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technology and its related consultancy services.
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
TOFD overcomes both techniques in terms of speed of examination
and higher accuracy.
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
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
The more accuracy guaranteed by TOFD in sizing thru wall extension of
flaws allows more reliable fracture mechanic calculation for the residual
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.