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What is TOFD (Time of Flight Diffraction)?

The TOFD technique was first applied in 1985 at the Harwell Center (UK) in response to insistent requests to size cracks in nuclear reactor welds.
The TOFD technique is a fully computerized system able to scan, store, and evaluate indications in terms of height, length, and position with a grade of accuracy never achieved by other ultrasonic techniques.
The TOFD technique is based on diffraction of ultrasonic waves on tips of discontinuities, instead of geometrical reflection on the interface of the discontinuities.
This phenomena makes TOFD ideal for identifying cracks, lack of fusion located along the vertical axis of the weld (in particular for narrow gap preparation) or with any other orientations, because detection is not affected by the negative consequence of ultrasonic beam deviation from the receiver due to unfavorable orientation of the discontinuity.
These features have extended the use of TOFD to replace Radiography and complex Ultrasonic inspection by tandem technique wherever planar defects (cracks, lack of fusion) are the main object of examination.
The main disadvantages to both the aforementioned techniques are expensive manipulation for Radiography on big parts and the amount of time required to perform the tandem technique due to the accuracy necessary for reliable testing.
TOFD overcomes both techniques in terms of speed of examination and higher accuracy.

TOFD Norms

Experience acquired in application of TOFD has fully demonstrated its reliability and detection capacity with respect to other methods. This has induced potential users to establish procedures for coding TOFD in recognized standardized norms.
The British Standard Institute has issued the first draft: BS No 7706, ASME Code has included in Section V, art. 4, Appendix E, the computerized ultrasonic system and TOFD technique for ultrasonic examination of welds.
An "ad hoc" Workgroup has been nominated from within the ASME Technical Committee to prepare a document establishing the conditions for using computerized systems, such as TOFD in replacing Radiography examinations of welds, in the over 4" thickness range (see Code Case).
CEN (Comite Europeen de Normalisation) has issued the document CEN/138/WG on practical application of TOFD for ultrasonic examination of welds.

TOFD Applications

One of the major applications of TOFD is the ultrasonic examination of welds after final heat treatment and/or hydraulic testing, to verify the absence of cracks not detectable by Radiography and to prove conformity with prior ultrasonic manual examination carried out during construction.
In addition to higher sensitivity, the TOFD technique allows the full examination data to be recorded on hard disk, displaying all discontinuities in C/scan images. This enables "off line" evaluation of indications by computer using dedicated software. Very accurate sizing of defects can be achieved and printed for documentation.
Another advantage and current application of TOFD is its use in monitoring welds during the service life of components. Stored data acquired from initial examinations, made during the final stage of construction, can be compared with new data obtained from in-service inspection. This allows the stability of existing indications to be determined with high accuracy and reliability.
The more accuracy guaranteed by TOFD in sizing thru wall extension of flaws allows more reliable fracture mechanic calculation for the residual life evaluation.
Companies like Exxon, Shell, Fluor Daniel, Texaco, Chevron etc. are using TOFD to replace radiography in examination of welds after PVMT or hydraulic testing. The main inspection agencies have approved procedures of the TOFD Technique and its validation.