Solution
The feasibility study was carried out on a manufactured fillet weld, with known embedded defects, which will serve as a benchmark for future ACFM inspections. The inspector scanned the sample, measured the indications and analysed the results, in accordance with the ASTM Designation E2261-07: Standard Practice for the Examination of Welds Using the ACFM Technique Standard. Due to the size of the weld cap, three scans were carried out; one scan on each toe and one scan on the weld centreline.
These three scan lines were sufficient to cover the entire cap and part of the adjacent weld parent material. The right-angled ACFM probe was used to adapt to the geometry of the sample. The technician inspected both a painted fillet weld (figure 1) and an unpainted fillet weld (figure 2).
Another ACFM inspection was conducted on a second sample that had three defects, confirmed by liquid penetrant inspection (LPI). In the unpainted sample, the defects were found very easily and their signature was easily defined. So, the client coated the sample, as per their procedure, and TWI repeated the inspection on the coated sample. The signature was less defined, but it was still sufficient enough to detect and measure the defects accurately (figure 3). The calculated depths differed between the coated and uncoated samples, which the inspector confirmed as a result of a decrease in amplitude, due to the coating.
Therefore, from the feasibility study, TWI concluded that ACFM was an effective technique for detecting defects in all parts of the weld, even with the decreased signal amplitude, as you can add compensation to the ACFM software, which improves the calculated depth. It is essential to confirm the thickness and nature of the paint and to include this in the scanning or analysis stage.
As it was proved feasible to leave the coating on, TWI felt confident in using ACFM to inspect the client’s tank floors, on-site, without removing any surface coating. During the inspection, the inspector followed a written procedure. On one tank, the inspector scanned 19.050m, including toes and weld centrelines, working clockwise from the circular entry plate. Due to the length of the tank, the inspector scanned 500mm sections, overlapping slightly to capture all areas. In some areas, the cap width was larger, so the inspector took additional scans on the cap. In one area, the inspector detected a defect (figure 4), and magnetic particle inspection (MPI) was used to confirm the indication.
TWI completed the data-analysis post-inspection and added compensation for the paint thickness to the analysis software during the characterisation process, as recommended by the feasibility study.
A report with defect sizing and defect depth was provided containing information of the indications and their measurements. The measurements obtained from the ACFM are calculated with the ASSIST software; the embedded algorithms in the software assume the indication is semi-elliptical. However as the depth with ACFM is estimated, the defect orientation, shape and value are to be review on the case-by-case basis.