Nondestructive Evaluation of Delamination in CFRP Laminates Using Electromagnetic Acoustic Transducers

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Electromagnetic acoustic transducers (EMATs) were applied to nondestructive evaluation of delamination in CFRP laminates. Since Lamb wave velocity depends on both frequency and plate thickness, the thickness reduction due to delamination results in the velocity change. In this study, the group velocity of Lamb wave in CFRP laminates was measured by the EMATs while introducing delamination stepwise.The EMAT, which consists of a coil and a permanent magnet, launches the Lamb wave using the eddy current, induced in the vicinity of the plate surface, and the static magnetic field. To enable the eddy current to be generated, we used plain-woven carbon prepreg sheets, which were stacked and were cured in an autoclave. To enhance the magnetic field strength, steel block was put below the plate at the opposed position of the transmitter. In spite of these improvements, Lamb wave signal was still obscured by the leakage of the excitation signal. By taking difference of signals obtained applying two opposite magnetic fields for receiver, Lamb wave signal was extracted. The availability of electromagnet in place of permanent magnet was also examined.Lamb wave signals were received and sampled for various path lengths at each stage of delamination. Distribution of amplitude, in the time domain was calculated for each frequency component. Time of amplitude peak was plotted against path length, the slope of which gives the inverse of group velocity. Comparing the group velocities in the sound and delaminated sections, we found that the group velocity increased in the delaminated area, as predicted theoretically. However, the velocity in the sound section differed from that measured before introduction of delamination. It was revealed that the delaminated part may be detected as the area of elevated velocity when the velocity in the sound part is known.