| 摘要 |
An improved method for defect detectability for the inspection of liquid filled pipes using magnetostrictive sensors. The improved method comprises first recognizing the liquid-induced changes in the dispersion properties of the second longitudinal wave mode, L(0,2). These liquid-induced changes include a severe dispersion at periodic branching frequencies that result in a pulse-like characteristic in the extended received signal. A trailing portion of a received signal component associated with a geometric irregularity, is shown to comprise the branching frequency components. The trailing portion of the extended signal may therefore be removed in order to improve defect detection. The removal process comprises one of three alternative methods. A first method includes creating a short duration pulse free of the frequency components that comprise the trailing signals, and applying the pulse to a magnetostrictive transmitter, thereby generating a longitudinal wave signal in the pipe wall free of the trailing signal frequency components. A second method comprises transmitting a broadband signal and processing the detected signal through a digital filter that eliminates those frequency components known to comprise the trailing signals. A third method involves using a signal whose bandwidth lies in the region between two adjacent branching frequencies in the dispersion curve. The result is a detected signal within which components representative of irregularities are less extended and therefore less likely to override subsequent defect components in the detected signal. |
