## 1 Introduction  
Guided waves, which are ultrasonic waves restricted by the boundaries of the solid waveguide during propagation [1], have been applied in the inspection of many industrial structures, such as pipes [2,3], rods [4], plates [5, 6, 7] and rails [8, 9, 10] with the advantages of long distance, full coverage and low cost [11]. The conversions between the electrical energy and mechanical energy are mainly involved in guide wave inspection, which are accomplished by the transducers as the key parts in the inspection system. Transducers with various characteristics, including vibration directions, frequency bandwidths and sizes, are generally customized to satisfy the demands under different conditions. Based on the physical effects, the guided wave transducers can be categorized into different types, including electromagnetic acoustic transducers (EMAT), laser transducers, piezoelectric transducers (PZT) and magnetostrictive transducers. The EMATs are able to work without contacting the waveguide using the Lorentz force [11; 12] although their applications are limited due to the low conversion efficiency, inapplicability for non-metal waveguides and signal instability caused by the coil vibration. The laser can be used to generate guided waves in a contactless manner based on the thermoelastic effect. However, the high power laser may burn the spot in the waveguide and the generated waves normally have very high frequencies (several MHz). With the high price, the inspection system with laser transducers still remain in laboratories without being widely used so far [13; 14]. The magnetostrictive transducers and PZTs are widely used at a low cost and with satisfactory performance [15]. The magnetostrictive transducers can be made into small patches, which are flexible and light for different conditions [16]. Compared with the PZTs, they have the advantages of good durability, low requirement in coupling, low cost and superiority in shear vibration generation. For the inspection of ferromagnetic materials, the magnetostrictive transducers can also be used without contact [17]. The dimensions of the ferromagnetic materials change during the process of magnetization and their magnetic state varies when external stress is applied to them. These effects are from the properties of ferromagnetic materials, which are called the magnetostrictive effect (Joule effect) and inverse magnetostrictive effect (Villari effect), respectively. These effects can be explained with magnetic domain theory [18]. The magnetostrictive effect works in the generation of guided waves while the inverse effect accounts for the measurement [19]. Many studies have been carried out for the improvement of energy conversion, directivity and mode selection since the first application of the magnetostriction in guided wave inspection. In early studies, the magnetostriction (\(\lambda_{s}\)) of the ferromagnetic waveguide was used to generate guided waves [20]. This method still works in cable inspections [21; 22]. However, the magnetostriction of these ferromagnetic waveguides, which are commonly made of iron (\(\lambda_{s}\) = 14 ppm), are very small and thus, the patches and strips made of materials with a large magnetostriction, such as Ni (\(\lambda_{s}\) = \(-\)50 ppm), Co (\(\lambda_{s}\) = \(-\)93 ppm) and Hiperco (\(\lambda_{s}\) = 60\(\sim\)100 ppm), are coupled with the waveguide for wave generation and detection [23; 24; 25], which can also be used for non-ferromagnetic waveguides [26]. There has been research carried out on the application of giant magnetostrictive materials, such as Galfenol (\(\lambda_{s}\) = 200\(\sim\)400 ppm) and Terfenol-D (\(\lambda_{s}\) = 1500\(\sim\)2000 ppm) in guided wave inspection [27; 28]. These materials have poor machinability and need large driving magnetic fields for an ideal performance. The undesirable temperature adaptability and high cost also restricts their applications in field tests.