Research On Active Lamb Wave-based Damage Identification For Impacted Laminates

Over the past two decades, Structural Health Monitoring(SHM) has gained significant interests as an alternative to the conventional nondestructive evaluation(NDE) techniques for thin-walled structures. Among several different methods investigated in SHM, Lamb Wave-based techniques have many advantages. For the excitation and detection of the Lamb Waves in thin-walled structures, piezoceramic patch transducers are typically used. By exciting Lamb Waves and measuring reflected signals from various places in the structure, one can determine the location and severity of the damage.Composite laminates have been widely applied to the aeronautics and astronautics industry and often subjected to impact load in the process of service. Therefore, it’s very meaningful to study structure health monitoring technique for composite laminates with impact damage. The main contents of this dissertation are as follows:The basic characteristics of Lamb wave is firstly introduced in detail. By constructing the movement equation of elastic wave in isotropic materials, the boundary conditions of plate substituted later, one can obtain the dispersion equation of Lamb wave in the isotropic material plate and the relationship between the phase velocity and group velocity. With aluminum and steel plate as the example, the dispersion curves are drawn. The expression form of the cutoff frequency is deduced, with pointing out its application significance.According to the partial wave theory, the dispersion equation of Lamb wave in the anisotropic material layer is deduced. This work laid the foundation for the later research, providing the theory basis.Then contents are introduced in detail which are related to the finite element simulation method of the propagation behavior of Lamb wave in composite laminates. Explicit dynamic algorithm is introduced first, and then PZT exciter model, damage model and the relevant theory are illustrated. The specific process of the finite element simulation and the interpretation of related parameters setting are presented, with simple analysis of the finite element simulation results. In general, this chapter provides a completed finite element simulation method of Lamb wave propagation behavior in composite laminates with impact damage.Finally based on the finite element simulation results, the author has carried on the simple signal processing and the damage location identification. Typical frequency domain method using fast Fourier transform and time frequency method using wavelet transform are applied in signal processing, then comparing the Lamb wave propagation under different frequencies. The existence of the damage is qualitatively judged and the advantage of time-frequency method compared to domain method is shown.Later based on the time of flight of Lamb wave signal, ellipse method is used to rough identify the location of impact damage. Considering the approximation algorithm, error range is acceptable. The correctness and validity of modeling and simulation method used in the front side is proved as well.