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  • 1.
    Afzal, Md
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
    Arteaga, I. Lopez
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
    Kari, Leif
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    Kharyton, V.
    INVESTIGATION OF DAMPING POTENTIAL OF STRIP DAMPER ON A REAL TURBINE BLADE2016In: PROCEEDINGS OF THE ASME TURBO EXPO: TURBINE TECHNICAL CONFERENCE AND EXPOSITION, 2016, VOL 7A, AMER SOC MECHANICAL ENGINEERS , 2016Conference paper (Refereed)
    Abstract [en]

    This paper investigates the damping potential of strip dampers on a real turbine bladed disk. A 3D numerical friction contact model is used to compute the contact forces by means of the Alternate Frequency Time domain method. The Jacobian matrix required during the iterative solution is computed in parallel with the contact forces, by a quasi-analytical method. A finite element model of the strip dampers, that allows for an accurate description of their dynamic properties, is included in the steady-state forced response analysis of the bladed disk. Cyclic symmetry boundary conditions and the multiharmonic balance method are applied in the formulation of the equations of motion in the frequency domain. The nonlinear forced response analysis is performed with two different types of boundary conditions on the strip: (a) free-five and (b) elastic, and their influence is analyzed. The effect of the strip mass, thickness and the excitation levels on the forced response curve is investigated in detail.

  • 2.
    Afzal, Mohammad
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Structural and vibroacoustics. kth.
    Lopez Arteaga, Ines
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Structural and vibroacoustics. Eindhoven University of Technology, the Netherlands.
    Kari, Leif
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Structural and vibroacoustics.
    A formulation of the Jacobian matrixfor 3D numerical friction contact model applied to turbine blade shroud contactIn: Journal of Sound and Vibration, ISSN 0022-460X, E-ISSN 1095-8568Article in journal (Other academic)
    Abstract [en]

    An analytical expression is formulated to compute the Jacobian matrix for 3D friction contact modelling that eciently evaluates the matrix while computing the friction contact forces in the time domain by means of the alternate frequency time domain approach. The developed expression is successfully used for thecalculation of the friction damping on a turbine blade with shroud contact interface having an arbitrary 3Drelative displacement. The analytical expression drastically reduces the computation time of the Jacobian matrix with respect to the classical finite dierence method, with many points at the contact interface. Therefore,it also significantly reduces the overall computation time for the solution of the equations of motion,since the formulation of the Jacobian matrix is the most time consuming step in solving the large set of nonlinear algebraic equations when a finite dierence approach is employed. The equations of motion are formulated in the frequency domain using the multiharmonic balance method to accurately capture the nonlinear contact forces and displacements. Moreover, the equations of motion of the full turbine blade model are reduced to a single sector model by exploiting the concept of cyclic symmetry boundary condition for aperiodic structure. Implementation of the developed scheme in solving the equations of motion is proved to be effective and significant reduction in time is achieved without loss of accuracy.

  • 3.
    Afzal, Mohammad
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    Lopez Arteaga, Ines
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    Kari, Leif
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    An analytical calculation of the Jacobian matrix for 3D friction contact model applied to turbine blade shroud contact2016In: Computers & structures, ISSN 0045-7949, E-ISSN 1879-2243, Vol. 177, p. 204-217Article in journal (Refereed)
    Abstract [en]

    An analytical expression is formulated to compute the Jacobian matrix for 3D friction contact modeling that efficiently evaluates the matrix while computing the friction contact forces in the time domain by means of the alternate frequency time domain approach. The developed expression is successfully used for the calculation of the friction damping on a turbine blade with shroud contact interface having an arbitrary 3D relative displacement. The analytical expression drastically reduces the computation time of the Jacobian matrix with respect to the classical finite difference method, with many points at the contact interface. Therefore, it also significantly reduces the overall computation time for the solution of the equations of motion, since the formulation of the Jacobian matrix is the most time consuming step in solving the large set of nonlinear algebraic equations when a finite difference approach is employed. The equations of motion are formulated in the frequency domain using the multiharmonic balance method to accurately capture the nonlinear contact forces and displacements. Moreover, the equations of motion of the full turbine blade model are reduced to a single sector model by exploiting the concept of cyclic symmetry boundary condition for a periodic structure. Implementation of the developed scheme in solving the equations of motion is proved to be effective and significant reduction in time is achieved without loss of accuracy.

  • 4.
    Afzal, Mohammad
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    Lopez-Arteaga, Ines
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    Kari, Leif
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    Adaptive control of normal load at the friction interface of bladed disks using giant magnetostrictive materialIn: Journal of Vibration and Control, ISSN 1077-5463, E-ISSN 1741-2986Article in journal (Other academic)
    Abstract [en]

    A novel application of magnetostrictive actuators in underplatform dampers of bladed disks is proposed for adaptive control of the normal load at the friction interface in order to achieve the desired friction damping in the structure. Friction damping in a bladed disk depends on many parameters such as rotational speed, engine excitation order, nodal diameter, contact stiffness, friction coefficient and normal contact load. However, all these parameters have a fixed value at an operating point. On the other hand, the ability to vary some of these parameters such as the normal contact load is desirable in order to obtain an optimum damping in the bladed disk at different operating conditions. Under the influence of an external magnetic field, magnetostrictive materials develop an internal strain that can be exploited to vary the normal contact load at the friction interface, which makes them a potentially good candidate for this application. A commercially available magnetostrictive alloy, Terfenol-D is considered in this analysis that is capable of providing magnetostrain up to 0.002 under prestress and a blocked force over 1500 N. A linearized model of the magnetostrictive material, which is accurate enough for a DC application, is employed to compute the output displacement and the blocked force of the actuator. A nonlinear finite element contact analysis is performed to compute the normal contact load between the blade platform and the underplatform damper as a result of magnetostrictive actuation. The contact analysis is performed for different mounting configurations of the actuator and the obtained results are discussed. The proposed solution is potentially applicable to adaptively control vibratory stresses in bladed disks and consequently to reduce failure due to high-cycle fatigue. Finally, the practical challenges in employing magnetostrictive actuators in underplatform dampers are discussed.

  • 5.
    Afzal, Mohammad
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    Lopez-Arteaga, Ines
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL. Eindhoven University of Technology.
    Kari, Leif
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    Numerical analysis of multiple friction contacts in bladed disksIn: International Journal of Mechanical Sciences, ISSN 0020-7403, E-ISSN 1879-2162Article in journal (Other academic)
    Abstract [en]

    The damping potential of multiple friction contacts in a bladed disk, tip shroud and strip damper is investigated, showing that friction damping effectiveness can be potentially increased by using multiple friction contact interfaces. Friction damping depends on many parameters such as rotational speed, engine excitation order and mode family and therefore it is not possible to damp all the critical resonances using a single friction contact interface. For example, a strip damper is more effective for the low nodal diameters, where blade/disk coupling is strong. The equations of motion of the bladed disk with multiple friction contacts are derived in the frequency domain for a cyclic structure with rotating excitations and a highly accurate method is used to generate the frequency response function (FRF) matrix. Furthermore, a finite element contact analysis is performed to compute the normal contact load and the contact area of the shroud interface at operating rotational speed. The multiharmonic balance method is employed in combination with the alternate frequency time domain method to find the approximate steady state periodic solution. A low-pressure turbine bladed disk is considered and the effect of the engine excitation level, strip mass, thickness and the accuracy of FRF matrix on the nonlinear response curve are investigated in detail.

  • 6. Alberdi, A.
    et al.
    Gil-Negrete, N.
    Vinolas, J.
    Kari, Leif
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
    Nieto, F. J.
    Dynamic characterisation of different magneto-sensitive natural rubbers for application in vibration isolation2010In: Proceedings of ISMA 2010 - International Conference on Noise and Vibration Engineering, including USD 2010, Katholieke Universiteit Leuven , 2010, p. 227-231Conference paper (Refereed)
    Abstract [en]

    Dynamic properties of magneto-sensitive natural rubber components were experimentally studied. Different magneto-sensitive rubbers were manufactured, consisting of irregularly shaped micron-sized iron particles embedded in a natural rubber matrix, and the influence of the hardness of the matrix material and the particle volume concentration were analyzed. Vibration isolators consisting of magneto-sensitive elastomers promise to have more functionality than conventional isolators as they can change their dynamic stiffness rapidly, continuously and reversibly under the application of an external magnetic field. Experimental measurements on MS components show that a better performance may be obtained at applications where small amplitudes are required, using soft matrix materials and with concentration close to a critical particle volume fraction.

  • 7.
    Alberdi-Muniain, Ane
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Structural and vibroacoustics.
    Gil-Negrete, N.
    Department of Applied Mechanics, CEIT and Tecnun (University of Navarra).
    Kari, Leif
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Structural and vibroacoustics.
    Direct energy flow measurement in magneto-sensitive vibration isolator systems2012In: Journal of Sound and Vibration, ISSN 0022-460X, E-ISSN 1095-8568, Vol. 331, no 9, p. 1994-2006Article in journal (Refereed)
    Abstract [en]

    The effectiveness of highly nonlinear, frequency, amplitude and magnetic field dependent magneto-sensitive natural rubber components applied in a vibration isolation system is experimentally investigated by measuring the energy flow into the foundation. The energy flow, including both force and velocity of the foundation, is a suitable measure of the effectiveness of a real vibration isolation system where the foundation is not perfectly rigid. The vibration isolation system in this study consists of a solid aluminium mass supported on four magneto-sensitive rubber components and is excited by an electro-dynamic shaker while applying various excitation signals, amplitudes and positions in the frequency range of 20-200 Hz and using magneto-sensitive components at zero-field and at magnetic saturation. The energy flow through the magneto-sensitive rubber isolators is directly measured by inserting a force transducer below each isolator and an accelerometer on the foundation close to each isolator. This investigation provides novel practical insights into the potential of using magneto-sensitive material isolators in noise and vibration control, including their advantages compared to traditional vibration isolators. Finally, nonlinear features of magneto-sensitive components are experimentally verified.

  • 8.
    Alberdi-Muniain, Ane
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Structural and vibroacoustics.
    Gil-Negrete, N
    Kari, Leif
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Structural and vibroacoustics.
    Indirect energy flow measurement in magneto-sensitive vibration isolator systems2013In: Applied Acoustics, ISSN 0003-682x, ISSN 0003-682x, Vol. 74, no 4, p. 575-584Article in journal (Other academic)
    Abstract [en]

    The indirect energy flow measurement method is extended to cover highly nonlinear, frequency, amplitude and magnetic field dependent magneto-sensitive natural rubber isolators applied in a real vibration isolation system. Energy flow is an effective measure of vibration isolation while being a single quantity that considers both force and velocity. The use of the indirect technique is of interest while requiring only accelerometers since it is usually difficult to directly measure the force in a real application. The vibration isolation system is composed of four magneto-sensitive rubber isolators that are inserted under a vibrating source consisting of a solid aluminium mass excited by an electro-dynamic shaker. Magneto-sensitive rubber isolators are more useful than conventional rubber isolators since the dynamic stiffness varies with the application of an external magnetic field, thus resulting in more effective vibration isolation. Various approximations regarding the indirect technique are investigated, concluding that average stiffness of magneto-sensitive isolators can be used and auto-spectrum of the foundation velocity ignored. In addition, various error analyses are performed. Finally, the indirect measurement of the energy flow is validated by direct measurements, showing very good agreement.

  • 9.
    Alberdi-Muniain, Ane
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Structural and vibroacoustics.
    Gil-Negrete, N.
    Department of Applied Mechanics, CEIT and Tecnun (University of Navarra).
    Kari, Leif
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Structural and vibroacoustics.
    Influence of carbon black and plasticisers on dynamic properties of isotropic magnetosensitive natural rubber2012In: Plastics, rubber and composites, ISSN 1465-8011, E-ISSN 1743-2898, Vol. 41, no 7, p. 310-317Article in journal (Refereed)
    Abstract [en]

    The dynamic shear modulus of magnetosensitive (MS) natural rubber composites is experimentallystudied, where influences of carbon black, plasticiser and iron particle concentrations areinvestigated at various dynamic shear strain amplitudes and external magnetic fields within thelower structure borne frequency range. The iron particles embedded in natural rubber areirregularly shaped and randomly distributed; the plasticisers simplify the iron particle blendingprocess, while carbon black reduces the production costs and improves the mechanicalproperties. The results show that the relative MS effect on the shear modulus magnitude increaseswith increased plasticiser and iron particle concentration and decreases with increased carbonblack concentration. Furthermore, their relative contributions are quantified. Consequently, thestudy provides a basis for optimising the composition of MS natural rubber to meet a variety ofrequirements, including those of vibration isolation, a promising application area for MS materials.

  • 10.
    Alberdi-Muniain, Ane
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Structural and vibroacoustics.
    Gil-Negrete, N
    Kari, Leif
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Structural and vibroacoustics.
    Modelling energy flow through magneto-sensitive vibration isolators2013In: International Journal of Engineering Science, ISSN 0020-7225, E-ISSN 1879-2197, Vol. 65, p. 22-39Article in journal (Refereed)
    Abstract [en]

    A highly nonlinear model of the energy flow in a magneto-sensitive (MS) vibration isolation system is developed where it is possible to investigate the influences of MS rubber material parameters; magnetic field strength; MS isolator dimension and position; excitation force magnitude, position and frequency; engine mass, inertia and dimension and, finally, foundation inertance. The MS vibration isolation system consists of an engine modelled by a solid mass, excited by a vertical force and mounted upon four MS isolators being connected to a relatively stiff foundation characterised by its driving-point and transfer inertances at and between the connection points. The energy flow into the foundation is the most appropriate indicator of the effectiveness of a real vibration isolation system while considering both foundation velocity and force. The MS isolator model applied is a nonlinear MS rubber model including frequency, dynamic amplitude and magnetic field dependence. The energy flow model results are compared to those of measurements, showing good agreement. Finally, parameter studies are carried out. The developed energy flow model provides a basis for designing MS vibration isolation systems to meet specific requirements.

  • 11.
    Alberdi-Muniain, Ane
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Structural and vibroacoustics.
    Gil-Negrete, N.
    Department of Applied Mechanics, CEIT and Tecnun (University of Navarra).
    Nieto, F.J.
    Department of Applied Mechanics, CEIT and Tecnun (University of Navarra).
    Kari, Leif
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Structural and vibroacoustics.
    Vinolas, J.
    Department of Applied Mechanics, CEIT and Tecnun (University of Navarra).
    An experimental study of magneto-sensitive natural rubber components applied in a vibration isolation system2009In: CONSTITUTIVE MODELS FOR RUBBER VI / [ed] Gert Heinrich, Michael Kaliske, Alexander Lion, London: Taylor & Francis, 2009, p. 99-104Conference paper (Refereed)
    Abstract [en]

    The effectiveness of magneto-sensitive natural rubber components applied in a vibration isolation system is experimentally investigated, where influences of excitation position, amplitude, frequency and magnetic field are examined. The magneto-sensitive elastomer consists of micron-sized, irregularly shaped iron particles blended in soft natural rubber at a concentration close to the critical particle volume fraction, shown to be the most favorable composition for optimum behaviour. A rigid aluminium mass supported on four vibration isolators is excited by an electro-dynamic shaker. Each component of this vibration isolation system is composed of two thin, square shaped, symmetrically positioned magneto-sensitive elements excited in simple shear with a magnetic field applied perpendicularly to the motion by an electromagnet. The magnetic field is varied by applying different intensities through the coil. The excitation position is either on the centre or on the edge of the surface of the mass, using step-sine excitation of various amplitudes in the frequency range of 0 to 300 Hz. The results show that it is possible to use magneto-sensitive rubber for vibration control purposes.

  • 12.
    Austrell, Per-Erik
    et al.
    Division of Structural Mechanics, Lund Institute of Technology.
    Kari, LeifKTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Structural and vibroacoustics.
    Constitutive Models for Rubber IV: proceedings of the 4th European Conference for Constitutive Models for Rubber, ECCMR 2005, Stockholm, Sweden, 27-29 June 20052005Collection (editor) (Refereed)
    Abstract [en]

    The unique properties of elastomeric materials are taken advantage of in many engineering applications. Elastomeric units are used as couplings or mountings between stiff parts. Examples are shock absorbers, vibration insulators, flexible joints, seals and suspensions etc.

     

    However, the complicated nature of the material behavior makes it difficult to accurately predict the performance of these units, using for example finite element modelling. It is therefore necessary that the constitutive model accurately capture relevant aspects of the mechanical behavior.

     

    The latest development concerning constitutive modelling of rubber is collected in these proceedings. It is the fourth ECCMR-European Conference on Constitutive Modelling in a series on this subject.

     

    Topics included in this volume are, Hyperelastic models, Strength, fracture & fatigue, Dynamic properties & the Fletcher-Gent effect, Micro-mechanical & statistical approaches, Stress softening, Viscoelasticity, Filler reinforcement, and Tyres, fiber & cord reinforced rubber.

  • 13.
    Azhdar, Bruska
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Stenberg, Bengt
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
    Kari, Leif
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Structural and vibroacoustics.
    Determination of dynamic and sliding friction, and observation of stick-slip phenomenon on compacted polymer powders during high-velocity compaction2006In: Polymer testing, ISSN 0142-9418, E-ISSN 1873-2348, Vol. 25, no 8, p. 1069-1080Article in journal (Refereed)
    Abstract [en]

    Dynamic friction, sliding friction, and the stick-slip phenomenon have been studied on compacted polymer powders during high-velocity compaction. It is particularly important from a practical point of view to distinguish the stick-slip mechanism and the sliding mechanism which occur concurrently. A practical experimental system has been successfully developed to study the dry frictional force and to measure the sliding coefficient between the polymer powder particles and the die wall during high-velocity compaction. Two new components have been introduced as relaxation assists to improve the compaction process by reducing the frictional forces. It was found that the relaxation assist device leads to an improvement in the polymer powder compaction process by giving a more homogeneous opposite velocity and a better locking of the powder bed in the compacted form with less change in dimensions. The subsequent movement of the particles can be reduced and the powder bed attains a higher density with a minimum total elastic spring-back. The relative time of the stick-slip phenomenon during the compacting stage is also reduced so that the time needed to transfer from an intermittent stick-slip state to a smooth sliding state is reduced and the powder bed slides smoothly. It was found that the dynamic, dry frictional force is intermittent (stick-slip mechanism) at low compaction rates but that at high compaction rates is becomes more smooth (sliding mechanism). Both mechanisms depend on the nature of the powder and on the compaction conditions. At the beginning of the compaction stage, the sliding coefficient decreases due to an increase in the radial to axial stress ratio until the maximum pressure has been reached. During the reorganization stage, more time is needed for large particles to move, rotate and slide due to their relatively large diameter and mass. As a result, the reorganization stage is extended and the stick-slip phenomenon is observed more with increasing particle size. Much better transfer of the pressure throughout the powder bed and less loss of pressure lead to a higher sliding coefficient due to the overall friction during the compaction process. It was found that the sliding coefficient is proportional to the density. A more homogeneous density distribution in the compacted powder and a smaller pressure loss during compaction has a major influence on the sliding coefficient and on the quality of the compacted material

  • 14.
    Azhdar, Bruska
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Stenberg, Bengt
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Kari, Leif
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Structural and vibroacoustics.
    Determination of springback gradient in the die on compacted polymer powders during high-velocity compaction2006In: Polymer testing, ISSN 0142-9418, E-ISSN 1873-2348, Vol. 25, no 1, p. 114-123Article in journal (Refereed)
    Abstract [en]

    A uniaxial high-velocity compaction process for polymer powder using a cylindrical, hardened steel die and a new technique with relaxation assist was tested with various heights. The influences of the relaxation assist device on the process characteristics are discussed. Two bonded strain gauges and a high-speed video camera system were used to investigate the springback phenomenon during the compaction process. It was found that the relaxation assist improves the compaction of the polymer powder by locking the powder bed in the compacted form. It is shown that the high-velocity compaction process is an interruption process and that the delay times between the pressure waves can be reduced by increasing the height of the relaxation assist device. The delay times between the pressure waves are also strongly dependent on the strain rate. If the height of the relaxation assist device is increased, the first gross instantaneous springback, and the total elastic springback, are reduced. In addition, the density of the powder bed is increased. The relative times of the compacting stage, decompacting stage and the reorganisation of the particles can be also controlled by altering the height of the relaxation assist.

  • 15.
    Azhdar, Bruska
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Stenberg, Bengt
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Kari, Leif
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Structural and vibroacoustics.
    Development of a High-Velocity Compaction process for polymer powders2005In: Polymer testing, ISSN 0142-9418, E-ISSN 1873-2348, Vol. 24, no 7, p. 909-919Article in journal (Refereed)
    Abstract [en]

    The High-Velocity Compaction (HVC) process for powder polymers has been studied, with a focus on the compactibility characteristics and surface morphology of the compacted materials, with and without relaxation assists, by increasing compacting quantity and direction. The basic phenomena associated with HVC are explained and the general energy principle is introduced to explain pull-out phenomena during the decompacting stage. Polyamide-11 powders with different particle size distributions have been compacted with the application of different compaction profiles, e.g. different energies and velocities. Scanning electron microscopy (SEM) and image computer board camera, (IC-PCI Imaging Technology) have been used to the study the morphological characteristics, the limit of plastic deformation and particle bonding by plastic flow at contact points, and pull-out phenomena. The relative green density is influenced more by the pre-compacting (primary compaction step) than by the post-compacting (secondary compaction step). The pressure and density distribution differences between the upper and lower surface are not uniform. Projectile supports or 'relaxation assists' are presented as a new technique to reduce pull-out phenomenon. Experimental results for different compaction profiles are presented showing the effect of varying the opposite velocity during the decompacting stage, and how to improve the homogeneous densification between the upper and lower surface and the evenness of the upper surface of the compacted powder bed by using relaxation assists.

  • 16.
    Azhdar, Bruska
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Stenberg, Bengt
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Kari, Leif
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Structural and vibroacoustics.
    Polymer-nanofiller prepared by high-energy ball milling and high velocity cold compaction2008In: Polymer Composites, ISSN 0272-8397, E-ISSN 1548-0569, Vol. 29, no 3, p. 252-261Article in journal (Refereed)
    Abstract [en]

    High-energy ball milling using comilling in a solid state by low-temperature mechanical alloying to prepare nickel-ferrite (NiFe2O4) nanopowders and ultrafine poly(methyl methacrylate) (PMMA), dispersing nanoparticles in a polymer matrix, and a uniaxial high-velocity cold compaction process using a cylindrical, hardened steel die and a new technique with relaxation assists have been studied. The focus has been on the particle size distributions of the nanocomposite powder during the milling and on the surface morphology of the nanocomposite-compacted materials after compaction with and without relaxation assists. Experimental results for different milling systems are presented showing the effects of milling time and material ratio. It was found that a longer mixing time give a higher degree of dispersion of the nanopowder on the PMMA particle surfaces. Furthermore, with increasing content of NiFe2O4 nanopowder, the reduction of the particle size was more effective. Different postcompacting profiles, i.e. different energy distributions between the upper and lower parts of the compacted powder bed, lead to different movements of the various particles and particle layers. Uniformity, homogeneity, and densification on the surfaces in the compacted powder are influenced by the postcompacting magnitude and direction. It was found that the relaxation assist device leads to an improvement in the polymer powder compaction process by reducing the expansion of the compacted volume and by reducing the different opposite velocities, giving the compacted composite bed a more homogeneous opposite velocity during the decompacting stage and reducing the delay time between the successive pressure waves.

  • 17.
    Blanco, Blas
    et al.
    KTH.
    Alonso, A.
    Gil-Negrete, N.
    Kari, Leif
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
    Gim´enez, J.G.
    Implementation of Timoshenko element local deflection for vertical track modellingArticle in journal (Refereed)
    Abstract [en]

    A vertical track model suitable for the study of the dynamic response and the interaction between wheel and rail in the time domain is developed by using Timoshenko beam elements, and its performance is optimized by accounting for the local deflection of these type of elements. Implementation of the local system enables to obtain an accurate description of the contact force in a more computational efficient way than other numerical methods, and it leads to an almost total elimination of the discontinuities caused by the moving nature of the load and the shear incompatibilities introduced by the conventional formulation with Timoshenko beam elements. The work presented here describes both static and dynamic approaches of the local system directly obtained through the resolution of the beam governing equations. Two resolution strategies for the timedomain response are proposed, along with the local system formulation. The first is based on numerical integration of the whole system by introducing a Newmark scheme followed by a Newton–Raphson iterative process. The second resolution strategy is based on a numerical convolution integration, which is able to reduce significantly the computational cost of the simulation. This last resolution methodology together with implementation of the local system approach, join a computationally efficient routine and an accurate description of the track dynamic, which are valuable features for dynamics simulations. The results are validated by comparison with those from previous works.

  • 18.
    Blanco, Blas
    et al.
    KTH.
    Alonso, A.
    Kari, Leif
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
    Gil-Negrete, N.
    Gim´enez, J.G.
    Distributed support modelling for vertical track dynamic analysisArticle in journal (Refereed)
    Abstract [en]

    The finite length nature of rail-pad supports is characterised by a Timoshenko beam element formulation over an elastic foundation, giving rise to the distributed support element (TEEF). The new element is integrated into a vertical track model, which is solved in frequency and time domain. The developed formulation is obtained by solving the governing equations of a Timoshenko beam for this particular case. The interaction between sleeper and rail via the elastic connection is considered in an analytical, compact and efficient way. The modelling technique results in realistic amplitudes of the ‘pin-pin’ vibration mode and, additionally, it leads to a smooth evolution of the contact force temporal response and to reduced amplitudes of the rail vertical oscillation, as compared to the results from concentrated connection support models. Simulations are performed for both parametric and sinusoidal roughness excitation. The model of support proposed here is compared with a previous finite length model developed by other authors, coming to the conclusion that the proposed model gives accurate results at a reduced computational cost.

  • 19.
    Blom, Peter
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Structural and vibroacoustics.
    Kari, Leif
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Structural and vibroacoustics.
    A non-linear constitutive audio frequency magneto-sensitive rubber model including amplitude, frequency and magnetic field dependence.2011In: Journal of Sound and Vibration, ISSN 0022-460X, E-ISSN 1095-8568, Vol. 330, no 5, p. 947-954Article in journal (Refereed)
    Abstract [en]

    A novel constitutive model of magneto-sensitive rubber in the audible frequency range is presented. Characteristics inherent to magneto-sensitive rubber within this dynamic regime are defined: magnetic sensitivity, amplitude dependence, elasticity and viscoelasticity. Prior to creating the model assumptions based on experimental observations concerning these components are formulated. The first observation is that not only does the rubber display a strong amplitude dependence even for small strains, but also the magnetic sensitivity is strongly amplitude dependent. The second and third are, respectively, that the elastic component is magneto-sensitive, whereas the viscoelastic dependence on magnetic induction appears to be small. Thus, the model is developed from these assumptions and parameters are optimized with respect to experimental values for one case and subsequently validated for others; a very good agreement is obtained.

  • 20.
    Blom, Peter
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Structural and vibroacoustics.
    Kari, Leif
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Structural and vibroacoustics.
    Amplitude and frequency dependence of magneto-sensitive rubber in a wide frequency range2005In: Polymer testing, ISSN 0142-9418, E-ISSN 1873-2348, Vol. 24, no 5, p. 656-662Article in journal (Refereed)
    Abstract [en]

    Two new aspects of the dynamic behaviour in the audible frequency range of magneto-sensitive (MS) rubber are highlighted: the existence of an amplitude dependence of the shear modulus - referred to as the Fletcher-Gent effect - for even small displacements, and the appearance of large MS effects. In order to illustrate these two features, results are presented of measurements performed in the audible frequency range on two different kinds of rubber: silicone and natural rubber with a respective iron particle volume concentration of 33%. The particles used are of irregular shape and randomly distributed within the rubber. An external magnetic field of 0-0.8 T is applied. Both kinds of rubber are found to be strongly amplitude dependent and, furthermore, displaying large responses to externally applied magnetic fields - a maximum of 115%. Also included are graphs of measurements on silicone and natural rubber devoid of iron particles. Those results support the conclusion that introducing iron particles in the rubber gives rise to a strong, non-negligible, amplitude dependence in the entire frequency range.

  • 21.
    Blom, Peter
    et al.
    KTH, Superseded Departments, Vehicle Engineering.
    Kari, Leif
    KTH, Superseded Departments, Vehicle Engineering.
    Magneto-rheological rubber isolators in the audible frequency range2004In: Nordic Vibration Research, 2004Conference paper (Other academic)
  • 22.
    Blom, Peter
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Structural and vibroacoustics.
    Kari, Leif
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Structural and vibroacoustics.
    Magneto-sensitive rubber in the audible frequency range2005In: CONSTITUTIVE MODELS FOR RUBBER IV / [ed] Per-Erik Austrell, Leif Kari, London: Taylor & Francis, 2005Conference paper (Refereed)
  • 23.
    Blom, Peter
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Structural and vibroacoustics.
    Kari, Leif
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Structural and vibroacoustics.
    Smart audio frequency energy flow control by magneto-sensitive rubber isolators2008In: Smart materials and structures (Print), ISSN 0964-1726, E-ISSN 1361-665X, Vol. 17, no 1Article in journal (Refereed)
    Abstract [en]

    A magneto-sensitive rubber isolator inserted between a source and an infinite plate is modelled in the audible frequency range, and the energy flow into the plate with the rubber subjected to a magnetic field applied perpendicular to the axial displacement is calculated. Subsequently the result is compared to the corresponding energy flow for zero magnetic induction; upon the application of an external magnetic field the rubber becomes stiffer, thus shifting the internal resonances of the isolator. This is a fast and reversible process enabling adaption of the isolator to rapidly changing audio frequency conditions by simply turning on and off a magnetic field. In the application example considered, the energy flow into the plate at the first internal dynamic peak stiffness frequency is reduced by approximately 7 dB-a large difference in a sound and vibration context-by inducing magnetic saturation of the rubber.

  • 24.
    Blom, Peter
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Structural and vibroacoustics.
    Kari, Leif
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Structural and vibroacoustics.
    The frequency, amplitude and magnetic field dependent torsional stiffness of a magneto-sensitive rubber bushing2011In: International Journal of Mechanical Sciences, ISSN 0020-7403, E-ISSN 1879-2162, Vol. 60, no 1, p. 54-58Article in journal (Refereed)
    Abstract [en]

    A dynamic torsional stiffness model of a magneto-sensitive circular annular rubber bushing is presented where influences of frequency, amplitude and magnetic field dependence are included. This is achieved by employing a newly developed non-linear magneto-sensitive audio frequency constitutive equation in an engineering formula for the torsional stiffness of a rubber bushing. The engineering stiffness formula predicts the frequency and amplitude dependent stiffness in a simple way, based on geometric dimensions and the shear modulus. The shear modulus is provided by the rubber model. The results from these calculations predict and clearly display the possibility of controlling over a large frequency range, through the application of a magnetic field, the magneto-sensitive rubber bushing stiffness.

  • 25.
    Coja, Michael
    et al.
    KTH, Superseded Departments, Vehicle Engineering.
    Kari, Leif
    KTH, Superseded Departments, Vehicle Engineering.
    A new computational method based on waveguides for modeling of preload nonlinear effects on cylindrical vibration isolators2004In: Nordic Seminar on Computational Mechanics, 2004Conference paper (Other academic)
  • 26.
    Coja, Michael
    et al.
    KTH, Superseded Departments, Vehicle Engineering.
    Kari, Leif
    KTH, Superseded Departments, Vehicle Engineering.
    A self similarity audio-frequency stiffness model of pre-compressed vibration isolators2004In: Nordic Vibration Research, 2004Conference paper (Other academic)
  • 27.
    Coja, Michael
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Structural and vibroacoustics.
    Kari, Leif
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Structural and vibroacoustics.
    A simple engineering audible-frequency stiffness model for a preloaded conical rubber isolatorManuscript (preprint) (Other academic)
  • 28.
    Coja, Michael
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Structural and vibroacoustics.
    Kari, Leif
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Structural and vibroacoustics.
    An analytical, closed-form stiffness model for preloaded conical rubber mounting in the audible frequency range2005In: CONSTITUTIVE MODELS FOR RUBBER IV / [ed] Per-Erik Austrell, Leif Kari, London: Taylor & Francis, 2005Conference paper (Refereed)
  • 29.
    Coja, Michael
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Structural and vibroacoustics.
    Kari, Leif
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Structural and vibroacoustics.
    An effective waveguide model for pre-compressed vibration isolatorsIn: Acta Acoustica united with Acustica, ISSN 1610-1928, E-ISSN 1861-9959Article in journal (Refereed)
  • 30.
    Coja, Michael
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Structural and vibroacoustics.
    Kari, Leif
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Structural and vibroacoustics.
    Axial audio-frequency stiffness of a bush mounting: the waveguide solution2007In: Applied Mathematical Modelling, ISSN 0307-904X, E-ISSN 1872-8480, Vol. 31, no 1, p. 38-53Article in journal (Refereed)
    Abstract [en]

    An axial, dynamic stiffness model of an arbitrary wide and long rubber bush mounting is developed within the audible-frequency range, where influences of audible frequencies, material properties, bush mounting length and radius, are investigated. The problems of simultaneously satisfying the locally non-mixed boundary conditions at the radial and end surfaces are solved by adopting a waveguide approach, using the dispersion relation for axially symmetric waves in thick-walled infinite plates, while satisfying the radial boundary conditions by mode matching. The rubber is assumed nearly incompressible, displaying dilatation elasticity and deviatoric viscoelasticity based on a fractional derivative, standard linear solid embodying a Mittag-Leffler relaxation kernel, the main advantage being the minimum parameter number required to successfully model wide-frequency band material properties. The stiffness is found to depend strongly on frequency, displaying acoustical resonance phenomena; such as stiffness peaks and troughs. The presented model agrees fully with a simplified, long-bush model while diverging from it for increased diameter-to-length ratios. To a great extent, the increased influences of higher order modes and dispersion explain the discrepancies reported for the approximate approach.

  • 31.
    Coja, Michael
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Structural and vibroacoustics.
    Kari, Leif
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Structural and vibroacoustics.
    Rubber versus steel vibration isolators - The audible frequency contest2005In: KGK Kautschuk Gummi Kunststoffe, ISSN 0948-3276, Vol. 58, no 11, p. 564-569Article in journal (Refereed)
    Abstract [en]

    The audible frequency axial dynamic stiffness of three vibration isolators plausibly used in the design of a ship main engine suspension system are examined and compared for different static preloads. An indirect measurement method is used to investigate the blocked dynamic transfer stiffness using a specially designed test rig displaying a strong frequency dependence where resonance and antiresonance phenomena appear in the form of troughs and peaks respectively. The significant influence of the preload effects is also assessed for each isolator. Clearly, the rubber isolator presents superior performances in comparison with the two others, steel and combined steel-rubber isolators, over the studied frequency range 200 to 1000 Hz for each preload 30, 40 and 50 kN.

  • 32.
    Enflo, Bengt Olof
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Theoretical and Applied Mechanics.
    Hedberg, Claes M.Blekinge Institute of Technology.Kari, LeifKTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Structural and vibroacoustics.
    Non-Linear Acoustics: Fundamentals and Applications2008Conference proceedings (editor) (Refereed)
  • 33.
    Felcsuti, G.
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    Liu, H.
    Kari, Leif
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    Effect of modal behaviour on dynamic transmission error of driveline gears2016In: Proceedings of ISMA 2016 - International Conference on Noise and Vibration Engineering and USD2016 - International Conference on Uncertainty in Structural Dynamics, KU Leuven, Departement Werktuigkunde , 2016, p. 865-875Conference paper (Refereed)
    Abstract [en]

    Dynamic transmission error (DTE) is widely accepted as the source of gear whine noise, therefore its reduction is essential for NVH optimization of drivelines. While most studies in hypoid gear dynamics concentrate on the gears only, this paper takes into account the additional effect of other driveline components and examines their contribution to the DTE. The analytical model developed in this paper consists of a hypoid gear pair connected to a simple structure with well-defined modal behaviour. A test rig is specifically designed to validate the model. In the rig, the pinion rotates only along one axis while the gear is also allowed to have translational motion induced by a rigid body modes on an elastic foundation. This model could help identifying the most influential parameters in terms of the mode and DTE interaction and thereby improve the NVH performance of driveline units.

  • 34.
    Feng, Leiping
    et al.
    KTH, Superseded Departments, Vehicle Engineering.
    Nilsson, A.
    KTH, Superseded Departments, Vehicle Engineering.
    Kari, Leif
    KTH, Superseded Departments, Vehicle Engineering.
    New designs of sandwich engine foundations1998In: International Conference on Sandwich Construction, 1998, p. 289-300Conference paper (Refereed)
  • 35.
    Feng, Leiping
    et al.
    KTH, Superseded Departments, Vehicle Engineering.
    Nilsson, A.
    KTH, Superseded Departments, Vehicle Engineering.
    Kari, Leif
    KTH, Superseded Departments, Vehicle Engineering.
    Vibration of engine foundations1997In: Modern Practice in Stress and Vibration Analysis / [ed] M D Gilchrist, Rotterdam: Balkema, 1997, p. 311-316Conference paper (Refereed)
  • 36.
    García Tárrago, Maria-José
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Structural and vibroacoustics.
    Kari, Leif
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Structural and vibroacoustics.
    Gil-Negrete, N.
    Department of Applied Mechanics, CEIT and Tecnun (University of Navarra).
    Vinolas, J.
    Department of Applied Mechanics, CEIT and Tecnun (University of Navarra).
    An effective engineering formula for rubber bush stiffness including amplitude dependence2005In: CONSTITUTIVE MODELS FOR RUBBER IV / [ed] Per-Erik Austrell, Leif Kari, London: Taylor & Francis, 2005Conference paper (Refereed)
  • 37.
    García Tárrago, Maria-José
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    Kari, Leif
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    Gil-Negrete, N.
    Viñolas, J.
    An effective engineering formula for torsion stiffness of rubber bush including amplitude dependence2005In: Constitutive Models for Rubber IV: Proceedings of the 4th European Conference for Constitutive Models for Rubber, ECCMR 2005, 2005, p. 319-323Conference paper (Refereed)
    Abstract [en]

    Effective engineering formula for torsion stiffness of a long rubber bush in the frequency domain including amplitude dependence is presented. The classical theory of elasticity is the tool used to find this relation between torque and torsion angle in the frequency domain and by assuming an equivalent amplitude strain for the whole bushing. The complex value of the shear modulus to be inserted into the stiffness formula is calculated by applying a separable elastic, viscoelastic and friction rubber model to that equivalent strain.

  • 38.
    García Tárrago, Maria-José
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Structural and vibroacoustics.
    Kari, Leif
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Structural and vibroacoustics.
    Vinolas, J.
    Department of Applied Mechanics, CEIT and Tecnun (University of Navarra).
    Gil-Negrete, N.
    Department of Applied Mechanics, CEIT and Tecnun (University of Navarra).
    Frequency and amplitude dependence of the axial and radial stiffness of carbon-black filled rubber bushings2007In: Polymer testing, ISSN 0142-9418, E-ISSN 1873-2348, Vol. 26, no 5, p. 629-638Article in journal (Refereed)
    Abstract [en]

    The frequency and amplitude dependent dynamic behavior of carbon-black filled rubber bushings is experimentally investigated for a commercially available bushing in the axial and radial directions. Based on measurement observations, models for the axial and radial dynamic stiffness of rubber bushings are developed. The amplitude dependence-referred to as the Fletcher-Gent effect and mainly caused by the presence of carbon-black fillers in the rubber-is included in the analytical models by means of equivalent shear moduli, which result from applying a separable elastic, viscoelastic and friction material model to equivalent strains of the non-homogeneous strain states inside the bushing when subjected to axial or radial deflections. Good correlations between measurements and the axial and radial models at amplitudes of 0.1, 0.2 and 0.5 mm from 5 to 155 Hz-when the material parameters are achieved from axial measurements at 0.1 mm-prove the accuracy of both stiffness models.

  • 39.
    García Tárrago, Maria-José
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Structural and vibroacoustics.
    Kari, Leif
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Structural and vibroacoustics.
    Vinolas, J.
    Department of Applied Mechanics, CEIT and Tecnun (University of Navarra).
    Gil-Negrete, N.
    Department of Applied Mechanics, CEIT and Tecnun (University of Navarra).
    Torsion stiffness of a rubber bushing: a simple engineering design formula including amplitude dependence2007In: Journal of Strain Analysis for Engineering Design, ISSN 0309-3247, E-ISSN 2041-3130, Vol. 42, no 1, p. 13-21Article in journal (Refereed)
    Abstract [en]

    An engineering design formula for the torsion stiffness of a filled rubber bushing in the frequency domain, including the amplitude dependence, is presented. It is developed by applying a novel separable elastic, viscoelastic, and friction material model to an equivalent strain of the strain state inside the bushing, thus leading to an equivalent shear modulus that is inserted into an analytical formula for the torsion stiffness. The rubber model is the result of extending the force-displacement relation established in a sound rubber component model to the stress-strain level. Unlike other simplified methods, this procedure takes into account the variation in the properties inside the bushing owing to non-homogeneous strain states. Moreover, as this formula depends on the bushing geometry in addition to the material properties, it is a fast engineering tool to design the most suitable rubber bushing to fulfil user requirements. Furthermore, it is shown-by dividing the considered bushing into several slices, consequently each equivalent shear modulus is closer to the true value - that the approach of working with only one equivalent shear modulus for the whole bushing is accurate enough.

  • 40.
    García Tárrago, Maria-José
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Structural and vibroacoustics.
    Vinolas, J.
    Department of Applied Mechanics, CEIT and Tecnun (University of Navarra).
    Kari, Leif
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Structural and vibroacoustics.
    Axial stiffness of carbon black filled rubber bushings frequency and amplitude dependence2007In: Kautschuk und Gummi, Kunststoffe, ISSN 0022-9520, Vol. 60, no 1-2, p. 43-48Article in journal (Refereed)
    Abstract [en]

    A simple engineering model for the axial dynamic stiffness of carbon black filled rubber bushings in the frequency domain including amplitude dependence is presented. The axial stiffness is calculated by applying a newly developed rubber model to an equivalent strain of the strain state inside the bushing, thus yielding an equivalent shear modulus which is inserted into an analytical formula for the stiffness. Therefore, this procedure includes the Fletcher-Gent effect inside the bushing due to non-homogeneous strain states. An experimental verification is carried out on a commercially available bushing. Firstly, the model parameters are obtained from two different experiments: elastic and friction parameters are achieved from one quasistatic test performed at 0.1 Hz and 1 mm amplitude, while frequency dependence parameters are obtained from one dynamic test at 0.1 mm of amplitude over a frequency range from 1 to 100 Hz. Secondly, new dynamic tests at five amplitudes are performed on the same bushing and the results are compared to those of the model showing good agreements.

  • 41.
    Gil-Negrete, N.
    et al.
    Department of Applied Mechanics, CEIT and Tecnun (University of Navarra).
    Vinolas, J.
    Department of Applied Mechanics, CEIT and Tecnun (University of Navarra).
    Kari, Leif
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Structural and vibroacoustics.
    A Nonlinear Rubber Material Model Combining Fractional Order Viscoelasticity and Amplitude Dependent Effects2009In: Journal of applied mechanics, ISSN 0021-8936, E-ISSN 1528-9036, Vol. 76, no 1, p. 1-9Article in journal (Refereed)
    Abstract [en]

    A nonlinear rubber material model is presented, where influences of frequency and dynamic amplitude are taken into account through fractional order viscoelasticity and plasticity, respectively. The problem of simultaneously modeling elastic, viscoelastic, and friction contributions is removed by additively splitting them. Due to the fractional order representation mainly, the number of parameters of the model remains low, rendering an easy fitting of the values from tests on material samples. The proposed model is implemented in a general-purpose finite element (FE) code. Since commercial FE codes do not contain any suitable constitutive model that represents the full dynamic behavior of rubber compounds (including frequency and amplitude dependent effects), a simple approach is used based on the idea of adding stress contributions from simple constitutive models: a mesh overlay technique, whose basic idea is to create a different FE model for each material definition (fractional derivative viscoelastic and elastoplastic), all with identical meshes but with different material definition, and sharing the same nodes. Fractional-derivative viscoelasticity is implemented through user routines and the algorithm for that purpose is described, while available von Mises' elastoplastic models are adopted to take rate-independent effects into account. Satisfactory results are obtained when comparing the model results with tests carried out in two rubber bushings at a frequency range up to 500 Hz, showing the ability of the material model to accurately describe the complex dynamic behavior of carbon-black filled rubber compounds.

  • 42.
    Gil-Negrete, N.
    et al.
    Department of Applied Mechanics, CEIT and Tecnun (University of Navarra).
    Vinolas, J.
    Department of Applied Mechanics, CEIT and Tecnun (University of Navarra).
    Kari, Leif
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Structural and vibroacoustics.
    A simplified methodology to predict the dynamic stiffness of carbon-black filled rubber isolators using a finite element code2006In: Journal of Sound and Vibration, ISSN 0022-460X, E-ISSN 1095-8568, Vol. 296, no 05-apr, p. 757-776Article in journal (Refereed)
    Abstract [en]

    A new and different approach to the inclusion of the amplitude-dependent effect, known as the Fletcher-Gent effect or Payne effect, in a linear viscoelastic rubber material model is presented to predict the dynamic stiffness of filled rubber isolators using a finite element (FE) code. The technique is based on providing a linear viscoelastic model with the adequate material data set, once the dynamic strain amplitude, to which the rubber mount is subjected, is estimated. A generalized Zener model is adopted to describe the frequency-dependent behaviour of the material through the use of hereditary integrals. The dynamic strain amplitude dependence is not modelled through any friction model or plasticity theory, as usually is in literature. It is introduced by considering the frequency-dependent properties of the compound at an adequate strain value, which enforces the estimation of an equivalent strain value. As a first approximation, a quasi-static value is used as the reference value at which material properties should be provided to the linear viscoelastic model. The technique works directly in frequency domain, the dynamic stiffness of the bushing being directly obtained. The methodology is applied to evaluate the dynamic stiffness of a real bushing in working conditions with very satisfactory results. Despite the assumptions made, especially regarding the estimation of the equivalent strain amplitude value, errors of the predictions fall within the limits usually accepted by rubber manufacturers.

  • 43. Gil-Negrete, N.
    et al.
    Viñolas, J.
    Kari, Leif
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
    Dynamic stiffness prediction of filled rubber mounts: Comparison between a fractional derivative viscoelastic-elastoplastic model and a simplified procedure2005In: Constitutive Models for Rubber IV: Proceedings of the 4th European Conference for Constitutive Models for Rubber, ECCMR 2005, 2005, p. 479-485Conference paper (Refereed)
    Abstract [en]

    A time domain non-linear model is presented to predict the dynamic stiffness of rubber isolators using a FE code. The problem of simultaneously modelling elastic, viscoelastic and friction contributions is removed by additively splitting them. Viscoelastic response is modelled via a fractional derivative model, while amplitude dependency is considered through Coulomb friction elements. Nevertheless, determination of the dynamic stiffness takes long time and requires large amount of memory, as well as the estimation of the transfer function between applied excitation and resulting forces at different frequency and amplitude values. This is why a frequency domain simplified procedure is also proposed to obtain the dynamic stiffness. The technique is based on providing a generalized Maxwell model with the adequate material data set, once the dynamic strain amplitude, to which the mount is subjected, is estimated from quasi-static strain values. The methodology has proved to be efficient and easy to be applied.

  • 44.
    Grishenkov, Dmitry
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Ultrasound.
    Kari, Leif
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    Brismar, Torkel B.
    Karolinska University Hospital.
    Paradossi, Gaio
    Università di Roma Tor Vergata.
    Acoustic properties of polymer-shelled ultrasound contrast agents. Bulk volume vs. microcapillary2009In: 16th International Congress on Sound and Vibration 2009, ICSV 2009, Krakow, 2009, p. 2515-2522Conference paper (Refereed)
    Abstract [en]

    The focus of contrast-enhanced ultrasound research has developed beyond detecting the blood pool to new areas such as perfusion imaging, drug and gene therapy, and targeted imaging. Polymer-shelled microbubbles are proposed as a new generation of ultrasound contrast agents (UCAs) which fulfil the requirements of these applications. With a shelf-life of several months and possibility to conjugate pharmacological molecules to their surface, these UCAs will allow not only to enhance the contrast of ultrasound images, but also to function as carriers of drugs to be delivered locally. In this study, the results of an experimental investigation of three types of UCAs stabilized by thick poly vinyl alcohol (PVA) shell are presented. These UCAs are synthesized from a PVA aqueous solution under varied pH values and temperature. The UCAs differ from each other in their average diameter, shell thickness and polydispersity. Knowledge of the peak negative pressure at which the solid shell fractures is paramount for a proper use of UCAs. Therefore, the dependence of this quantity on temperature and number of cycles in the incident pulse is examined. Much of the blood volume resides in the microcirculation, with capillaries playing a particularly important role in patho-physiology and drug delivery. In this sense in vitro characterization of the UCAs oscillation was moved from bulk volume to the capillary scale, where tissue-bubble interaction takes place. The main conclusion to be drawn from these results is that the shell of the UCAs begin to fracture at values of mechanical index (MI) approved for clinical applications. The fatigue, i.e. the accumulation of damage within the shell of the UCAs, is found to play an important role in fracturing the shell. Finally adhesion of the UCAs to the elastic wall is studied and correlated with estimates of the shell’s visco-elastic constants. Open questions arising from this comparison are briefly discussed.

  • 45.
    Grishenkov, Dmitry
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    Kari, Leif
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Structural and vibroacoustics.
    Brodin, Lars-Åke
    KTH, School of Technology and Health (STH), Medical Engineering.
    Brismar, Torkel B.
    CLINTEC, Department of Radiology, Karolinska Institutet.
    Paradossi, Gaio
    Dipartimento di Chimica, THE UNIVERSITY OF ROME.
    In vitro contrast-enhanced ultrasound measurements of capillary microcirculation: Comparison between polymer- and phospholipid-shelled microbubbles2011In: Ultrasonics, ISSN 0041-624X, E-ISSN 1874-9968, Vol. 51, no 1, p. 40-48Article in journal (Refereed)
    Abstract [en]

    The focus of contrast-enhanced ultrasound research has developed beyond visualizing the blood pool and its flow to new areas such as perfusion imaging, drug and gene therapy, and targeted imaging. In this work comparison between the application of polymer- and phospholipid-shelled ultrasound contrast agents (UCAs) for characterization of the capillary microcirculation is reported. All experiments are carried out using a microtube as a vessel phantom. The first set of experiments evaluates the optimal concentration level where backscattered signal from microbubbles depends on concentration linearly. For the polymer-shelled UCAs the optimal concentration level is reached at a value of about 2 x 10(4) MB/ml, whereas for the phospholipid-shelled UCAs the optimal level is found at about 1 x 10(5) MB/ml.

    Despite the fact that the polymer shell occupies 30% of the radius of microbubble, compared to 0.2% of the phospholipid-shelled bubble, approximately 5-fold lower concentration of the polymer UCA is needed for investigation compared to phospholipid-shelled analogues. In the second set of experiments, destruction/replenishment method with varied time intervals ranging from 2 ms to 3 s between destructive and monitoring pulses is employed. The dependence of the peak-to-peak amplitude of backscattered wave versus pulse interval is fitted with an exponential function of the time gamma = A( 1 - exp(-beta t)) where A represents capillary volume and the time constant beta represents velocity of the flow. Taking into account that backscattered signal is linearly proportional to the microbubble concentration, for both types of the UCAs it is observed that capillary volume is linearly proportional to the concentration of the microbubbles, but the estimation of the flow velocity is not affected by the change of the concentration. Using the single capillary model, for the phospholipid-shelled UCA a delay of about 0.2-0.3 s in evaluation of the perfusion characteristics is found while polymer-shelled UCA provide response immediately. The latter at the concentration lower than 3.6 x 10(5) MB/ml have no statistically significant delay (p < 0.01), do not cause any attenuation of the backscattered signal or saturation of the receiving part of the system. In conclusion, these results suggest that the novel polymer-shelled microbubbles have a potential to be used for perfusion evaluation.

  • 46.
    Jerrelind, Jenny
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Vehicle Dynamics. KTH, School of Engineering Sciences (SCI), Centres, VinnExcellence Center for ECO2 Vehicle design.
    Lopez Arteaga, Ines
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Vehicle Dynamics. KTH, School of Engineering Sciences (SCI), Centres, VinnExcellence Center for ECO2 Vehicle design.
    Drugge, Lars
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Vehicle Dynamics. KTH, School of Engineering Sciences (SCI), Centres, VinnExcellence Center for ECO2 Vehicle design.
    Kari, Leif
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    Effects of non-linear wheel suspension bushing on vehicle response2012In: Proceedings of the ASME Design Engineering Technical Conferences And Computers And Information In Engineering Conference, Vol 6, ASME Press, 2012, p. 615-622Conference paper (Refereed)
    Abstract [en]

    This work presents an analysis of the effects of non-linear characteristics of a top mount bushing in the wheel suspension of a vehicle when evaluating vehicle characteristics such as comfort and handling. The investigation is performed by comparing simulation results from a quarter car model when using a non-linear bushing model and an approximated linear bushing model. It is revealed when analysing the results that there are differences in the response when comparing measures such as sprung mass acceleration, rattle space ratio and tyre-ground contact force. The conclusion is that the more detailed bushing model mainly affects the acceleration levels especially at high frequencies where the linear model underestimates the acceleration. The rattle space ratio and tyre-ground contact force are also affected but not to the same extent.

  • 47.
    Jurado, F.J.
    et al.
    Department of Applied Mechanics, CEIT and Tecnun (University of Navarra).
    Mateo, A.
    Department of Applied Mechanics, CEIT and Tecnun (University of Navarra).
    Gil-Negrete, N.
    Department of Applied Mechanics, CEIT and Tecnun (University of Navarra).
    Vinolas, J.
    Department of Applied Mechanics, CEIT and Tecnun (University of Navarra).
    Kari, Leif
    KTH, Superseded Departments, Vehicle Engineering.
    Testing and FE modelling of the dynamic properties of carbon black filled rubber1999In: EAEC, 1999Conference paper (Other academic)
  • 48.
    Kari, Leif
    KTH, Superseded Departments, Vehicle Engineering.
    A non-linear analytical model for preloaded rubber cylinders2000In: Modern Practice in Stress and Vibration Analysis / [ed] A A Becker, Engineering Materials Advisory Services , 2000, p. 51-62Conference paper (Refereed)
  • 49.
    Kari, Leif
    KTH, Superseded Departments, Vehicle Engineering.
    A nonlinear dynamic stiffness model of a vibration isolator at finite deformations2003In: Modern Practice in Stress and Vibration Analysis, 2003, Vol. 440-4, p. 475-480Conference paper (Refereed)
    Abstract [en]

    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.

  • 50.
    Kari, Leif
    KTH, Superseded Departments, Vehicle Engineering.
    An analytical temperature-dependent collocation model for preloaded rubber cylinders2002In: Journal of Strain Analysis for Engineering Design, ISSN 0309-3247, E-ISSN 2041-3130, Vol. 37, no 4, p. 289-299Article in journal (Refereed)
    Abstract [en]

    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 -60 to +60degreesC, where the shift function factor directly proportional to the temperature is found to play the greatest role. Contrary to other semi-inverse models, this model coincides at vanishing preloads with a well-known linear formula while extending the applicable shape factor range to cover shape factors typically found for vibration isolators.

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