Sensors & Actuators: A. Physical 386 (2025) 116353 0924-4247/© 2025 Elsevier B.V. All rights are reserved, including those for text and data mining, AI training, and similar technologies. Development of HBVD-EMAT and its application in surface crack defect detection Zenghua Liu a , Yanhong Guo b , Xin Zhao a,* , Jinjie Cheng b , Mengqi Su b , Kunsong Zheng b , Xiaoyu Liu b , Cunfu He a a School of Information Science and Technology, Beijing University of Technology, Beijing, China b College of Mechanical and Energy Engineering, Beijing University of Technology, Beijing, China A R T I C L E INFO Keywords: EMAT Surface wave Pulse compression Crack detection Halbach magnet ABSTRACT Electromagnetic acoustic transducers (EMATs) operate on the principle of electromagnetic induction, resulting in low energy conversion efficiency. The amplitude of the surface wave signal excited by EMATs is low, and the signal pulse width is large. Additionally, the signal used by conventional EMATs (surface wave EMAT in meander-line coil form) is typically a narrow-band signal with a single frequency, which limits the available frequency information for defect evaluation. To address these issues, this research proposes a novel EMAT integrating a Halbach magnet with a variable-distance meander-line coil (HBVD-EMAT). The HBVD-EMAT generates a magnetic field using a Halbach magnet and employs pulse compression techniques to guide the design of the coil and excitation signal. Finite element simulation is used to optimize the meander point where the current direction in the coil changes and to design the Halbach magnet length, improving the match between the frequency range of the HBVD-EMAT excited surface wave and the ideal frequency range. Detection experi- ments demonstrate that the HBVD-EMAT can excite a wideband pulse compression surface wave signal. Compared to the conventional EMAT, the signal amplitude of the HBVD-EMAT is enhanced by 1018 % due to the combined effect of the Halbach magnet and pulse compression technique. The main lobe width is reduced by 80 %, and the signal-to-noise ratio (SNR) is increased from 29.1 dB to 42.5 dB. These enhancements significantly improve the EMAT’s ability to detect surface cracks in metal materials. The use of HBVD-EMAT enables accurate detection of closely spaced adjacent defects, and for multiple defects, it ensures the detection of all defects while precisely localizing each one. The time domain and frequency domain characteristic parameters extracted from the detection signal of HBVD-EMAT can quantitatively evaluate the crack depth. After these parameters had been combined to evaluate the crack depth, the prediction error of crack depth had been reduced. 1. Introduction Metal materials are used in the manufacture of components in various industrial fields. However, in the process of service, the material surface is prone to crack defects [1]. Crack defects will not only reduce the strength and durability of components, but also threaten the service life of components [2]. At present, the commonly used methods for crack detection are ultrasonic detection, ray detection and eddy current detection [3]. X-ray detection has a highly accurate detection ability, but there is a very serious problem of ionizing radiation [4]. Eddy current detection is excellent in finding tiny surface defects, but it is limited by skin depth effect, so the detection sensitivity of deep cracks is insuffi- cient [5]. Ultrasonic detection has the characteristics of convenience, rapidity and high repeatability of detection results, so ultrasonic surface waves are widely used in surface (or near surface) crack detection [2]. Ultrasonic surface waves have the characteristics of long-distance propagation and wide detection range, and are very sensitive to sur- face and near-surface defects when propagating in thick samples [6,7]. There are various methods for generating surface waves, with electro- magnetic acoustic transducer (EMAT) being one of them. EMAT com- bines magnet and meander-line coil. Under the condition that the magnet provides a static magnetic field, the particles on the surface of the material can vibrate alternately to form surface waves after alter- nating current is introduced into the meander-line coil [8,9]. EMAT has the characteristics of flexible configuration, no coupling agent and high temperature resistance, and is widely used in the field of ultrasonic * Corresponding author. E-mail address: zhaoxin@bjut.edu.cn (X. Zhao). Contents lists available at ScienceDirect Sensors and Actuators: A. Physical journal homepage: www.journals.elsevier.com/sensors-and-actuators-a-physical https://doi.org/10.1016/j.sna.2025.116353 Received 3 January 2025; Received in revised form 11 February 2025; Accepted 19 February 2025