Most of metal materials show such negligible internal damping that only elastic modulus which contributes to the stiffness is treated important. But viscoelastic materials show both of the elastic and viscous characteristics, which must betreated in the dynamic system.Elasticity acts as storing the energy as the spring behaviors. Viscosity acts as dissipating the energy by internal friction. The damping characteristic of the rubber mount or sheet is very important in evaluating the performance of the rubber madeproducts. So there has been many efforts among engineers measure the damping performance of the viscoelastic materials. There are two methods for measuring the loss factor; one is resonant method and the other is nonresonant method. The resonant method is by measuring the modal damping in the system which the mass and the dynamic stiffness of the viscoelastic material make. To measure the loss factor in the different frequencies, the mass have to be adjusted. The nonresonant method is divided into two methods, the impedance method and the transfer function method. The impedance method is measuring the loss factor by measuring dynamic displacement which is occurred by dynamic force.The transfer function method is by measuring the dynamic displacement of each side of the viscoelastic material. In this paper, the loss factor of the natural rubber is measured by the impedance method using the simple exciting test instrument, and the test results are compared to those of resonant method. The theory and the test procedure for measuring the loss factor are introduced with some test recommendations.
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