Insight Vol 54 No 4 April 2012 217 GUIDED WAVES The magnetostriction means of generation and reception of ultrasonic guided waves is being used for the long- range inspection of pipelines for in-service damage such as corrosion. The generation of higher-strength ultrasonic guided waves in pipes using a magnetostrictive transducer, due to increased magnetostriction using a fux concentrator, is reported in this paper. A soft magnetic ribbon with negligible magnetostriction was used as a fux concentrator in the magnetostrictive sensor. The amplitude of the signal from a sensor with the fux concentrator at a power level of 10% (of the maximum instrument power level) was found to be comparable with the signal obtained from the sensor without fux concentrator at power levels of 50% of the maximum. The effect of the fux concentrator on different magnetostrictive sensors is also reported in this paper. Introduction Magnetostrictive sensors (MsS) are a type of guided wave sensor considered useful for the structural health monitoring (SHM) of systems. The main advantages of a MsS over other guided wave sensors, such as piezoelectric sensors, are that it requires no couplant, can be operated with a gap to the material under test and it has a good sensitivity in frequencies up to a few hundred kHz. A MsS uses a magnetostrictive material having a large magnetostriction (λ s ), such as Terfenol-D (λ s = 1000 ppm), Hyperco (λ s = 70 ppm), Ni (λ s = –36 ppm) and Ni 58 Fe 42 (λ s = 27 ppm), etc. The dimensions of a ferromagnetic material change in the presence of a magnetic field, known as magnetostriction [2] . An alternating magnetic field excites the magnetostrictive material causing a change in the dimensions of the material. The magnetostriction of the material changes with the same frequency as the excitation frequency. When an appropriate bias magnetic field is applied in addition to the alternating field, the MsS generates the ultrasonic waves in objects such as plates, pipes and tubes. This effect has a wide range of applications in: (a) non-destructive testing (NDT); (b) process measurement; (c) acoustic telemetry; and (d) robotics, etc. Its inverse effect, known as the Villari effect, detects the transmitted waves [3] . The efficiency and accuracy of defect detection using a MsS depends on: (a) the magnetostriction of the material used; (b) the uniformity of the bias field; (c) the excitation field, which depends on the power level of the signal applied to the MsS; (d) the frequency; (e) the directionality; and (f) the mode of the wave generated. Several earlier attempts have been made to improve the performance of magnetostrictive sensor systems viz. in the selection of the mode of the guided waves [3] , by increasing the amplitude of torsional wave generation [4] , by providing a uniform bias field using V-type yokes and solenoid coils [5,6] , and using pre-magnetised strips instead of permanent magnets [7,8] . The torsional T(0,1) mode is very attractive for non-destructive evaluation (NDE) because of its non-dispersive nature and it has the fastest group velocity (although the phase velocity is the slowest) [9] , in addition to the limited mode conversions when interacting with defects and edges (which improves the interpretation of the signals). The transduction efficiency of the torsional mode can be increased by bonding the magnetostrictive material at an angle of 45° to the longitudinal axis of the pipe [9,10] . The excitation field is one of the factors that affect the amplitude of the guided waves generated [9] . The field strength can be increased by increasing the number of turns in the excitation coil or the excitation power. High power is typically used to excite the transmitter or drive coils and thereby generate strong ultrasonic waves in the structure. The use of high-power electronics is expensive, bulky and unsafe. In order to increase the excitation field at a lower power, a soft magnetic material that has zero magnetostriction and which acts as a flux concentrator (FC) can be used between the magnetostrictive strip and the drive coil. In this work, we used different magnetostrictive materials, viz. Ni 58 Fe 42 (λ s = 27 ppm), Ni (λ s = –36 ppm) and Co 50 Fe 50 (λ s = 70 ppm), as transducers to generate ultrasonic guided waves. Fe-rich soft magnetic ribbon has been used as a FC to increase the excitation field. A large increase in the amplitude of the signal strengths has been observed with all the transducers. The strength of the signals is very large for the MsS with FC, compared with the signal amplitudes without FC. Experimental details An aluminium pipe 3.65 m long, with 72 mm outer diameter and 6 mm wall thickness, was used as the test-piece. Three types of magnetostrictive strips, such as: (a) Ni 58 Fe 42 – 15 mm wide and 0.1 mm thick; (b) Ni – 20 mm wide and 0.15 mm thick; and (c) Co 50 Fe 50 – 25 mm wide and 0.2 mm thick, were circumferentially attached on the outer diameter of the pipe and were used as transmitters. A copper wire of SWG 26 wound on the transmitter strip was used as the excitation coil. To compare the amplitudes of the transmitted signals of different MsSs, a single MsS receiver (Ni 58 Fe 42 – 15 mm width and 0.1 mm thickness) was used throughout the experiment. Copper wire of SWG 41 was used for the receiver coil. No couplant was used between the strips and the pipe. Improvement in the signal strength of magnetostrictive ultrasonic guided wave transducers for pipe inspection using a soft magnetic ribbon-based fux concentrator K Sathish Kumar, V Satya Narayana Murthy and K Balasubramaniam Submitted 14.10.11 Accepted 09.03.12 K Sathish Kumar and Krishnan Balasubramaniam* are with the Center for Nondestructive Evaluation, Department of Mechanical Engineering, Indian Institute of Technology Madras, Chennai – 600 036, India. V Satya Narayana Murthy is with the Department of Physics, BITS Pilani, Hyderabad Campus, Hyderabad – 500 078, India. *Corresponding author. Email: balas@iitm.ac.in DOI: 10.1784/insi.2012.54.4.217