Elastic properties of anisotropic monolithic samples of compressed expanded graphite studied with ultrasounds

The elastic properties of cubic samples of compressed expanded graphite det ermined by means of ultrasonic velocity measurements are presented. These m aterials are highly porous and exhibit porosity-dependent anisotropic modul i. The results are analyzed according to two approaches. The first involves semi-empirical equations fitted to the experimental data, resulting in inf ormation about the shape and the connectivity of pens. It is found that por es are oblate ellipsoids, connected parallel to their direction of flatness . The second approach is based on application of the percolation theory nea r the rigidity threshold. The value of the critical exponent indicates that compressed expanded graphites behave like elastic networks in which centra l forces are predominant. Results of this study give evidence that ultrasou nd is a convenient and accurate method for investigation of the critical be havior of the elastic properties in highly tenuous structures.