Rubber Properties : Damping Properties

Damping is the difference between deforming work and elastic recovery. The energy loss is caused by friction and is determined by the loading and unloading phase of a process. Damping due to surface friction between the rubber product and a mating surface may be present. But there is also internal friction within the elastic body itself, call material damping. With internal friction, unlike surface friction, no appreciable variations occur in the fraction forces. In both cases the work of damping is changed into heat, which thus account for the energy lost in vibration. The case of oscillating loading is presented by Figure 1. The area of the loop U1 – U2 is a measure of damping. It is equal to the energy loss per oscillation and is called absolute damping in joules but published values so far are given mainly in kgf cm . The ratio of the area of the strip U1-U2 to the area under the upward curve U1 is the percentage damping. The area U1 is the total strain energy, also called absorbed energy. The Roelig percentage damping is thus given by

damping[%] = 100x(U1-U2)/U1

Figure 1 Damping Properties

A percentage damping of, for example, damping = 30% mean that 30% of the total energy imposed on the rubber material is absorbed by the rubber material, i.e. damped. The percentage damping of natural rubber grows from 6 to 30% with increase rubber hardness. With synthetic rubber, e.g. butadiene rubber, the value are higher for soft grades but are almost identical for harder grades. The damping capacity of rubber is considerably greater than that of steel. The percentage damping is not greatly used in the calculation of dynamic problems. The damping has no constant value. It depends on the grade of the rubber, temperature, strain rate(velocity) and acceleration, shape, and type of stress (compression, tensile, or shear). Generally valid damping values which would be approximately dependent on IRHD can't be stated. The magnitude of damping has to be obtained in given cases by enquiry from the rubber product manufacturer or ascertained dynamically with the help of a suitable apparatus. The determination of dampingand dynamic modulus is governed by ASTM D945-55. A preloaded probe is stressed dynamically. The damping and the dynamic modulus are determined from the area of the hysteresis loop and its position. The Yerzley oscillograph is particularly well suited for determination of damping under impact load when the oscillation curves are plotted. The behaviour of rubber product which are subject to forces caused by oscillating masses is tested on the Barry impact machines. If energy tires is designed , its damping properties is lower than general tires becuase energy tires must have lower energy lost.