Ultrasonic harmonic imaging in nondestructive evaluation: preliminary experimental study

Harmonic imaging technology (HIT) has emerged as a major imaging trend in medical ultrasound that has enabled a substantial improvement of the image quality in medical ultrasonic imaging. The HIT, however, has seldom been used in nondestructive evaluation (NDE). In an attempt to exploit the HIT for the NDE purpose an experimental study has been conducted. The experiments were made using transducer consisting of two concentric elements with the same geometrical focus. The outer element with lower center frequency was used as a transmitter, and the inner with higher frequency used as a receiver. A narrow band excitation was applied to the transmitter, and the measurements were performed on two copper blocks in the immersion case. The first block with side-drilled holes at different depths was used to examine propagation of harmonics in a solid. The second block with an electron beam (EB) weld and with side-drilled, flat-bottom and round-bottom holes was used for imaging and characterizing the weld. The harmonic images were constructed from the harmonics extracted from the measured data. The results have revealed that the 2nd and 3rd harmonics were clearly seen, the 4th harmonics were detectable about the focal zone, and the image of higher order harmonic had a better resolution. From the study it can be concluded that the HIT is applicable to NDE and worthwhile further exploiting its potential benefits