Study On Structural Health Monitoring Based On Ultrasonic Lamb Wave Under Varying Temperature And The Temperature Compensation

Structural Health Monitoring?SHM?based on Lamb wave can effectively improve the safety degree of structure operation and reduce the maintenance cost.It has been widely used in aerospace engineering,high-speed trains,ships and other fields.The working environment of these structures is usually at the large range of temperature,and Lamb wave is easy to be affected by the temperature effect in the process of propagation,which will cause the misjudgment of damage to SHM.It seriously affects the stable operation of SHM system.Therefore,the development of stable and efficient temperature compensation method is one of the important challenges for SHM.The traditional temperature compensation methods are limited to multi-sample collection,tedious operation and small compensation interval.In order to solve these problems,a parameter model temperature compensation method is proposed in this paper,which based on Hilbert transform and Ordinary Least Square method.It effectively improves the compensation interval of single baseline signal and speeds up the operation speed of the system,and also is suitable for the mode of A₀ and S₀.The specific research work of this thesis is as follows:Firstly,the background and the advantages of SHM based on Lamb wave are introduced.Then the influence of temperature effect on the propagation of Lamb wave is introduced.The research progress of temperature compensation technology at home and abroad is reviewed.On the basis of this,the main research contents,routes and methods of this paper are put forward.Secondly,according to the analysis of the active Lamb wave SHM signal equation based on the piezoelectric sensors,figure out that,signals are affected by the following factors:First,the change of characteristic parameter of structural material will cause the change of propagation speed and propagation distance,which will lead to the change of arrival time of signal.Second,the change of dielectric constant of PZT will cause the fluctuation of output voltage and the change of signal amplitude.The last,the response of bond layer with different thickness to different frequency excitation signals is different.Thirdly,an experimental platform is set up to study the relationship between Lamb wave signals and temperature.Then,the linear relationship between the instantaneous phase difference and temperature difference,as well as the change trend of signal amplitude with temperature are found.It is concluded that when the temperature difference is greater than 2?,the signal fluctuation caused by the temperature effect is larger than that of the signal caused by the damage,and the temperature compensation is required.Fourthly,a novel compensation method is proposed,which is based on the Hilbert transform and Ordinary Least Square?OLS?method.The phase factors at each temperature point are obtained by the linear relationship between the instantaneous phase difference and the temperature difference.Then,the amplitude factors are acquired by the trend of signal amplitude with temperature and OLS.Finally,the parameter model of temperature compensation is established by combining the phase factors and amplitude factors.The current signal is compensated by reconstruction.Finally,the proposed method is demonstrated by different modes of Lamb wave signals.The results show that the maximum residual amplitude between the compensated signal and baseline signal tends to zero,and the maximum normalized error decreases greatly when the temperature difference is within 10?.For the modulated sine signal with central frequency of250kHz,which on behave of A₀ mode and S₀ mode,the baseline signal is selected every 12?and 14?,and the threshold is-22dB and-26dB,respectively.It is proved that the damage hot spot area decreases and the damage center position is closer to the real damage center after compensation combined with ellipse location principle.The hot spot area of A₀ mode damage imaging is reduced by 46%,and the relative error between damage location and real damage center is reduced to 2.83%when the compensation is applied.And also,the hot spot area of S₀is reduced by 56%,and the relative error is reduced to 1.81%when the compensation is applied.The above results show that the temperature compensation proposed in this paper effectively reduces the baseline signals collection and expands the compensation interval,and improves the robustness and effectiveness of the SHM system.