Research On Propagation Mechanism Of Low Frequency Nonlinear Lamb Waves And Damage Evaluation Of Plate Materials

The plate structure is widely applied in aircrafts and other important equipments.Due to fatigue load,corrosion environment and other factors,the mechanical properties of materials gradually deteriorate and invisible early damages might occur.The early damages should be early detected before they quickly develop and lead to structural failure that would result in incredible losses of life and property.Thus,there is an increasing demand to evaluate those early damages by non-destructive testing methods.Because of non-destructive,non-pollution,high detection efficiency and large detection range,the ultrasonic technologies based on nonlinear Lamb waves attract more and more attention.Although there have been massive research outcomes of nonlinear Lamb waves in high frequency domain,the mechanism of low frequency Lamb wave propagation in thin plates with material nonlinearity or early tiny damages,has rarely been studied by researchers.Thereby,the nonlinear effect and behavior of the low frequency Lamb waves were investigated by the theory,simulation and experiments.The main work and conclusions in this dissertation are listed in the following:(1)Based on the constitutive model with material nonlinearity,the characteristics of the second order harmonic were analyzed,which included the second harmonic and the resonant mixing harmonic.We obtained the mode types and cumulative conditions of the second order harmonic,and built the relationship between them and the three-order elastic constants.Especially,the condition of the A₀ resonant wave generation by a pair of S₀ mode and A₀ mode Lamb mixing was derived for the first time.(2)Based on the wave theory,the propagation characteristics of low frequency S₀mode nonlinear Lamb waves in metal materials were studied by numerical simulations and experimental methods.The quantitative relationship between the normalized acoustic nonlinear parameter of Lamb waves and the primary frequency,propagation distance,material nonlinearity,frequency-thickness were discussed.Especially,the cumulative distance of the second harmonic was related to the frequency even when the frequency-thickness was the same.Meanwhile,the relationship between the normalized acoustic nonlinear parameter and the size of local damage was discussed.It was proposed for the first time that the normalized acoustic nonlinear parameter was clearly related with the position of the local damage in the plate thickness direction.The result may offer a potentiality for locating the local damage in the thickness direction by using S₀ mode nonlinear Lamb waves.(3)The propagation mechanism and characteristics of the resonant wave,generated by a pair of A₀ and S₀ mode primary Lamb waves propagation in a thin plate with distributed micro-cracks,were investigated by numerical simulations.It verified the resonant condition,the characteristics of the resonant wave and the relationship between the resonant normalized acoustic nonlinear parameter and the features of the micro-cracks.Furthermore,the resonant wave can be used to locate the position and evaluate the length of the micro-crack region.It reveals that the one-way Lamb mixing method is feasible to quantitatively identify and locate the micro-crack damage in thin plates.(4)We studied the properties of primary wave,higher harmonic(second harmonic and third harmonic)and resonant harmonic in a plate with corrosion damage.The A₀mode Lamb wave method,S₀ mode Lamb wave method and one-way S₀/A₀ mode Lamb wave mixing method were experimentally investigated.The relationship between the different Lamb waves and the different damage levels,which were made by different times of hydrochloric corrosion,were qualitatively discussed.For the first time,we study the evaluation of the primary wave amplitude,higher harmonic and resonant harmonic on the corrosion,and it was found that the A₀ mode resonant wave was consistent with the theoretically analyzing results,which experimentally proved that the resonant wave could be used to locate the corrosion damage.The results of this dissertation are of great significance to explore the propagation mechanism of low frequency nonlinear Lamb wave in thin plates with material nonlinearity and micro-damages,and can provide a theoretical,numerical and experimental basis for nonlinear Lamb-wave detection techniques used in the evaluation and location of damages.