Investigation of a novel multiresonant beam energy harvester and a complex conjugate matching circuit

The aim of the work described in this thesis is firstly to improve the collection of vibration energy for piezoelectric cantilever harvesters, by a mechanical technique, so that the devices can harvest energy over a wider bandwidth. Secondly to investigate a new circuit topology for achieving complex conjugate load matching to the piezoelectric harvester. The thesis has been divided into two parts - the mechanical approach and the electrical approach. For the mechanical approach, a novel multiresonant beam, comprising piezoelectric fiber composites on a clamped-clamped beam and side mounted cantilevers, was proposed. The side cantilevers are tuned by tip masses to be resonant at different frequencies. A Rayleigh-Ritz model was developed to predict the vibration response of the proposed model multiresonant beam. This model showed that the bandwidth of the multiresonant beam was increased over that of a single cantilever harvester. A multiresonant beam for energy harvesting was experimentally tested and compared with a single cantilever energy harvester. The transmissibility and voltage responses were investigated, the beam showed a wide frequency response between 14.5Hz and 31Hz, whereas the single cantilever only showed one resonant frequency. Therefore the multiresonant beam system is feasible for wide band energy harvesting. For the electrical approach, the task was to investigate complex conjugate impedance matching for the piezoelectric energy harvesters, so that the output impedance from the piezoelectric harvester can be reduced, and maximum energy extracted from the device with a possibility of frequency tuning. A new amplified inductor circuit was proposed to enable the capacitive output impedance of the piezoelectric device to be cancelled. Experimental and software simulations are provided to verify the theoretical predictions. A prototype amplified inductor circuit was simulated and tested. The results showed that a variable effective inductance was achieved. However the circuit is lossy due to imperfections within the system, and needs further work to eliminate these imperfections.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

A nonlocal nonlinear stiffened shell theory with stiffeners modeled as geometrically-exact beams

In the present work, we have developed a nonlocal and nonlinear structure mechanics theory and computational formulations for the nonlinear stiffened shell with its stiffeners modeled by the three-dimensional geometrically exact beam model while the shell part is modeled by three-dimensional geometrically exact shell model. This hybrid beam-shell geometrically exact stiffened shell model is used for solving dynamic problems of stiffened shell structures, including dynamic fracture of the shell. We first formulate the nonlocal equations for geometrically nonlinear beams using the kinematics of geometrically exact beam theory. Then, we develop a stiffened shell model by coupling the nonlocal geometrically exact beam and shell formulations. Several numerical examples are presented to validate and verify both the nonlocal and nonlinear beam formulation and the nonlocal/nonlinear stiffened shell formulation. We have demonstrated that the proposed nonlocal stiffened shell theory can model the crack growth in stiffened shell structures.Q. Zhang, A.M Zhang, and Y.X. Peng are supported by grants from National Natural Science Foundation of China (Grant number 51925904, 51909042)

New Insights into the Role of Portlandite in the Cement System: Elastic Anisotropy, Thermal Stability, and Structural Compatibility with C-S-H

Portlandite, Ca(OH)2, is a primary product in cement hydration, on which some biases or misconceptions persist, such as portlandite has inferior mechanical properties because of its layered microstructure and crystal brittleness. In this work, the mechanical properties of portlandite are systematically investigated by using first-principles methods and atomic force microscopy. No dramatic difference among average elastic constants is observed between portlandite crystal and C-S-H gel, but only a slight drop in elastic modulus, at ∼12%. In fact, portlandite even has better performance under uniaxial tension loading in the a and b directions in comparison to that of C-S-H. However, portlandite is quite anisotropic with its Young\u27s modulus anisotropy reaching 3.50, because of its stacking layers in the c axis. We have found that because of its small yield stress in the c direction (1.5 GPa) and softening behavior, portlandite can be classified as a very soft material. By using atomic force microscopy, we find that the indentation modulus of a perfect portlandite is about 15.32 \ub1 4.25 GPa. Moreover, we have also studied the size effects as well as defects in the portlandite crystal by using both molecular dynamics simulations and electron microscopy. On the basis of the results, we proposed a new C-S-H gel growth model by postulating an "epitaxial growth on monolayer portlandite template", which explains and elucidates the reactive hydration process, in a dynamic sequence of "protonating, attracting, and stacking", during C-S-H nucleation and growth

Nanoengineering Microstructure of Hybrid C-S-H/Silicene Gel

Two-dimensional (2D) materials have been incorporated into calcium silicate hydrate (C-S-H) gel to enhance its mechanical performance for decades, while the modified C-S-H gel exhibits poor toughness, tensile strength, and ductility. In this work, we report a new design strategy and synthesis route to strengthen C-S-H interface by intercalating a silicene sheet of one atom thickness. The hybrid C-S-H/Silicene gel shows superb mechanical properties, with a remarkable enhancement in strength and other functional properties. By using density functional theory (DFT) and molecular dynamics (MD) simulations, we have demonstrated that Si-O bonds between silicene and C-S-H are stable and covalent, and the interaction energy of this bilayer gel nearly doubles by forming a 3D covalent network with a strong bridging effect. Owing to its better crystallinity enrichment and its induced dislocation dissipation mechanism, the hybrid C-S-H/Silicene gel possesses a higher tensile ductility (similar to 118% average enhancement and similar to 228% in the c direction) and a much smaller elastic stiffness (59.04 GPa for average Young\u27s modulus). This work offers an ingenuous route in turning brittle C-S-H gel into a soft gel, which provides opportunities for fabricating ultrahigh performance cementitious materials

Higher-order nonlocal operator theory for phase-field modeling of ductile fracture in elasto-plastic materials

In this work, we propose a novel approach based on the higher-order nonlocal operator for the phase-field modeling of ductile fracture in elasto-plastic materials. The present method introduces the total energy function consisting of the elastic, plastic, and fracture terms. The plasticity is coupled with the fracture through the degradation function which is applied to the tensile part of the elastic strain energy. The proposed higher-order nonlocal operator method brings several advantages over the original nonlocal operator method that requires only first-order terms. Moreover, the proposed method does not require the direct computation of kernel function or moment matrix derivatives. Therefore, this approach can improve the computational efficiency and simplify in implementation. The accuracy and effectiveness of the proposed method are demonstrated through various numerical examples, which have the ability to detect complex patterns of ductile fracture, such as crack propagation and plastic localization.National Research Foundation (NRF)This work was supported by the National Research Foundation, Prime Minister’s Office, Singapore under its Medium-Sized Centre funding scheme through the Marine and Offshore Program