Research On On-line Monitoring Method Of Helicopter Rotor Blade Damage Based On Acoustic Emission

Long time working under high dynamic aerodynamic environments with alternative bending and twisting loads,helicopter rotor blades are prone to fatigue damage,which will result in cracks,delamination or other damages.These internal damages,if not promptly detected and repaired,will cause serious consequences.However,the structures of the blades are comprehensive and the skins of them are thickness varying mixed fiber composite materials,such that traditional non-destructive testing method is not suitable.It is necessary to exploit new monitoring methods for the composite material rotor blades.This paper focus on damage monitoring technologies for the helicopter rotor blades using acoustic emission(AE)technologies.Firstly,the results of the wavelet transform of the composite material acoustic emission signals under different wavelet base functions are compared and analyzed,and the suitable wavelet base in this application is selected.An example of the wavelet transform using the selected base is set for validation of its effectiveness.Afterwards,the propagation characteristics of the acoustic emission signal on the commonly used composites of the helicopter blades are analyzed by pencil lead break test method.The attenuation features of the AE signals' transmission on the composite articles under different frequency bands are analyzed employing harmonic wavelet transform method;Next,signal de-noising and rebuilding measures are exploit.A finite impulse response(FIR)digital filter is designed using MATLAB,and the primary signals are filtered with the FIR filter.Then wavelet energy spectrum coefficient method is used to find the frequency band of the maximum signals energy of the signal,which is used to reconstruct the signal(so called secondary filtering).Finally,the reconstructed damage signal is analyzed using acoustic emission characteristic parameter method,and the characteristic parameters of the damage signal are extracted.Then,the relationship between the characteristics of different damage types and their processes of damage evolution are obtained.The research results show that,the signal after twice filtered by the FIR filter and the wavelet energy coefficient signal reconstruction can represent accurately the AE signal characteristics.By signal analyses of the filtered signal,the features of the four stages of the helicopter blades damage process(matrix cracking,fiber fracture,fiber and matrix degumming and delamination)are obtained,which lay the foundation for the next health monitoring of helicopter rotor blades.