A Research On Nonlinear Ultrasonic Guided Wave Method For Micro-Damage Identification And Location

In engineering applications,plate-type structural materials are widely used in various mechanical equipment.During the service of these mechanical equipment,such as impact impact,chemical corrosion,alternating load,high temperature creep and other factors will cause damage inside the structural components.If these structural damages are not discovered in time,it will affect the safe use of the equipment and even cause a safety accident.Therefore,it is important to test and evaluate the damage of the plate structure.Nonlinear ultrasonic guided wave technology is an effective means to detect early micro-damage in plate structures.In this paper,the micro-damage identification and localization method based on nonlinear ultrasonic guided wave mixing is studied for the deficiency of the second harmonic method.The main contents are as follows;(1)The wave equations in the isotropic material plate are derived theoretically,and the conditions for the mixing of the two ultrasonic waves in the plate to produce the second-order harmonics of the mixing are summarized.Taking 1060 aluminum as the research object,several sets of modal combinations satisfying the mixing conditions were found.The detection and localization methods for local micro-damage of two ultrasonic guided waves in the same direction collinear mixing and opposite collinear mixing are deduced.(2)The finite element simulation model is established to study the generation and propagation of second-order harmonics in three cases:single primary Lamb wave excitation,two primary Lamb wave collinear co-directional excitation,and two primary Lamb wave collinear opposite excitation.The effects of the second-order harmonic method under single harmonic excitation and the wave mixing method under mixed harmonic excitation on the evaluation and localization of local micro-damage are compared.The results show that the mixing of two primary Lamb waves may generate back-propagating mixed second order harmonics.Compared with the second-order harmonic method under single harmonic excitation,the wave mixing method of ultrasonic guided wave can effectively eliminate the influence of external nonlinear sources of materials,and local micro-damage can be located by controlling the overlapping positions of two primary guided waves.(3)The mixing effect of two ultrasonic guided waves in aluminum plates was studied experimentally,and different degrees of thermal damage were detected and evaluated by wave mixing.It is found that the mixing effect of two primary guided waves satisfying the mixing condition in the aluminum plate can effectively generate the second harmonic of the sum frequency and the difference frequency.The generated second-order harmonics cumulatively increase as the propagation distance increases.The nonlinear coefficient corresponding to the second-order harmonic of mixing increases as the degree of thermal damage increases.