The Experts below are selected from a list of 31503 Experts worldwide ranked by ideXlab platform
R S Lakes - One of the best experts on this subject based on the ideXlab platform.
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lumped negative stiffness damper for absorption of flexural waves in a rod
Smart Materials and Structures, 2017Co-Authors: Samuel P Balch, R S LakesAbstract:A damper based on negative stiffness from column tilt buckling was used to achieve enhanced Mechanical Damping of bending vibration of a rod. Damping increased by a factor of three, via a damper column that was only about 0.2% of the mass of the aluminum alloy rod to be damped.
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column dampers with negative stiffness high Damping at small amplitude
Smart Materials and Structures, 2013Co-Authors: H Kalathur, R S LakesAbstract:High structural Damping combined with high initial stiffness is achieved at small amplitude via negative stiffness elements. These elements consist of columns in the vicinity of the post-buckling transition between contact of flat surfaces and edges of the ends for which negative incremental structural stiffness occurs. The column configuration provides a high initial structural stiffness equal to the intrinsic stiffness of the column material. Columns of the polymers polymethyl methacrylate (PMMA) and polycarbonate were used. By tuning the pre-strain, a very high Mechanical Damping was achieved for small amplitude oscillations. The product of effective stiffness and effective Damping as a figure of merit |Eeff|tanδeff of about 1.5 GPa was achieved for polymer column dampers in the linear domain and about 1.62 GPa in the small amplitude nonlinear domain. For most materials this value generally never exceeds 0.6 GPa.
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negative stiffness and enhanced Damping of individual multiwalled carbon nanotubes
Physical Review B, 2008Co-Authors: R S Lakes, Robert W CarpickAbstract:The Mechanical instabilities and viscoelastic response of individual multiwalled carbon nanotubes and nanofibers (MWCNTs/Fs) under uniaxial compression are studied with atomic force microscopy. Specific buckling events are evident by regimes of negative stiffness, i.e., marked drops in force with increasing compression. Uniaxial cyclic loading can be repeatedly executed even in initially postbuckled regimes, where the CNTs/Fs display incremental negative stiffness. Increases in Mechanical Damping of $145\text{\char21{}}600\phantom{\rule{0.2em}{0ex}}%$ in these initially postbuckled regimes, as compared to the linear prebuckled regimes, are observed. Increased Damping is attributed to frictional energy dissipation of walls in buckled configurations of the MWCNTs/Fs. This represents the extension of the concept of negative stiffness to the scale of nanostructures and opens up possibilities for designing nanocomposites with high stiffness and high Damping simultaneously.
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anomalies in stiffness and Damping of a 2d discrete viscoelastic system due to negative stiffness components
Thin Solid Films, 2007Co-Authors: Yunche Wang, J G Swadener, R S LakesAbstract:The recent development of using negative stiffness inclusions to achieve extreme overall stiffness and Mechanical Damping of composite materials reveals a new avenue for constructing high performance materials. One of the negative stiffness sources can be obtained from phase transforming materials in the vicinity of their phase transition, as suggested by the Landau theory. To understand the underlying mechanism from a microscopic viewpoint, we theoretically analyze a 2D, nested triangular lattice cell with pre-chosen elements containing negative stiffness to demonstrate anomalies in overall stiffness and Damping. Combining with current knowledge from continuum models, based on the composite theory, such as the Voigt, Reuss, and Hashin-Shtrikman model, we further explore the stability of the system with Lyapunov's indirect stability theorem. The evolution of the microstructure in terms of the discrete system is discussed. A potential application of the results presented here is to develop special thin films with unusual in-plane Mechanical properties.
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composites with inclusions of negative bulk modulus extreme Damping and negative poisson s ratio
Journal of Composite Materials, 2005Co-Authors: Yunche Wang, R S LakesAbstract:The effect of a negative bulk modulus phase in elastic composites is studied. Negative bulk modulus K<0 is shown to be possible in selected unit cells. In isotropic solids, K<0 can be attained when negative Poisson’s ratio is sufficiently small, below the stability limit (for stress control) 1/4 1. Such materials, if used as inclusions, are predicted to be stable with respect to the band formation, even if they are large. Composites with spherical inclusions of negative bulk moduli are shown to exhibit negative Poisson’s ratio and anomalies in composite bulk modulus and Young’s modulus (and in the corresponding Mechanical Damping) but not in the shear modulus.
Jeanmaurice Nyobeyome - One of the best experts on this subject based on the ideXlab platform.
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prediction of Mechanical shaft failures due to pulsating torques of variable frequency drives
IEEE Transactions on Industry Applications, 2010Co-Authors: Joseph Songmanguelle, Stefan Schroder, Tobias Geyer, Gabriel Ekemb, Jeanmaurice NyobeyomeAbstract:Mechanical damage of rotating shafts has been reported for several years from various high-power applications. This paper shows that the variable frequency drive incorporated in a rotating shaft is one of the main root causes of Mechanical-shaft failures. Simple analytical relationships show that the frequencies of the motor air-gap torque have a more significant impact on the Mechanical-shaft failure than their magnitudes. Effects of Mechanical Damping are analytically derived and analyzed. Motor air-gap torque is successfully reconstructed using only the motor's voltage and current, thus avoiding torque sensors, which are subject to failure and errors. Simple relationships between frequencies of current harmonics and frequencies of motor pulsating torques are proposed. For pulsewidth-modulated inverters (two and multilevel), possible drive operating points that might excite the shaft's eigenmodes are predicted. Simulation results of four interleaved three-level neutral-point-clamped converters are analyzed for validation purposes. Experimental tests up to 35 MW are performed on a compressor test bed. The presented results confirm the accuracy of the proposed approach, which is particularly valuable for multimegawatt drive applications.
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prediction of Mechanical shaft failures due to pulsating torques of variable frequency drives
Energy Conversion Congress and Exposition, 2009Co-Authors: Joseph Songmanguelle, Stefan Schroder, Tobias Geyer, Gabriel Ekemb, Jeanmaurice NyobeyomeAbstract:Mechanical damage of rotating shafts has been reported for several years from various high-power applications. This paper shows that the variable frequency drive incorporated in a rotating shaft is one of the main root causes of Mechanical shaft failures. Simple analytical relationships show that the frequencies of the motor airgap torque have a more significant impact on the Mechanical shaft failure than their magnitudes. Effects of Mechanical Damping are analytically derived and analyzed.
Daniel J Inman - One of the best experts on this subject based on the ideXlab platform.
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the bandwidth of optimized nonlinear vibration based energy harvesters
Smart Materials and Structures, 2014Co-Authors: Andrea Cammarano, Simon A Neild, Steve G Burrow, Daniel J InmanAbstract:In an attempt to improve the performance of vibration-based energy harvesters, many authors suggest that nonlinearities can be exploited to increase the bandwidths of linear devices. Nevertheless, the complex dependence of the response upon the input excitation has made a realistic comparison of linear harvesters with nonlinear energy harvesters challenging. In a previous work it has been demonstrated that for a given frequency of excitation, it is possible to achieve the same maximum power for a nonlinear harvester as that for a linear harvester, provided that the resistance and the linear stiffness of both are optimized. This work focuses on the bandwidths of linear and nonlinear harvesters and shows which device is more suitable for harvesting energy from vibrations. The work considers different levels of excitation as well as different frequencies of excitation. In addition, the effect of the Mechanical Damping of the oscillator on the power bandwidth is shown for both the linear and nonlinear cases.
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resistive impedance matching circuit for piezoelectric energy harvesting
Journal of Intelligent Material Systems and Structures, 2010Co-Authors: Na Kong, Alper Erturk, Daniel J InmanAbstract:A two-stage power conditioning circuit consisting of an AC-DC converter followed by a DC-DC converter is proposed for a vibration-based energy harvesting system. The power conditioning circuit intends to maximize the amount of power extracted from a piezoelectric energy harvester by matching the source impedance with the circuit by adaptively adjusting the duty cycle. An equivalent electrical circuit representation derived from a distributed-parameter piezoelectric energy harvester model is adapted to enable the impedance matching method proposed here. For a given piezoelectric energy harvester, there is a theoretical maximum power output that is determined by the Mechanical Damping, base acceleration, and the effective mass of the harvester structure under base excitation. Experimental results are given to validate the effectiveness of the proposed resistive impedance matching circuit around the first resonance frequency of a cantilevered piezoelectric energy harvester.
Paulo Alexandre Justino - One of the best experts on this subject based on the ideXlab platform.
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modelling control and pontryagin maximum principle for a two body wave energy device
Renewable Energy, 2011Co-Authors: Jose J Cândido, Paulo Alexandre JustinoAbstract:Frequency-domain analysis is applied to a wave energy device composed of two coaxial axisymmetric bodies. For each frequency optimal Damping coefficient values which maximize absorbed power are obtained. Several displacement amplitude restriction scenarios are considered. A stochastic model to describe the device’s behaviour in irregular waves is developed. Optimal Mechanical Damping and spring coefficients are computed. Considering different sea state conditions, probability density functions are defined for relevant parameters and time-averaged absorbed power values are obtained.
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modelling control and pontryagin maximum principle for a two body wave energy device
Renewable Energy, 2011Co-Authors: Jose J Cândido, Paulo Alexandre JustinoAbstract:Abstract Frequency-domain analysis is applied to a wave energy device composed of two coaxial axisymmetric bodies. For each frequency optimal Damping coefficient values which maximize absorbed power are obtained. Several displacement amplitude restriction scenarios are considered. A stochastic model to describe the device’s behaviour in irregular waves is developed. Optimal Mechanical Damping and spring coefficients are computed. Considering different sea state conditions, probability density functions are defined for relevant parameters and time-averaged absorbed power values are obtained. A time domain model is also developed for the device. A non-linear power take-off mechanism configuration, consisting in a hydraulic circuit with low-pressure and high-pressure gas accumulators, is devised. Time-averaged absorbed power is maximized in terms of characteristic mechanism parameter. A sub-optimal method of phase control by latching is applied to the device in order to improve its performance. Analytical development of Pontryagin Maximum Principle is used to establish an algorithm for device’s control.
Joseph Songmanguelle - One of the best experts on this subject based on the ideXlab platform.
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prediction of Mechanical shaft failures due to pulsating torques of variable frequency drives
IEEE Transactions on Industry Applications, 2010Co-Authors: Joseph Songmanguelle, Stefan Schroder, Tobias Geyer, Gabriel Ekemb, Jeanmaurice NyobeyomeAbstract:Mechanical damage of rotating shafts has been reported for several years from various high-power applications. This paper shows that the variable frequency drive incorporated in a rotating shaft is one of the main root causes of Mechanical-shaft failures. Simple analytical relationships show that the frequencies of the motor air-gap torque have a more significant impact on the Mechanical-shaft failure than their magnitudes. Effects of Mechanical Damping are analytically derived and analyzed. Motor air-gap torque is successfully reconstructed using only the motor's voltage and current, thus avoiding torque sensors, which are subject to failure and errors. Simple relationships between frequencies of current harmonics and frequencies of motor pulsating torques are proposed. For pulsewidth-modulated inverters (two and multilevel), possible drive operating points that might excite the shaft's eigenmodes are predicted. Simulation results of four interleaved three-level neutral-point-clamped converters are analyzed for validation purposes. Experimental tests up to 35 MW are performed on a compressor test bed. The presented results confirm the accuracy of the proposed approach, which is particularly valuable for multimegawatt drive applications.
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prediction of Mechanical shaft failures due to pulsating torques of variable frequency drives
Energy Conversion Congress and Exposition, 2009Co-Authors: Joseph Songmanguelle, Stefan Schroder, Tobias Geyer, Gabriel Ekemb, Jeanmaurice NyobeyomeAbstract:Mechanical damage of rotating shafts has been reported for several years from various high-power applications. This paper shows that the variable frequency drive incorporated in a rotating shaft is one of the main root causes of Mechanical shaft failures. Simple analytical relationships show that the frequencies of the motor airgap torque have a more significant impact on the Mechanical shaft failure than their magnitudes. Effects of Mechanical Damping are analytically derived and analyzed.