Research On Air-coupled Ultrasonic Testing System For Composite Materials Based On Lamb Wave

Composite materials have become an essential tool for enhancing the mechanical properties of equipment in various fields,including aerospace and automobile,etc.Unfortunately,these materials are prone to defects such as delamination,cracks,and breaks,which can significantly impact the mechanical properties.Consequently,it is imperative to effectively detect and evaluate these defects to ensure optimal performance of composite materials.This thesis employs air-coupled ultrasonic Lamb wave detection technology to perform non-destructive testing and defect imaging of internal and surface flaws in composite material thin plates.The specific research contents are as follows:Firstly,starting from the propagation characteristics of ultrasonic Lamb waves,the propagation law of the isotropic free boundary plate is studied,and the dispersion equation of ultrasonic Lamb waves is deduced.On this basis,the propagation model in the multi-layer composite plate is further established,and the dispersion curve of the experimental plate is drawn.The multimodal characteristics of ultrasonic Lamb waves are analyzed through dispersion curves.To obtain a pure mode Lamb wave,this thesis chooses a 200 k Hz air-coupled ultrasonic transducer.Combined with the frequencythickness product parameters,the theoretical optimal incidence angle is calculated,which provides theoretical support for the subsequent experimental design.Then,the denoising algorithm and reconstruction algorithm for probabilistic inspection of damage(RAPID)are studied.The discrete wavelet transform is used to decompose the original signal,and the low-frequency coefficients are used to reconstruct the signal,to achieve signal denoising.To improve the image quality,this paper improves the traditional RAPID,specifically in the optimization of damage index(DI)and probability distribution function.Among them,the threshold and the square term are introduced in the DI,which can suppress the fluctuation of the DI caused by the energy fluctuation of the Lamb wave,and increase the difference between the damaged area and the non-damaged area,which is conducive to improving the measurement accuracy of the defect area;In terms of the probability distribution function,two sets of non-standard normal distributions are used to optimize the imaging quality of the left and right sides of the defect,and then the two sets of data are multiplied together to obtain the probability damage index optimized for both sides.Finally,a Lamb wave air-coupled ultrasonic detection system is built,which realizes the automatic signal excitation and acquisition functions.Based on the detection system,the improved RAPID is verified experimentally,which proves that the algorithm can improve the accuracy of defect measurement and the imaging quality of both sides of the edge.