Fatigue Damage Detection In Aluminum Alloy Plate By Nonlinear Mixing Method Of Lamb Wave

Metallic materials are often subjected to periodic loads such as heat and force in the engineering application.The structures are prone to generate fatigue crack,resulting in fracture failure of material.Nonlinear acoustics technology is very sensitive to the changes of material microstructure and mechanical properties,which provides a new idea and method for detecting and evaluating the micro-scale damage of metallic materials.In this thesis,the identification ability of Lamb wave nonlinear mixing technology for detecting low cycle fatigue crack is explored in A7N01P-T4 aluminum alloy,which has important academic value and engineering application significance.First,this thesis explains the basic principles of Lamb waves.The wave equation of Lamb wave in isotropic plate is derived,and the numerical solution of the dispersion equation is solved iteratively by dichotomy;The interaction model between Lamb waves and "opening and closing" crack is studied,revealing the reason for the emergence of sum frequency and difference frequency sideband generated by mixing.Moreover,the parameter for evaluating the effect of nonlinear mixing is clarified.This thesis also discusses the microscopic mechanism of fatigue damage in metallic materials,and lists the mechanical equations of a single crack using the theory of the unilateral contact method of cracks.Secondly,the finite element simulation software ABAQUS was used to simulate the fatigue damage detection by Lamb wave nonlinear mixing method in the aluminum alloy plate,in which randomly distributed microcracks were generated by python script.The calculated displacement nephogram clearly shows the crack surface clapping behavior caused by the interaction between Lamb wave and crack.On this basis,the effects of propagation distance,crack length and width,crack number and crack area on the nonlinear mixing of Lamb wave were analyzed.The results show that the difference frequency and sum frequency sidebands clearly exist in the spectrum of the transmitted wave.The acoustic nonlinearity parameter based on the sum frequency amplitude increases monotonously with the length and number of crack,and decreases with the crack width.In addition,the waveform of the mixed wave does not change much in the process of propagation,whereas the signal amplitude and nonlinear parameter are greatly reduced.Then,the experimental device of ultrasonic Lamb wave nonlinear mixing method was setup to detect low cycle fatigue damage in A7N01P-T4 aluminum alloy plate.The influence of propagation distances and detection positions on the detection results was analyzed.The experimental results show that the nonlinear ultrasonic parameter exhibits a monotonous upward trend with the degree of fatigue damage,and decreases after reaching the peak value when the material is fracture fatigue.Both the simulation and experiment of ultrasonic Lamb wave nonlinear mixing show that the micro-scale fatigue damage in the material lead to an increase in nonlinear effect.Relevant fatigue model can be established based on the relationship between the nonlinear ultrasonic parameter and the degree of fatigue damage,which can be used to evaluate the fatigue damage and predict the residual life of metallic materials.