Is damping additive? : On the accumulated effect of interface damping and material damping

Traditionally the structural loss factor of a vibrating structure at resonance is defined by the half power bandwidth which then incorporates all mechanisms, e. g. material damping, interface damping, added viscoelastic damping, acoustic damping and damping in joints, contributing to the total damping. Damping measures based on vibration response data and vibration energy dissipated locally at interfaces and in material volumes may be computed in post processing finite element computations. Appropriately defined measures may then localize the damping to interfaces and material volumes where the dissipation actually occurs. It is also possible to separate and quantify the damping contributions from different damping sources. The interaction and combined effect of mixed material and interface damping is studied for a simple, three dimensional, build-up structure containing overlapping contact interfaces and internal regions with material damping. Results are presented based on damping measures which separate damping contributions from different contact interfaces and internal material volumes. By comparison of the total structural damping with partial contributions, from the specific damping sources occurring in each case, the combined, accumulated, effect of the different sources is investigated. The question whether different kinds of damping is additive or not is finally addressed.