The Influence of Voids in the Vibrations of Bodies with Dipolar Structure

In our study we analyse the vibration of a right cylinder which consists of an elastic material with dipolar structure and has pores. One end of this cylinder is subjected to an excitation, harmonically in time. The other end of the cylinder and its lateral surface are free of loads. We prove that the presence of the voids does not affect the spatial decay of effects away from the excited end, if the harmonic excitation level is below a predetermined threshold.

Analysis, Design, and Optimisation of an Electromagnetic Energy Harvester

One of two major limitations of a vibration energy harvester (VEH), concerns its limited performance due to its confined physical enclosure. The maximum span realizable is attained at a specific excitation level. This excitation level provides the maximum energy harvested by the VEH device. Due to span constraints, VEHs are designed to operate at the maximum span achievable at the maximum excitation level existing within the region of interest. In this study, a constrained optimisation problem (for the VEH) is formulated and investigated. This paper focuses on the analysis, design and optimisation of a nonlinear VEH device.

Characteristics of a Magnetic Bulk Thermostat for Granular Gas Investigations in Microgravity

A magnetic thermostat employing soft-ferromagnetic particles and a varying magnetic field has been developed to investigate a homogeneous granular gas system in microgravity. While the thermostat’s mechanism of creating homogeneous distribution of the particles was shown earlier, its characteristics have not been understood well due to limited access to a microgravity environment. Therefore, a parametric study by numerical simulation based on the discrete element method is carried out in this paper to evaluate effects of tunable parameters in the thermostat. The result shows the capability of the system and provides a wide range of options and improvements for future experiments. Moreover, it predicts that the thermostat allows variation of homogeneity and excitation level of the granular gas just by changing the magnetic parameters without using any mechanical means. In addition, the suggested improvement is experimentally implemented and evaluated in a drop tower test.

Nonlinear excitation of hypersound vibrations in ferrite plate in conditions of combine influence in two frequencies. Part 1. Resonance on difference frequency

The task about nonlinear excitation of hypersound vibrations in ferrite plate in conditions of combine influence in two frequencies is investigated. As a preliminary task the investigation of only magnetic vibrations by two-frequency excitation is carried out. The possibility of description of forced linear vibrations on the basis of single nonuniform linear second order equation with arbitrary excitation is shown. It is found the analytical solution of task about excitation of oscillator by two signals which frequencies are distinguishes up and down from central frequency on the same frequency interval. It is shown the equivalency of representation of magnetic vibrations in linear regime and model vibrations on the basis of oscillator. It is found that in the general case the vibrations have view as beating which rounding frequency is equal to difference between excitation frequencies. The whole positing of task about excitation of nonlinear magnetoelastic vibrations in normal magnetized ferrite plate by two-frequency excitation is proposed. It is found that in conditions of large nonlinearity when the own elastic resonance of plate is equal to the difference frequency the powerful elastic vibrations are excited. It is found the nonlinear excitation of powerful non-resonance vibrations which take place also in the case of large elastic dissipation. It is shown that the non-resonance vibrations are determined precisely two-frequency character of excitation. It is found that the amplitude of non-resonance vibrations by increasing of plate thickness also is increased. By the small level of excitation, the low of increasing is linear, by middle – quadratic, by large – again approaches to linear with saturation and non-permanent sudden jumps. The character of excitation in conditions of resonance on difference frequency is investigated. It is shown that this resonance has powerful determined nonlinear character because it arises only by enough large excitation level. It is shown that the further increasing of resonance amplitude by the increasing the excitation level is realized by the low which is near to quadratic. But after this increasing when excitation level reaches determined value the resonance amplitude is saturated and remains constant. It is drawn attention to some discrepancy between the realization on nonlinearity by magnetic and elastic systems. For the de-scription of this discrepancy the empirical quadratic dependence is proposed. In brief is proposed some remarks about further development of work.

Shaker Testing with Simultaneous Control of PSD and FDS

The fatigue damage spectrum (FDS) model characterizes how the damage potential is distributed over the excitation frequency range, similarly to how the power spectral density (PSD) characterizes the distribution of the excitation level. However, reproducing the operational PSD during in-house shaker testing does not necessarily mean that the FDS would be also reproduced because some unusually distinctive peaks, higher than those in a signal generated from the PSD, occur in vibrations of automobiles and railway vehicles. Presence of these peaks in real operational vibrations and their absence in the PSD-based random testing is the reason why the shaker testing FDS obtained by the ordinary PSD control is different in shape and usually lies below the operational FDS. It is shown in this paper that the FDS shape as a function of frequency can be controlled by manipulating some of the IFFT phases instead of making all of them random. Since the phase manipulation does not affect the excitation PSD, the FDS and the PSD can be controlled simultaneously, which is demonstrated for an example of operational vibrations of an automobile. This new concept of shaker testing with the PSD+FDS control can also be used for accelerated testing when the FDS target needs to be artificially increased. It can be done without scaling up the PSD, thereby avoiding concerns about the test exaggeration factor being too high.

Nonlinear vibration of knitted spacer fabric under harmonic excitation

Knitted spacer fabrics can be an alternative material to typical rubber sponges and polyurethane foams for the protection of the human body from vibration exposure, such as automotive seat cushions and anti-vibration gloves. To provide a theoretical basis for the understanding of the nonlinear vibration behavior of the mass-spacer fabric system under harmonic excitation, experimental, analytical and numerical methods are used. Different from a linear mass-spring-damper vibration model, this study builds a phenomenological model with the asymmetric elastic force and the fractional derivative damping force to describe the periodic solution of the mass-spacer fabric system under harmonic excitation. Mathematical expression of the harmonic amplitude versus frequency response curve (FRC) is obtained using the harmonic balance method (HBM) to solve the equation of motion of the system. Parameter values in the model are estimated by performing curve fit between the modeled FRC and the experimental data of acceleration transmissibility. Theoretical analysis concerning the influence of varying excitation level on the FRCs is carried out, showing that nonlinear softening resonance turns into nonlinear hardening resonance with the increase of excitation level, due to the quadratic stiffness term and the cubic stiffness term in the model, respectively. The quadratic stiffness term also results in biased vibration response and causes an even order harmonic distortion. Besides, the increase of excitation level also results in elevated peak transmissibility at resonance.

Experimental Configuration to Determine the Nonlinear Parameter β in PMMA and CFRP with the Finite Amplitude Method

Parameters to measure nonlinearity in polymethylmethacrylate (PMMA) and carbon fiber reinforced polymer (CFRP) materials have been determined with nonlinear ultrasound (NLUS). The nonlinear parameter β has been determined using the variation of the Finite Amplitude Method (FAM) with harmonic generation. Using this as a reference, the first contribution of this work consists of deducting the experimental configuration necessary to measure this nonlinear parameter in a correct and feasible way. Excitation level, frequency of the wave generated, number of cycles analysed and the distances transducer-specimen and specimen-hydrophone have been determined in both materials. The second contribution is a semi-analytical model that allows to obtain the nonlinear parameter in materials by removing water contribution and considering geometric and viscous attenuation, using the data obtained in an immersion tank. Finally, an application of this model has been carried out in PMMA in order to determinate the nonlinear parameter in this material. From the results, we confirm that the configuration determined in this paper to obtain the parameter β decreases the noise in the measurements.

Экситонное излучение гетероструктур CdTe/ZnTe с двойными ультратонкими узкозонными слоями

The exciton luminescence spectra of CdTe/ZnTe heterostructures containing two thin CdTe layers separated by barriers of different thickness were studied. The complex temperature dependence of the intensity of luminescence from these layers in its underbarrier and abovebarrier excitation led to the conclusions about the effect of the thickness of the barriers on the energy transfer between the CdTe layers and gave the possibility to observe, at specific temperatures, the implementation of a resonant-type excitation. The dependence of the shape of the exciton emission contours on the excitation level gives information about the real structure of CdTe layers with the monolayer thickness of 1.5 and 4.0 monolayers.