Damage Identification Of Composite Laminates Based On Active Lamb Wave

Carbon fiber composite materials are more and more used in the main structural components of high performance fields such as aviation and aerospace industry because of its advantages of high specific stiffness and strength,corrosion resistance and fatigue resistance.They are susceptible to defects and slag inclusions in the manufacturing process,which may affect their best performance.In addition,they are vulnerable to low-speed impact and other threatening behaviors,and there may be serious damage inside the material,while there is no trace on the impacted side.They may lead to matrix cracking and delamination,fiber fracture and even fatigue fracture.The invisible damage level corresponds to the structural degradation,which reduces the residual strength and durability of the structure,leads to catastrophic material failure and threatens the safe operation of the aircraft.Therefore,it is urgent to carry out structural health monitoring for carbon fiber composite structures.Because of its long propagation distance,low cost and good sensitivity to various defects,Lamb wave has become a research hotspot of composite material detection and evaluation,and is the most promising health monitoring technology.Although the health monitoring technology based on Lamb wave has attracted people's extensive attention and achieved some research results,it still faces a series of challenges:positioning accuracy,structural damage identification and degree evaluation methods are still in the preliminary exploration stage,far from mature and so on.In view of the existing problems in the current structural health monitoring research,this topic takes the composite plate structure as the research object,through the research of sensor system construction,signal processing,damage location and degree identification technology,to achieve the accurate judgment and evaluation of structural damage.The specific innovative research work is described as follows:(1)Firstly,the basic theory of Lamb wave and the principle of damage detection are introduced.Through the theory and finite element simulation,the propagation characteristics of Lamb wave in plate structure and the effect between damage and Lamb wave are studied.According to the relationship between the amplitude and frequency spectrum of the damage scattering signal and the damage size,the optimal excitation frequency of the carbon fiber composite structure is determined,which lays the foundation for the research of damage location method.(2)In this paper,a new damage location method based on the minimum variance distortion response(MVDR)is proposed.The near-field MVDR array signal model is established.The array signal is obtained based on the linear array,and the narrow-band signal is extracted by "Sym8" wavelet transform as the input signal,so as to reduce the influence of dispersion effect on positioning.According to the wave speed,the direction vector of the search point in the monitoring area relative to the reference sensor is determined,and the damage location is realized by the near-field MVDR focusing beam algorithm.(3)A lamb wave tomographic technique based on difference signal is proposed.The algorithm does not need to analyze the complex multimode propagation characteristics of Lamb waves,nor to understand and model the characteristics of materials or structures.The effect that the wave velocity is not constant is eliminated.The damage factor can be obtained by calculating the square sum of the difference signals,and the damage location can be realized.Compared with the traditional method,this method has the advantages of simple calculation and fast imaging speed.(4)The Lamb wave tomography technology based on Hilbert energy spectrum is proposed.By comparing the maximum energy spectrum of the signal before and after the damage,the damage factor is obtained to realize the structural damage imaging.The single damage and multiple damage cases are simulated by finite element method to verify the correctness of the method.Through single damage and multiple damage experiments,the structure damage can be accurately located.(5)An intelligent assessment method of composite damage based on deep learning algorithm is proposed.The damage identification system based on Lamb wave is constructed to realize the accurate monitoring of structural response signal.Wavelet denoising is used to eliminate the influence of noise.The damage location method based on depth neural network is studied,and the relationship model between signal spectrum characteristics and damage location is established.The problem of the influence of the wave velocity of signal propagation on the location accuracy is solved,and the damage location accuracy is improved.Based on convolution neural network algorithm,a method of damage location and quantitative simultaneous identification for composite materials is proposed.The relationship model between the damage location and damage degree and the signal spectrum is established.The test results show that the proposed method can accurately identify the damage location and damage degree at the same time.(6)The damage identification method of composite materials in strong noise environment based on chaos theory is proposed.The damage factor is calculated by box dimension,and the structure damage imaging under different noise levels is realized.This method does not need to preprocess the signal,but directly uses the original signal for damage imaging,avoiding the complex denoising process of traditional methods(7)A composite damage assessment method based on the combination of synchronous compression wavelet transform and intelligent algorithm is proposed.The synchronous compressed wavelet transform(SWT)has the advantages of high time-frequency resolution,strong anti noise performance and insensitivity to wavelet generating function.It is a promising new feature extraction algorithm in the field of structural health monitoring based on vibration characteristics.By combining the wavelet transform of synchronous compression and stack self encoder,the influence of strong noise interference can be restrained,and the feature extraction of weak damage under strong noise can be realized.Finally,the accurate evaluation of structural damage can be realized.