Showing papers by "Leif Kari published in 2017"

Constitutive Models for Rubber IV : proceedings of the 4th European Conference for Constitutive Models for Rubber, ECCMR 2005, Stockholm, Sweden, 27-29 June 2005

Abstract: 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 ...

Dynamic stiffness of chemically and physically ageing rubber vibration isolators in the audible frequency range

Abstract: The constitutive equations of chemically and physically ageing rubber in the audible frequency range are modelled as a function of ageing temperature, ageing time, actual temperature, time and frequency. The constitutive equations are derived by assuming nearly incompressible material with elastic spherical response and viscoelastic deviatoric response, using Mittag-Leffler relaxation function of fractional derivative type, the main advantage being the minimum material parameters needed to successfully fit experimental data over a broad frequency range. The material is furthermore assumed essentially entropic and thermo-mechanically simple while using a modified William–Landel–Ferry shift function to take into account temperature dependence and physical ageing, with fractional free volume evolution modelled by a nonlinear, fractional differential equation with relaxation time identical to that of the stress response and related to the fractional free volume by Doolittle equation. Physical ageing is a reversible ageing process, including trapping and freeing of polymer chain ends, polymer chain reorganizations and free volume changes. In contrast, chemical ageing is an irreversible process, mainly attributed to oxygen reaction with polymer network either damaging the network by scission or reformation of new polymer links. The chemical ageing is modelled by inner variables that are determined by inner fractional evolution equations. Finally, the model parameters are fitted to measurements results of natural rubber over a broad audible frequency range, and various parameter studies are performed including comparison with results obtained by ordinary, non-fractional ageing evolution differential equations.

Dynamic stiffness of chemically and physically ageing rubber vibration isolators in the audible frequency range: Part 2—waveguide solution

Abstract: The dynamic stiffness of a chemically and physically ageing rubber vibration isolator in the audible frequency range is modelled as a function of ageing temperature, ageing time, actual temperature, time, frequency and isolator dimension In particular, the dynamic stiffness for an axially symmetric, homogeneously aged rubber vibration isolator is derived by waveguides where the eigenmodes given by the dispersion relation for an infinite cylinder satisfying traction free radial surface boundary condition are matched to satisfy the displacement boundary conditions at the lateral surface ends of the finite rubber cylinder The constitutive equations are derived in a companion paper (Part 1) The dynamic stiffness is calculated over the whole audible frequency range 20–20,000 Hz at several physical ageing times for a temperature history starting at thermodynamic equilibrium at $$+25\,^{\circ }\hbox {C}$$ and exposed by a sudden temperature step down to $$-60\,^{\circ }\hbox {C}$$ and at several chemical ageing times at temperature $$+25\,^{\circ }\hbox {C}$$ with simultaneous molecular network scission and reformation The dynamic stiffness results are displaying a strong frequency dependence at a short physical ageing time, showing stiffness magnitude peaks and troughs, and a strong physical ageing time dependence, showing a large stiffness magnitude increase with the increased physical ageing time, while the peaks and troughs are smoothed out Likewise, stiffness magnitude peaks and troughs are frequency-shifted with increased chemical ageing time The developed model is possible to apply for dynamic stiffness prediction of rubber vibration isolator over a broad audible frequency range under realistic environmental condition of chemical ageing, mainly attributed to oxygen exposure from outside and of physical ageing, primarily perceived at low-temperature steps

Adaptive control of normal load at the friction interface of bladed disks using giant magnetostrictive material

Abstract: 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 desi ...

Torsional wave propagation in tough, rubber like, doubly crosslinked hydrogel

Abstract: Torsional wave amplitude for tough, rubber like, doubly crosslinked hydrogel decreases rapidly with propagation distance where maximum decrease after one wave length is at frequency with maximum lo ...