13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- they can be used for ultrasonic measurements without a coupling medium between the transmitter/receiver and the test object. This set-up allows measurements on sensitive surfaces and objects also at elevated temperatures. Another advantage of EMATs is their very efficient generation of shear waves with both vertical and horizontal polarization for oblique angles of incidence as well as the generation of linearly, radially and elliptically polarized shear waves for the normal incidence [9]. Figure 1. Schematic drawing of the experimental setup: ambient temperature (a) and T = 300 °C (b) EMATs generate ultrasonic waves by means of Lorentz forces, magnetostriction or by magnetic forces. Ultrasound generation by means of magnetostriction or magnetic forces requires ferromagnetic materials. Thus, since the investigated material in this project is a paramagnetic austenitic steel, the ultrasonic waves are generated only by Lorentz forces. The excitation of ultrasound by EMATs is performed by the superposition of high frequency eddy currents, induced by a coil with a geometry matched to the wave pattern, and a static or low-frequency magnetic field (Figure 2). The waves are picked up by a receiving coil geometrically identical to the transmitter coil. In this project radially polarized shear waves are generated by the transmitter at the one end and the receiver at the other end [9]. Figure 2. Assembling of an electromagnetic acoustic transducer for radially polarized shear waves: schematic picture (a), photo of a probe (b)
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