ADVANCED MATERIALS & PROCESSES | OCTOBER 2024 19 clearer images of indications that better represent their actual shape and orientation. It is important to note that TFM pulse-echo modes (such as TT, LL, TTTT, and LLLL) are like standard PAUT in that part geometry variations could affect the precision of the provided position of detected indications. Indirect modes (such as TTT, TTL, LLL, LTL, and TTTTT) yield additional scan sets compared to standard PAUT. However, results are very sensitive to geometry variations. ORTHOTROPIC DECK CASE STUDY In the rib-to-deck assembly of the top part of the orthotropic steel deck, ribs are connected to the deck using partial joint penetration (PJP) in a half V at a 60° to 80° angle. The connection angle to the deck and the rib’s wall thickness depend on the specifications. The Load and Resistance Factor Design standard of the American Association of State Highway and Transportation Officials (AASHTO) states that, “The one-sided weld between the web of a closed rib and the deck plate shall have a target penetration of 80%, with 70% minimum and no blow-through, choosing the proper probe and wedge combination, positioning the probe as close as possible to the weld, using a sectorial scan with optimal focus and gain, validating accuracy through technique qualification, and maintaining probe position along the scanned distance with an auxiliary scanner. When used properly, phased array ultrasonic testing (PAUT) offers several advantages over conventional methods including penetration percentage measured at all positions in the weld, good repeatability due to less subjectivity in acquisition and analysis, and faster acquisition times. Further, PAUT can be adapted for different joint configurations with accuracy of the weld penetration percentage achieving ±5% compared to macro etch testing. That said, PAUT also has limitations. These include sensitivity to the index offset and weld profile (e.g., an oversized weld cap), the tedious nature of interpreting certain flaws such as undercuts, and inconsistent penetration for long welds. TOTAL FOCUSING METHOD: HOW IT WORKS The total focusing method (TFM) is an image reconstruction process that uses the elementary A-scans generated through full matrix capture (FMC) data acquisition. The FMC pulse/receive sequence is designed to gather a large amount of waveform data using a single phased array probe, which the TFM algorithm can then process to create a fully focused image (Figs. 1 and 2). PROS AND CONS OF PAUT AND TFM The recommended PAUT scan type for partial penetration inspections is the sectorial scan, which generates focal laws of different angles pulsed by the same elements. The A-scan density and coverage is defined by the range of angles (45° to 70°) and angle resolution (45°, 46°, 47°, and so on). This scan strategy is advantageous because despite the probe’s small surface footprint, multiple beam angles provide a large coverage area within the inspected part. Having multiple angles also eases flaw characterization, and only a few focal laws are required to cover the entire weld. However, beam-to-beam resolution worsens as the sound path increases, which means that inherent difficulties arise when calibrating for longer sound paths. TFM is engineered to be focused everywhere in the TFM zone provided that the probe configuration and zone settings enable focusing in the near field. Thus, when inspecting in the near field, geometrical echoes are reduced compared to phased array results. TFM is also known for its capacity to provide Fig. 1 — In the FMC/TFM process, data is collected by pulsing one element and receiving on all elements (repeated until each element is pulsed), creating A-scan data for every receiving element. Fig. 2 — The TFM image, or zone, is composed by calculating the time of flight according to inspection mode (pulse echo or indirect) and selected wave type (longitudinal and/or transverse). Fig. 3 — Scan configuration and S-scan example.
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