Can the spatial distribution of damping be measured?

Our knowledge regarding the detailed description of damping in a vibrating structure is very primitive. The most common approach involves assuming proportional viscous damping, then determining damping factors for each mode. The next step, followed by many authors, is to obtain a full nonproportional viscous damping matrix. Both methods may misrepresent the true damping behavior, as viscous damping is assumed from the outset. In this study a new method for identification of the spatial distribution of damping is proposed. The approach is based on general linear damping models and is not confined to the traditional viscous damping model. Algorithms have been developed for taking measured frequencies, damping factors and complex mode shapes, and fitting the parameters of a damping model with an exponential relaxation function. The viscous damping model is a special case of this model. Simulation studies show that the method seems to perform quite robustly. Preliminary experimental results on a beam with a localized patch of damping are very encouraging and show that the method can indeed indicate the spatial distribution of damping correctly. The approach also gives some information on the question of whether the chosen exponential damping mechanism is correct or not.