Bulk Modulus and Density Measurements of Small Compressible and Incompressible Samples

A novel system is described for determining physical properties, including bulk modulus and density, of small solid samples. The system consists of a chamber in which the sample weight is measured by a weighing device immersed in a gas of controllable density. Thus, the method of measuring density is based on Archimedes’ principle where the weight of an object is reduced by the weight of the displaced fluid. This particular device has been designed for examining the density of disk-shaped samples 3 mm in diameter and 0.4 mm thick. The weighing device has a repeatability of 4 nN and sample densities can be determined to 0.5%. A significant feature of this device is the ability to measure buoyancy forces at a plurality of gas densities, which allows one to capture nonlinear behaviors associated with closed-cell compressible media. Results are presented for a quasi-closed cell foam that experiences volume reduction as the gas pressure is increased. Volumetric strains are determined as the difference between the observed behavior and the linear behavior of incompressible media. Plots of hydrostatic stress versus volumetric strain are initially linear, as described by the bulk modulus, and exhibit a “kink” at high pressures, presumably due to the complete compression of internal cells.