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Reinforced fiber polymer composites are a class of materials that are composed of multiple constituents that work together to create a material specific for applications. By combining different fibers and matricies, laminates can be created that meet demands for high specific stiffness, damping specifications, and electrical resistance. However, their internal complexity subjects them to a number of internal failure modes that have the potential to fail the laminate. Those failure mechanisms are fiber breaking, microcracking in the matrix, debonding of the fibers from matrix, and delamination of ply layers. To assess these failures, nondestructive evaluation methods have been developed to detect internal damage before catastrophic failure occurs. This dissertation investigates an in-situ magnetostrictive based nondestructive method for monitoring delaminations in carbon fiber reinforced polymer laminates by using embedded Terfenol-D particles. The objective is to characterize how laminate ply count and delamination presence affect sensing through the mechanical and magnetic parameters that influence the induced voltage or sensing signal. In addition, the effect of magnetostriction on the formation and propagation of cracks on the sensor boundaries are also investigated. Methods used to characterize this behavior involve experimental testing, analytical, and numerical modeling. From the results, a threedimensional finite element analysis model reveals how the sensor interacts mechanically with the host structure through lower stresses in the delaminated region due to the absence of adhesive forces. The stress variation results in a local magnetic permeability change which influences the induced voltage. The experimental nondestructive testing show that the key parameter influencing the sensing signal for this setup was the particle density, which is controlled by fabrication process. An attempt to analytically model the experimental sensing signal with a first order differential equation using a multi-step process was successful, but there is poor correlation with the experimental results. Finally, analytical mechanics are developed to evaluate the interlaminar failure under a magnetostrictive stress of 55MPa, and was found to not cause interlaminar failure or delamination propagation in Section-A.

High strength, light weight and flexibility have made fabrics the preferred material for personal body armor and other impact protection applications such as passenger airbags, turbine blade containment systems, military and motorcycle helmets, and space debris shields. Recently, a shear thickening fluid has been used to treat a Kevlar fabric for an additional enhancement to the ballistic resistance of the neat fabric. Motivated by this technique of dissipation augmentation to high strength fabrics, this research aims at investigating the incorporation of other energy-dissipative materials into high strength fabrics. Specifically, two magnetic field-responsive materials (a magnetorheological fluid and Terfenol-D) have been used as a dissipation augmentation of Kevlar fabrics. No previous work has reported either experimental or computational research on the impact dynamics of Kevlar fabric treated with magnetorheological fluids or magnetostrictive solids. This research has investigated both computational modeling and experimental evaluation of the impact dynamics of textile composite armor, treated with magnetic field-responsive materials. Fragment simulating projectile impact tests have been conducted for the fabricated composite targets under an applied magnetic field. A computational model based on a hybrid particle-element method has been developed, to simulate the impact dynamics of composite fabric targets embodying magnetorheological fluids. This model is a mesoscale multiphysics model which can simulate impact dynamics including complex magneto-thermo-mechanical coupling effects as well as interactions among a projectile, fabric yarns, and magnetorheological fluid particles. Computer simulations have been performed to validate the hybrid particle-element method against experimental results. The computational method developed in this research has shown good agreement with the experimental data, in terms of the ballistic limit and residual velocity of a striking projectile. As fabric impact protection systems become more complex, and more expensive materials are introduced, computation may play a more important role in design. Therefore, the hybrid particle-element model in this dissertation may contribute to the improvement of the computational capability for virtual prototyping of fabric-interstitial fluid composites. / text

Thin films of Strontium Ruthenate have been grown on Strontium Titanate and Lanthanum Aluminate (100) substrates by pulsed laser deposition. X-ray diffraction results show that the films grown on the Strontium Titanate are amorphous and polycrystalline on the Lanthanum Aluminate. Resistances versus temperature measurements show that the films exhibit semiconducting characteristics. In addition to the growth of Strontium Ruthenate thin films, multilayer heterostructures of Terfenol-D thin films on polycrystalline Lead Titanate thin films were grown by pulsed laser deposition. By using a novel experimental technique called magnetic field assisted piezoelectric force microscopy it is possible to investigate the magnetoelectric coupling between the electrostrictive Lead Titanate and magnetostrictive Terfenol-D thin film. Upon examination of the produced thin films the phase and amplitude components of the piezoelectric signal experience changes in response to an applied in-plane magnetic field. These changes provide experimental evidence of a magnetoelectric coupling between the Terfenol-D and Lead Titanate layers.

A nonlinear active vibration absorber to control the vibrations of plates is investigated. The absorber is based on the saturation phenomenon associated with dynamical systems with quadratic nonlinearities and a two-to-one internal resonance. The technique is implemented by coupling a second-order controller with the plate's response through a sensor and an actuator. Energy is exchanged between the primary structure and the controller and, near resonance, the plate's response saturates to a small value. Numerical as well as experimental results are presented for a cantilever rectangular plate. For numerical studies, finite-element methods as well as modal analysis are implemented. The commercially available software ABAQUS is used in the finite-element analysis together with a user-provided subroutine to model the controller. For the experimental studies, the plate is excited using a dynamic shaker. Strain gages are used as sensors, while piezoelectric ceramic patches are used as actuators. The control technique is implemented using a dSPACE digital signal processing board and a modeling software (SIMULINK). Both numerical and experimental results show that the control strategy is very efficient. A numerical study is conducted to optimize the location of the actuators on the structure to maximize its controllability. In this regard, the control gain is maximized for the PZT actuators. Furthermore, a more general method is introduced that is based on a global measure of controllability for linear systems. Finally, the control strategy is made adaptive by incorporating an efficient frequency-measurement technique. This is validated by successfully testing the control strategy for a non-conventional problem, where nonlinear effects hinder the application of the non-adaptive controller. / Ph. D.

We tackle the problem of suppressing high-amplitude vibrations of cantilever beams when subjected to either primary external or principal parametric resonances. Guided by results of previous investigations into the nonlinear dynamics of single- and multi-degree-of-freedom structures, we design mechatronic systems of sensors, actuators, and electronic devices and implement nonlinear active feedback control. In the case of external excitation, we devise two vibration absorbers based on either quadratic or cubic feedback. We conduct theoretical analyses and demonstrate that when a two-to-one (one-to-one) internal resonance condition is imposed between the plant and the quadratic (cubic) absorber, there exists a saturation phenomenon. When the plant is forced near its resonant frequency and the forcing amplitude exceeds a certain small threshold, the nonlinear coupling creates an energy-transfer mechanism that limits (saturates) the response of the plant. Our theoretical studies reveal that the cubic absorber creates regimes of high-amplitude quasiperiodic and chaotic responses, thereby limiting its utility. However, we show that superior results can be achieved when the natural frequency of the quadratic absorber is set equal to one-half the excitation frequency. Consequently, we apply the quadratic technique through a variety of linear and nonlinear actuators, sensors, and electronic devices. We design and build second-order analog circuits that emulate the quadratic absorber. Using a DC motor, piezoelectric ceramics, and Terfenol-D struts as actuators and potentiometers, strain gages, and accelerometers as sensors, we demonstrate successful single- and multi-mode vibration control. In order to realize a more versatile implementation of the control strategy, we resort to a digital signal processing (DSP) board. We compose a code in C and design a digital absorber by developing algorithms that, in addition to replacing the analog circuit, automatically detect the amplitude and frequency of oscillation of the plant and fine-tune the absorber parameters. We take advantage of the digital realization, implement a linear absorber, and compare the performance of the quadratic absorber with that of its linear counterpart. In the case of parametric excitation, we investigate two techniques. First, we explore application of the quadratic absorber. We prove theoretically and demonstrate experimentally that this control scheme is not reliable. Then, we propose an alternate approach. We devise a control law based on cubic velocity feedback. We conduct theoretical and experimental investigations and show that the latter strategy leads to effective vibration suppression and bifurcation control. / Ph. D.

This thesis deals with the idea of energy harvesting from mechanical vibration. It describes the magnetostrictive principle as a possibility to obtain an electrical energy. It is about a generator made of a beam with Terfenol-D material and a coil. The model of this device is created in Matlab/Simulink and FEMM application. For various values of measured vibration, these methods are then compared. In FEMM, you can improve energy gain by modeling of various environmental conditions and with using of another materials (e.g. by adding of permanent magnets). The outcome of the this thesis expresses the ability to harvest the energy with designed magnetostrictive generator compared to the already created models of the piezoelectric and electromagnetic generator. Based on these results, it is possible to determine which generator is more suitable for particular application.

A mechatronic system of actuators, sensors, and analog circuits is demonstrated to control the self-excited oscillations known as chatter that occur when single-point turning a rigid workpiece with a flexible tool. The nature of this manufacturing process, its complex geometry, harsh operating environment, and poorly understood physics, present considerable challenges to the control system designer. The actuators and sensors must be rugged and of exceptionally high bandwidth and the control must be robust in the presence of unmodeled dynamics. In this regard, the qualitative characterization of the chatter instability itself becomes important. Chatter vibrations are finite and recognized as limit cycles, yet modeling and control efforts have routinely focused only on the linearized problem. The question naturally arises as to whether the nonlinear stability is characterized by a jump phenomenon. If so, what does this imply for the "robustness" of linear control solutions? To answer our question, we present an advanced hardware and control system design for a boring bar application. Initially, we treat the cutting forces merely as an unknown disturbance to the structure which is essentially a cantilevered beam. We then approximate the structure as a linear single-degree-of-freedom damped oscillator in each of the two principal modal coordinates and seek a control strategy that reduces the system response to general disturbances. Modal-based control strategies originally developed for the control of large flexible space structures are employed; they use second-order compensators to enhance selectively the damping of the modes identified for control. To attack the problem of the nonlinear stability, we seek a model that captures some of the behavior observed in experiments. We design this model based on observations and intuition because theoretical expressions for the complex dynamic forces generated during cutting are lacking. We begin by assuming a regenerative chatter mechanism, as is common practice, and presume that it has a nonlinear form, which is approximated using a cubic polynomial. Experiments demonstrate that the cutting forces couple the two principal modal coordinates. To obtain the jump phenomena observed experimentally, we find it necessary to account for structural nonlinearies. Gradually, using experimental observation as a guide, we arrive at a two-degree-of-freedom chatter model for the boring process. We analyze the stability of this model using the modern methods of nonlinear dynamics. We apply the method of multiple scales to determine the local nonlinear normal form of the bifurcation from static to dynamic cutting. We then find the subsequent periodic motions by employing the method of harmonic balance. The stability of these periodic motions is analysed using Floquet theory. Working from a model that captures the essential nonlinear behavior, we develop a new post-bifurcation control strategy based on quench control. We observe that nonlinear state feedback can be used to control the amplitude of post-bifurcation limit cycles. Judicious selection of this nonlinear state feedback makes a supplementary open-loop control strategy possible. By injecting a harmonic force with a frequency incommensurate with the chatter frequency, we find that the self-excited chatter can be exchanged for a forced vibratory response, thereby reducing tool motions. / Ph. D.

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