Over the past decades, the development of electronics resulted in an expansion of ultrasonic phased array systems in the field of nondestructive testing (NDT). Several imaging algorithms were also developed to help interpret the data and characterize flaws. Among them, the total focusing method (TFM) stands out as it allows the generation of the highest possible imaging resolution of subsurface defects. TFM synthetically focuses the array on each image pixels in the region of interest by post-processing all possible pairs of transmit/receive elements (full matrix capture - FMC). However, the downsides of TFM are (1) the amount of time required to acquire the FMC and (2) the amount of data processed to generate an image. Those downsides make real-time TFM imaging only possible with high-end instruments. Sparse array techniques are prominent candidates to overcome some of these limitations. The method proposed in this presentation consists of using a reduced portion of array elements in transmission, while the reception is performed with all elements. The method can therefore improve acquisition speed and reduce the data to be processed. This presentation investigates the minimum number of transmission events when using a linear array probe of 64 elements. Due to the limited number of firings, a simplified and straightforward approach to explore the best sparse array layout can be applied, without using complex stochastic optimization algorithms as found in previous sparse array research works [1,2]. The procedure consists of comparing the beam pattern (BP) of all possible sparse array combinations against the beam pattern of the FMC in terms of main lobe width (MLW) and the sidelobe levels (SLL). At the end of the exploration process, the best layout is selected according to the solution that is closest to the FMC on those terms. The method is assessed experimentally using an aluminum block containing 5 side drill holes (SDH) of 1 mm drilled at different depths. The block was measured in the horizontal and vertical positions to evaluate both axial and lateral resolutions. Ultimately, the best sparse array layout is a compromise between the scanning speed and the image quality. The results were assessed in terms of the array performance indicator (API). It was found that a sparse-TFM layout with 5 firing events (92.2% reduction in firing events) obtained from the proposed algorithm can produce images with API comparable to TFM images generated using the FMC.[1]&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Chen, P., Zheng, Y., and Zhu, W., 2011, “Optimized Simulated Annealing Algorithm for Thinning and Weighting Large Planar Arrays in Both Far-Field and Near-Field,” IEEE J. Ocean. Eng., 36(4), pp. 658–664.[2]&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Hu, H., Du, J., Xu, N., Jeong, H., and Wang, X., 2017, “Ultrasonic Sparse-TFM Imaging for a Two-Layer Medium Using Genetic Algorithm Optimization and Effective Aperture Correction,” NDT E Int., 90, pp. 24–32.

Minimum Transmission Events for Fast Ultrasonic Imaging Using Sparse-Tfm Approaches

Paper Proceedings: https://asmedigitalcollection.asme.org/qnde/QNDE2021/volume/85529

ASME 2021 48th Annual Review of Progress in Quantitative Nondestructive Evaluation (QNDE2021)

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