to the hierarchical nature of the resulting composite displacement field, the overlay mesh and the original mesh need not have compatible discretization. Thus the specifications and superimposed location of the overlay mesh can be tailored to fit the needs of the analyst regardless of the global mesh topology. The resulting model is used to analyze a number of laminated composite plate problems that contain localized subregions where significant 3-D stress fields exist (e.g. free edge effects, delamination fronts, and adhesive bonds). / Ph. D.

Thin, elastic, shallow shells having uniform thickness and rectangular boundaries are investigated. The boundary conditions are either simply supported or claped, and the shell is subjected to a uniformly distributed load applied over either the full shell area or a central region. The thickness, material properties, edge lengths, and surface area of the shell are specified, and the objective is the determination of the shell shape which will maximize the buckling load. Marguerre's two, coupled, non-linear equations of equilibrium are used to describe prebuckling deformations and stresses. Considering small vibrations about the equilibrium state, two, coupled, linear equations of motion are derived. Subsequently, by recognizing that at buckling the lowest frequency of vibration goes to zero, the buckling equations are obtained. Finally, the Lagrange multiplier technique is employed to formulate an augmented objective function, and the calculus of variations is applied in order to derive the governing set of equations. The resulting system of equations is solved numerically by the finite difference method. Results for shells with various surface areas are presented. For each surface area the investigation is performed on shells having either clamped or simply supported boundary conditions and either a square or a rectangular boundary. The applied uniform load covers either the full shell area or a partial central region. The shell form, buckling load, and buckling modes of the optimal forms are compared with those of the reference form (double sine) having the same surface area, and changes are noted. Also, comparisons with respect to forms, buckling load, and type of buckling are made between the optimal form of a shell subjected to a full uniform load and the optimal form of the same shell subjected to a partial uniform load. In some cases, the buckling load of the optimal form is sensitive to imperfections in the form or in the loading distribution as well as to changes in the design. In these cases, some of the apparent advantages of the optimum form may be diminished. Thus, the frequencies of vibration at buckling, the corresponding buckling modes, and the presence of adjacent equilibrium states are monitored in order to evaluate the sensitivity of the optimal form to imperfections and to design changes. / Ph. D. / incomplete_metadata

This work investigates the potential of reducing stresses in the region of stress concentration by applied induced strains. A thin 30 inch square plate with a 1 inch diameter circular hole under uniaxial load was used for this investigation. This investigation considered first an ideal case with a few physical limitations for the purpose of probing the limits of active stress reduction. Applied induced strains were applied over the region A ≤ r ≤ 1.5A, where A is the radius of the hole. It was found that the axisymmetric applied induced strains could reduce the stress concentration factor (SCF) from 3 to 2. With non-axisymmetric applied induced strain distribution the SCF could be reduced to 1.45. Numerical optimizations based on finite element simulations were also carried out for a composite plate with a hole and similar reductions in stress concentration factors were obtained. Next, a more realistic case, consisting of bonded and embedded piezoelectric actuators was considered. It was found that partial-thickness actuators produce large radial stresses which erase any benefits associated with axisymmetric actuation. With non-axisymmetric actuation, the actuators with present technology limitations were found to be effective in reducing stress concentration factor for a fatigue load case. However, due to repetitive nature of the load, the energy expenditure may be large. For an extreme load case, actuators with present technology limitations were not very effective in reducing SCF. Actuators were needed to be placed over a larger area to achieve a larger reduction in SCF. Also, passive stiffening was found to be more effective in reducing stress concentration than bonded actuators with present technology limitations. / Ph. D.

Typescript (photocopy). / Digitized by Kansas Correctional Industries

Thesis. 1975. M.S.--Massachusetts Institute of Technology. Dept. of Civil Engineering. / Bibliography: leaves 124-129. / M.S.

Soil nail walls are a form of mechanical earth stabilization for cut situations. They consist of the introduction of passive inclusions (nails) into soil cut lifts. These nailed lifts are then tied together with a structural facing (usually shotcrete) . The wall lifts are constructed incrementally from the top of cut down. Soil nail walls are being recognized as having potential for large cost savings over other alternatives. The increasing need to provide high capacity roadways in restricted rights of way under structures such as bridges will require increasing use of techniques such as combined soil nail and piling walls. The Swift Delta Soil Nail wall required installing nails between some of the existing pipe piling on the Oregon Slough Bridge. This raised questions of whether the piling would undergo internal stress changes due to the nail wall construction. Thus, it was considered necessary to understand the soil nail wall structure interaction in relation to the existing pile supported abutment. The purpose of this study was to investigate the Swift Delta Wall using finite element (FE) modeling techniques. Valuable data were available from the instrumentation of the swift Delta Wall. These data were compared with the results of the FE modeling. This study attempts to answer the following two questions: 1. Is there potential for the introduction of new bending stresses to the existing piling? 2. Is the soil nail wall system influenced by the presence of the piling? A general purpose FE code called ABAQUS was used to perform both linear and non-linear analyses. The analyses showed that the piling definitely underwent some stress changes. In addition they also indicated that piling influence resulted in lower nail stresses. Comparison of measured data to predicted behavior showed good agreement in wall face deflection but inconsistent agreement in nail stresses. This demonstrated the difficulty of modeling a soil nail due to the many variables resulting from nail installation.

This thesis focuses on the developing of statistical methodology in reliability and quality engineering, and to assist the decision-makings at enterprise level, process level, and product level. In Chapter II, we propose a multi-level statistical modeling strategy to characterize data from spatial logistics systems. The model can support business decisions at different levels. The information available from higher hierarchies is incorporated into the multi-level model as constraint functions for lower hierarchies. The key contributions include proposing the top-down multi-level spatial models which improve the estimation accuracy at lower levels; applying the spatial smoothing techniques to solve facility location problems in logistics. In Chapter III, we propose methods for modeling system service reliability in a supply chain, which may be disrupted by uncertain contingent events. This chapter applies an approximation technique for developing first-cut reliability analysis models. The approximation relies on multi-level spatial models to characterize patterns of store locations and demands. The key contributions in this chapter are to bring statistical spatial modeling techniques to approximate store location and demand data, and to build system reliability models entertaining various scenarios of DC location designs and DC capacity constraints. Chapter IV investigates the power law process, which has proved to be a useful tool in characterizing the failure process of repairable systems. This chapter presents a procedure for detecting and estimating a mixture of conforming and nonconforming systems. The key contributions in this chapter are to investigate the property of parameter estimation in mixture repair processes, and to propose an effective way to screen out nonconforming products. The key contributions in Chapter V are to propose a new method to analyze heavily censored accelerated life testing data, and to study the asymptotic properties. This approach flexibly and rigorously incorporates distribution assumptions and regression structures into estimating equations in a nonparametric estimation framework. Derivations of asymptotic properties of the proposed method provide an opportunity to compare its estimation quality to commonly used parametric MLE methods in the situation of mis-specified regression models.

This dissertation presents several methods for improving statistical and first principles modeling capabilities, with an emphasis on nonlinear, unsteady state batch processes. Batch process online monitoring is chosen as a main research area here due to its importance from both theoretical and practical points of view. Theoretical background and recent developments of PCA/PLS-based online monitoring methodologies are reviewed, along with fault detection metrics, and algorithm variations for different applications. The available commercial softwares are also evaluated based on the corresponding application area. A detailed Multiway PCA based batch online monitoring procedure is used as the starting point for further improvements. The issue of dynamic batch profile synchronization is addressed. By converting synchronization into a dynamic optimization problem, Dynamic Time Warping (DTW) and Derivative DTW (DDTW) show the best performance by far. To deal with the singularity point and numerical derivative estimation problems of DTW and DDTW in the presence of noise, a robust DDTW algorithm is proposed by combining Savitzky-Golay filter and DDTW algorithm together. A comparative analysis of robust DDTW and available methods is performed on simulated and real chemical plant data. As traditional Multiway PCA-based (MPCA) methods consider batch monitoring in a static fashion (fail to consider time dependency between/within process variables with respect to time), an EWMA filtered Hybrid-wise unfolding MPCA (E-HMPCA) is proposed that considers batch dynamics in the model and reduce the number of Type I and II errors in online monitoring. Chemical and biochemical batch examples are used to compare the E-HMPCA algorithm with traditional methods. First principles modeling is known to be time consuming for development. In order to increase modeling efficiency, dynamic Design of Experiments (DOE) is introduced for Dynamic Algebraic Equation (DAE) system parameter estimation. A new criterion is proposed by combining PCA and parameter sensitivity analysis (P-optimal criterion). The new criterion under certain assumptions reduce to several available criteria and is suitable for designing experiments to improve estimation of specific parameter sets. Furthermore, the criterion systematically decomposes a complex system into small pieces according to PCA. Two engineering examples (one batch, one continuous) are used to illustrate the idea and algorithm. / text

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