Time of Flight Diffraction (ToFD)

Time of Flight Diffraction (ToFD) is a highly sensitive ultrasonic technique specifically designed for accurate flaw detection and height sizing in welded joints. QMS Asset Integrity is NATA-accredited to perform ToFD on ferritic weld joints using both automatic and manual techniques.

Unlike conventional UT which relies on reflected amplitude, ToFD measures the time taken for diffracted signals from crack tips. This gives highly accurate through-wall sizing of planar defects - data that is essential for fitness-for-service and engineering critical assessments (ECA) under standards such as AS 3788 and API 579.

ToFD is frequently paired with PAUT to provide a comprehensive weld inspection package. Together they offer full volumetric coverage with both detection and sizing capability, reducing the need for radiography and its associated radiation safety exclusion zones.

ToFD uses a pair of probes - one transmitter and one receiver - positioned either side of the weld centreline. The transmitter emits a broad, unfocused ultrasonic beam. The receiver detects two reference signals (lateral wave and back-wall reflection) plus any diffracted signals generated at the tips of through-wall flaws.

By measuring the difference in arrival times of these diffracted signals, the technician can accurately calculate the vertical position and height of a defect within the weld - typically to an accuracy of ±1 mm. Data is recorded as a D-scan (greyscale image) for full traceability.