Effect of central and non-central frequency components on the quality of damage imaging

Accurate image reconstruction of damage through Lamb wave diffraction tomography (LWDT) requires substantial information of scatter field. This can be achieved using transducer network to collect the scatter field data. However, this requires a large number of transducers that creates logistical constraints for the practical applications of the technique. Various methods have been developed to improve the practicability of LWDT. One of the main approaches is to employ data at multiple frequencies within the bandwidth of the excitation signal. The objective of this study is to investigate the performance of using the data at non-central frequencies to reconstruct the damage image using LWDT. This provides an understanding on the influence of data at each individual frequency in the damage image reconstruction.In this paper, a series of numerical case studies with consideration of different damage sizes and shapes are carried out. Different non-central frequencies data is used to reconstruct the damage image. The results show that using the data at different non-central frequencies leads to different qualities of the reconstructed damage images. The quality of these reconstructed damage images are then compared to investigate the information contained of the data at each individual frequency. The study shows that the non-central frequencies data can provide additional information in the damage image reconstruction. Overall, the results of this study provide insights into the influences of the data at different frequencies, which is essential to advance the developments of the LWDT.Gnana Teja Pudipeddi, Ching-Tai Ng, Andrei Kotouso

Guided Wave Testing

Guided waves can propagate long distances in thin-walled structures, such as pipelines or plates. This allows for the efficient monitoring and testing of large structures and for the detection of hidden or inaccessible defects. Guided wave propagation is dispersive and multi-modal, requiring a thorough understanding of the wave propagation and scattering phenomena from simulations. Guided wave dispersion diagrams, mode shapes, and typical signals are illustrated for the example of isotropic plates. Both low and high frequency guided waves have been used for the testing of plate structures, with different wave modes and applications including tomography and arrays for the detection and localization of defects. For multilayered and anisotropic structures, guided wave propagation becomes more complex, and often the fundamental guided wave modes are employed for defect detection. For pipelines different commercially available testing systems have been developed and long propagation distances up to 100 m have been achieved. Careful selection of guided wave mode and excitation frequency allows the minimization of attenuation due to viscoelastic coatings and in buried pipelines. Synthetic focusing using non-axisymmetric modes improves defect imaging and localization. Experimental methods differ from standard ultrasonic testing, as good control of the excited guided wave mode shape and signal are required to achieve improved sensitivity for small defects. In addition to contact piezoelectric transducers, electromagnetic and laser techniques allow for noncontact measurements. Finite Element Analysis is one of the numerical simulation techniques used to obtain a better understanding of guided wave testing and to improve defect characterization