New Electromagnetic Transducer Applications

Thompson, R
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A number of recently identified applications of electromagnetic transducers (EMAT's) are identified. These have been made possible by the development of optimized electronics which allow one to obtain high signal-to-noise ratios in EMAT systems despite the fact that the transduction efficiencies are somewhat lower than those of piezoelectric transducers. It has been demonstrated that EMAT's can be used to excite horizontally polarized shear waves which have the property that the particle motion is always parallel to the surface of the part in which they are excited, independent of the direction of propagation. Such waves have a number of attractive features. They do not mode convert on reflection from obstructions parallel to the shearing motion, they are not accompanied by the excitation of surface waves, they can be scanned in angle with little change in amplitude by varying the drive frequency, and they can be excited and detected at surfaces that may be irregular, painted or at high temperatures. Application of these transducers to the inspection of welds in thick plates at welding temperatures is discussed. An ultrasonic ellipsometer, which excites a shear wave of controlled, elliptical polarization which can be used in a fashion analogous to optical ellipsometry to measure the properties of thin layers, such as adhesive bonds, is presented. The use of EMAT's to detect residual stress in ferromagnetic materials is also reviewed.