Development of Electromagnetic Acoustic Transducers for Degradation Evaluation of FRP

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Electromagnetic acoustic transducer (EMAT) was developed for glass fiber reinforced plastics (GFRP). Delamination is one of the defects in FRP laminates, which lowers the compressive strength. Thus the nondestructive evaluation is necessary in order to assure the structural safety. For metals, the EMAT is already applied in the nondestructive inspection. The EMAT consists of a coil and magnets, and it can generate and detect the ultrasonic wave without any couplant between transducer and the metal using the Lorentz force or magnetostriction. However, since the GFRP is nonconductive, the EMAT for metals cannot be applied. In this study, we suggested to embed a coil in the prepreg stacking and to compose the EMAT with a magnet on the coil.First, we ensured that the EMAT can transmit and receive the bulk shear wave, using the conventional piezoelectric transducer. We also found that the measurement of bulk wave by EMATs in pitch-catch mode is impossible because of the induction current. Then we switched to measure the guided SH wave velocity using two coils aligned in a line. The guided SH wave signal was extracted from leakage of the excitation signal by taking difference of two signals obtained for reversed magnetic fields. Accuracy of the velocity measurement was examined using the square plate specimen.We also proposed the detection of delamination in GFRP laminates using the velocity of Lamb wave. Meander line coil was used, pitch of which is adjusted to the wavelength of the lowest symmetric mode Lamb wave. It was found that the transmitter coil should be embedded in the medial interlayer to obtain the good detectability. We revealed that the position of the delamination can be evaluated if the change in the slope between path length and transit time is detected accurately.