Improved impulse response decay method (irdm) for structural damping measurements

This paper presents an experimental method to estimate the structural damping. In aerospace, but also in other sectors, the study of structural damping is very important be- cause it characterizes the structural dynamic behaviour. The vibrations and more generally how they are transmitted in the structure are a topic of great interest for aircraft noise sup- pression (Structure-Borne noise) and also for other aerospace applications (eg. space launch and satellite) wherein the high frequencies vibration are very dangerous. Many methods for damping measurements in literature are based on modal behaviour of the structure, so the maximum frequency at which they can operate is very limited. The proposed method is able to overcome this limitation and brings some advantages in the study of damping parameters. This method has been implemented starting from the Impulse Response Decay Method (IRDM) and allows the study at high frequencies and high modal density conditions. The method is based on the application of Hilbert transform directly on the time history of the signal also when it is not possible to isolate the structural modes, thus overcoming the limitations of the methods mentioned above. During the studies it has been shown how, in the tests, the choice of an impulsive excitation is very signicant in order to reduce the computational complexity of the extraction algorithm. In fact this choice allows to release the study by the frequency response of the structure, allowing to operate directly in time domain. Some devices operating on the function returned by the Hilbert transform permitted to achieve a reduction of numerical errors obtaining results much more reliable and closer to reality. Finally, some results obtained for a fuselage composite skin have been reported, as example of application of aforesaid method.