Tanju Yildirim

Development of a nonlinear adaptive absorber based on magnetorheological elastomer

The resonance shift property of magnetorheological elastomer is important for developing adaptive absorbers. However, it is well-known that passive nonlinear absorbers have wider effective frequency bandwidths. This article combines these two characteristics in order to develop a hybrid magnetorheological elastomer absorber which can shift its natural frequency and has a wider absorption bandwidth under each constant current. The adaptability and nonlinearity were fully verified experimentally. Afterwards, the absorption ability of the hybrid magnetorheological elastomer absorber was investigated and analyzed. The results show that the effective bandwidth of this absorber is broadened under certain levels of current than linear absorber, and this is caused by the presence of nonlinearity; and the adaptability induced by the magnetorheological elastomer undoubtedly empowers the absorber possible to trace the excitation frequency changing in real time. A short-time Fourier transform was finally used to control the magnetorheological elastomer absorber to verify its controllability, showing that an optimal absorption transmissibility was achieved by the controlled hybrid absorber.

Experimental nonlinear model identification of a highly nonlinear resonator

In this work, two model identification methods are used to estimate the nonlinear large deformation behaviour of a nonlinear resonator in the time and frequency domains. A doubly-clamped beam with a slender geometry carrying a central intra-span mass when subject to a transverse excitation is used as the highly nonlinear resonator. A nonlinear Duffing equation has been used to represent the system for which the main source of nonlinearity arises from large mid-plane stretching (i.e. geometric extensibility at the centerline). The first model identification technique uses the free vibration of the system and the Hilbert transform to identify a nonlinear force-displacement relationship in the large deformation region (compared to the thickness of the beam). The second method uses the frequency-response of the system at various base accelerations to relate the maximum resonance frequency to the nonlinear parameter arising from the centerline extensibility. Experiments were conducted using the doubly-clamped slender beam and an electrodynamic shaker to identify the model parameters of the system using both of the identification techniques. It was found that both methods produced near identical model parameters; an excellent agreement between theory and experiments was obtained using either of the identification techniques. This follows that two different model identification techniques in the time and frequency domains can be employed to accurately predict the nonlinear response of a highly nonlinear resonator.

Design, Fabrication, and Test of a Coupled Parametric–Transverse Nonlinearly Broadband Energy Harvester

A coupled parametric–transverse nonlinearly broadband energy harvester utilizing mechanical stoppers has been designed, fabricated, experimentally tested, and in some cases theoretically verified. An energy harvester with coupled parametric and transverse cantilever beams with additional tip-masses was excited using an electrodynamic shaker. A piezoelectric bimorph has been attached to each cantilever beam; when the excitation frequency was in the vicinity of the parametric or transverse resonances, the mechanical strain developed in the piezo-bimorphs was converted into electrical energy across a purely resistive ac load. For the cases involving no stoppers, a weak softening-type nonlinear frequency–voltage behavior was observed for the parametrically excited cantilever beam; however, with the addition of mechanical stoppers, both the transverse and parametrically excited cantilever beams displayed a strong hardening-type nonlinear frequency–voltage behavior. The stoppers substantially increased the operating bandwidth for both the parametric and transversely excited cantilever beams compared to the case without stoppers. For the theoretical investigations, a good agreement for both the fundamental frequencies and frequency–response curves was obtained. It is shown that by coupling transverse and parametric cantilevers with mechanical stoppers, the nonlinear energy harvested by the system takes place over a much broader frequency-bandwidth when compared to the singular transverse cantilever mechanism (by about 163.5%).

Many-Body Complexes in 2D Semiconductors.

2D semiconductors such as transition metal dichalcogenides (TMDs) and black phosphorus (BP) are currently attracting great attention due to their intrinsic bandgaps and strong excitonic emissions, making them potential candidates for novel optoelectronic applications. Optoelectronic devices fabricated from 2D semiconductors exhibit many-body complexes (exciton, trion, biexciton, etc.) which determine the materials optical and electrical properties. Characterization and manipulation of these complexes have become a reality due to their enhanced binding energies as a direct result from reduced dielectric screening and enhanced Coulomb interactions in the 2D regime. Furthermore, the atomic thickness and extremely large surface-to-volume ratio of 2D semiconductors allow the possibility of modulating their inherent optical, electrical, and optoelectronic properties using a variety of different environmental stimuli. To fully realize the potential functionalities of these many-body complexes in optoelectronics, a comprehensive understanding of their formation mechanism is essential. A topical and concise summary of the recent frontier research progress related to many-body complexes in 2D semiconductors is provided here. Moreover, detailed discussions covering the aspects of fundamental theory, experimental investigations, modulation of properties, and optoelectronic applications are given. Lastly, personal insights into the current challenges and future outlook of many-body complexes in 2D semiconducting materials are presented.

An experimental investigation into nonlinear dynamics of a magneto-rheological elastomer sandwich beam

An experimental investigation has been carried out on the nonlinear dynamics of a clamped–clamped Magneto-Rheological Elastomer (MRE) sandwich beam with a point mass when subjected to a point excitation. Three sets of experiments have been conducted namely for (i) an aluminium beam, (ii) a MRE sandwich beam in the absence of a magnetic field and (iii) a MRE sandwich beam in the presence of a magnetic field. An electrodynamic shaker was used to excite each system and the corresponding displacement of the point mass was measured: for the third experiment (iii), an array of magnets has been placed at various distances away from the centre of the point mass to investigate the effect of changing stiffness and damping properties on the nonlinear dynamical behaviour. An interesting feature for the third group is the beam point mass displacement was no longer symmetric as the stiffness and damping of the system are increased when moving towards the magnets. Both the first and second groups exhibited distinct nonlinear behaviour; however, for the third group this work shows that for a low magnetic field the sandwich beam exhibits two distinct resonance peaks, one occurring above and the other below the fundamental natural frequency of the transverse motion, with the right one larger. For a larger magnetic field, these peaks even out until the magnetic force was large enough that the hardening-type nonlinear behaviour changes to a softening-type; a significant qualitative change in the nonlinear dynamical behaviour of the system, due to the presence of the magnetic field, was observed.