The use of tests on high-shape-factor bearings to estimate the bulk modulus of natural rubber

The bulk modulus of elastomeric materials such as natural rubber is an extremely difficult property to measure since the bulk modulus is several orders of magnitude larger than their shear modulus, so that the material will deform only in shear if at all possible.In most applications the deformation is assumed to be a constant volume one and the material is assumed to be incompressible, but there are situations where the compressibility of the material can play an important role and where it is necessary to have an accurate estimate of the bulk modulus.It will be shown in this paper that one way to determine the bulk modulus is to use the measured vertical stiffness of bearings used as seismic isolators to estimate its value.Seismic isolators are usually made with compounds, known as high damping rubbers, that are nonlinear and have large hysteresis, which can make interpretation of the measurements difficult; but in some cases the compounds used, for example linear natural rubber, have almost no hysteresis and are very linear in shear up to very large shear strains.In this paper, test results from a particular seismic isolation project were analyzed using the theory of bearing mechanics to provide an estimate of the bulk modulus for this particular compound and to show how, if tests on bearings with other compounds are available, to interpret the data for this purpose.