Experimental evaluation of vibro-acoustic behaviour of composite fuselage structures realized with embedded viscoelastic damping treatments

This paper presents an approach to characterize the vibro-acoustic behavior of composite fuselage structures developed using embedded viscoelastic damping treatments; it was assessed within the framework of a national research project named A.R.C.A where the Department of Aerospace En-gineering of the University of Naples (ITALY) acted as developer of experimental and numerical damping characterization procedures. The experimental tests were performed in a climatic room in order to simulate a cruise flight temperature level for commercial transport aircraft. The real influ-ence of viscoelastic materials on the damping of composite structures is currently unknown. For this reason experimental acquisitions and vibrational data analyses have been performed on both damped (with embedded viscoelastic damping treatments) and untreated composite fuselage skin coupon. The tests have been performed at temperatures of -35°C for no-treated composite fuselage structure, and 15°C, -20°C, -25°C and -35°C for treated composite fuselage structure. The extrac-tion of the parameters useful to characterize the damping behavior was carried out by a modified Impulse Response Decay Method (IRDM). The first analysis step consisted in exciting the struc-ture by means of a mechanical impulse. Within the scientific literature, the commonly employed excitation systems are impulse Hammers or Shaker systems; the first one excites the structure gen-erally only up to 1000Hz while the second, because directly connected to the structure, influences its structural behavior. Within this activity, an innovative excitation system has been used, which has been conceived and manufactured by the authors. It is able to excite the structure up to 3000÷3500Hz without any influence on the structural behavior since it generates an impulse signal that approaches to the Dirac signal; for these reasons it allows a direct measurement of Impulse Re-sponse Function (time history)[3]. The best temperature interval for the treated composite fuselage skin coupon was evaluated, and the different structural behaviors for both composite fuselage struc-ture (treated and untreated) were characterized. Moreover, the high number of tests performed per-mitted to evaluate the confidence interval of the damping estimations and to validate the IRDM modified method by statistical analysis. Obtained results show that composite fuselage structures realized with embedded viscoelastic damping treatments can be utilized to reduce the transmitted vibration (main source of interior structure borne noise) and consequently to increase passengers comfort.