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Results of picture frame shear tests with optical registration of the strain fields are presented for glass (plain and twill, three types) and glass/PP woven (plain and twill) fabric reinforcements for composite materials. Four problems were investigated. (1) How does the shear diagram vary with differences in test conditions? The major factor is the sample pretension, which is influenced by its gripping, removing/preserving yarns near the grips and “conditioning” in the shear cycles. (2) Does the shear of the fabric differ from the pure shear prescribed by the frame? The differences are normally negligible. (3) How large are the variations of the local fabric shear? The scatter of the local fabric shear does not exceed 2°. (4) How is shear of the fabric translated into deformations of the yarns on the micro-scale? Different stages of the fabric deformation are identified: rotation of the yarns followed by their lateral compression.

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A vibration energy harvester using magnet/piezoelectric composite transducer J. Appl. Phys. 115, 17E522 (2014); 10.1063/1.4867599Bi-stable energy harvesting based on a simply supported piezoelectric buckled beam

This study presents a shoe-mounted nonlinear piezoelectric energy harvester (PEH) with intent to capture energy from human walking. The PEH consists of a piezoelectric cantilever beam magnetically coupled to a ferromagnetic ball and a crossbeam. A sleeve is included to guide the travel of the ball. Experimental measurements and theoretical simulations demonstrate that the proposed design can collect energy from diverse excitation sources with different directions produced by the foot, including vibrations, swing motions, and the compressive force. The ball and the crossbeam sense the swing motion and the compressive force, respectively, and then actuate the piezoelectric beam to function. The piezoelectric beam senses the vibration along the tibial axis and generates electricity. The proposed PEH achieves the superposition of these excitations and generates multiple peaks in voltage output within one gait cycle. The output power generated by the fabricated prototype ranges from 0.03 mW to 0.35 mW when the walking velocity varies from 2 km/h to 8 km/h.

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This letter presents a monostable piezoelectric energy harvester (PEH) for achieving enhanced energy extraction from low-level excitations. The proposed PEH is realized by introducing symmetric magnetic attraction to a piezoelectric cantilever beam and a pair of stoppers to confine the maximum deflection of the beam. The lumped parameter model of such a system is presented and experimentally validated. Theoretical simulations and experimental measurements demonstrate that the proposed design can bring about a wider operating bandwidth and higher output voltage than the linear PEH. Under a sinusoidal vibration with an amplitude of 3 m/s2, a 54% increase in the operating bandwidth and a 253% increase in the magnitude of output power are achieved compared to its linear counterpart. Moreover, the proposed PEH exhibits rich dynamic features, including the tunable operating bandwidth, adjustable voltage and power levels, and softening hysteresis.

Harvesting ambient vibration energy is a promising method for realizing self-powered autonomous operation for low-power electronic devices. Most energy harvesters developed to date employ bending-beam configurations and work around the resonant points. There are two critical problems that have hindered the widespread adoption of energy harvesters: insufficient power output and narrow working bandwidth. To overcome these problems, we proposed a novel energy harvester, called a high-efficiency compressive-mode piezoelectric energy harvester (HC-PEH). The HC-PEH delicately synthesizes the merits of the force amplification effect of the flexural motion and the dynamic properties of elastic beams, and thus is capable of high power output with wide working bandwidth. In this paper, theoretical and experimental studies were performed on the HC-PEH. Taking nonlinear stiffness, nonlinear damping, and nonlinear piezoelectricity into account, we developed an analytical model that provides comprehensive insight into the nonlinear mechanical and electrical behaviors of the system. The analytical results closely render the experimental data and demonstrate great performance enhancement. In the experiment, a maximum power output of 54.7 mW is generated at 26 Hz under an acceleration of 4.9 m s −2 , which is over one order of magnitude higher than other stateof-the-art systems.