Non-destructive Testing Research Based On Laser Ultrasonic Technique For Laminated Structure

The guided ultrasonic wave is widely acknowledged as one of the most encouraging tools for quantitative identification of damage in composite structures, because ultrasonic waves are sensitive to small defects and can travel a relatively long distance in the case of attenuation. In laser ultrasonic detection method, thermos-elastic effect and ablation mechanism are used in ultrasonic excitation in order to detect the damage of the structure. Due to its non-contact excitation, the characteristics of high spatial resolution and the low demand of the surface quality, laser ultrasonic technology has become a hotspot in the field of Non-Destructive Testing. In order to detect the damage of laminated material effectively, of which the structure is complex, this thesis focuses on the laser ultrasonic based testing method for laminated materials.Firstly, an in-depth insight about the principle of laser ultrasonic technology is made. The propagation characteristics of Lamb wave and the characteristics of frequency dispersion are also analyzed in this thesis. On this basis, the signal processing methods including Fourier transform and wavelet transform are introduced in detail. The wavefield separation of Lamb wave and the visualization of structural damage based on energy spectrum are both realized.Secondly, the method of one-dimensional short space truncation is used to analyze the change law of wavenumber along with space based on Lamb wave dispersion equation. As a consequence, the position of damage is evaluated. Based on this, the relationship between the wavenumber and the characteristics of structure is discussed. The severity of aluminium plate damage is analyzed quantitatively and the damage degree of carbon fiber composite is evaluated.Finally, signal processing methods based on wavenumber analysis are used in two-dimensional space. Local wavenumber estimation in both time and frequency domain is used to make the damage visible by short-space Fourier transform. Wavenumber analysis is used for wavefield at different time. Therefore, healthy signal is filtered by window function in wavenumber domain according to the weight of different wavenumber. As a result,damage visualization is realized by using energy diagram in two-dimensional wavefield.