Comparison of Morphology Characterization for Regular Cracks with Multi-mode Total Focusing Method

The multi-mode total focusing method (MTFM) is used for characterization of crack morphology, whose imaging quality is closely related to crack orientation. In this paper, the signal acquisition was performed using the 39° wedge and 16-element linear array probe with center frequency of 10 MHz. The morphology of regular cracks was characterized and compared by MTFM imaging in direct, half-skip and full-skip modes, respectively. The established simulation model was a 9.0 mm thickness aluminum plate containing a series of regular bottom cracks, whose lengths were all 2.0 mm and orientation angles were 0°, ±15°, ±35° and ±60°, respectively. The simulated results showed that direct, half-skip and full-skip modes were suitable for morphology characterization of regular cracks with orientation angles of 35°, 0° and -35°, respectively. The quantitative errors of crack lengths and orientation angles were no more than 0.22 mm and 0.8°, respectively. The measurement errors in experiment were within 0.28 mm and 0.9° for crack lengths and orientation angles, being in good agreement with simulated results. On the basis, the increment of element pitch is helpful for enlarging receiving area of ultrasonic beam and characterization range for crack morphology.