| Wide-chord fan blades are widely used in large bypass-ratio turbofan engines,which are the most front-end components of large bypass-ratio turbofan engines and providing 80% propulsion.Compared with the traditional titanium fan blade,composite fan blades are characterized by light weight,low noise,anti-flutter,high efficiency.They also have strong flutter resistance,damage resistance,shock resistance and low maintenance cost,which represent the development direction of fan blades.In operation,the fan blades are excited by the external excitation from the air inlet and the internal flow field of the fan stage during operation.The complex loads will exert multi-frequency component and multi-mode excitations on the fan blade.The vibration of fan blades have great influence on engine stability and reliability.However,interiorly there are few researches on the vibration characteristics of aero-engine composite fan blades.MFCs are characterized by good damage resistance,flexibility,reliability and be on hand.They have excellent plane driving performance,fast output response and wide excitation frequency range.In this paper,the muti-MFCs modal test method and the vibration characteristics of composite fan blades under multi-MFCs complex excitation were studied as follows:1.The multi-MFCs modal test system was designed and constructed.With the scanning laser vibrometer,the vibration response of the concerned points on the blade was measured.Experimental results show that the MFC excitation method is simple to operate and has high signal-to-noise ratio.In order to excite the blade modes as much as possible and obviously,the MFC should be placed at the position where the modal strain of blade is large.With the different placements of the MFC,the modal responses of the blades are significantly different.The response size of the blade is related to the piezoelectric fiber polarization direction strain,and the direction of MFC fiber polarization.2.Combined with the Campbell diagram,the vibration characteristics of composite fan blade under multi-MFCs superposition excitation was studied.There is a linear relationship between the vibration amplitudes of the blades before and after superposition,the superposition coefficients related to the modal order.3.To study the effect of the harmonic excitation on blade vibration,the experiment used a single MFC shaker to perform the sinusoidal excitation.The results show that the effect of harmonics excitation on the response amplitude of the blade cannot be negligible.In the design of fan blades,since the frequency multiplication of low-order modes is close to the natural frequency of the blades,the low orders natural frequency should be avoided to have an evident multiple relationship.4.To study the influence of the excitation signal phase difference of different exciters on the vibration characteristics of the fan blade,MFC shakers are performed to the sinusoidal excitation with different phase.The influence of the phase of the excitation on the vibration amplitude of the fan blade is related to the mode order and the strain at the position of the MFC exciter.By arranging the position of the MFC and adjusting the phase of the MFC excitation signal,the vibration damping effect can be achieved.This paper provides a new way of measuring the vibration of the blade.The results will be used to further study the vibration characteristics of the blade under complex excitation. |
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