Configuration optimization of magnetostrictive transducers for longitudinal guided wave inspection in seven-wire steel strands Zenghua Liu n , Jichen Zhao, Bin Wu, Yinong Zhang, Cunfu He College of Mechanical Engineering and Applied Electronics Technology, Beijing University of Technology, Beijing 100124, China article info Article history: Received 10 November 2009 Received in revised form 14 May 2010 Accepted 14 May 2010 Available online 21 May 2010 Keywords: Magnetostrictive transducer Longitudinal mode Steel strand Coil Permanent magnet abstract The configuration of magnetostrictive transducers for both transmitter and receiver was optimized for the generation and reception of ultrasonic longitudinal guided waves in seven-wire steel strands in a pitch catch arrangement. Three axisymmetric permanent magnets significantly improved the capability of magnetostrictive transducers compared to two permanent magnets, and effectively increased the amplitude of the longitudinal guided wave mode, L(0, 1) at 160 kHz. Experimental results show that the maximum amplitude of a received guided wave signal could be obtained by using a receiver with a three-layer coil in parallel and a transmitter with a three-layer coil in series. The amplitudes of the defect-reflected signal increased by as much as 50% or more as compared with those when both transducers used a single layer coil. As a result, magnetostrictive transducers with an optimized configuration, including permanent magnet distribution and multilayer coil connection, could be efficiently used for the inspection of seven-wire steel strands by using ultrasonic guided waves in a pitch catch arrangement. & 2010 Elsevier Ltd. All rights reserved. 1. Introduction Seven-wire steel strands are key load-carrying components. With high strength and low relaxation, they are widely used to strengthen prestressed structures, such as high-rise concrete buildings and long span cable-stayed and suspension bridges. They are of primary importance in securing the safety of the entire structure. However, with continuing loading and other unforeseen circumstances, prestress loss and defects in steel strands are possible, potentially affecting the safety performance of an entire prestressed structure [1]. With the growing number of applications of the use of prestressing technique in civil engineering, a significant amount of research attention has been paid to the monitoring of the condition of seven-wire steel strands. Various monitoring methods have been applied to these spiral structures [2,3]. The ultrasonic guided wave technique is an emerging non- destructive testing method which has great potential in the inspection of many waveguides, such as pipes, plates, rods and rails [4–8]. In recent years, this technique has been applied to the inspection of seven-wire steel strands due to its prominent advantages which include long distance inspection capability, mode and frequency tuning, cost effectiveness and stress sensitiv- ity. Relatively complete inspection, including stress measurement and defect detection, of seven-wire steel strands can be achieved using an ultrasonic guided wave technique. Nowadays, several types of transducers are used for ultrasonic guided wave inspection of steel strands. Rizzo [9] attached piezoelectric wafer transducers at the end of an entire steel strand or one of its wires in a through transmission configuration to study the response of the lowest- order longitudinal mode, L(0, 1), at different levels of load in the steel strand. It has been identified that the ultrasonic features associated with the transmitted ultrasonic energy are sensitive to variation in the applied load. Chen et al. [10,11] and Chaki et al. [12] mounted acoustic emission piezoelectric transducers at the end of a steel strand to measure the time shift of ultrasonic guided waves detected at the center wire of the seven-wire strand. However, due to the complex helical configuration of seven-wire steel strands, inevitable mechanical coupling strongly limited the application of piezoelectric type transducers in an ultrasonic guided wave inspection of in-service steel strands, or even made it impossible. Rizzo et al. [13] used a non-contact laser to generate ultrasonic guided wave signals in seven-wire steel strands. Due to the lack of mode control capability, the laser could only excite broad-band signals which consisted of both longitudinal and flexural guided wave modes. For the identification of signal characterization, time–frequency wavelet transform processing was used. Kwun et al. [14] first used non-contact magnetostrictive transducers to excite and receive ultrasonic guided waves in prestressed steel strands and achieved applied stress measurement by using the acoustoelastic effect of ultrasonic guided waves. It was found that a certain portion of the frequency components of ARTICLE IN PRESS Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/ndteint NDT&E International 0963-8695/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.ndteint.2010.05.003 n Corresponding author. E-mail address: liuzenghua@bjut.edu.cn (Z. Liu). NDT&E International 43 (2010) 484–492