Research On Crack Quantitative Method For Aluminum Alloy Plate Based On High-resolution Lamb Waves

In the fields of railway transportation,waterway transportation,aerospace and other fields,the structures of large-scale transportation equipment always work in high-intensity load and complex environment,which will make cracks appear in same structures,resulting in the decline of the mechanical properties of the structures and ultimately threatening the safety operation.Moreover,there are a large number of aluminum alloy plate-like structures in the equipment,so it is of great significance to research the crack quantitative method of aluminum alloy plate for monitoring the structural state,providing maintenance information and ensuring the safe operation of the equipment.As a guided wave propagating in a plate-like structure,Lamb wave is very sensitive to crack damage.In this thesis,high-resolution Lamb wave was used to quantitatively monitor cracks in aluminum alloy plates,and the Lamb wave nonlinear and surrogate models were combined to improve the diagnostic accuracy of micro-cracks monitoring and the engineering applicability of crack quantitative method.Firstly,according to the propagation process of Lamb wave,a pulse compression algorithm suitable for Lamb wave was designed,and the design criterion of high-resolution Lamb waves was determined.At the same time,in order to meet the requirement of ignoring dispersion in pulse compression algorithm,a dispersion compensation method based on distance domain mapping was designed,which laid the foundation for crack monitoring using high-resolution Lamb waves.Secondly,the interaction mechanism between high-resolution Lamb wave signal and crack was studied by finite element simulation,and a variety of features were extracted from the simulation signal to establish a crack diagnosis model.By comparing the crack diagnosis results of different signals,the superiority of using high-resolution Lamb wave for crack diagnosis was proved,and the high-resolution Lamb wave signal with the best diagnosis effect was determined,and a verification test was carried out.The quantitative monitoring of aluminum alloy plate cracks was realized,and the accuracy of crack monitoring was improved.Thirdly,the micro-crack diagnosis research was carried out based on Lamb wave nonlinearity.The excitation signal was designed to monitor the second harmonic of Lamb wave stably,the pulse inversion technique was used to separate the fundamental wave and the second harmonic,and the nonlinear damage feature was constructed combined with the pulse compression technique.The relationship between the designed nonlinear damage eigenvalue and the micro-crack length was determined through fatigue tests,and the micro-crack diagnosis model was established,which improved the monitoring accuracy of the crack monitoring method for micro-cracks.Finally,a crack diagnosis method that can overcome the influence of material uncertainty was investigated.The high-resolution Lamb wave with different material parameters and cracks was simulated by finite element.The damage features were extracted as a training data set,a surrogate model was established to obtain the response surface of the damage features,and the effect of Lamb waves was obtained.The crack diagnosis model was established by fusing multiple damage feature response surfaces,and the ability of the method to overcome the uncertainty of material parameters was verified by experiments,which improved the engineering applicability of the crack monitoring method and reduced the construction cost of the crack diagnosis model.