TOFD is a field-proven advanced ultrasonic inspection technique developed in the 1970s for accurate crack-sizing. 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.
TOFD requires access to both sides of the weld, typically on the OD surface and can be performed on ferrous and non-ferrous alloys and on non-metallic materials, depending on their acoustic transmission capabilities.
The processed results are presented as picture of a vertical slice through the weld that is similar to a B-scan in conventional ultrasonic. The software displays the TOFD data and shear wave results together in a strip chart. Scan lengths are measured by an encoder and the scan images are labelled 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.