Short-circuit Frequencies Research Articles

Energy harvesting using variable stiffness piezolaminated shallow shell panels exposed to hygrothermal environments

The effect of hygrothermal environment on the energy harvesting capabilities of piezolaminated shallow shell panels made of variable stiffness composite layers (VSCL) is investigated. The use of curvilinear fibers to tailor the characteristics of the energy harvester is also explored. To model the deformation of the harvester, a finite element model based on a first-order normal shear deformation theory (FNSDT) is developed. Nonlinear strain–displacement relations based on Green–Lagrange strains are considered for nonuniform stress distribution due to temperature and moisture. A nine noded isoparametric element is used to discretize the domain. The effects of temperature and moisture concentration change on Power frequency response functions (FRFs), Voltage FRFs and Motion FRFs are presented. Power FRFs, Voltage FRFs and Motion FRFs are plotted for optimum load resistance when the shell structure is excited at short and open circuit frequencies. The current study is expected to help design and optimize the vibrational energy harvester to power sensors used in different automotive, aerospace, nuclear applications, subjected to elevated temperature and moisture conditions.

Tri‐band radar cross‐section reduction based on optimised multi‐element phase cancellation

This study presents a planar single-layer metasurface, with tri-band 10 dB radar cross-section (RCS) reduction under normal incidence for both polarisations, by employing optimised multi-element phase cancellation. The designed artificial magnetic conductor (AMC) unit cell is composed of an upper metal structure, a dielectric layer, and a bottom metal ground. When the dielectric constant and thickness of the substrate are fixed, regardless of any upper structure, the reflection coefficients of AMC are the same as the perfect electric conductor (PEC) at the short-circuit frequencies. Taking into account the multiple reduction bands that avoid short-circuit frequencies, a RCS reduction surface with a large frequency ratio is achieved. The optimised planar metasurface obtains 10 dB RCS reduction in the tri-band frequency (3.81–7.33, 19.16–22.83, and 35.63–38.37 GHz) with bandwidths of 63, 17, and 7.4%, respectively. Moreover, the bistatic scattering patterns between the proposed surface and the equal-sized PEC are investigated. The theoretically predicted, simulated, and measured results are in good agreement and provide a new strategy for achieving multi-band RCS reduction.

On the vibration analysis of coupled transverse and shear piezoelectric functionally graded porous beams with higher-order theories

This investigation aims to perform a detailed natural frequency analysis of functionally graded porous beams integrated with transverse ( d31) and shear ( d15) piezoelectric layers under short circuit and open circuit electrical conditions. It is assumed that the core layer is made of functionally graded materials containing porosities. Due to the existence of internal pores, the mechanical properties of functionally graded materials are considered according to the modified power-law rule which includes the effect of porosity. The distribution of electric potential within the d31 and d15 piezoelectric layers is modeled based on nonlinear variation for both short circuit and open circuit conditions. Employing the classical, the first-order, and the higher-order beam theories incorporated with the virtual work principle as well as Maxwell’s equation, the electromechanical equations of motion are derived. The governing equations are then solved analytically for simply supported boundary condition and a parametric study is presented. After validation of the results, some new interesting conclusions covering the effects of porosity volume fraction, porosity distribution, various piezoelectricity modes, power-law index, and the beam theories on short circuit and open circuit resonance frequencies are reported. It is believed that the presented numerical results could provide a benchmark to check the accuracy of the approximated approaches.

Piezoelectric energy harvesting using macro fiber composite patches

Over the last decade, vibration energy harvesting has received substantial attention of many researchers. Piezoelectric materials are able to capture energy from ambient vibration and convert it into electricity which can be stored in batteries or utilized to power small electronic devices. In order to benefit from the 33-mode of the piezoelectric effect, interdigitated electrodes have been utilized in the design of macro fiber composites which are made of piezoelectric fibers of square cross sections embedded into an epoxy matrix material. This paper presents an analytical model of a macro fiber composite bimorph energy harvester using the 33-mode. The mixing rule is applied to determine the equivalent and homogenized properties of the macro fiber composite structures. The electromechanical properties of a representative volume element composed of piezoelectric fibers and an epoxy matrix between two successive interdigitated electrodes are coupled with the overall electro-elastodynamics of the harvester utilizing the Euler–Bernoulli theory. Macro fiber composite bimorph cantilevers with diverse widths are simulated for power generation when a resistive shunt loading is applied. Stress components in the Kapton layers, which are typically a part of any macro fiber composite patch, and in the bonding layers have been included in the model contrary to previously published studies. Variable tip mass, attached at the free end of the beam, is utilized in this paper to tune the resonance frequency of the harvester. The generated power at the fundamental short circuit and open circuit resonance frequencies of harvesters having three different widths is analyzed. It has been observed that higher electrical outputs are produced by the wider macro fiber composite bimorph using (M8528-P1 patches).

Multi-mode piezoelectric shunt damping with residual mode correction by evaluation of modal charge and voltage

A resonant, piezoelectric shunt tuning procedure is based on the limiting eigenvalue problems associated with short and open circuiting of the piezoelectric domains attached to a vibrating structure. Whereas short circuit and open circuit frequencies are directly obtained from the eigenvalues, the associated electrical equation further determines a modal charge as an short circuit reaction force and a new modal voltage from the electric deflection in the open circuit limit. By a modal representation with short circuit mode shapes the structural equation fully decomposes, while the inherent contributions from non-resonant vibration modes in the corresponding electrical equation consistently define an effective modal capacitance, conveniently estimated by the modal charge to voltage ratio. The shunt tuning is obtained from the governing characteristic equation for the targeted vibration mode, in which the residual mode correction is explicitly represented by the effective modal capacitance. The tuning procedure with an effective modal capacitance is generalized to multiple piezoelectric domains with independent shunts for simultaneous damping of multiple vibration modes. The accuracy of the proposed shunt tuning methods is demonstrated by a numerical beam example with three piezoceramic patch pairs and independent resistive–inductive shunts. The analysis is carried out in a commercial finite element program, in which the required modal frequencies, charge and voltage are readily available as output to the short circuit and open circuit eigenvalue problems.

Robust electromechanical finite element updating for piezoelectric structures effective coupling prediction

A robust electromechanical updating methodology for piezoelectric structures effective coupling prediction is first presented. It combines a finite element design experiment–based a priori sensitivity analysis, a response surface method–based meta-modelling and a genetic algorithm–based multi-objective optimization procedure. Then, this methodology is applied to cantilever aluminium thick plate and thin beam structures that are bonded, respectively, with two oppositely poled large and oppositely and same poled small piezoceramic patches on their upper and lower surfaces. In order to correlate corresponding first few effective modal electromechanical coupling coefficients, a mechanical updating is first considered; it consists of identifying the stiffness parameters of linear springs, modelling the clamp, so that relative deviations between the first few experimental and finite element short-circuit frequencies are minimized; then, an electric updating is considered; it consists of finding the patches’ relative transverse blocked dielectric constants that minimize the first few experimental and finite element open-circuit frequencies relative deviations. Free vibrations are simulated using ANSYS®-coupled piezoelectric three-dimensional finite element. The obtained results have partially confirmed those from a former ad hoc updating method since the sensitivity analyses led to same stiffness parameters’ irreducible number and only some effective modal electromechanical coupling coefficient test–model correlations were enhanced.

Research on short circuiting transfer mode of ultrasonic assisted GMAW method

Ultrasonic assisted gas metal arc welding (U-GMAW) has been recently developed to improve the metal transfer characteristics. The ultrasonic wave is applied on the metal transfer process by means of an acoustic field. Welding electrical signal measurement and high speed camera are employed to study the differences of short circuiting metal transfer between conventional GMAW and U-GMAW. Compared with the conventional GMAW, the short circuit frequencies of U-GMAW are obviously increased under the same welding parameters. Moreover, the voltage range of the stable short circuiting transfer is enlarged, and the weld appearances become more uniform with the action of the ultrasonic wave. The high speed video images indicate that the U-GMAW arc is compressed and the electrical field intensity is increased. The decrease in the arc length is the main reason for the increase in the short circuit frequency.

Tuning of sol–gel derived PZT MEMS resonators

This paper demonstrates the tunability of resonant frequencies for MEMS piezoelectric resonators acting in the d33 mode by experiment and theoretical analysis. Thin-film MEMS beams made by sol–gel PZT processing are first fabricated and tested to investigate the tuning capability. The three-layered interdigitated-electrode trapezoidal beams are then modeled by finite element analysis for validation. Beam curl and undercutting are also examined to present an alternative way of finding the stress gradient in comparison to Stoney's formula. The experimental and analytical results indicate that piezoelectric MEMS resonators have the ability to passively tune their resonant frequencies between open-circuit and short-circuit frequencies. Tuning of 0.2–0.6% is demonstrated experimentally, which compares with finite element predictions of 1.02–1.08%. Consideration is given to the reason for the differences in experimental percentages versus those predicted numerically, including the use of bulk PZT values in the ANSYS simulations, the undercut and curling effects of fabrication, and the low percentage of piezoelectric poling in the length direction.