The ultrasonic phased array total focusing method (TFM) has the advantages of high imaging resolution and high sensitivity to small defects. However, it has a long imaging time and cannot realize near-distance defect imaging, which limits its application for industrial detection. A sparse-TFM algorithm is adopted in this work to solve the problem regarding rapid imaging of near- distance defects in thin plates. Green’s function is reconstructed through the cross-correlation of the diffuse full matrix captured by the ultrasonic phased array. The reconstructed full matrix recovers near-distance scattering information submerged by noise. A sparse array is applied to TFM for rapid imaging. In order to improve the imaging resolution, the location of active array elements in the sparse array can be optimized using the genetic algorithm (GA). Experiments are conducted on three aluminium plates with near-distance defects. The experimental results confirm that the sparse-TFM algorithm of Lamb waves can be used for near-distance defects imaging, which increases the computational efficiency by keeping the imaging accuracy. This paper provides a theoretical guidance for Lamb wave non-destructive testing of the near-distance defects in plate-like structures.

Guided wave tomography has shown great potential for quantitative nondestructive evaluation in structural health monitoring. An improved simultaneous iterative reconstruction technique (SIRT) combining genetic algorithm (GA) is presented in order to improve image quality of guided wave tomography. The simulated reconstructed images of flawed plate and pipe using usual SIRT and improved SIRT methods have been compared quantitatively and qualitatively.

Signal enhancement of ultrasonic guided waves is studied using time reversal technique. The defect in pipe is detected by activating single mode guided waves L(0,2) at the center frequency of 70 kHz. Time-reversal technique is used to further enhance the defect detection capability. A method to identify the defect is described on a finite element model. Numerical results show that the time reversal technique has the ability to evidently enhance the amplitude of damage reflected wave and improve the detection efficiency of defect.

Localized flaws such as corrosions in petroleum pipelines often cause fragility, impairing integrity and shortening service lifetime of the structures. There has been much interest recently in monitoring the integrity of the pipe structures. Ultrasonic guided waves provide a highly efficient technique for rapid pipe inspection because they can be made to propagate significant distances in pitch-catch configurations. Crosshole tomographic geometry is formed in such pitch-catch configurations when transmits and receivers are respectively laid along two parallel circumferential belts around the pipe. Considering the pipe as an unwrapped plate, we investigate the adapation of the tomographic reconstruction in seismology to the guided wave inspection of a pipe. Various effects such as transducer arrangement, mesh precision, sampling interval and iterative algorithm on tomographic reconstruction are analyzed. The results provide a theoretical basis for quantitative detection of pipeline flaw using guided wave tomography.

The detection of localized defects such as cracks and corrosion in pipes using guided waves has been shown to be an effective nondestructive evaluation technique for structural health monitoring (SHM). Cross borehole tomography in seismology is introduced into the guided wave inspection of a pipe when the pipe is considered as an unwrapped plate. Guided waves propagating in pipe with a crack defect are simulated using the finite element model and the arrival times for the fastest modes are extracted and sent to the tomographic algorithm. The tomographic reconstruction is based on the simultaneous iterative reconstruction technique (SIRT). For some cylindrical shell geometries such as stacked storage tanks, access to the entire circumference of the structure could be impractical or even impossible, three different image fusion techniques are used to enhance the image equality reconstructed from the incomplete datasets. The results show that the defect is more pronounced after imaging fusion.