Mechanical Systems and Signal Processing 173 (2022) 109010 Available online 14 March 2022 0888-3270/© 2022 Elsevier Ltd. All rights reserved. Sign coherence factor-based search algorithm for defect localization with laser generated Lamb waves Honglei Chen a , Kailiang Xu a, b, * , Zenghua Liu c , Dean Ta a, b a Academy for Engineering & Technology, Fudan University, Shanghai, China b Center for Biomedical Engineering, School of Information Science and Technology, Fudan University, Shanghai, China c Faculty of Materials and Manufacturing, Beijing University of Technology, Beijing 100124, China A R T I C L E INFO Communicated by Marc Rebillat Keywords: Sign coherence factor Laser ultrasound Lamb waves Defect localization Nondestructive testing ABSTRACT Techniques based on laser generated Lamb waves (LGLW) and compact array can realize far field, noncontact localization of defects in plate structures. Fully utilizing the information of LGLW signals is beneficial for improving the robustness and image contrast of the localization. However, the classical imaging algorithms are performed by mapping the signal parameters (amplitude, polar sign) into each discrete grid based on the time-of-flight principle. It is time consuming and the performance is sensitive to the frequency. In this study, a sign coherence factor (SCF)-based search algorithm is developed for defect localization with the LGLW signals. In the algorithm, the defect locations are identified by an adaptive search process of individual scatterer. The char- acteristics of defect localization with compact array-based Lamb waves are analyzed, including the statistical and diversity characteristics, lateral effect of temporal-spatial mapping trajectories. A mathematical model of the individual scatterer and its searching algorithm are designed by incorporating the signal sign with the characteristics. The evolutionary strategy and the clustering algorithm are combined to develop the search algorithm of the individuals in the imaging zone. The SCF of individuals kept based on the analysis with multiple frequencies of LGLW signals are added for high contrast and robustness defect imaging. Two full laser defect detection experi- ments were carried out to verify the effectiveness of the developed algorithm. Compared with the SCF algorithm, the developed SCF-based search algorithm shows better performance in improving the signal-to-noise ratio of images and its execution time is less affected by the number of discrete grids in the plate. 1. Introduction Compact array-based Lamb wave defect detection techniques are baseline-free in which signals from intact samples are not necessary [1]. The lasers, based on the thermo-elastic mechanism and the two-waves interference principle, have been widely used for noncontact excitation and reception of Lamb waves. The incorporating of the laser generated Lamb waves (LGLW) and compact array results many attractive characteristics for nondestructive testing (NDT) and structural health monitoring (SHM), including the noncontact, baseline-free and rich signal information [2–6]. However, the classical imaging algorithms are processed with an ergodic mapping process of signal parameters (amplitude, polar sign) that is time-consuming. In addition, their performance is sensitive to the * Corresponding author. E-mail address: xukl@fudan.edu.cn (K. Xu). Contents lists available at ScienceDirect Mechanical Systems and Signal Processing journal homepage: www.elsevier.com/locate/ymssp https://doi.org/10.1016/j.ymssp.2022.109010 Received 25 August 2021; Received in revised form 26 December 2021; Accepted 1 March 2022