In this thesis, we present theoretical and experimental investigation into the nonlinear statics and dynamics of clamped-clamped in-plane MEMS arches when excited by an electrostatic force. Theoretically, we first solve the equation of motion using a multi- mode Galarkin Reduced Order Model (ROM). We investigate the static response of the arch experimentally where we show several jumps due to the snap-through instability. Experimentally, a case study of in-plane silicon micromachined arch is studied and its mechanical behavior is measured using optical techniques. We develop an algorithm to extract various parameters that are needed to model the arch, such as the induced axial force, the modulus of elasticity, and the initially induced initial rise. After that, we excite the arch by a DC electrostatic force superimposed to an AC harmonic load. A softening spring behavior is observed when the excitation is close to the first resonance frequency due to the quadratic nonlinearity coming from the arch geometry and the electrostatic force. Also, a hardening spring behavior is observed when the excitation is close to the third (second symmetric) resonance frequency due to the cubic nonlinearity coming from mid-plane stretching. Then, we excite the arch by an electric load of two AC frequency components, where we report a combination resonance of the summed type. Agreement is reported among the theoretical and experimental work.

Composite laminate structures offer promising aerospace applications owing to their multi-functional capabilities. These composites exhibit the unique ability to transition between stable geometries, passing through an intermediate, predicted unstable geometry. Actuation is achieved through the application of external loading conditions, encompassing both mechanical and thermal stimuli, with a particular emphasis on the latter. The nonlinear response induced by such transitions experimentally motivates this research due to the minimal amount of data on the transition section between geometries. Application of these composites under loading conditions proves interesting for a thermal management application with additional thermal loading present. This use would require the composite laminate to withstand various loads, as well as to satisfy varying geometry constraints. Boundary conditions play a pivotal role in governing the composite's response, with certain configurations proving ideal for maximizing deformation. Understanding the dynamic response due to these various conditions allows for the implementation of the bistable composites in our thermal management case. Investigation of the total response and the speed of actuation during the loading process highlights the composite's viability. Through the repeated testing, both analytically and experimentally, the composite laminates have proven that they are viable for the thermal management application. Experimental research into the application, through the addition of the flat plate installed directly into the thermal management case, or the addition of the various other geometries is of much interest as this study continues to move forward.

The postbuckling load carrying capacity of composite plates offers immense potential to their applications for loads exceeding their primary buckling load. However, such an efficient and economical usage of these plates can be reliable only if the nonlinear postbuckling behavior of these plates, which includes a good understanding of secondary buckling, is understood thoroughly. The present investigation is an attempt to understand secondary buckling of almost square composite clamped-simply supported plates, both unstiffened as well as stiffened, in some detail. With the help of the finite element method, a large number of numerical studies have been conducted to understand the secondary buckling characteristics. The sensitivity of these characteristics to variations in boundary conditions, lamination sequence, imperfections, and stiffener geometry has been considered. It has been found that the occurrence of secondary buckling in clamped-simply supported plates under uniform end shortening critically depends on the intensity of restrictions imposed on the inplane normal displacements along the unloaded simply supported edges of the plate. These restrictions could be due to the actual boundary conditions at these edges, or due to the presence of stiffeners along these edges. It has also been found that the presence of imperfections significantly delays the event of secondary buckling. Finally, it has been found that changes in lamination sequence of the plate alter its secondary buckling characteristics in ways that are, in general, quantitative in nature. The numerical investigations were followed by a limited number of experiments involving the testing of unstiffened as well as stiffened composite plates with the intent of augmenting the confidence in the numerical predictions made. Three different lamination sequences were considered during the testing phase of this investigation. It was found that the agreement between experimental data and numerical predictions was quite good. The occurrence of secondary buckling followed the predictions closely. / Ph. D.

Sandwich structures constitute basic components of advanced supersonic/hypersonic flight and launch vehicles. These advanced flight vehicles operate in hostile environments consisting of high temperature, moisture, and pressure fields. As a result, these structures are exposed to large lateral pressures, large compressive edge loads, and high temperature gradients which can create large stresses and strains within the structure and can produce the instability of the structure. This creates the need for a better understanding of the behavior of these structures under these complex loading conditions. Moreover, a better understanding of the load carrying capacity of sandwich structures constitutes an essential step towards a more rational design and exploitation of these constructions. In order to address these issues, a comprehensive geometrically non-linear theory of doubly curved sandwich structures constructed of anisotropic laminated face sheets with an orthotropic core under various loadings for simply supported edge conditions is developed. The effects of the radii of curvature, initial geometric imperfections, pressure, uniaxial compressive edge loads, biaxial edge loading consisting of compressive/tensile edge loads, and thermal loads will be analyzed. The effect of the structural tailoring of the facesheets upon the load carrying capacity of the structure under these various loading conditions are analyzed. In addition, the movability/immovability of the unloaded edges and the end-shortening are examined. To pursue this study, two different formulations of the theory are developed. One of these formulations is referred to as the mixed formulation, While the second formulation is referred to as the displacement formulation. Several results are presented encompassing buckling, postbuckling, and stress/strain analysis in conjunction with the application of the structural tailoring technique. The great effects of this technique are explored. Moreover, comparisons with the available theoretical and experimental results are presented and good agreements are reported. / Ph. D.

General unsymmetric laminates exhibit large natural curvatures at room temperature. Additionally, inherent to most unsymmetric laminates is the presence of two stable configurations. Multiple configurations and stability issues arise because of the geometric nonlinearities associated with the large curvatures. The laminate can be changed from one stable configuration to the other by a simple snap-through action. This situation offers the opportunity to use shape memory alloys (SMA) attached to the laminate to generate the snap-through forces and change the shape of the laminate on command. Presented is a model which can predict SMA-induced deformations in general unsymmetric laminates and, particularly, the occurrence of the snap through. First, a methodology is developed to predict the deformations of flat general unsymmetric epoxy-matrix composite laminates as they are cooled from their elevated cure temperature. Approximations to the strain fields are used in the expression for the total potential energy, and the Rayleigh-Ritz approach is used to study equilibrium. To further study the laminate deformations, finite-element analyses are performed. Experimental results are presented which confirm the predictions of the developed theory and the finite-element analyses regarding the existence of multiple solutions and the magnitude of the deformations. Results are compared with those of several other investigators. Next, the deformation behavior of general unsymmetric laminates subjected to applied forces is studied. The principle of virtual work is used to derive the equilibrium equations relating the laminate deformations to the applied forces. By solving the equilibrium equations as a function of the force level, relations between the laminate deformations and the applied force are derived, and the force level at which the laminate changes shape is determined. Finally, an existing SMA constitutive model is implemented into the developed theory to predict the deformations of simple structures to SMA-induced forces. Experiments on a narrow aluminium plate with an externally attached SMA actuator are conducted. The experimental results show good agreement with the predictions from the developed theory. Next, the deformation behavior of general unsymmetric laminates subjected to SMA actuators is predicted using the developed theory. Experiments using SMA actuators to generate the snap through of nsymmetric laminates are conducted. Good correlation with the developed theory is obtained. / Ph. D.

The geometrically nonlinear response of a shallow, circular, cylindrical panel under a midspan line load and induced strain actuation is presented. The panel is a laminate of piezoelectric material perfectly bonded to the convex and concave surfaces of a core of passive material. Since the curved edges are free and the straight edges are pinned a fixed distance apart, the response of the panel is independent of the axial coordinate. Hence, the governing ordinary differential equations are of the same form as for a shallow circular arch. Without induced strain actuation, the panel exhibits snap-through behavior under the midspan load. Induced strain distributions are determined at a constant midspan load to displace the panel to an inverted configuration in a stable manner. This adaptive structure may find application as an electromechanical, nonlinear spring with a digital-like, load-displacement response characteristic. / Master of Science

<p>The primary focus of this dissertation is the characterization of snap-through buckling of discrete and continuous systems. Snap-through buckling occurs as the consequence of two factors, first the destabilization, or more often the disappearance of, an equilibrium position under the change of a system parameter, and second the existence of another stable equilibrium configuration at a remote location in state space. In this sense snap-through buckling is a global dynamic transition as the result of a local static instability.</p><p> </p><p>In order to better understand the static instabilities that lead to snap-through buckling, the behavior of mechanical systems in the vicinity of various local bifurcations is first investigated. Oscillators with saddle-node, pitchfork, and transcritical bifurcations are shown analytically to exhibit several interesting characteristics, particularly in relation to the system damping ratio. A simple mechanical oscillator with a transcritical bifurcation is used to experimentally verify the analytical results. The transcritical bifurcation was selected since it may be used to represent generic bifurcation behavior. It is shown that the damping ratio may be used to predict changes in stability with respect to changing system parameters.</p><p>Another useful indicator of snap-through is the presence of chaos in the dynamic response of a system. Chaos is usually associated snap-through, as in many systems large amplitude responses are typically necessary to sufficiently engage the nonlinearities that induce chaos. Thus, a pragmatic approach for identifying chaos in experimental (and hence noisy) systems is also developed. The method is applied to multiple experimental systems showing good agreement with identification via Lyapunov exponents.</p><p>Under dynamic loading, systems with the requisite condition for snap-through buckling, that is co-existing equilibria, typically exhibit either small amplitude response about a single equilibrium configuration, or large amplitude response that transits between the static equilibria. Dynamic snap-through is the name given to the large amplitude response, which, in the context of structural systems, is obviously undesirable. This phenomenon is investigated using experimental, numerical, and analytical means and the boundaries separating safe (non-snap-through) from unsafe (snap-through) dynamic response in forcing parameter space are obtained for both a discrete and a continuous arch. Arches present an ideal avenue for the investigation of snap-through as they typically have multiple, often tunable, stable and unstable equilibria. They also have many direct applications in both civil engineering, where arches are a canonical structural element, and mechanical engineering, where arches may be used to approximate the behavior of curved plates and panels such as those used on aircraft.</p> / Dissertation

No description available.

Arch-shaped microelectromechanical systems (MEMS) have been used as mechanical memories, micro-sensors, micro-actuators, and micro-valves. A bi-stable structure, such as an arch, is characterized by a multivalued load deflection curve. Here we study the symmetry breaking, the snap-through instability, and the pull-in instability of bi-stable arch shaped MEMS under steady and transient electric loads. We analyze transient finite electroelastodynamic deformations of perfect electrically conducting clamped-clamped beams and arches suspended over a flat rigid semi-infinite perfect conductor. The coupled nonlinear partial differential equations (PDEs) for mechanical deformations are solved numerically by the finite element method (FEM) and those for the electrical problem by the boundary element method. The coupled nonlinear PDE governing transient deformations of the arch based on the Euler-Bernoulli beam theory is solved numerically using the Galerkin method, mode shapes for a beam as basis functions, and integrated numerically with respect to time. For the static problem, the displacement control and the pseudo-arc length continuation (PALC) methods are used to obtain the bifurcation curve of arch's deflection versus the electric potential. The displacement control method fails to compute arch's asymmetric deformations that are found by the PALC method. For the dynamic problem, two distinct mechanisms of the snap-through instability are found. It is shown that critical loads and geometric parameters for instabilities of an arch with and without the consideration of mechanical inertia effects are quite different. A phase diagram between a critical load parameter and the arch height is constructed to delineate different regions of instabilities. The local water slamming refers to the impact of a part of a ship hull on stationary water for a short duration during which high local pressures occur. We simulate slamming impact of rigid and deformable hull bottom panels by using the coupled Lagrangian and Eulerian formulation in the commercial FE software LS-DYNA. The Lagrangian formulation is used to describe planestrain deformations of the wedge and the Eulerian description of motion for deformations of the water. A penalty contact algorithm couples the wedge with the water surface. Damage and delamination induced, respectively, in a fiber reinforced composite panel and a sandwich composite panel and due to hydroelastic pressure are studied. / Ph. D.

As a new concept for morphing structures, the use of piezoelectric actuators to effect snap-through behavior of simple unsymmetric cross-ply composite laminates is examined. Many unsymmetric laminates have more than one stable room-temperature shape and can be snapped through from one stable shape to another. In this new concept for morphing structures, one or more piezoelectric actuators are bonded to unsymmetric laminates, and are then used to snap the laminate from one shape to another. The actuator would be used to change shape, but would not be required to maintain the shape. Using the Rayleigh-Ritz technique, several models are developed to predict the interaction between the base laminate and the actuator. In particular, the voltage (applied to the actuator) needed to snap the laminate is predicted. The NASA-LaRC Macro-Fiber Composite&174; (MFC&174;) actuator is chosen as the actuator of choice for this work. A laminate is manufactured, an actuator is bonded to the laminate, and experiments are performed. Since the agreement between the initial models and experimental results was not good, the models were revised. Good agreement between the predictions of the revised model and experiment is reached. Suggestions for future research directions are presented. / Ph. D.