A PEAK FREQUENCY SHIFT METHOD FOR GUIDED WAVE THICKNESS MEASUREMENT AND ITS REALIZATION BY DIFFERENT TRANSDUCER TECHNIQUES

摘要

Ultrasonic guided wave inspection is much more efficient than traditional point-by-point examination. Guided waves can propagate over long distances from a single position, and therefore is a good method of evaluating thickness degradation, especially for large-area structures such as pipes and vessels. In this paper, guided wave thickness measurement potential is studied utilizing a peak frequency approach. Because guided wave velocities are functions of the product of the frequency and the structure thickness, the dispersion curve of phase velocity vs. frequency will shift as the thickness changes, and the mode excitation frequency, which is called peak frequency, will also shift. The relationship between peak frequency shifts and thickness changes can be used for guided wave thickness measurement. Both Lamb waves and shear horizontal (SH) guided waves were studied. Experiments were carried out on plates with different thicknesses. Phase velocity dispersion curves and their changes with the thicknesses variations were calculated theoretically. Peak frequency shifting information was acquired experimentally by signal analysis for establishing an algorithm of calculating thicknesses from peak frequency shifts. Different guided wave transducer techniques, such as piezoelectric and electromagnetic acoustic transducer (EMAT) technique, were studied and compared for realization of the thickness measurement method.