The magnetoelectric (ME) effect is a property that results in power/energy conversion between magnetic and electric forms. Two-phase composites consisting of magnetostrictive and piezoelectric materials have been developed that show remarkable ME voltage/charge coefficients. This extrinsic ME effect is achieved by using mechanical coupling as a medium between the magnetostrictive and piezoelectric phases. As described in this thesis, I investigated the optimization of the material properties of sensors/gradiometers, transmitters, and gyrator applications using ME heterostructures with a multi-push-pull structure. In applications, ME sensors will need to work in an open environment where there will be a mix of magnetic signals and microphonic noises. Prior research has determined that both passive and active mode ME sensors are affected by vibrational noise in the open environment. Therefore, as described herein, an ME gradiometer consisting of a pair of ME sensors working under H-field modulation (active mode) was developed to address the issue of microphonic noise. The common mode rejection ratio of my ME gradiometer was determined to be 74. Gradiometer curves were also measured, which presented the gradiometer outputs as a function of the normalized distance between the magnetic source and the ME gradiometer. Based on resulting data, the proposed ME gradiometer was confirmed to be capable of significant vibration noise rejection. However, this method is not appropriate for rejecting longitudinal vibrations due to the propagation direction being the same as the magnetic field. To resolve this dilemma, a new ME laminate structure was designed that could better reject vibrational noise. Additionally, two different configurations were developed to measure the gradiometer curve. Second, in order to understand how much energy can be wirelessly transmitted by ME laminates within a local area, a portable (area ~ 16 cm^2), a very low-frequency transmitter was developed using ME laminate with Metglas/PZT structure. The proposed strain-driven ME laminate transmitter functions as follows: (a) a piezoelectric layer is first driven by alternating current electric voltage at its electromechanical resonance (EMR) frequency; (b) subsequently, this EMR excites the magnetostrictive layers, giving rise to a magnetization change; (c) in turn, the magnetization oscillations result in oscillating magnetic fluxes, which can be detected through the use of a search coil as a receiver. The prototype measurements revealed an induction transmission capabilities in the near field. Furthermore, the developed prototype evidenced a 10^4 times higher efficiency in the near field over a small-circular loop of the same area, exhibiting its superiority over the class of traditional small antennas. Next, recent efforts in our group resulted in the development of an ME gyrator based on ME heterostructures. Such gyrators facilitate current-to-voltage conversion with high power efficiency. ME gyrators working at their resonance frequency are capable of converting power with an efficiency of > 90 %, which show potential for use in power convertors. Here, we found that the resonance frequency could be tuned through the use of a frequency-modulation technique. Accordingly, this method can be utilized to match the frequency difference between the power supply and the piezoelectric transducer in actual applications, which will increase the power efficiency. Another problematic issue is that the electromechanical coupling factor of piezoelectric transducers is limited by bandwidth. Typically, transducers cannot be impedance matched to a power supply, which significantly reduces power efficiency. Our initial studies have shown that an improved impedance match can be realized by using an ME gyrator to geometrically tune a transducer, which will substantially enhance power efficiency. The last chapter will mainly focus on ME gyrator applications. Designing linear power amplifiers that operate reliably at high frequency is quite challenging, which is mainly due to the fact that the parasitic impedances of their electronic components tend to dominate at higher frequencies, thereby leading to significant power-efficiency loss. Therefore, ME gyrator may play an important role between the power amplifier and the acoustic transducer to reduce the power loss. In this chapter, we achieved the impedance matching between a piezoelectric transducer and a power supply by implementing geometric changes to the gyrator. Both the power efficiency of an individual ME gyrator and a piezoelectric transducer are > 90%. Therefore, the total power efficiency of the ME gyrator and the piezoelectric transducer also approach > 80% when they got connected together. The second aspect of this chapter pertains to resonance-frequency tuning using three method. Since an ME gyrator will be used to achieve impedance matching, the resonance frequency of the ME gyrator and a piezoelectric transducer may not exactly match. This limitation will be overcome through capacitance tuning of the piezoelectric transducer in order to achieve frequency matching. Finally, an equivalent circuit will be developed that connects a piezoelectric transducer with a gyrator, thereby enabling the impedance of the output port of the transducer and the shifted EMR frequency of the transducer to be modified. / Doctor of Philosophy / In my dissertation, I focus on the magnetoelectric (ME) effect is a property that results in power/energy conversion between magnetic and electric forms. Two-phase composites consisting of magnetostrictive and piezoelectric materials have been developed that show remarkable ME voltage/charge coefficients. As described in this dissertation, I investigated the optimization of the material properties of sensors/gradiometers, transmitters, and gyrator applications using ME heterostructures with a multi-push-pull structure. An ME gradiometer consisting of a pair of ME sensors working under H-field modulation (active mode) was developed to address the issue of microphonic noise. The common mode rejection ratio of my ME gradiometer was determined to be 74. Gradiometer curves were also measured, which presented the gradiometer outputs as a function of the normalized distance between the magnetic source and the ME gradiometer. Besides that, a new ME laminate structure was designed that could better reject vibrational noise. Second, in order to understand how much energy can be wirelessly transmitted by ME laminates within a local area, a portable, a very low-frequency transmitter was developed using ME laminate with Metglas/PZT structure. The prototype measurements revealed an induction transmission capability in the near field. Furthermore, the developed prototype evidenced a 10^4 times higher efficiency in the near field over a small-circular loop of the same area, exhibiting its superiority over the class of traditional small antennas. In the last chapter, we achieved the impedance matching between a piezoelectric transducer and a power supply by implementing geometric changes to the gyrator. The total power efficiency of the ME gyrator and the piezoelectric transducer approach > 80% when they got connected together. The second aspect of this chapter pertains to resonance-frequency tuning using three methods. Finally, an equivalent circuit will be developed that connects a piezoelectric transducer with a gyrator, thereby enabling the impedance of the output port of the transducer and the shifted EMR frequency of the transducer to be modified.

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Tensile static and tension-tension fatigue behaviour has been studied in coupon specimens made from continuous fibre reinforced glass/epoxy and carbon/epoxy laminates of various lay-ups, including a series of GFRP and CFRP 0,90,0 cross-ply laminates with different 90° ply thicknesses and CFRP 0, 90, +/-45 laminates with different ply stacking sequences. A variety of techniques has been used to monitor the accumulation of damage; microscopy on the polished edge of coupons, penetrant-enhanced X-radiography for CFRP laminates and visual observations for the transparent GFRP laminates. Under static loading, mechanical properties and damage thresholds are established for the onset of events such as cracking in the 90' and 45° plies and delamination. The experimentally determined 90° ply cracking threshold strains agree with predictions based on fracture mechanics, provided residual thermal strains are taken into account. Fatigue failure data are obtained for the CFRP laminates and plotted as conventional S-logN curves. The fatigue behaviour of the CFRP laminates has also been described qualitatively using a form of fatigue life representation in terms of the predominant damage mechanisms observed during cyclic loading. A detailed study of transverse ply matrix cracking showed that the mode of crack propagation depended on the type of loading. Crack growth across the width of the ply was instantaneous under static loading and at high cyclic stresses. At low cyclic stresses, i.e. below the static cracking threshold, cracks initiated only after a number of cycles (dependent on the stress level) and then grew slowly across the width of the ply throughout the course of loading. Slow crack growth was also observed at high cyclic stresses when the density of full width cracks was high and the crack spacing was small. The crack growth rate was found to be independent of crack length and to depend on crack spacing and hence was strongly influenced by interactions between neighbouring cracks. Fatigue growth of individual cracks was modelled using an approach based on an expression for the stress intensity factor at the tip of a transverse ply crack and a Paris fatigue crack growth relationship.

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This thesis presents novel research in the area of energy harvesting from broadband vibra-tions. The aim of energy harvesting is to recover energy wasted or unused in the environmentto power low-consumption devices on the order of hundreds of microwatts to milliwatts. The motivation is twofold. In providing a localized, self-contained power source, device reliability, flexibility of installation location can be improved, and maintenance costs can be reduced. Furthermore, reduced reliance on batteries will mitigate the environmental impact associated with resource extraction, and disposal. To this end, this thesis investigates bistable laminates with piezoelectric transduction as broadband energy harvesters. Hitherto, a wealth of literature exists in which narrowband energy harvesters have been studied and optimized to operate over a small frequency interval. While these have been successful to the point of having devices commercially available, many situations exist where the dominant frequencies from which energy is to be harvested change with respect to time, or may be dominated by noise, thus not having a truly dominating frequency. Energy harvesters with nonlinear frequency responses have attracted substantial research interest because of their ability to respond over a broaderfrequency band. Due to complexities of the response of these harvesters, particularly when the intensity of the vibrational input is high, modeling their behavior is difficult. Designing these harvesters is therefore challenging as the relationships between the various design parameters and power output can be highly involved, or require numerical solutions as analytical solutions may not be possible. This thesis helps to address this knowledge gap. Bistable laminates ofboth cantilever and plate configuration are studied. Parametric studies are undertaken to empirically demonstrate the relationship between power output and parameters such as resistance load, proof mass addition, operation orientation, different shapes, ply angles, and introduction of adjustable magnetic compression. Modeling work is also undertaken to capture the mainfeatures of the nonlinear response such as subharmonics, superharmonics, and snap-through. A study is also carried out to quantify the differences of performance between a linear harvester and an equivalent bistable counterpart. As a practical demonstration, some plate-type harvesters are subjected to excitation patterns based on measured train data. Ultimately, thisthesis provides an in depth understanding of bistable shape, layup, and design on harvesting performance.

Multifunctional hybrid composites are proposed as novel solutions to meet the demands in various industrial applications ranging from aerospace to biomedicine. The combination of carbon fibers and/or fabric, metal foil and carbon nanotubes are utilized to develop such composites. This study focuses on processing of and fracture toughness characterization of the carbon fiber reinforced polymer matrix composites (PMC) and the CNT modified interface between PMC and a metal foil. The laminate fabrication process using H-VARTM, and the mode I interlaminar fracture toughness via double cantilever beam (DCB) tests at both room temperature and high temperature are conducted. The cross-sections and fracture surfaces of the panels are characterized using optical and scanning electron microscopes to verify the existence of CNTs at the interface before and after fracture tests. The experimental results reveal that CNT’s improve bonding at the hybrid interfaces. Computational models are developed to assist the interpretation of experimental results and further investigate damage modes. In this work, analytical solutions to compute the total strain energy release rate as well as mode I and mode II strain energy release rates of asymmetric configurations layups are utilized. Finite element models are developed in which the virtual crack closure technique is adopted to calculate strain energy release rates and investigate the degree and effect of mode-mixity. Results from analytical solutions agree well with each other and with results obtained from finite element models.

The fatigue response of mechanical properties and life due to constant stress amplitude tension-tension(T-T)cyclic loading at elevated temperature in nanocomposite APC-2 laminates was investigated. From the basic testing the total amount of 1% by weight of SiO2 spreaded in the interfaces was proved optimally. Related experiments on unidirectional nanocomposite APC-2 laminates were completed, including static tension tests in [0]16¡B[30]16¡B[45]16¡B[60]16 and [90]16 and T-T cyclic tests in [0]16¡B[45]16 and [90]16 specimens at room temperature. After obtaining experimental data, such as ultimate strength and elastic modulus, which were found improved significantly, and then comparing with the basic theory of mechanics of composites, rule of mixtures was adopted to estimate the properties of cross-ply and quasi-isotropic nanocomposite APC-2 laminates and found the largest errors were within 25%. In the consideration of heterogeneous and anisotropic properties of the matrix and the reinforced fibers in nature, the results are reasonably acceptable. On the other hand, the S-N curves according to the experimental data of the fatigue tests were plotted. The vertical axis shows the nondimensional stress level, i.e., the applied maximum stress normalized by ultimate strength at room temperature, and the horizontal axis represents the logarithm of applied cycles. The S-N curves at room and elevated temperatures were also expressed by curve fitting from top to bottom as temperature increasing from RT to 150¢J for both cross-ply and quasi-isotropic nanocomposite laminates. However, when applied maximum stress was normalized by the corresponding ultimate strength, the positions of S-N curves were reverse, i.e., the curves were shown from bottom to top as temperature increasing from RT to 150¢J. That strongly hints us the resistance to fatigue at elevated temperature in both lay-ups of nanocomposite laminates is indeed significantly improved.

This thesis aims to fabricate the high performance Magnesium/Carbon-Fiber/PEEK five-layer hybrid nanocomposite laminates. The adopted Mg thin sheets are 0.5mm thick. The Carbon-Fiber/PEEK prepregs were stacked into two lay-ups, such as cross-ply [0/90]s and quasi-isotropic [0/45/90/-45], with the adding of nanoparticles SiO2 spreaded among the laminates. After etching of Mg foils by CrO3-base etchants, a five-layered Mg/Carbon-Fiber/PEEK nanocomposite laminate was made according to the modified diaphragm curing process. Then, the mechanical properties, such as stress-strain curve, strength and stiffness were obtained by tensile test at room temperature (25¢J), 50, 75, 100, 125 and 150¢J and the fatigue properties were also obtained under constant stress amplitude tension-tension cyclic loading elevated at room and elevated temperatures 25, 75, 100, 125 and 150¢J. Finally, the Mg sheets and fractured laminates were observed by the SEM and OM. The results according to the experiments were summarized as follows: 1.The slope of stress-strain curve dropped at strain¡Ü0.0015. It can be inferred that fracture occurred in the laminates at this time. Stiffness approached the theoretical value by curve fitting with the strain range of 0 to 0.0015. 2.The mechanical properties decreased with the environmental temperature rise. 3.The resistance to the temperature effect of the quasi-isotropic Magnesium/Carbon-Fiber/PEEK laminate is superior to that of the cross-ply Magnesium/Carbon-Fiber/PEEK laminate. 4.The cross-ply Magnesium/Carbon-Fiber/PEEK laminate is brittler than that of the quasi-isotropic laminate generally. 5.The irregular bright lines were found in the third etched Mg sheet and that resulted in the delamination of Mg sheet after treatment. The unetched part maybe the defect of Mg sheet. 6. It was found that AZ31 has the precipitation hardening effect at 50¢J and 75¢J.

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