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TIME OF FLIGHT DIFFRACTION (TOFD)Time of Flight Diffraction (TOFD) is a field-proven advanced ultrasonic inspection technique developed in the 1970s for accurate crack-sizing in nuclear applications. TOFD examinations rely on the time of flight, rather than the amplitude of the ultrasound signal to size the defect and are therefore not as sensitive to defect orientation. A minimum of 1 set of 2 TOFD probes are used for a scan. However, when examining a weld, 2 extra sets of shear wave probes pointed at either side of the weld root and cap are often used, especially for thicker material. The software displays the TOFD data and shear wave results together in a strip chart.
TOFD can be performed on ferrous and non-ferrous alloys and on non-metallic materials, depending on their acoustic transmission capabilities.
TOFD is commonly performed on welds of piping or vessels. TOFD examinations:
- Give a computerized image and record of the inspection results.
- Allow more accurate sizing of defects than traditional ultrasonic inspection techniques.
- Facilitate characterizing lack of fusion defects or other planar flaws (cracks) since sizing is not sensitive to defect orientation.
- Are typically very fast once all of the probes have been positioned and setup on a rail. In many cases, an entire weld can be inspected with a single pass of the rail.
TOFD is also used for inspecting fabricated piping and pressure vessel welds. It can be used in lieu of radiography or manual ultrasonic inspection. Weld acceptability is determined based on the:
- ASME B31.3 specified rejection criteria for piping.
- ASME Code Case 2235 for pressure vessels.
TOFD requires access to both sides of the weld, typically on the OD surface. For extremely thick materials (4 or 5 inches) and depending on the setup it may be necessary to have as much as 10 or 12 inches of clean, smooth surface on either side of the weld. Clearance of about 5 inches around the pipe or vessel is necessary to allow the TOFD probes to pass. High temperatures require use of high temp probes, specialized couplant and a duty cycle to protect the equipment.
The processed results are presented as picture of a vertical slice through the weld that is similar to a B-scan in conventional ultrasonics. Scan lengths are measured by an encoder and the scan images are labeled accordingly. This allows for easy reference and repeatability of future inspections. The data file created is accessible at any time for additional analysis or reference. Test results are interpreted in accordance with current specifications and/or customer requirements. The reports are prepared from standard laptops and are normally presented to the customer at the end of the shift before the crew has left the site; a formal report is issued later after peer quality assurance reviews.
