Leif Kari

Bio: Leif Kari is an academic researcher from Royal Institute of Technology. The author has contributed to research in topics: Natural rubber & Audio frequency. The author has an hindex of 21, co-authored 105 publications receiving 1371 citations.

An analytical temperature-dependent collocation model for preloaded rubber cylinders

Abstract: The non-linear temperature-dependent stiffness of an axially preloaded rubber cylinder is examined by an analytical collocation model, where influences of temperature, cylinder diameter and length, material parameters and prestrain are investigated. The rubber is assumed to be incompressible with the deviatoric response determined by an extended neo-Hookean free energy function, embodying a temperature shift function, being directly proportional to the temperature and to the temperature-dependent rubber density. The model is based on a semi-inverse method where the motion is split into two deformations: the first, a homogeneous temperature expansion, while the second, a preload deformation where material planes parallel to the bonded metal plate in the rubber cylinder are assumed to remain parallel, with the boundary conditions on the free rubber surface satisfied by collocation. The stiffness depends strongly on the preload—particularly for larger diameter-length ratios—and on the temperature covering —6...

Non-linear elasto-plastic shock wave simulation in high-velocity compaction by discrete element method

Abstract: Non-linear elasto-plastic shock waves in particle systems at high-velocity compaction are simulated by the discrete element method. The contact laws applied between the spherical metal particles include non-linear elastic and plastic loading, adhesion and elastic unloading. Of particular interest is to study the transmission and reflection of non-linear shock waves through a particle system with and without voids between the particles. Interesting results are presented in the paper including the particle deformation at incident and reflected shocks and particle velocity oscillations as a result of voids. The study of non-linear elasto-plastic shock waves in particle systems has a strong practical relevance including high-velocity compaction of metal powder.

Numerically Exploring the Potential of Abating the Energy Flow Peaks through Tough, Single Network Hydrogel Vibration Isolators with Chemical and Physical Cross-Links.

Abstract: Traditional vibration isolation systems, using natural rubber vibration isolators, display large peaks for the energy flow from the machine source and into the receiving foundation, at the unavoidable rigid body resonance frequencies. However, tough, doubly cross-linked, single polymer network hydrogels, with both chemical and physical cross-links, show a high loss factor over a specific frequency range, due to the intensive adhesion–deadhesion activities of the physical cross-links. In this study, vibration isolators, made of this tough hydrogel, are theoretically applied in a realistic vibration isolation system, displaying several rigid body resonances and various energy flow transmission paths. A simulation model is developed, that includes a suitable stress–strain model, and shows a significant reduction of the energy flow peaks. In particular, the reduction is more than 30 times, as compared to the corresponding results using the natural rubber. Finally, it is shown that a significant reduction is possible, also without any optimization of the frequency for the maximum physical loss modulus. This is a clear advantage for polyvinyl alcohol hydrogels, that are somewhat missing the possibility to alter the frequency for the maximum physical loss, due to the physical cross-link system involved—namely, that of the borate esterification.

A nonlinear dynamic stiffness model of a vibration isolator at finite deformations

Abstract: A nonlinear dynamic model of a vibration isolator is presented where influences of precompression and dynamic amplitude are investigated within the frequency domain. It is found that the dynamic stiffness at the frequency of a harmonic displacement excitation is strongly dependent on those parameters. The problems of simultaneously modeling the elastic, viscous and friction forces are removed by additively splitting them, where the elastic force is modeled by a nonlinear, shape factor based approach, the viscous force by a fractional derivative model while the friction force is modeled by a generalized friction element. The dynamic stiffness magnitude is shown to increase with static precompression and frequency while decreasing with dynamic excitation amplitude, with its loss angle displaying a maximum at an intermediate amplitude. The dynamic stiffness at a static precompression, using a linearized elastic force response model, is shown to agree with the fully nonlinear model except at the highest dynamic amplitudes. The latter model is displaying an increased stiffness magnitude at the highest amplitudes due to nonlinear elastic effects. Furthermore, a harmonic displacement excitation is shown to result in a force response containing the excitation frequency and all higher-order harmonics, whereas every other higher-order harmonics vanish for the elastically linearized case.

Resilient mounting of engines

Abstract: In any type of vehicle a number of mechanical sources generate structure‐borne noise. The energy is transmitted from excitation points to surrounding structures. These radiate noise externally and internally. The most commonly used method to reduce the radiated noise is to mount the sources resiliently to the supporting foundation. The insertion loss due to the resilient mounting can in many cases be extremely poor. This could be due to the break down of the stiffness of the foundation in the audible frequency range. At the same time the stiffness of the mounts could be increased drastically as compared to the static stiffness. A test rig and measurement procedures for determining the dynamic properties of mounts are described. Measured and predicted results are compared for rubber mounts with simple geometries. The insertion loss of a resilient mounting very much depends on the location of the engine feet on a foundation. Case studies relating to passenger vessels and catamarans built with aluminum and sandwiched are presented. Reduction of both noise and weight is discussed.

Application of Fractional Calculus for Dynamic Problems of Solid Mechanics: Novel Trends and Recent Results

Abstract: The present state-of-the-art article is devoted to the analysis of new trends and recent results carried out during the last 10 years in the field of fractional calculus application to dynamic problems of solid mechanics. This review involves the papers dealing with study of dynamic behavior of linear and nonlinear 1DOF systems, systems with two and more DOFs, as well as linear and nonlinear systems with an infinite number of degrees of freedom: vibrations of rods, beams, plates, shells, suspension combined systems, and multilayered systems. Impact response of viscoelastic rods and plates is considered as well. The results obtained in the field are critically estimated in the light of the present view of the place and role of the fractional calculus in engineering problems and practice. This articles reviews 337 papers and involves 27 figures. DOI: 10.1115/1.4000563

Formulas For Natural Frequency And Mode Shape

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