Research On Metal Microcrack Detection Method Based On Nonlinear Ultrasound

Sheet metal structures are widely used in various types of equipment,and fatigue damage is one of the main defects.In the actual industrial production process,even if a small crack,under the action of alternating stress,high temperature and corrosion,if the defect is not recognized and measures are not taken in time,the microcrack will expand rapidly,causing structural fracture,which will eventually lead to safety accidents.Therefore,testing early fatigue damage(such as fatigue microcracks)is the key to metal structure life assessment and failure analysis.This paper mainly studies the finite amplitude method and mixing-frequency modulation detection method based on electromagnetic ultrasonic transducers for microcrack,including:1)Based on the principle of electromagnetic ultrasonic surface wave detection,the electromagnetic ultrasonic surface wave simulation model was established,and the propagation characteristics of the surface wave in the aluminum plate was studied.Based on the spring model,an interface force model introducing hysteresis effect and damping force was further established to make it more consistent with the basic characteristics of fatigue microcrack.2)The electromagnetic ultrasonic mixing-frequency modulation simulation model with microcrack interface force model was established,analyzing the influence of excitation frequency and amplitude on the modulation effect,and the excitation parameters was determined.Fatigue microcrack detection characteristics under mixing-frequency excitation was analyzed by simulation,the effect of the depth of microcracks was studied.3)A nonlinear ultrasonic detection system based on harmonic theory was built,and the nonlinear characteristics of intact specimens and fatigue specimens were compared and analyzed;The variation of harmonic amplitude was studied when the amplitude of electromagnetic ultrasound excitation changed,and the second harmonic component was enhanced by the pulse inversion technique to increase the nonlinear coefficient.4)The excitation parameters of the mixing-frequency ultrasound modulation were experimentally studied by the constructed nonlinear ultrasonic detection system,including the influence of excitation frequency and amplitude on the sum of side-frequency components.The nonlinear characteristics of fatigue defects under mixing-frequencyexcitation were analyzed;The effect of different excitation delays on the modulation effect was studied;the variation of the side-frequency components of different fatigue loading period specimens was studied.