The Loss Factor as a Measure of Mechanical Damping

The problem of damping representation and measurement is investigated. Among the many parameters found in literature, the most comprehensive is loss factor η. Several definitions of η are feasible, but in linear problems they all should reduce to the ratio of the in-phase and quadrature parts of the associated complex modulus. This work surveys measuring methods for materials and structures with respect to the way they express η, either by approximated or correct expressions. Since some commonly used techniques such as the Oberst method do not follow the definition of η, special care is required when dealing with damping values obtained by different methods and in different environments. On the whole, non-homogeneous values must be expected, owing to the physical differences among the phenomena that enable damping measurement.