The design and performance of non-linear vibration isolating materials.

The mechanical properties of resilient cellular materials, such as dynamic stiffness and damping, depend on several physical parameters characteristic of the material and the conditions of use, eg permeability, elastic modulus, cellular structure, static pre-strain. In many end use situations the pre-compressions and dynamic amplitudes are large and the material operates in a non-linear regime. The effects of non-linear material behaviour on the performance of systems employing cushion foams has not previously been reported on.In this work the influence of non-linear material behaviour on the vibration isolation characteristics of the material is examined. Previous theoretical and experimental studies have been confined to small strain conditions where the material behaves in a linear fashion and the properties are independent of deformation. This work extends the theoretical analysis to allow the study of the variation of the mechanical properties and vibration isolation performance with pre-strain. The fluid flow model proposed by Gent and Rusch is shown to be inadequate and an alternative proposed which conforms closely to experiment. This is extended to non-Newtonian fluids and incorporated in a model for fluid flow damping in the non-linear regime.The response of cushion foams in transportation situations is studied for small and large amplitude dynamic excitations. A multi-degree of freedom model of the person-seat system is presented and used to reproduce the responses of real vehicle seats measured in the field. The model is capable of being used to predict the optimum cushion behaviour, such as stiffness, viscoelastic damping and fluidic damping, required to enhance the ride comfort provided by a particular seat system. At higher vibration amplitudes experimental determinations show that the cushion foam behaves in a non-linear manner with strain dependent properties and several degrees of freedom. Above a certain critical excitation amplitude the classical theories of vibration isolation are shown to break down with the appearence of subharmonic frequencies in the power spectrum of the motion of the isolated mass. The resulting period doubling bifurcation cascade is similar to that found by workers in other fields. The motion of the isolated mass is complex and has not been reported previously. The behaviour is interpreted as a manifestation of chaos.