Increasing Blade Damping Through Inclusion of Cavities Filled with Viscoelastic Materials

Abstract

In order to simulate hollow fan blades, hollow test specimens in the form of flat trapezoidal plates were designed and mounted as cantilevers. An open-ended cavity of uniform thickness comprised 71% of the plan form area. Two viscoelastic materials were developed and used as fillers. Each was modified for reduced density and then aged after application to simulate the influence of 4000 service hours at a characteristic mission profile. Values of the loss factor (reciprocal of the system quality factor) determined from the resonant responses of the filled plates in the first bending, first torsion and second bending modes at 75 C (170F) were found to be in the range  = 0.025 - 0.045 (Q = 22-40),  = 0.06 (Q = 16), and  = 0.09 0.11 (Q = 9-11) for the three modes, respectively; an improvement by 1.5 to 2 orders of magnitude over values measured with the unfilled specimens. A finite element analysis, using temperature and frequency dependent properties of the filler material as determined by separate tests, showed satisfactory agreement with observed values for temperatures up to 120 C (250 F). This study confirmed that the addition of viscoelastic filler to a hollow plate can increase damping significantly and that the system response could be predicted by finite element analysis. Through appropriate selection of the filler material, such systems can be designed to have maximum damping at a desired specific combination of temperature and frequency.