Abstract: When the high frequency focused ultrasound technology is used to detect internal defects in metal materials, the defects near the surface can’t be accurately located due to the blind zone. Thus it is difficult to evaluate the performance of the material. Therefore, the method of the defect feature extraction in ultrasonic blind zone based on mathematical morphology is proposed. Firstly, signal of the blind zone is processed by morphological filter via the flat structure element, so the characteristic signal of the defect is extracted. After that, the accumulated energy of the characteristic signal is calculated, which can be used to locate the depth of the defect. In the experiment, the cold rolled galvanized sheet is detected using a high frequency focused ultrasonic transducer of 100 MHz. The ultrasonic signal is processed with morphological filter via the flat structuring elements with length of 30. The distance between the defect and the surface of the material is 288.5 μm, and the position of the defect is 275.6 μm determined by the proposed method, so the relative error of the defect position is 4.5%. In order to verify the effectiveness of the method, wavelet packet method is compared. The results show that the method of mathematical morphology has smaller error and improves the B-scan imaging effect. So the defect feature can be more obvious.

Key words: blind zone of ultrasonic, defect extraction, mathematical morphology, ultrasonic detection