This article presents a comprehensive review of the laser-based guided ultrasonic waves propagation imaging (G-UPI) system and respective signal/data processing methods related to the nondestructive testing and evaluation of thin-walled structures. The primary goal of this study is to review and recognize various processing methods, explain the working principles of the most influential methods, and highlight outstanding capabilities. In addition, the suitability of the methods for multiple types of damage and defect in various materials and structures are presented. At the same time, success stories of difficult-to-inspect cases are highlighted. Its secondary goal is to compare and discuss the merits and demerits of the laser-scanning part of the system for ultrasound generation and acquisition to provide a guideline for scanning scheme or hardware selection. Finally, the potential challenges and prospects of the G-UPI are discussed. It is expected that this review would serve as an entrance key for newcomers and a reference point for researchers to explore the opportunities for further improvement in the laser ultrasound-based evaluation of critical engineering structures.

Guided ultrasonic waves propagation imaging: a review

ABSTRACT The feasibility of using laser-generated ultrasonic Total Focusing Method (TFM) was assessed for guided ultrasonic waves in finite plates. The application under consideration is for inspection of ultrasonically welded battery tab-to-electrode foil stack joints. The testing constraints for this weld necessitate couplant-free, remote, guided-wave conditions making laser ultrasonic TFM an ideal inspection technique. It was determined that laser-generated guided wave TFM can be used to remotely assess defects in a finite plate when the defects are strong reflectors in the plane of wave propagation. The finite dimensions of the tab require a strong understanding of the edge reflection effects on the TFM image. The guided wave modes used in this study were strongly affected by scattering due to the complex weld geometry, which most resembles that of a periodic triangular grated wave guide. Future work will investigate methods to compensate for the strong scattering/guided wave effects, the use of other guided wave geometries, out of plane TFM reconstruction for other weld defect types, as well as apodization effects.

Assessment of Laser-Generated Ultrasonic Total Focusing Method for Battery Cell Foil Weld Inspection

ABSTRACT Among many damage imaging methods based on Lamb wave, phased array Delay and Sum (DAS) has attracted much attention because of its high efficiency. However, inherent dispersion effect leads to the inaccurate acquisition of time-domain features, which affects imaging accuracy. The imaging index based on the amplitude of scattering wave ignores the interaction between Lamb wave and damage, which is difficult to reflect the physical characteristics of damage scattering, and further affects imaging accuracy. We use the physical characteristic parameter-reflectivity of the damage surface as the imaging index and establish an inverse scattering model of damage in frequency domain with dispersion relationship. The simulation and experiment results show that the DAS method combined with the frequency-domain inverse scattering model can realise the high-quality imaging of the damage in the isotropic waveguide structure.

Damage Imaging of Lamb Wave in Isotropic Plate Using Phased Array Delay and Sum Based on Frequency-domain Inverse Scattering Model

Fiber waviness and voids may be produced in thick composites due to improper manufacturing conditions and consequently pose a risk of structural failure. A proof-of-concept solution for imaging fiber waviness in thick porous composites was proposed from both numerical and experimental studies, via calculating ultrasound non-reciprocity along different wave paths in a sensing network constructed by two phased array probes. Time-frequency analyses were conducted to reveal the cause of ultrasound non-reciprocity in wavy composites. Subsequently, the number of elements in the probes and excitation voltages was determined for fiber waviness imaging using the ultrasound non-reciprocity with a probability-based diagnostic algorithm. The fiber angle gradient was observed to cause ultrasound non-reciprocity and fiber waviness in the thick wavy composites were successfully imaged regardless of presence of voids. This study proposes a new feature for the ultrasonic imaging of fiber waviness and is expected to contribute to processing improvement in thick composites without prior knowledge of material anisotropy.

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Imaging of Fiber Waviness in Thick Composites with Unknown Material Properties Using Probability-Based Ultrasound Non-Reciprocity

Thin-walled structures, like aircraft skins and ship shells, are often several meters in size but only a few millimeters thick. By utilizing the laser ultrasonic Lamb wave detection method (LU-LDM), signals can be detected over long distances without physical contact. Additionally, this technology offers excellent flexibility in designing the measurement point distribution. The characteristics of LU-LDM are first analyzed in this review, specifically in terms of laser ultrasound and hardware configuration. Next, the methods are categorized based on three criteria: the quantity of collected wavefield data, the spectral domain, and the distribution of measurement points. The advantages and disadvantages of multiple methods are compared, and the suitable conditions for each method are summarized. Thirdly, we summarize four combined methods that balance detection efficiency and accuracy. Finally, several future development trends are suggested, and the current gaps and shortcomings in LU-LDM are highlighted. This review builds a comprehensive framework for LU-LDM for the first time, which is expected to serve as a technical reference for applying this technology in large, thin-walled structures.

https://www.mdpi.com/1424-8220/23/6/3183/pdf?version=1679297340

A Review of Laser Ultrasonic Lamb Wave Damage Detection Methods for Thin-Walled Structures

Total Focusing Method Damage Imaging in Frequency Domain Using Laser-Ultrasonic Lamb Wave Based on Time-domain Filtering in Multi-band

Conventional critically refracted longitudinal (LCR) wave technique seriously relies on the first critical angle (θCR), showing poor adaptability to the wave velocity variations of materials in defect and stress characterization. We propose an enhanced strategy based on the phased array ultrasonic, termed PA-LCR. Three typical beam profiles, deflection, focusing and their combination, are studied on the excitation of LCR wave. With one fixed wedge and time delay law, the wave can be excited within a wild angle range relative to θCR, about ±7° for −6 dB amplitude, and the adaptable longitudinal wave velocity is ranged about 6000 m/s. In addition, a calculation method of the LCR wave velocity is also proposed and verified. 

Fengzhong Li

Papers

Total Focusing Method Damage Imaging in Frequency Domain Using Laser-Ultrasonic Lamb Wave Based on Time-domain Filtering in Multi-band

A phased array ultrasonic-based enhanced strategy of critically refracted longitudinal (LCR) wave technique

Damage Imaging of Lamb Wave in Isotropic Plate Using Phased Array Delay and Sum Based on Frequency-domain Inverse Scattering Model