1,721,043 research outputs found
Correction to: Asymmetric vibrations and chaos in spherical caps under uniform time-varying pressure fields (Nonlinear Dynamics, (2022), 107, 1, (313-329), 10.1007/s11071-021-07033-7)
No full textThe original article was published with erroneous author information. The correct authorship is as it stands in this correction. The original article has been corrected
Thermal effects on dynamics of circular cylindrical shell
No full textIn this paper, an experimental study on the dynamic of cylindrical shells made of Polyethylene terephthalate (PET) is presented; a thermic gradient has been applied on a specimen of the present work to obtain a functionally gradient material (FGM) equivalent properties: the PET shell had been exposed at a thermal temperature gradient in the range of its glass transition temperature of 79°C. A complex setup has been specifically designed and built to characterise, with dynamic tests, the structural properties of the specimen on temperature change from -10°C up to about 90°C and under thermic gradient with different forcing load. Predicting the mechanical properties of shells, panels and plates is one of the main concern of structural engineers; since shell elements present complicated stability behaviours, rich linear vibration spectra (high modal density), high sensitivity to perturbations and strong interactions with surrounding elements. The linear and dynamic behaviour have been investigated. The shell behaviour is also investigated by means of a finite element model, in order to enhance the comprehension of experimental results
Chaotic dynamics of spiral bevel gears
Full text linkThis study investigated the nonlinear dynamics of a Spiral Bevel Gear (SBG) system used in helicopter transmissions. Two approaches were employed to determine the Mesh Stiffness (MS): the average slope method (average-MS) and a hybrid analytical-computational method (adaptive-MS), which accounts for Hertzian contact nonlinearity. The dynamic mesh force was computed using polynomial interpolation techniques and loaded tooth contact analysis. The accuracy of the adaptive-MS approach was validated through comparison with a verified nonlinear finite element method (Nonlinear-FEM) simulation, demonstrating strong agreement. The GearDyns-SBG program was used to solve the system's dynamic model under varying working conditions. Backward and forward simulations were conducted to track stable branches, providing insights into the system's behavior. The study evaluated the dynamic responses based on both mesh stiffness approaches using tools such as FFT spectrum, amplitude-frequency analysis, nonlinear time series analysis, Poincare maps, phase diagrams, and recurrence plots. The results revealed complex behaviors, including tooth separation, backside contact, boundary crises, and period-doubling cascades. Additionally, the largest Lyapunov exponent and fractal dimension were used to characterize the dynamics and 3D bifurcation analysis captured transitions between regular and chaotic regimes. The periodicity characteristics of the system were evaluated by recurrence quantification analyses. These findings enhance the understanding of nonlinear gear dynamics of SBG and provide reliable methods for predicting and analyzing their behavior
Asymmetric vibrations and chaos in spherical caps under uniform time-varying pressure fields
Full text linkThis paper presents a study on nonlinear asymmetric vibrations in shallow spherical caps under pressure loading. The Novozhilov’s nonlinear shell theory is used for modeling the structural strains. A reduced-order model is developed through the Rayleigh–Ritz method and Lagrange equations. The equations of motion are numerically integrated using an implicit solver. The bifurcation scenario is addressed by varying the external excitation frequency. The occurrence of asymmetric vibrations related to quasiperiodic and chaotic motion is shown through the analysis of time histories, spectra, Poincaré maps, and phase planes
Synchronicity Phenomena in Circular Cylindrical Shells Under Random Excitation
No full textIn many engineering areas, the structures are subjected to external forcing with a prevalent harmonic component in conjunction with significant likely non-deterministic contribution; moreover, the coupling between loading conditions and extreme environmental temperatures can lead to complex dynamic phenomena. In this paper, an experimental study on the nonlinear dynamics of a thin polymeric circular cylindrical shell, carrying a top mass, subjected to thermal gradients and random excitation is presented. Tests have been performed in controlled temperature conditions and the shell has been excited through an electrodynamic shaker. The experimental results pointed out that a broadband random excitation at the base of the shell can give rise to the synchronicity of the response to the resonance corresponding to the first axisymmetric mode of the shell; the vibration energy is conveyed to specific harmonics, some of which are outside and far from the excitation band
Seismic vibration of shells with non-Newtonian fluid interaction: Experiments
No full textIn the presented study the nonlinear vibrations of a fluid-filled circular cylindrical shell under seismic excitation is investigated. A PET thin shell with an aluminum top mass is harmonically excited from the base through an electrodynamic shaker in the neighborhood of the natural frequency of the first axisymmetric mode. The dilatant fluid is composed of a cornstarch-water mixture with 60% cornstarch and 40% water of total weight. The preliminary results show a strong non-linear response due to the coupling between the fluid and structure and the shaker-structure interaction that leads to a very interesting dynamic response of the system. The specimen is a polymeric circular cylindrical shell: an aluminum cylindrical mass is glued on the shell top edge; conversely, the bottom edge of the shell is clamped to a shaking table. The following sensors have been adopted: three triaxial accelerometers placed on the top mass at 120°, a monoaxial accelerometer at the base of the shell, a laser vibrometer to measure the lateral velocity on the mid-height of the shell. The test article has been excited in the axial direction through a harmonic load, with a step-sweep controlled output, the voltage signal sent to the shaker amplifier is closed-loop controlled; to avoid interaction between the control system and the specimen under study, no controls have been used for controlling the shaker base motion. The harmonic forcing load consists of a stepped-sine sweep of frequency band 100-500 Hz with a step of 2.5 Hz. All the tests have been performed with the shell full filled with quiescent fluid. The dynamic scenario is carefully analyzed by means of time histories, spectra, phase portraits and Poincaré maps. The experiments show the onset of complex dynamics: subharmonic and quasiperiodic responses, Chaos
Time series analysis of arm and forearm measurement for functional electrical stimulation control
No full textIdentify the dynamics of the muscular group of arm and forearm related to accelerometric and surface electromyographic signals is not an easy task especially in persons affected by pathological tremor, in which irregular tremor is not voluntary. The vibrational phenomena studied in this work regards the arm and forearm vibration that are monitored by means of SEMG (surface electro-myographic) sensor and accelerometer sensor with the purpose to detect and recognize the dynamic properties and correlations of onset of pathological tremor in patients affected by Parkinson disease and essential tremor. These pathologies that affect skeletal muscles present a typical characteristic vibration frequency between 1Hz and 12Hz, this property is monitored in out-patient tests. A condition monitoring system has been developed to monitor the tremor and produce an electrical stimulation to reduce the tremor: the system allows data monitoring with a portable microcontroller board powered with low voltage batteries and is based on a cDAQ9191 data acquisition system with a chassis controller module designed for data input, controlling and output generation. The algorithm generates functional electrical stimulation signals for control purpose. Experimental measurement data on parkinsonian patient are presented. Data are analyzed, and the results are presented
Identification of dynamic behaviour of forearm for active control of pathological tremor
No full textPathological tremor is defined as an oscillatory and rhythmic movement of a segment of the body caused by the alternating activation of muscle groups in antagonism between them; a typical pathological condition known as essential tremor, which is the most common form of tremor also having a family character (2-5% of the population) have a characteristic frequency between 5 Hz and 7 Hz, such as Parkinsonian tremor that arises at frequencies higher than 8 Hz. The purpose of this study is to investigate the dynamic behavior of the forearm induced by the hand movements, measured due to the electromyographic sensors and accelerometers. The relationships of accelerometric and electromyographic signals are compared between a control group of healthy subjects and patients affected by Parkinson disease (PD) and essential tremor (ET). The vibrational phenomena analyzed in this experimental investigation are observed by means of SEMG (surface electro-myographic) sensors placed on flexor and extensor muscle compartment of forearm and accelerometer sensors for the hand vibration with the aim to detect and recognize the dynamic properties of onset of pathological tremor in patients affected by (PD) and (ET). A real-time monitoring and data logger system have been developed and deep described to monitor the tremor: the system allows data monitoring with a portable lightweight microcontroller board based on a CompactRio data acquisition system with a chassis controller module designed for data input, controlling and output generation, powered by 12 Volt battery. The CRio System provides an algorithm to generate functional electrical stimulation signals with different waveforms for control purpose. Experimental measurement data on healthy control subjects are presented. Data are analyzed, and the results are discusse
Experimental investigation on spur gears with novel coatings and surface micro texturing
No full textGears fault diagnostic and prognostic techniques have been the significant subjects of the condition-based monitoring systems in recent time due to the potential advantages that could be gained from. The present work is part of a more extensive study regarding efficiency and durability of novel tungsten carbide coatings and surfaces laser micro-texturing applied on carburized and nitride gears for several industrial application. In this paper, the effect of the different gear coatings and texturing on the vibration level is investigated experimentally. The durability and the pitting arising have been examined. The test rig consists of two electric motors with an electric power recirculating layout and is equipped with a temperature monitoring of the oil-jet type lubrication system. Vibrations phenomena are measured through triaxial accelerometer mounted on the shaft frame, close to the bearings, in order to detect spectral characteristics of vibration signals due to gear fault. Tests have been conducted far from the resonances of the system at a fixed speed and torque load; Vibration measurements have been performed at planned intervals. Results are presented and discusse
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