Browsing by Author "Dr. Fuh-Gwo Yuan, Committee Chair"

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Atomistic Simulations of Fracture of 2d Graphene Systems and the Elastic Properties of Carbon Nanotubes
(2004-11-15) Jin, Yun; Dr. Kara Peters, Committee Member; Dr. Eric C. Klang, Committee Member; Dr. Jeffrey W. Eischen, Committee Member; Dr. Fuh-Gwo Yuan, Committee Chair
The essential feature of all the macroscopic mechanical properties of a material are governed by constitutive atoms and the basic laws of physics. The traditional descriptions of fracture phenomena in solids have been developed almost exclusively from continuum concepts. These continuum approaches have successfully described many fracture mechanism in solid materials and continue to be of great use. Nevertheless, they still have some shortcomings, such as the spurious singularity of the stress field at crack tip. Hence, atomistic predictions may open new avenues in the studies of microscopic origins of material failure behavior. Also the highly active researches on the lightweight nano-structured materials in the past decade revealed the emergence of interests in predicting the properties of materials in the atomic level before they are synthesized. In this dissertation the fracture mechanism of a nanostructure material has been investigated by atomistic simulations. Macroscopic fracture parameters have been examined from both atomistic simulation and continuum models. There is a very good agreement between atomistic simulation and theoretical results from LEFM for the energy release rate in a semi-infinite graphene sheet containing cracks. The atomic description of the stress field in this case is also obtained and matches very well with the linear elastic solutions. Then another case in which a center-cracked graphene sheet with finite width is proposed. The energy release rates are obtained in both global energy approach and local force approach. These simulations show that, in macroscopic fracture mechanics under small deformation, linear elastic fracture mechanics is sufficient for the description of cracking behavior for this covalently bonded material. The results merge the discrete (atomistic) and continuum (macroscopic) description of facture. Meanwhile, the method to calculate J integral in the atomic system is successfully developed. The numerical results of J integral agree very well with the energy release rate in the same systems. Then after a necessary modification on the Tersoff-Brenner potential, the critical value of J integral, denoted by , is eventually reached as the measure of the fracture toughness of graphene sheet. The mechanical properties of single-walled carbon nanotube (SWNT) have also been evaluated in this dissertation. Several elastic moduli of SWNTs using the MD simulations were obtained at the atomic scale. The values of the in-plane Young¬°¬Øs modulus, rotational shear modulus, and in-plane Poisson's ratio are in the range of existing theoretical and experimental results. It has been shown from simulations that overall the elastic constants of SWNTs are insensitive to the morphology pattern such as nanotube radius and thus the effect of curvature on the elastic constants can be neglected. Assumption of the transversely isotropic properties on the cylinder surface of the single-walled nanotube was confirmed by numerical calculations. Besides the conventional energy approach, a new method, which denoted as force approach in the dissertation, has been developed to analysis the elastic properties of carbon nanotubes. The results from two approaches matched very well. The advantage of the force approach is that it can provide more accurate prediction than the energy approach. Furthermore, the force approach can predict the nonlinear behavior without assumption of assumed total potential energy in quadratic form described for small-strain deformation in the energy approach.
Elastic Wave Propagation in Composites and Least-squares Damage Localization Technique
(2004-07-29) Wang, Lei; Dr. Fuh-Gwo Yuan, Committee Chair; Dr. Kara Peters, Committee Member; Dr. Jeffrey W. Eischen, Committee Member
The main objective of Structural Health Monitoring (SHM) is to be able to continuously monitor and assess the status of the integrity of a structure or its components with a high level of confidence and reliability. In general, the common techniques employed in SHM for monitoring structures and detecting damages can be divided into two categories: (1) vibration-based approach and (2) wave-based approach. Since wave-based approach can provide better local health status information and has higher sensitivity to damages than vibration-based approach, this thesis focuses on damage localization of plate structure using wave-based approach by first characterizing elastic waves in composite laminates; then using a time-frequency signal processing technique to analyze dispersive stress waves; and lastly a least-squares technique is proposed for damage localization. Exact solutions of dispersive relations in a composite lamina and composite laminate are first deduced from three-dimensional (3-D) elasticity theory. The dispersion relations containing infinite number of symmetric and antisymmetric wave modes are numerically solved. Then, to make dispersive wave solutions tractable in composites, a higher-order plate theory is proposed. The dispersion relations of three antisymmetric wave modes and five symmetric wave modes can be analytically determined. The dispersion curves of phase velocity and group velocity are obtained from the two theories. From the results of the 3-D elasticity theory and higher-order plate theory, it can be seen from dispersion curves that the higher-order plate theory gives a good agreement in comparison with those obtained from 3-D elasticity theory in the relatively high frequency range; and especially for the lowest symmetric and antisymmetric modes, dispersion relation curves obtained from the two theories match very well. In the Chapter of time-frequency analysis of dispersive waves, a Wavelet Transform (WT) is directly performed on a transient dispersive wave to extract the time-frequency information of transient waves. Consequently, the dispersion relations of group velocity and phase velocity can be mathematically obtained. Experiments are set up to verify the proposed WT method, in which a lead break is used as a simulated acoustic emission source on the surface of an aluminum plate. The dispersion curves of both phase and group velocities of the lowest flexural wave mode obtained from the experiments by using WT show good agreement with theoretical prediction values. Having group velocities verified from the experiments, a least-squares method is proposed to SHM field for iteratively searching damage location based on elastic wave energy measurements. The method is suitable for achieving automated SHM system since the proposed method is based on active damage detection technique and deals with the entire sensor data in the least-squares algorithm without the need of ambiguously measuring the time-of-flights. The simulated data are obtained from finite difference method in conjunction with Mindlin plate theory. Simulated examples for damage detection are demonstrated by using the least-squares method. Moreover, an active SHM system is set up to validate the feasibility of the least-squares damage localization technique. From the simulated and experimental results, it is shown that the estimated damage position by least-squares method gives good agreement with the targeted damage location.
Vibration Energy Harvesting by Magnetostrictive Material for Powering Wireless Sensors
(2008-05-11) Wang, Lei; Dr. Gianluca Lazzi, Committee Member; Dr. Gregory D. Buckner, Committee Member; Dr. Kara J. Peters, Committee Member; Dr. Fuh-Gwo Yuan, Committee Chair
Wireless Sensor Networks (WSN) have been increasingly applied to Structural Health Monitoring (SHM). For WSN to achieve full potential, self-powering these sensor nodes needs to be developed. A promising approach is to seamlessly integrate energy harvesting techniques from ambient vibrations with the sensor to form a self-powered node. The objective of this study is to develop a new magnetostrictive material (MsM) vibration energy harvester for powering WISP (Wireless Intelligent Sensor Platform) developed by North Carolina State University. Apart from piezoelectric materials which currently dominate in low frequency vibration harvesting, this new method provides an alternate scheme which overcomes the major drawbacks of piezoelectric vibration energy harvesters and can operate at a higher frequency range. A new class of vibration energy harvester based on MsM, Metglas 2605SC, is deigned, developed, and tested. Compared to piezoelectric materials, Metglas 2605SC offers advantages including ultra-high energy conversion efficiency, high power density, longer life cycles, lack of depolarization, and high flexibility to survive in strong ambient vibrations. To enhance the energy conversion efficiency and shrink the size of the harvester, Metglas ribbons are transversely annealed by a strong magnetic field along its width direction to eliminate the need of bias magnetic field. Governing equations of motion for the MsM harvesting device are derived by Hamilton's Principle in conjunction with normal mode superposition method based on Euler-Bernoulli beam theory. This approach indicates the MsM laminate wound with a pick-up coil can be modeled as an electro-mechanical gyrator in series with an inductor. Then a generalized electrical-mechanical circuit mode is obtained. Such formulation is valid in a wide frequency range, not limited to below the fundamental natural frequency. In addition, the proposed model can be readily extended to a more practical case of a cantilever beam element with a tip mass. The model resulting in achievable output performances of the harvester powering a resistive load and charging a capacitive energy storage device, respectively, is quantitatively derived. An energy harvesting circuit, which interfaces with a wireless sensor, accumulates the harvested energy into an ultracapacitor, is designed on a printed circuit board (PCB) with plane dimension 25mm*35mm. It mainly consists of a voltage quadrupler, a 3F ultracapacitor, and a smart regulator. The output DC voltage from the PCB can be adjusted within 2.0˜5.5V which is compatible with most wireless sensor electronics. In experiments, a bimetallic cantilever beam method is developed to determine the piezomagnetic constant d from the measured Lambda-H curve. The maximum output power and power density on the resistor can reach 200 uW and 900 uW⁄cm3, respectively. For a working prototype, the average power and power density during charging the ultracapacitor can achieve 576 uW and 606 uW⁄cm3 respectively, which compete favorably with the piezoelectric vibration energy harvesters.