Probe Standoff Optimization Method for Phased Array Ultrasonic TFM Imaging of Curved Parts

Abstract

The reliability of the ultrasonic phased array total focusing method (TFM) imaging of parts with curved geometries depends on many factors, one being the probe standoff. Strong artifacts and resolution loss are introduced by some surface profile and standoff combinations, making it impossible to identify defects. This paper, therefore, introduces a probe standoff optimization method (PSOM) to mitigate such effects. Based on a point spread function analysis, the PSOM algorithm finds the standoff with the lowest main lobe width and side lobe level values. Validation experiments were conducted and the TFM imaging performance compared with the PSOM predictions. The experiments consisted of the inspection of concave and convex parts with amplitudes of 0, 5 and 15 λAl, at 12 standoffs varying from 20 to 130 mm. Three internal side-drilled holes at different depths were used as targets. To investigate how the optimal probe standoff improves the TFM, two metrics were used: the signal-to-artifact ratio (SAR) and the array performance indicator (API). The PSF characteristics predicted by the PSOM agreed with the quality of TFM images. A considerable TFM improvement was demonstrated at the optimal standoff calculated by the PSOM. The API of a convex specimen’s TFM was minimized, and the SAR gained up to 13 dB, while the image of a concave specimen gained up to 33 dB in SAR.

The Effect of the Position Determination Error for Flexible Linear Array Elements on the Tomogram Focusing

“…An incorrect determination of the delay law leads to a change the focal spot dimensions in more than two times and a change in the amplitude of the ultrasonic signal in the range from −2.5 dB to +2.0 dB [ 14 ]. In addition, the signal-to-noise ratio decreases and information about small reflectors may be lost or underestimated, artifacts may appear on the tomogram, as well as increasing the size of the reflectors and incorrect determining of its coordinates [ 25 , 26 , 27 , 28 , 29 ].…”

Studies of Angular Resolution for Acoustic Arc Arrays

“…However, in this case an individual prism is required for each surface; this becomes impractical for many applications as the surface profile of an object under inspection changes. A more common approach involves joining a flat acoustic array with a flexible prism [14] or with a water-immersed camera or flexible membrane [15][16][17][18][19]. In these cases, first, the object surface profile is identified; then, the required delay laws are calculated for each transducer-receiver couple using the Fermat principle [20,21] for the restoration of the inner structure of an object under inspection using the TFM.…”

Adaptive Ultrasonic Full Matrix Capture Process for the Global Imaging of Complex Components with Curved Surfaces

“…Previous researchers have used flexible probes [ 9 ], ice coupling [ 10 ] and other methods to conduct FMC-based ultrasonic testing of curved surfaces. The most economical and convenient method is to immerse the workpiece and a flat probe in water and use an imaging algorithm adapted to the curved surface for imaging [ 11 , 12 , 13 ]. Among the existing FMC imaging methods, frequency domain reverse time migration (FD-RTM) is a new technology with the smallest artifacts and the highest resolution [ 14 ], and it is the imaging technology used in this article.…”