The effect of changes in sample dimensions in the direction of an applied magnetic field was discovered and described by Joule in 1942 and was called Joule magnetostriction . The principle of magnetostriction is based either on the boundaries shifting between the domains or the domain oscillation/rotation due to the applied magnetic fields. Typically, a permanent magnetic field is used to give the domains a preferred orientation and then variable magnetic fields are applied to initiate the rotation of the domains causing the dimensional changes. Depending on the mutual orientation of magnetic fields (in plane or out of plane), oscillation of domains can produce longitudinal or transverse vibrations.
For practical guided wave NDE, the fundamental transverse vibration mode was found to be best due to the low dispersion rate (the shear SH (0,1) mode in plates and torsional T (0,1) mode in pipes). Low dispersion implies that the group and phase velocities of guided waves are not frequency-dependent. This feature of SH guided waves significantly simplifies the interpretation of signals. Also, these wave modes do not show any out-of-plane particle displacement, and as a consequence, they do not produce any coupling effect (and accompanying trailing signals) with liquids as compared to compressional (longitudinal) mode guided waves. Therefore, the magnetostrictive EMATs that produce transverse and horizontally polarized vibrations are utilized for guided wave testing more frequently.
Figure 1. Configuration of magnetostrictive EMAT used for generation of transverse vibrations in plate
The physical effect for generation of transverse vibration using orthogonally oriented permanent and variable magnetic fields was discovered by Wiedemann in 1881 for generation of T mode guided waves in rod shaped components . In a similar evaluation on a plate, a SH wave transducer consisting of a meander coil and static bias magnetic field parallel to the coil elements was described by Thompson in 1979 . In this configuration, the SH wave was propagating in the direction perpendicular to the permanent magnetic bias direction as shown on Figure 1. The direction of wave propagation is shown as “Shear horizontal wave 1” (direction parallel to “y” axis). Since 2002, this particular configuration of magnetostrictive EMAT has been extensively utilized by Southwest research Institute (SwRI) in different practical applications of MsS technology for both generation of SH and T modes of guided waves directly in components (if ferromagnetic) or via the ferromagnetic strip coupled to components [4,5].
In 2008, an additional option for generation of transverse vibrations was investigated and accommodated in MsS sensors design. This approach utilized the transverse vibration propagating parallel to the direction of permanent bias field (the wave direction is shown on Figure 1 as “Shear horizontal wave 2” and it is parallel to “x” axis). The predominant direction of wave propagating was found to be related mostly to the anisotropy of the ferromagnetic material. In the case where the material is predominantly isotropic, transverse vibrations of similar amplitude propagating in both the “x” and “y” direction could be detected together with multi-directional components formed by superposed waves. Since the magnetostrictive sensors are mostly required to send the guided wave in pre-defined direction (“x” or “y”), either cold rolled strip materials (with the pronounced anisotropy due to the rolling texture) or the strip material heat treated in the presence of magnetic fields enforcing the magnetic anisotropy are typically utilized in sensors design.
The sensors utilizing transverse vibrations propagating in the bias direction were referenced in publications as MsT (magnetostrictive transducer) and were described in different configurations for inspection pipes and tubing in petrochemical and nuclear industry [6-9]. Depending on application, one or another type of magnetostrictive transduction mechanism (MsS or MsT) was found to be superior to the other and both of them have been utilized in custom sensors design.
1. Joule, James (1842). “On a new class of magnetic forces,”. Annals of Electricity, Magnetism, and Chemistry 8: 219–224.
2. Wiedemann, Gustav (1881), Electrizitat 3: 519.
3. Thompson, R.B., “Generation of horizontally polarized shear waves in ferromagnetic materials using magnetostrictively coupled meander-coil electromagnetic transducers,” Appl Phys Lett 1979;34:175–7.
4. Kwun, H., Dynes, H., “Long Range Guided Wave Inspection of Pipe Using the Magnetostrictive Sensor Technology-Feasibility of Defect Characterization,” Nondestructive Evaluation of Utilities and Pipelines II, 3398, pp. 28–34, 1998.
5. Kwun et all. “Method and Apparatus Generating and Detecting Torsional Wave Inspection of Pipes or Tubes,” U.S. Patent 6,917,196, July 12, 2003.
6. S.Vinogradov, “Method and System for Generating and Receiving Torsional Guided waves in a Structure,” U.S. Patent 7,821,258, B2, October 26, 2010.
7.S. Vinogradov, “Magnetostricive Transducer for Torsional Mode Guided Wave in Pipes and Plates," Materials Evaluation, Vol. 67, N 3, 2009, pp. 333–341.
8.S. Vinogradov, C.Barrera, “Development of Guided Wave Examinations of Piping and Tubing Using Magnetostrictive Sensor Technology", 8th International Conference on NDE in Relation to Structural Integrity for Nuclear and Pressurized Components, Maritim Hotel, Berlin, Germany, September 29 - October 1, 2010.
9.S. Vinogradov, “Development of Enhanced Guided Wave Screening Using Broadband Magnetostrictive Transducer and Non-Linear Signal Processing ,” Fourth Japan-US Symposium on Emerging NDE Capabilities for a Safer World, Maui Island, Hawaii, USA, June 7-11, 2010