# New Magnetstrictive Transducer Designs for Emerging Application Areas of NDE
## 4 Sensors Utilizing Direct Wiedemann Effect  
Some common implementations of direct Wiedemann effect sensors for industrial structures are magnetostrictive sensors (MsS) for pipeline inspection and EMAT implementations for ferromagnetic tube inspection. Figure 2 shows (a) the design of an EMAT probe for heat exchanger tubing and (b) the design of an MsS probe that is used for guided wave testing of pipes. The EMAT configuration for tubing utilizes a biasing magnetic field created by passing a direct current (of the order of 15 Amperes) along the length of the tube, and a time-dependent axial magnetic field produced by passing a pulsed current through a coil inside or outside the tube. Since the magnetostrictive properties of carbon steel tubes can vary significantly, even at different locations in the same tube, the signal quality of this arrangement might vary. Based on field experience, the presence of oxides on the tube surface typically improves the probe performance. When the ferromagnetic properties are favorable, good signal quality has been reported in a wide frequency range (32-128 kHz) on carbon steel heat exchanger tubes [22]. It should be noted that these probes cannot be used in magnetically soft materials such as Duplex(r) or Seacure(r) because the RF coils introduce a residual magnetic field along the length of a tube. This residual field interacts with the dynamic magnetic field created by the same coil to create unwanted longitudinal mode guided waves. Due to this effect, reversed-Weidemann style sensors are used in these alloys, as discussed in a later section.  
MsS sensors for pipeline testing can conceptually employ the same configuration as an EMAT (circumferential bias field and axial time-varying field). The main difference is the addition of a magnetostrictive material that is attached to the pipeline, accentuates the magnetostrictive effect, and also allows operation on non-ferromagnetic materials. Typically, the circumferential field is created by moving a magnet along the strip material to induce a residual magnetic field.  
Although sensors that have been designed as described above have been used for many years for tube and pipeline inspection, there are some challenges that exist with these sensors:* For larger sensors, the winding of the RF coils for the time-varying field becomes prohibitively long, and the electrical impedance becomes very high; this can be challenging for existing electronics, because the dynamic field strength is proportional to the current in the RF coil at a fixed frequency.
* It can be difficult to introduce a strong and/or uniform magnetic bias for larger sensors, given the size of the area to magnetize.
* Most commercial MsS sensors use residual magnetic biasing that is vulnerable to external magnetizing fields and stresses applied to the strip.  
Some of these problems have been addressed with more advanced designs. For example, the problem with maintaining the magnetic bias was resolved in the MsS sensor configurations by including a DC winding to act as an electromagnet [23; 24]. Not only does this allow larger signal amplitudes, these sensors were also shown to stay functional at temperatures close to the Curie point, 930 \({}^{\circ}\)C [9].