Research On Detection And Localization Of Metal Fatigue Damage Based On Nonlinear Ultrasonic

Metal parts and components inevitably suffer from metal fatigue under long-term cyclic loading.If the metal fatigue is not handled in a timely manner,it will result in metal failure and sudden rupture,causing catastrophic accidents and huge human and financial losses.In this paper,the 6061-t6 aluminum alloy sheet and 5A06 aluminum alloy sheet are taken as the research object,and nonlinear ultrasonic nondestructive testing technology was used to evaluate the fatigue state of aluminum plate by extracting the nonlinearity parameter ? from high-order harmonics which are the nonlinear characteristics of ultrasonic waves propagating in aluminum plates.It can diagnose the early fatigue of aluminum plates and predict the remaining service life,thus discovering potential threats and preventing fatigue fracture accidents.Ultrasonic Lamb wave has many advantages such as long transmission distance,strong detection ability,rich signal information,and so on.So it is suitable for research on fatigue damage detection and localization algorithm of aluminum alloy sheet based on nonlinear ultrasonic technology.The concrete content includes:Propagation modes and dispersion characteristics of Lamb wave were in-depth understanding,and the phase velocity dispersion curve,group velocity dispersion curve and excitation angle curve of two aluminum plates were drawn using Matlab numerical methods.So Lamb waves of a particular mode can be excited.The second harmonic accumulation conditions were studied,and the expression of ultrasonic nonlinear coefficient was deduced.Overall solution of nonlinear ultrasonic testing of metal fatigue damage was studied.Finite element software,ABAQUS,was used to simulate the nonlinear ultrasonic testing of fatigue damage in aluminum alloy sheet.The propagation of Lamb waves in aluminum plates and the ultrasonic nonlinear phenomena caused by fatigue damage were observed.Two specimens,6061-t6 aluminum plate and 5A06 aluminum plate,were prepared.The nonlinear ultrasonic experimental system was set up to determine the parameters of each module and the form of excitation signal.Suitable piezoelectric transducers and coupling agent were selected.With the increase of propagation distance,the relative nonlinear coefficient increases approximately linearly,which proves the cumulative effect of the second harmonic nonlinearity with the propagation distance.The relative nonlinear coefficient decreases drastically with the increase of the offset distance of the sensor path from the fatigue position,which provides a theoretical basis for the subsequent localization algorithm.The surface properties of the aluminum plate have a great influence on the nonlinear ultrasonic testing,so in the experiment,the uniformity of the aluminum plate surface properties should be ensured.Pulse inversion method can effectively extract the second harmonic signal in the nonlinear ultrasonic testing.The relative nonlinear coefficient is sensitive to the change of fatigue damage state,so the relative nonlinear coefficient can be used to characterize the fatigue damage state of the specimen.The localization of fatigue damage location in the specimen was studied,and three localization algorithms were proposed.The advantages and disadvantages of each localization algorithm were compared and analyzed.