Применение метода максимальной энтропии в ультразвуковой дефектоскопиис учетом переменной формы эхосигнала

Abstract

In order to reconstruct the image of reflectors with super resolution for more accurately determining the size of discontinuities in the examined item, it is suggested to use the maximum entropy (ME) method for processing echo signals measured in the TOFD mode. The effect of super resolution will make it possible to simplify the procedure for determining the phase of echoes from the crack upper and lower edges, a circumstance that is of much importance for their recognition. Conventionally, application of the ME method involves the need to calculate the circulant matrix correlating the reflection coefficient in the examined item and the echo signal measured in the TOFD mode. However, the echo signal shape depends on the reflector occurrence depth, a circumstance due to which phantom flare spots appear in the reconstructed image. A program for calculating the modified matrix has been developed, which takes into account the altered shape of the echo reflected from a point reflector lying at an arbitrary depth. For checking the efficiency of the echo signal calculation program, the obtained echo signals were compared with the echo signals calculated using the CIVA program. The obtained echoes have been found to differ from each other by less than 5%. The images obtained in a numerical experiment were used to study the effect the echo signal variation factor has on the results obtained from applying the ME method. It has been found that the echo signal waveform variability imposes certain limitation on using the ME method with a circulant matrix: the "side lobes" make around 40% of the reflector flare amplitude on the reconstructed image. The use of the modified ME method for reconstructing the reflector image made it possible to significantly reduce the level of "side lobes" and increase the resolving power by more than a factor of 3. Model experiments have also confirmed the effectiveness of the ME method with the modified matrix. The head wave impulse and the impulse reflected once from the examined item bottom have become narrower by more than a factor of 5 with a small level of "side lobes". Owing to this circumstance, the impulses were easily found to be in antiphase.