Time Of Flight Diffraction (TOFD)

Time of Flight Diffraction (TOFD) is an advanced method of Ultrasonic inspection that leverages on existing UT methodologies thereby bringing a whole new range of testing and detection capabilities to the NDT forefront. TOFD is an excellent and widely accepted pre-service and in-service inspection tool. It does not rely on the reflectivity of the flaw but uses the diffracted sound initiating from the flaw tips. This means it can detect flaws irrespective of their type or orientation.

Two ultrasonic probes are used in TOFD. One transmitter and one receiver on either side of the weld. A longitudinal sound beam is transmitted into the test object. Part of this will be reflected or diffracted by any obstacles along its path before it reaches the receiver. TOFD provides a wide area of coverage with a single beam by exploiting ultrasonic beam spread theory inside the wedge and the inspected material. The probes are moved in a parallel motion along the weld. The resultant waveforms are digitized, stored on hard disk and displayed on a video-screen as a grey scale image. The image that we get is, in effect, a cross-sectional view of the examined weld. This can be used for the accurate measurement and detection of indications. All the data that is stored and can be used in future also.

Applications

Can inspect Weld, forge, wrought parts
Pre-service and In-service inspection
Weld inspection of pressure vessels, pipelines, plates, storage tanks and spheres
Detection of corrosion underneath reinforcement rings of nozzles
Cladding thickness measurement
Monitoring crack propagation
In-service inspection at ambient or high temperatures for the detection of damage mechanisms

Advantages

TOFD accurately detect and size cracks, slag, fusion faults and other defects, relatively indifferent to weld bevel angles and flaw orientation
Complements PAUT
Quick to set up and perform an inspection, as a single beam offers a large area of coverage
The conventional nature and use of conventional probes makes it economical
Rapid scanning with imaging and full data recording
No safety hazards and no interruptions or delays in other operations because of lack of radiation
Inspection rate is nearly independent of the wall thickness of the object

Codes and Standards

ISO/DIS 10863:11, Welding – Use of time-of-flight diffraction technique (TOFD) for examination of welds
EN 583-6, Non-destructive testing – Ultrasonic examination – Part 6: Time-of-flight diffraction technique as a method for detection and sizing of discontinuities
EN 15617, Non-destructive testing of welds – Time-of-flight diffraction technique (TOFD) – Acceptance levels
BS7706
EN14751
ASME SEC V Article IV Mandatory Appendix III, Non mandatory Appendices N,O
ASTM E-2373