The Quantitative Study of TOFD influenced by the Frequency Window of Autoregressive Spectral Extrapolation

摘要

The time of flight diffraction (TOFD) technique can be employed to measure flaw height by use of the time lag of diffraction signals which come from the top and bottom edges of defect. However, the small flaw size or the poor temporal resolution will lead to signal aliasing in time domain, so it is hard to quantify the defect. To separate the mixed signals, Autoregressive Spectral Extrapolation (ARSE) technique takes advantage of the data in frequency window, which has a high signal-to-noise radio(SNR), to estimate the data outside this frequency window by prediction error principle. Particularly, the choice of frequency window plays an important role in the above process. In this paper, combining ARSE technique with TOFD, the cracks with the height of 1.00mm, 1.20mm and 1.50mm in depth of 50.0mm were detected quantitatively in carbon steel block whose thickness was 120.0mm. Meanwhile, the influence of frequency window to the detection result was studied by simulation and experiment methods, and different quantitative results of crack height were presented. The relative error was less than 5% when the maximum in frequency spectrum of reference signal decreased 6dB or 7dB.