| Metal alloy materials are widely used in the field of advanced industrial major equipment.During the service process of metal alloy material structure,under the action of fatigue load,its structural stress concentration area is prone to early fatigue damage,resulting in gradual degradation of mechanical properties,and if its safety and health status is not evaluated in time,the failure of its structure may cause major safety accidents.Therefore,it is of great theoretical and engineering application significance to carry out early fatigue damage detection of metal alloy material structures.In recent years,the nonlinear Lamb wave has attracted much attention because of its sensitivity to early fatigue damage.In this paper,a nonlinear Lamb wave detection technique based on phase velocity matching is proposed for the early fatigue damage of metal alloy material structures.The relationship between the nonlinear coefficients and the internal microstructure evolution during the formation of fatigue damage in metal alloy materials is further analyzed,and an evaluation method for early fatigue damage is developed.The main research contents and results are as follows:(1)The propagation characteristics of Lamb waves are studied with the condition of nonlinear accumulation of higher harmonics and the phase velocity matching mode.Based on the nonlinear wave equation,the propagation law of Lamb wave in solid flat plate is analyzed,and the dispersion curve is obtained by solving the Rayleigh-Lamb equation based on the numerical analysis method.Based on the higher-order strain energy analysis if the nonlinear wave equation,the cumulative growth condition of the higher harmonic signal is derived.Three types of Lamb wave modes satisfying phase velocity matching are analyzed from the dispersion curves,and it is concluded that the longitudinal wave type modes are more sensitive to internal damage.Three theoretical models of ultrasonic propagation nonlinearity caused by dislocation changes in the microstructure and the model of interaction between early fatigue microcrack damage and Lamb waves are described,and the physical mechanism of contact nonlinearity is simulated by three-dimensional finite element simulation.(2)The higher harmonic response characteristics of different longitudinal wave type modes satisfying phase velocity matching to fatigue damage are investigated.The piezoelectric excitation and reception models are described,and two Lamb modes satisfying the second and third harmonic phase velocity matching are established as the excitation signals by the Lamb wave dispersion curves.The second harmonic and third harmonic response signal amplitudes of the two phase velocity matching modes under different fatigue damage states were compared,and the sensitivity of the second-order nonlinear coefficients and third-order nonlinear coefficients to different damage was further analyzed.The results show that the Lamb wave S1 mode is more sensitive to the damage than the S2 mode and has less modal interference.In addition,the third harmonic is more sensitive to the fatigue microcrack damage than the second harmonic,and is more suitable as the excitation signal mode for the detection of early fatigue damage.(3)The third harmonic response characteristics of S1 mode to fatigue damage under approximate phase velocity matching are studied.Based on the theory of second harmonic approximate phase velocity matching,the third harmonic approximate phase velocity principle is analyzed.A fatigue damage detection method based on nonlinear Lamb wave third harmonic approximate phase velocity matching is proposed.The nonlinear responses of two approximate phase velocity matching based modes for different fatigue damages are analyzed and further experimental validation is carried out.The results show that the nonlinear response of the approximate phase velocity matching mode differs little from that of the phase velocity matching mode in a small frequency deviation range,and the third harmonic and third-order nonlinear coefficients change consistently,and the approximate phase velocity mode can also be used for fatigue microcrack damage assessment.(4)The relationship between the third harmonic and third-order nonlinear coefficients and the microstructure evolution inside the structure is investigated.The nonlinear accumulation characteristics of waves with different fatigue damage types of metal alloy material structures are described.Based on the third harmonic phase velocity matching mode,the relationship between the third harmonic and the thirdorder nonlinear coefficients and the variation of the fatigue life of the structure is further investigated.The results show that when the fatigue life of the structure reaches 80%90%,the third harmonic has a significant decreasing trend;the third-order nonlinear coefficient increases sharply when the fatigue life of the structure exceeds 50%,and when the fatigue life reaches 80%-90%,the increase of the third-order nonlinear coefficient slows down and has a decreasing trend.Third harmonic and third-order nonlinear coefficients are proposed as indicators for early fatigue damage detection.The microstructural changes in the formation of structural fatigue damage in metal alloy materials were analyzed with the help of metallographic analysis methods,and the results showed that the appearance of internal microcracks led to the decrease of the third harmonic signal and the decreasing trend of the third-order nonlinear coefficient.(5)Based on the probabilistic imaging algorithm,the localization of two different fatigue damage states was studied.The damage probability imaging algorithm with the third-order nonlinear coefficient as the damage index was proposed,and the fatigue specimens with two different initial states were designed to complete the localization of different fatigue damage states.The variation law of the third-order nonlinear coefficient with fatigue damage under different sensing paths was analyzed.The results show that the damage probability imaging algorithm based on the third-order nonlinear coefficients can achieve not only the damage localization of fatigue cracks,but also the localization of early fatigue damage.The factors affecting damage localization are further analyzed. |
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