Vibration, Volume 3, Issue 2 (June 2020) – 7 articles

Track foundation stiffness (also referred as the track modulus) is one of the main parameters that affect the track performance, and thus, quantifying its magnitudes and variations along the track is widely accepted as a method for evaluating the track condition. In recent decades, the train-mounted vertical track deflection measurement system developed at the University of Nebraska–Lincoln (known as the MRail system) appears as a promising tool to assess track structures over long distances. Numerical methods with different levels of complexity have been proposed to simulate the MRail deflection measurements. These simulations facilitated the investigation and quantification of the relationship between the vertical deflections and the track modulus. In our previous study, finite element models (FEMs) with a stochastically varying track modulus were used for the simulation of the deflection measurements, and the relationships between the statistical properties of the track modulus and deflections were quantified over different track section lengths using curve-fitting approaches. The shortcoming is that decreasing the track section length resulted in a lower accuracy of estimations. In this study, the datasets from the same FEMs are used for the investigations, and the relationship between the measured deflection and track modulus averages and standard deviations are quantified using artificial neural networks (ANNs). Different approaches available for training the ANNs using FEM datasets are discussed. It is shown that the estimation accuracy can be significantly increased by using ANNs, especially when the estimations of track modulus and its variations are required over short track section lengths, ANNs result in more accurate estimations compared to the use of equations from curve-fitting approaches. Results also show that ANNs are effective for the estimations of track modulus even when the noisy datasets are used for training the ANNs. Full article

Nonlinear dissipative phenomena are common features of many dynamical systems and engineering applications, and their experimental characterization has always been a challenge among the research community. Within the wide range of nonlinear damping mechanisms, friction is surely one of the most common, and with a high impact on the dynamical behavior of structures. In this paper, the nonlinear identification of friction in a negative stiffness oscillator is pursued. The structure exhibits a strong nonlinear behavior, mainly due to its polynomial elastic restoring force with a negative stiffness region. This leads to an asymmetric double-well potential with two stable equilibrium positions, and the possibility of switching between them in a chaotic way. Friction plays a crucial role in this context, as it derives from the continuous sliding between the central guide and the moving mass. The system is driven through harmonic tests with several input amplitudes, in order to estimate the variations in the energy dissipated per cycle. The identification of the frictional behavior is then pursed by minimizing the errors between the experimental measurements and the model predictions, using the harmonic balance method in conjunction with a continuation technique on the forcing amplitudes. Full article

Minor differences in the vibration characteristics of a vehicle may greatly influence the comfort experienced by the driver. Therefore, such characteristics are significant in the process of vehicle development. In this experimental study, just-noticeable differences were determined for sinusoidal vertical whole-body vibrations at the frequencies 1.3 Hz and 6.0 Hz, and for the vibration amplitudes 0.2 m/s², 0.5 m/s² and 1.2 m/s². The stimulation set up was realised using a test rig constituting a seating position similar to that in a real vehicle environment. A transformed one-up-three-down method, in conjunction with a two-interval forced choice procedure, was used to determine difference thresholds, in accordance with Weber’s Law, for 14 test subjects. Median relative difference thresholds in the range of 6.7% to 11.0% were observed, and were examined for statistical significance (α < 0.05) and practical importance on amplitude and frequency, with respect to this law. The results showed a frequency-dependence at the lowest vibration amplitude and an amplitude-dependence for both frequencies from a statistical point of view. However, the amplitude-dependence at 6.0 Hz was considered as negligible for practical use. Full article

Regenerative suspension systems, unlike traditional passive, semi-active or active setups, are able to convert the traditionally wasted kinetic energy into electricity. This paper discusses flexible multi-objective control design strategies based on LMI formulations to suitably trade-off between the usual road handling and ride comfort performance and the amount of energy to be harvested. An electromechanical regenerative vehicle suspension system is considered where the shock absorber of each wheel is replaced by a linear electrical motor which is actively governed. It is shown by simulations that multivariable centralized control laws designed on the basis of a full-car model of the suspension system are able to achieve larger amount of harvested energy under identical ride comfort prescriptions with respect to scalar decentralized control strategies, designed on the basis of a single quarter-car model and implemented independently on each wheel in a decentralized way. Improvements up to $40\%$ and $20\%$ of harvested energy are respectively achievable by the centralized multivariable $H_2$ and $H_{\infty }$ optimal controllers under the same test conditions. Full article

The purpose of the present investigation was to examine the acute effects of whole body vibration (WBV) on isometric mid-thigh pull force–time curve (FTC) characteristics. Eleven recreationally trained subjects were randomly assigned to three treatment conditions: sham no vibration protocol (T1), vibration protocol 30 Hz 2–4 mm amplitude (T2), and vibration protocol 30 Hz 2–4 mm (T3). After completing a standardized warm-up, the subject stood on a vibration platform with the knee at a 120° angle and performed one of the three interventions. Each treatment condition required the subject to stand on the platform for thirty-second treatments, each separated by thirty seconds of recovery. Five minutes after the completion of the treatment conditions, the subjects performed the isometric mid-thigh pull. All FTCs were analyzed with standardized procedures for peak force (PF) and peak rate of force development (PRFD). A 1 × 3 repeated measures analysis of variance (ANOVA) was used to compare the three treatments. Additionally, coefficients of variance (CV), as well as intraclass and interclass correlations, were performed. There were no significant differences (p > 0.05) for any of the FTC analyses performed in this investigation. The CV and the 95% confidence interval (CI) indicate that the WBV protocol resulted in trivial changes in PF and beneficial changes in PRFD. A 30 Hz 2–4 mm amplitude WBV does not result in a significant increase in isometric mid-thigh pull performance. Full article

This paper introduces a nonparametric, nonlinear system identification toolbox called SAMI (simplified analysis for multiple input systems) developed for industrial measurements of vibro-acoustic systems with multiple inputs. It addresses the questions related to the user-friendly (semi-)automatic processing of multiple-input, multiple-output measurements with respect to the design of an experiment and the analysis of the measured data. When the proposed toolbox is used, with minimal user interaction, it is easily possible (a) to decide whether the underlying system is linear or not, (b) to decide whether the linear framework is still adequate to be used, and (c) to tell an inexperienced user how much can be gained using an advanced nonlinear framework. The toolbox is illustrated on openly accessible F-16 ground vibration testing measurements. Full article

Functionally graded materials (FGMs) have wide applications in different branches of engineering such as aerospace, mechanics, and biomechanics. Investigation of the mechanical behaviors of structures made of these materials has been performed widely using classical elasticity theories in micro/nano scale. In this research, static, dynamic and vibrational behaviors of functional micro and nanobeams were investigated using non-local theory. Governing linear equations of the problem were driven using non-local theory and solved using an analytical method for different boundary conditions. Effects of the axial load, the non-local parameter and the power index on the natural frequency of different boundary condition are assessed. Then, the obtained results were compared with those obtained from classical theory. These results showed that a non-local effect could greatly affect the behaviors of these beams, especially at nano scale. Full article