Research On Micro-crack Detection Method Of Aluminum Sheet Based On Nonlinear Lamb Wave

Aluminum alloy sheet has the characteristics of small density,easy processing,corrosion resistance,good electrical and thermal conductivity,and is widely used in industrial production.The most common form of damage is fatigue microcracks,which can seriously affect the mechanical properties of the components.Therefore,the non-destructive evaluation of the early micro-damage of aluminum alloy sheets is of great significance for structural health testing.At present,the ultrasonic evaluation method for early micro-damage of metal materials is mainly nonlinear ultrasonic detection technology.For thin-plate members,Lamb waves are characterized by easy excitation,small attenuation,long propagation distance,and high detection efficiency.Therefore,combining the sensitivity requirements and operability,this paper will use the nonlinear Lamb wave to evaluate the early micro-damage of aluminum alloy sheets.The specific research contents are as follows:Firstly,the nonlinear Lamb wave response mechanism of early fatigue micro-damage of metal materials is studied theoretically.Based on the theoretical analysis,the research scheme is designed and the feasibility of the scheme is demonstrated.Secondly,an aluminum alloy sheet test block designed to simulate the early micro-damage of metal materials was designed.Lamb wave nonlinear parameter test system is built to verify the performance of the test system and develop relevant test processes.Finally,according to the research scheme,the process of detecting the degradation of the mechanical properties from the initial state to the microcrack initiation and propagation is traced.The correspondence between the nonlinear parameters of Lamb wave and the early micro-damage of the material is established.The influence of residual stress on the nonlinear effect of Lamb wave during observation was observed by X-ray diffraction.The microcrack size was quantitatively characterized by metallographic test method,and the quantitative evaluation of microcrack size by Lamb wave nonlinear parameters was realized.The results show that the nonlinear Lamb wave has obvious advantages in evaluating the early micro-damage of the metal sheet,and the feasibility is high.The nonlinear test system constructed has good performance and the test results are stable and reliable.The results of the comparison test show: With the increase of the degree of early micro-damage,the nonlinear parameters of Lamb wave first increase and then decrease;Combined with the results of metallographic observation,the quantitative evaluation of fatigue microcrack size by Lamb wave nonlinear parameters is realized.The minimum size of measurable microcracks is 28.3?m,and by comparing the change of the slope of the Lamb wave nonlinear parameter curve,the critical point of microcrack generation can be roughly determined;Combined with the X-ray diffraction measurement results,the residual stress will increase the Lamb wave nonlinear effect of the material,and before the damage reaches 50% of the total life,the effect will increase,and then the influence will gradually decrease.