Effect of boundary conditions in the experimental determination of structural damping

Abstract

Building a digital twin representing the dynamical behaviour of a structure requires knowledge of its material and geometrical properties. While geometry, mass and stiffness can often be characterised quite accurately, at least for homogeneous isotropic materials, the experimental quantification of structural damping is a time consuming endeavour. Furthermore, the chosen experimental set-up subjects the obtained damping values to uncertainties. In this context, the present study takes a closer look at the influence of the boundary conditions on experimentally obtained damping values. A solid plate made out of a commonly used aluminium alloy has been measured repeatedly with different boundary conditions. The plate is excited by an automated impulse hammer and a laser scanning vibrometry has been employed in order to record the structural response at a total of 165 equally distributed points per measurement. Hence, the modal damping values do not only correspond to a segment but to the entire structure. For each boundary condition, the specimen has been measured 20 times consecutively in order to determine the underlying variance. These data point out that experimentally obtained damping values are much more sensitive to the applied boundary conditions than the natural frequencies. Furthermore, it is indicated that the lowest damping values are obtained when suspending the structure at the nodal lines of the corresponding mode. In the second part of this study, the experimentally determined damping values are compared to numerically obtained values for acoustic radiation damping. This comparison suggests that at higher frequencies, the overall damping of the plate is more affected by energy dissipation due to sound radiation than at lower frequencies.