Increasing Excitation Level Research Articles

Nonlinear vibrations of a ballistic vehicle with friction joints subjected to vibroacoustic excitation - experiments, modeling and simulations

This study presents the experiments, modeling and numerical simulations of an industrial aeronautical structure subjected to vibroacoustic excitation. In flight, especially during atmospheric re-entry, an aeronautical structure is exposed to major pressure fluctuations on its external surface. These fluctuations are described as a dynamic excitation which generates structural vibrations of the outer surface and of the components inside the aeronautical structure. Due to some of these internal components such as joints, the assembly may exhibit a nonlinear vibration response. The simulation of these nonlinear vibrations requires reliable modeling of the wall pressure fluctuations and a nonlinear vibration simulation method adapted to the nonlinear modeling of the structure. In this study, a modeling and simulation method is developed to compute the nonlinear vibration response of an industrial assembly to such a surface, random, correlated dynamic excitation. More specifically, numerical simulations are performed by proposing an extension of the well-known Harmonic Balance Method for nonlinear mechanical systems subjected to complex vibroacoustic excitation. The method is validated using a dedicated ground experiment. A metallic industrial assembly representing a ballistic vehicle and including friction joints is used. This structure is subjected to controlled vibroacoustic excitation: diffuse acoustic loading in a reverberant chamber. The structure exhibits a nonlinear vibration response due to friction. The finite element model is validated through an experimental modal analysis with an electrodynamic shaker. The vibroacoustic modeling of the excitation is then validated through a test-simulation comparison using diffuse acoustic field testing at low excitation level. Then the global nonlinear simulation process is validated using diffuse acoustic field testing at increasing excitation levels. Test and simulation results exhibit the same nonlinear behavior: increase of dissipation and softening effect at the main resonances. This work thus represents a new step towards the use of nonlinear vibration simulation methods with real industrial structures and real-life loading.

Experimental study of granular particle stacks’ circumferential edge-constraint effect and the level- and time-dependent acoustical behavior

Granular materials, such as activated carbon, have shown advantages in absorbing low-frequency sounds, but they also exhibit significant nonlinearities. In this article, three unique types of granular particle stacks’ acoustical behavior are described and experimentally analyzed. First, when measuring the sound absorption spectra of particle stacks with varying depths and sample holder sizes, it was found that the frictional interaction with the tube circumferential wall can significantly affect the measurements, and a 1-D plane-wave acoustical model is only valid when the sample is thin. Secondly, when granular samples were tested with increasing excitation levels, the results indicated a decreased modulus and an increased damping. By further varying excitation’s level and bandwidth, a fluid-displacement-related metric was identified to describe this behavior. Thirdly, when measuring particle stacks over a longer period, the stack’s resonance peak gradually shifts to a higher frequency as the sample consolidates. This frequency shift was proportional to log(Time).

Нестационарные колебания в системе из двух связанных осцилляторов в условиях кубической нелинейности и кубической связи. Часть 2. Вариация параметров системы

The task about the excitation of magnetoelastic vibrations in geometry of magnetostriction transducer which contains normal magnetized ferrite plate excited by alternating magnetic field is investigated. The dynamics of transducer is investigated on the basis of equations system for nonlinear vibrations of two connected oscillators – magnetic and elastic. The equation for magnetic oscillator is nonlinear and for elastic oscillator – linear. The nonlinearity and connections between oscillators have cubic character. In connection with excitation level there are five basal regimes: synchronize, three-multiplication of frequency, chaos, gigantic oscillations and delayed stabilization. The first two regimes are stable, without-threshold and completely determined. The remaining three regimes are non-stable and separated form stable regimes by sharp threshold by the excitation level. It is shown that by the increasing excitation level the vibrations amplitude approach to saturation experiences in value jumps about one and half or two times. The dependence of vibrations threshold of non-stable vibrations from excitation frequency is investigated. It is shown that by increasing the excitation frequency the threshold on non-stable vibrations in very high precision increases by power low of third ore fourth order. The character of vibrations by variation of oscillator dissipation is investigated. It is found that the primary influence on the character first oscillator vibrations exercise the dissipation not first but second oscillator. The variation of own oscillators frequencies is investigated. It is shown that the increasing of own oscillator frequencies leads the system to chaos and further to regularization of vibrations. The decreasing of own oscillator frequencies leads to compressed sine with centre displaced regime and leaving of system to infinity. The variation of cubic nonlinearity of first oscillator is investigated. The large nonsymmetry of variation of vibration character by nonlinearity parameter to both sides is established. The decreasing of nonlinearity parameter leads to compressed sine with displaced centre and further leaving of system to infinity. The increasing of nonlinearity parameter leads to chaos and regularization. The variation of connection between oscillators is investigated. It is found that the variation of connection parameters any of oscillators leads to the same order of regime replacement – from regularization to chaos, gigantic oscillations, delayed stabilization, compressed sine and leaving to infinity. On the plane in coordinates first and second connection parameters the regions of existence different regimes of vibrations is constructed. For the interpretation of described phenomena the model of dynamic potential is proposed. It is shown that the potential of first oscillator has minimum the place of which is determined by value of displacement of second oscillator. The course of chaotic vibrations in non-stable regimes is the difference of phase between vibrations of oscillator and jumps of potential in which these vibrations take place. The ring character of vibrations excitation in system as a whole is considered. The analogy with autovibrations in radioengineering system with positive opposite connection is proposed. The recommendations for further development of investigation are proposed.

SEISMIC TORSIONAL BEHAVIOR OF A TESTED FULL-SCALE STEEL BUILDING

Based on shaking experiment results of a full-scale 4-story steel building conducted on E-Defense – the world largest shaking table in Japan, the characterization of twisting behavior can be achieved. Exploration on twisting motion of the specimen building is presented in this paper, addressing global rotational responses in various increasing excitation levels. Study results show that the torsion movement of the building is mainly due to story mass eccentricity. On the other hand, the shaking-table also causes considerable influence on the building twisting response. System identification and mode superposition of response translational acceleration at floor mass centers are carried out. Some justifications for stiffness of non-structural are also presented, through which we can get a more precise basis to determine stiffness contribution of each component.

Limits and Opportunities for Miniaturizing Ultrasonic Surgical Devices Based on a Langevin Transducer.

Minimally invasive surgery offers opportunities for reduced morbidities, faster postoperative recovery, and reduced costs, and is a major focus of surgical device innovation. For ultrasonic surgical devices, which offer benefits of high precision, low force, and tissue selectivity in surgical procedures, there exist laparoscopic ultrasonic shears for minimally invasive surgeries that combine tissue cutting with vessel hemostasis and sealing functions. Another approach to laparoscopy that could enable new procedures, and increase the sites of surgeries that could be reached by an ultrasonic device, involves integrating a miniature ultrasonic tool with a flexible surgical robot. However, miniaturization presents challenges in delivering the ultrasonic vibrational energy required to cut hard and soft tissues, partly due to the concomitant small volume of piezoelectric material. This article aims to provide insights into the trade-offs between transducer size, volume of piezoceramic material, resonance frequency, and the achievable displacement amplitude of devices that, consistent with current ultrasonic surgical tools, are based on a bolted Langevin transducer (BLT) and tip. Different configurations of BLTs are studied, including a cascaded version, simple bar versions, and BLTs with different front mass geometries. Results show that a BLT with a larger number of piezoceramic rings exhibits a higher coupling coefficient [Formula: see text] but with the compromise of a lower mechanical Q and stronger nonlinear response at increasing excitation levels. Displacement amplitude is reduced considerably when a BLT is excited at a higher harmonic, where the PZT rings are maintained at a nodal plane, and the resonance frequency shift at increasing excitation levels increases significantly. The electromechanical and dynamic characteristics of a cascaded transducer excited in its third longitudinal mode (L3) are almost equivalent to a much shorter version of a BLT driven at the same frequency but in its first longitudinal mode (L1), showing that a cascaded BLT can be a realistic proxy for studying the dynamics of small BLT devices. A new figure of merit is proposed that is the product of Q , [Formula: see text], and gain, which [Formula: see text] accounts for the gain of cylindrical BLTs which is shown not to be unity. It also proves effective as it incorporates the key factors affecting the achievable displacement amplitude of a BLT, including for BLTs with gain profiles in the front mass. The order of highest to lowest amplitude of a series of six gain-profile BLTs matches the order estimated by the figure of merit. It is shown that a BLT with a stepped profile front mass can achieve displacement that has the potential to cut hard or soft tissue and exhibits the smallest shifts in resonance frequency at increasing excitation levels.

A new concept of frequency-excitation-up conversion to improve the yield of linear piezoelectric generators

A new and novel frequency-excitation up-conversion (FEUC) mechanism achieved by converting low-frequency excitation into friction-induced vibration (FIV) is proposed to improve the output power of linear piezoelectric energy harvesters at ultra-low ambient frequencies. For comparison, a conventional resonant energy harvester and an impact-driven energy harvester are created, the output power is measured at the resonant frequency under a 0.5 g driving acceleration. Then, a friction system driven by reciprocating sliding and capable of generating FIV is established. The driving reciprocating is equivalent to the low-frequency ambient vibration, and a single piezoelectric cantilever beam is added to the friction system for energy recovery. The results show that, compared with the impact-driven energy harvester, the vibration frequency can be increased hundreds of times, and the excitation level can be improved several orders of magnitude by the friction nonlinearities. The impedance of the energy harvester is reduced significantly due to a sharp increase in the vibration frequency. In addition, the open-circuit voltage amplitude is not substantially lowered due to the significantly increased excitation level, even though the piezoelectric cantilever beam operates in a high-frequency condition and in the absence of resonance. Therefore, the output power is dramatically improved. The effectiveness and superiority of the FEUC strategy proposed in this work has been demonstrated.

Revealing photoluminescence mechanisms of single CsPbBr3/Cs4PbBr6 core/shell perovskite nanocrystals.

CsPbBr3 nanocrystals (NCs) encapsulated by Cs4PbBr6 has attracted extensive attention due to good stability and high photoluminescence (PL) emission efficiency. However, the origin of photoluminescence (PL) emission from CsPbBr3/Cs4PbBr6 composite materials has been controversial. In this work, we prepare CsPbBr3/Cs4PbBr6 core/shell nanoparticles and firstly study the mechanism of its photoluminescence (PL) at the single-particle level. Based on photoluminescence (PL) intensity trajectories and photon antibunching measurements, we have found that photoluminescence (PL) intensity trajectories of individual CsPbBr3/Cs4PbBr6 core/shell NCs vary from the uniform longer periods to multiple-step intensity behaviors with increasing excitation level. Meanwhile, second-order photon correlation functions exhibit single photon emission behaviors especially at lower excitation levels. However, the PL intensity trajectories of individual Cs4PbBr6 NCs demonstrate apparent “burst-like” behaviors with very high values of g2(0) at any excitation power. Therefore, the distinguishable emission statistics help us to elucidate whether the photoluminescence (PL) emission of CsPbBr3/Cs4PbBr6 core/shell NCs stems from band-edge exciton recombination of CsPbBr3 NCs or intrinsic Br vacancy states of Cs4PbBr6 NCs. These findings provide key information about the origin of emission in CsPbBr3/Cs4PbBr6 core/shell nanoparticles, which improves their utilization in the further optoelectronic applications.

Shake table test of a masonry building retrofitted with shotcrete

This paper presents the results of a series of shake table tests carried out on a half-scale single-story unreinforced masonry building with asymmetric openings. First, the unretrofitted building is subjected to seven increasing stepsof bidirectional seismic excitation. The damaged building is then rehabilitated using steel mesh and shotcrete layer with two walls retrofitted from the exterior face and the other two from the interior face. Afterward, the shake table test is again conducted on the retrofitted specimen in nine increasing excitation levels. Three cases of interior-to-interior, interior-to-exterior, and exterior-to-exterior shotcrete connections are considered at the intersection of perpendicular walls, and for each case, the development of cracks is investigated. Moreover, the effects of fixity of the shotcrete vertical rebars in the foundation are investigated through releasing the rebar-foundation connection in the last three steps of the test on the rehabilitated specimen. The results indicate the adequacy of the retrofit method in creating a strong bond that leads to an acceptable composite action between the brick layer and shotcrete; though, concentrated cracks were observed at the connections of the perpendicular walls. Furthermore, the shotcrete layer has proven to be able to prevent further propagation of the previously developed cracks, which can provide a practical solution for rehabilitation of damaged masonry buildings.

Interactions of large-scale structures in the near field of round jets at high Reynolds numbers

A possible interaction between vortical structures in a round jet shear layer, viz. vortex rings and streamwise vortices, is explored following Davoust et al. (J. Fluid Mech., vol. 709, 2012, pp. 408–444). These authors reported a radial organization of streamwise vorticity in a jet at high diameter-based Reynolds number ( ), contrary to the classically observed azimuthal organization. They hypothesized that the observed weaker vortex rings in such jets could be deformed by streamwise vortices and further reoriented and stretched in the streamwise direction. As this hypothesis was based on the observations of one configuration of a jet flow, our study aims at assessing it by varying the key parameter, i.e. the relative strength of the vortex rings and the streamwise vortices, through forcing, along with various jet configurations. We first analyse a low-Mach-number, transitional jet, using high-speed stereo particle image velocimetry in a cross-sectional plane at two jet exit diameters from the nozzle exit. The axisymmetric mode is acoustically excited at various amplitudes to increase the strength of the rings relative to streamwise vortices, at a Strouhal number ( ) of 0.49, the most energetic frequency in the unforced jet. Starting from a radial array in the unexcited jet, a gradual shift towards an azimuthal configuration is obtained with increasing excitation level. Quantification of the relative strengths of the streamwise vortices and vortex rings confirms that a radial array is observed whenever the streamwise vortices are more intense than the rings, and conversely for the azimuthal configuration. We then extend the analysis to other jet cases in terms of , and state of the exiting boundary layer. We observe that the correlation between the radial or azimuthal organization of streamwise vortices and the relative strengths of the vortical structures holds systematically, confirming the possibility of the proposed interaction. A detailed analysis of the forced jets also sheds light on some interesting effects of acoustic excitation on the vortical organization in round jets.

Effects of acoustic nonlinearities on the ultrasonic backscatter coefficient estimation.

The backscatter coefficient (BSC) is a fundamental property of tissues and can be used to classify tissues. Two BSC calibration methods are the planar reflector method and the reference phantom method. In both methods, linear acoustic propagation is assumed. In this study, the calibration methods were evaluated when acoustic nonlinear distortion was present. Radio frequency data were acquired from two physical phantoms using a 5 MHz single-element transducer and low power (one excitation level) and high power (six increasing excitation levels) excitation signals. BSCs estimated from the high power settings were compared to the BSCs estimated using the low power by calculating the root mean square error (RMSE). The BSCs were parameterized by fitting the BSC curve to a power law and estimating the power law exponent and by estimating the effective scatterer diameter (ESD). When using the planar reflector method, estimates of the exponent were observed to monotonically increase in value versus increasing excitation level and the ESD decreased with increasing excitation level. The RMSE increased monotonically versus excitation level using the planar reflector method but did not increase using the reference phantom method. The results suggest that the effects of nonlinear distortion are minimized using the reference phantom method.

Experiments and simulations of an industrial assembly with different types of nonlinear joints subjected to harmonic vibrations

The structure Harmony-Gamma is a metallic assembly representative of an industrial structure for which the vibratory response is influenced by the apparition of nonlinear phenomena within two specific types of joints, the first corresponding to friction joints and the second to elastomer joints. Firstly, the paper presents the experimental procedure and results obtained for this structure. Secondly, a global methodology for modeling and simulation of the nonlinear vibrational response is set up.The experiments are performed for longitudinal and transverse swept sine experiments. First of all, swept sine experiments are performed at low excitation levels on the structure in order to update a linear finite-element model. Then, the evolution of the frequency response function is studied at increasing excitation levels in order to identify the contribution of the nonlinear effects on the global vibrational response of the system.The methodology for modeling and simulation consists of five steps. Firstly, a finite-element model of the structure is presented and updated in order to be representative of the structure when it is excited at low excitation levels. This model is then reduced using a hybrid sub-structuring technique. Thirdly, the nonlinear models of the friction and elastomer joints are added. The resulting optimization problem is solved by means of the Harmonic Balance Method (HBM) coupled with a Newton-Raphson continuation and a condensation process. Lastly, the simulation results are compared to the experimental results in order to validate the development of the finite element model for the nonlinear industrial structure Harmony-Gamma with nonlinear joints of different natures and to achieve a refined understanding of the nonlinear phenomena and their origin.

Recombination and diffusion processes in electronic grade 4H silicon carbide

Carrier dynamics in n-type 4H-SiC epilayers of varying thicknesses and low Z1/2 defect concentrations are investigated here in wide ranges of excess carrier density and temperature. Several techniques are employed to monitor carrier diffusion and recombination processes, including light induced transient grating (LITG), microwave photoconductance decay (MPCD) and free carrier absorption (FCA) using ps-laser pulses at 355 nm. The observed increase of the diffusion coefficient with the increasing excitation level is explained by the transition from the minority to the bipolar transport regime. Its subsequent decrease with even higher excitations is found to be governed by band-gap renormalization and degeneracy effects. The bulk lifetime, limited by hole traps at 0.19–0.24 eV above the valence band at lower excitations, was found to decrease from few microseconds to hundreds of nanoseconds during the transition regime from the minority to the bipolar transport. Our temperature-dependent measurements confirmed the trap activation energy and provided the approximate functional form of electron and hole lifetimes as τe = 340 × (T/300 K)3/2 ns and τh = 100 × (T/300 K)–1/2 ns, for the temperature T range 80–800 K. It was found to hold for 65 and 120 μm sample thicknesses, while the lifetimes were found to be twice shorter for the sample 35 μm thick.

Comparison of two ultrasonic backscatter coefficient methods under nonlinear distortion

The backscatter coefficient (BSC) is a fundamental property of tissues and can be parameterized for tissue characterization. The BSC requires a reference signal, which is estimated through either the planar reflector method (PRM) or the reference phantom method (RPM). In both methods, linear acoustic propagation is assumed. In this work, the BSC estimation methods are evaluated when nonlinear distortion is present. RF data were acquired from two physical phantoms, labeled A and B, with a 5 MHz single-element transducer using low power (1 excitation level) and high power (6 increasing excitation levels) excitation signals. Phantom A contained glass beads with diameters ranging from 75 to 90 μm and phantom B had glass beads with diameters ranging from 9 to 43 μm. The BSCs estimated using the low power setting and high power settings were compared for the both the PRM and RPM through a root mean square error (RMSE). Estimates of the effective scatterer diameter (ESD) were obtained using each method for each high power setting and compared to the low power setting. The RMSE increased as the power setting increased with much higher RMSEs using the PRM compared to RPM, i.e., a maximum of 7.3 times larger for phantom A and 8.6 times larger for phantom B. Estimates of ESD matched the ranges of glass beads sizes for both phantoms except when using the PRM at higher power settings. These findings suggest that the RPM is more robust to nonlinear distortion effects compare to the PRM.

Criteria for Best Performance of Pre-Optimized Solid Dampers

This paper furthers recent research by these authors. The starting point is the pre-optimization of solid dampers, which ensures that all dampers bound to “misbehave” are excluded since the early design stage. The authors now enlarge the scope of their investigations to explore those damper configurations selected inside the admissible design area. The purpose of the paper is to present a set of criteria apt to select a damper configuration which not only avoids unwanted situations, but in addition guarantees high performance under different design conditions. The analysis starts with the definition of a set of requirements a high performance damper should meet. In detail, the present investigation seeks to answer the following questions: in the low excitation regime, what is the frequency shift and the stiffening effect each damper can provide? for increasing excitation levels, which damper will start slipping sooner? in the high excitation regime, which damper provides the maximum dissipation? Like pre-optimization, it does not involve nonlinear finite element calculations, and unlike existing optimization procedures, is not linked to a specific set of blades the damper may be coupled to. The numerical prediction of the blade-damper coupled dynamics is here used only for validation purposes. The approach on which this paper rests is fully numerical; however, real contact parameters are taken from extensive experimental investigations made possible by those purposely developed test rigs which are the distinctive mark of the AERMEC Lab of Politecnico di Torino.

Experimental analysis of the linear and nonlinear vibration behavior of flax fibre reinforced composites with an interleaved natural viscoelastic layer

In this paper, linear and nonlinear vibration analyses are carried out on flax fibre reinforced composites with and without an interleaved natural viscoelastic layer. Experimental analysis was done in a clamped-free configuration on two stacking sequences of composites, [0]8 and [02/902]s. The procedure consists on exciting the specimens with different amplitude levels for the first six bending modes. The dynamic properties as frequencies and damping factors were studied, and the results obtained were analyzed and compared. Then, the amplitudes of deformations were determined and discussed for the different excitation levels. The presence of the non-linearity generated by the viscoelastic layer was clearly identified by the shift observed in the values of natural frequencies and loss factors with increasing excitation levels. The linear and nonlinear, elastic and dissipative parameters were calculated and compared.

Acoustic characterization of a nonlinear vibroacoustic absorber at low frequencies and high sound levels

A nonlinear vibroacoustic absorber (Nonlinear Energy Sink: NES), involving a clamped thin membrane made in Latex, is assessed in the acoustic domain. This NES is here considered as an one-port acoustic system, analyzed at low frequencies and for increasing excitation levels. This dynamic and frequency range requires a suitable experimental technique, which is presented first. It involves a specific impedance tube able to deal with samples of sufficient size, and reaching high sound levels with a guaranteed linear response thank's to a specific acoustic source. The identification method presented here requires a single pressure measurement, and is calibrated from a set of known acoustic loads. The NES reflection coefficient is then estimated at increasing source levels, showing its strong level dependency. This is presented as a mean to understand energy dissipation. The results of the experimental tests are first compared to a nonlinear viscoelastic model of the membrane absorber. In a second step, a family of one degree of freedom models, treated as equivalent Helmholtz resonators is identified from the measurements, allowing a parametric description of the NES behavior over a wide range of levels.

Relationship between isokinetic muscle strength and functional test in chronic ankle instability

This work studies the break-up process of a jet into droplets at low non-dimensional wavenumbers. The work is based on a novel droplet generator suitable for high temperature fluids including molten metals employing a piston resonant structure capable of generating high amplitude pressure excitations in the fluid medium. The relation between the excitation strength and the resulting jet break-up characteristics such as break-up length and in particular the temporal stability of the resulting droplet stream is studied. It is found that a given minimum excitation voltage is required for uniform droplet generation and that the temporal stability ameliorates further with increasing excitation levels. This analysis is complemented with in situ measurements of the generated pressure excitation signal in a molten metal. It is shown that the novel droplet generator can produce a continuous monodisperse stream of micrometre-sized molten tin droplets at low non-dimensional wavenumber of ≃0.12. This work is of particular importance for Laser Plasma produced (LPP) Extreme Ultraviolet (EUV) light sources, which require a highly stable stream of monodisperse micrometre-sized molten tin droplets as well as for 3D metal printing.

Dynamics of monodisperse micrometre-sized metal droplets at low non-dimensional wavenumbers

This work studies the break-up process of a jet into droplets at low non-dimensional wavenumbers. The work is based on a novel droplet generator suitable for high temperature fluids including molten metals employing a piston resonant structure capable of generating high amplitude pressure excitations in the fluid medium. The relation between the excitation strength and the resulting jet break-up characteristics such as break-up length and in particular the temporal stability of the resulting droplet stream is studied. It is found that a given minimum excitation voltage is required for uniform droplet generation and that the temporal stability ameliorates further with increasing excitation levels. This analysis is complemented with in situ measurements of the generated pressure excitation signal in a molten metal. It is shown that the novel droplet generator can produce a continuous monodisperse stream of micrometre-sized molten tin droplets at low non-dimensional wavenumber of ≃0.12. This work is of particular importance for Laser Plasma produced (LPP) Extreme Ultraviolet (EUV) light sources, which require a highly stable stream of monodisperse micrometre-sized molten tin droplets as well as for 3D metal printing.

Parameter sensitivity of an in-plane gap closing electrostatic energy harvester with end-stop impacts

Conventionally modeled as spring–damper system, the end-stops in vibration energy harvesters set a limit to the displacement of the proof mass at sufficiently high excitation levels. In some studies, it is seen that the end-stop parameters needed adjustment to fit the simulations to the measurements at particular operating condition. In this article, the discrepancy between the simulation and measurement results on varying the operating condition is investigated in detail. A check on sensitivity of an electrostatic in-plane gap closing energy harvester to the parameters end-stop stiffness, end-stop damping, and end-stop position at various biases and excitation levels is performed. The simulations at 3-V bias and root mean square (RMS) acceleration amplitude 0.6 g show a remarkable variation of 30 Hz in up-sweep jump-down frequency on varying end-stop position by 0.12 µm. The simulation results also show a significant increase in sensitivity of up-sweep jump-down frequency to end-stop damping on increasing excitation level at fixed bias. The article also discusses the sensitivity in jump frequencies to perturbations in the excitation signal due to the presence of noise, where the jump-down frequency becomes smaller as the noise level increases. The trajectories studied at 8-V bias and RMS acceleration amplitude 0.6 g with different end-stop parameters show a strong influence of the end-stop model parameters on the motion of the proof mass. A lumped model of the device is fitted to the measurements for a whole range of operating conditions with one fixed set of model parameter, where asymmetric end-stop positions and their effect on the device behavior are shown to be crucial. The results presented in this article show that in order to reproduce and analyze the measured behavior of the harvester over a range of operating conditions, very fine details in the model are significant.

Experimental analysis of the linear and nonlinear behaviour of composites with delaminations

An analysis of the linear and nonlinear vibration responses of composites with delaminations is presented. The effect of delamination size on the linear and nonlinear vibration response is studied. The composite material used in this paper is a glass fibre reinforced plastic (GFRP) having delaminations at the plies interfaces. The experimental procedure consists in inducing the specimen on its resonance flexural modes with different excitation levels (amplitudes) for six bending modes and for each delamination length. The presence of the nonlinearity introduced by the delamination was clearly identified by the variation of natural frequencies for increasing excitation levels. Then, nonlinear elastic parameters for progressive delamination length were determined and discussed for the first six bending modes. The linear and the nonlinear elastic parameters were compared in their sensitive modes.