Abstract: The conventional magnetostrictive sensor(MsS) employs a hard sensing coil which is made by winding electrical wires onto a plastic bobbin. The conventional MsS is hard to apply for practical inspections due to the wire winding is very time-consuming and it must be installed to the seven-wire steel strand via free ends. Meanwhile, a hard coil-based MsS is difficult to have a central frequency in megahertz-range with a higher accuracy in defects detection. An innovative flexible printed meander coil(FPMC)-based MsS is presented for exciting megahertz-range longitudinal guided waves in seven-wire steel strands. The FPMC employs optimal structural parameters after analyzing the work principle of the multi-group coils configuration, and it has 36 groups of coils which can increase the excited and received guided waves energy based on superposition principle. It can be wrapped onto the twisted surface of the strands. Therefore, this FPMC-based MsS is potential to be applied for in situ inspection. The obtained experimental results verify the proposed FPMC-based MsS can excite and receive megahertz-range longitudinal guided waves in seven-wire steel strands, having a central frequency of 1.20 MHz. In the excited guided waves, L(0, 1) and L(0, 3) mode are the dominated components. The proposed method provides a certain foundation for the megahertz-range MsS design.

Key words: Flexible printed meander coil, Magnetostrictive, Megahertz-range, Steel strand