The Time-of-Flight Diffraction (TOFD) method can be regarded more and more as a standard NDT technique in
an increasing number of countries. Nevertheless, there is still a considerable amount of development work going
on. An overview will be given of the most relevant recent developments, including advances in TOFD
transducer technology, modelling and interpretation tools, inspection of complex geometries and latest
applications. In addition the current status of appropriate codes and standards will be reviewed.
Although most TOFD inspections are performed on welds of simple geometry, such as longitudinal and
circumferential welds, it is of great advantage if also the nozzles present in e.g. a pressure vessel can be
inspected in the same go, and with the same technique. Therefore, systems for TOFD inspections must be
capable of inspecting such geometries, which requires special software tools. Modelling software for the
inspection of complex geometries will be discussed.
General acceptance of the TOFD technique will only be possible if also the formal side of the matter is
adequately organised. Initiatives in the Netherlands to establish acceptance criteria for weld imperfections
detected with TOFD are now being materialised, and a qualification scheme for TOFD operators is being
implemented. The status of these developments will be discussed.
The Time-of-Flight Diffraction (TOFD) technique is rapidly gaining importance as a stand-alone inspection
technique. The TOFD technique is an advanced ultrasonic inspection technique that fulfils a need for reliable
inspections. It is a powerful technique because it can simultaneously detect and size defects. TOFD is now
routinely used in a wide range of applications such as inspection of piping and pressure vessels.
What type of information will TOFD generally provide? A routine inspection provides the operator with the
Information on the type of defect is limited, similar to both manual and mechanised ultrasonic inspection. This is
not necessarily a problem, because from a fracture mechanics point of view, many defects that are normally
regarded as volumetric, such as slag inclusions, may be sharp enough to be able to act as crack initiators.
Also, this aspect is compensated, to a certain extent, by the ability of TOFD to distinguish between "defects without
measurable height" and "defects with measurable height".
- location of indications along the weld
- position of indications with respect to the scanning surface (depth information)
- through thickness extent, if exceeding a certain value (typical 2-mm), which means that porosity and slag will
generally be detected as "indications without measurable height"
- information on "embedded" or "surface breaking" character of a defect
This, in combination with the high probability of detection (POD) and a low false call rate (FCR), makes TOFD a
valuable tool for routine NDT, in the sense of "Good Workmanship evaluation supported by fracture mechanics
considerations" . Recent developments of codes and defect acceptance standards for use with the technique support
this tendency and is resulting in a wider acceptance every year. Apart from the growing realisation by the market
that TOFD offers unique advantages, these developments are also stimulated by the realisation of the five main
- Equipment improvement
- Transducer performance improvement
- Initiatives to establish the qualification of TOFD operators
- Acceptance of TOFD in weld and vessel inspection
- Implementation of new codes and standards
2. Short introduction to the technique
The principle of the TOFD technique is based on mapping of the position of the edges of defects. This is in marked contrast with conventional ultrasonics, which relies on the amplitude of specular reflections received from defects. To determine the position of the defect edges, a wide-beam transmitter probe is placed on one side of the weld under inspection. An identical receiver probe is placed on the other side of the weld (figure 1). In the case of a
flawless weld, the relevant ultrasonic signal will consist of a so called "lateral wave", corresponding to the direct surface path between transmitter and receiver, and a back wall echo. Between the lateral wave and the back wall echo there will be no signals. If defects are present in the weld, however, the ultrasonic waves will be diffracted by
the edges of the defects. The depth of the defect edge can be calculated from the time of flight of the corresponding
ultrasonic wave. Defect height can be readily measured by subtraction of the depth of lower and higher defect edge.
Fig 1: the principle of TOFD inspection on a welded plate|
Like any other technique, TOFD also has its limitations. In general the technique is less suitable for coarse grained
materials such as many types of austenitic steel. Inspectability with TOFD is usually assessed on a case-by-case
basis. In addition, inspection reliability close to the scanning surface is hampered by the presence of the lateral
wave, which may obscure defects present in this area. Specialised software algorithms can be used to minimise this
effect. Similarly, small defects located very close to the root of the weld may sometimes be obscured by irregular-ities
in the root such as mismatch.
General acceptance of the TOFD technique will only be possible if it is also properly embedded in codes and
standards. With more and more experienced gathered the potential for the technique is rapidly increasing.
Present developments in the codes, defect acceptance standards and personnel qualifications support this
3. Key factors in the evolution of TOFD
During the last few years much effort has been invested in activities intended to place TOFD in the market as a
stand-alone NDT technique. One of the factors that drove this process was the development of acceptance
criteria in the Dutch PMP joint industry project. For a technique to become generally accepted, the following
three main elements have to be available:
Once established, these should result in general acceptance of the technique for an increasingly wide range of
applications. Recently, new TOFD applications on 13%Cr steels, Titanium welds and even welds in some Duplex
steels were successfully validated. Special software has been developed, to enable the use of TOFD on welds with
complex geometry such as nozzles and offshore nodes  .
- A procedure that describes how is data generated. This is the work procedure, which can be validated in
combination with a reference block. The new generation of reference blocks contain an extra small hole just
below the near surface. Detection of this hole (upper and lower signals) demonstrates that the object is
inspected properly, which means that even the weak area of the technique is covered; if not, sometimes an
extra scan of the surface zone may be required.
- A standard that includes acceptance criteria: these prescribe how recorded inspection data should be
interpreted. A set of acceptance criteria is therefore needed. These have been developed in the said joint
- Personnel qualification. Initiatives in the Netherlands to establish a qualification scheme for TOFD-operators
are currently being implemented. Inspection companies in the Netherlands have joined together in
a committee to establish a procedure that rules the theoretical and practical TOFD training as well as the
The current generation of Portequip equipment was designed for flexible operation with various configurations
ranging from a lightweight package for immediate mobilisation to a fully extended set-up for large and difficult
inspections, including complex geometries under extreme weather conditions. The developments in the
equipment were primarily driven by the need for portability, signal quality and operational flexibility. This
means faster hardware, higher pulse repetition frequency (which means higher scanning speed) and compact less
power consuming hardware. In addition new windows-based software makes the analysis and processing of
recorded data a lot more flexible. This in combination with improved filtering techniques and higher bandwidth,
allow maximum advantage of high frequency probes. Mil-spec notebooks have replaced the large computers, see
Fig 2: new generation RTD TOFD equipment|
The development of TOFD equipment also included a new release of RTD PTOFD software. From the new
Windows-based version 8.0, tools such as database, spreadsheet and word processors for field reporting are
directly accessible. This set of tools also includes a ray-tracing module for the inspection of geometries such as
nozzles and nodes. Software tools such as straightening, lateral wave removal and hyperbolic cursors that were
previously available have remained.
A new feature is that the inspection can be made available to the client on a CD-ROM, which can also include a
special viewer version of the software. This enables the client to view inspection results on their own PC, and
includes interpretation facilities and the possibility to export pictures for his own documentation and storage in a
database. If required, the client's personnel can be trained in TOFD data interpretation.
6. Codes and Standards
TOFD has always been the best NDT method to provide information for fracture mechanics and engineering
critical analysis (ECA) calculations. It was traditionally used to back up more conventional inspection methods.
Its increasing use as a primary inspection method has historically been limited by a lack of nationally or
internationally recognised codes and standards that can be used with the method. This situation has progressively
improved over the years, to the point where the inspection, interpretation and acceptance are well controlled.
As the method was first devised in the UK, it seems appropriate that the UK should have been the first country to
produce a standard for inspection using the method, British Standard, BS7709. This was essentially a 'how to'
document, fitting for the first National standard in any method. Standardisation has progressed from this national
code to an international effort involving RTD, as the work on TOFD spread beyond the British shores to Europe.
The change was marked by the issue of a European prenorm prENV 583 Part 6, a co-operative international
With these in place, the development of suitable acceptance criteria became vital. This has progressed by both
the suitable adaptation of existing radiographic and ultrasonic codes and the introduction of specifically drafted
acceptance criteria. Examples of the former include Stoomwezen Rules T-0117 and British Standard BS5500.
Though both of these were originally drafted for use with conventional pulse-echo inspection (and include echo
amplitude requirements), the specification of flaw heights and other dimensions has allowed them to be used for
TOFD in their current form. Finally, there is the issue of the joint industry project mentioned earlier, that gives
TOFD a consistent set of criteria agreed across a range of industries.
The two basic requirements for suitable TOFD probes are high sensitivity and high bandwidth. The use of
alternative crystal materials like Piezo Composites offers a significant improvement. Piezo Composites are
especially appreciated in broadband applications.
Another improvement is the development and design changes for high temperature probes. New wedge materials
show less attenuation and new crystal designs show higher sensitivities. High frequency probes (up to 20MHz)
can reduce the "dead zone" and increase the part of the volume of the material that can be inspected. The use of
probes with frequencies higher than 15MHz strongly depends on smooth surface conditions, sound attenuation
and ultrasonic material noise of the object to inspect.
Reducing the dead zone increases inspectability of the zone just below the scanning surface. Although this
indeed improves detection of test holes just below the surface of test blocks, according to recent code
requirements, one should be careful in establishing validity under practical circumstances. The theoretical
limitations in near surface flaw detection will always remain, because these are directly dependent on probe
separation and probe centre frequency. In addition, the capabilities of the technique are influenced by
geometrical factors such as surface condition and the presence of mismatch. Therefore extremely small dead-zones
such as 1 mm can only theoretically be reached. Nevertheless it is without doubt that current software
tools, high-resolution probes and miniaturised front-end electronics close to the probes help in achieving the best
possible resolution close to theoretical limits.
Over the years, many fingerprints of installations have been taken during their construction. The re-inspection
after years of service shows that TOFD is highly reproducible, which makes the technique extremely suitable for
monitoring of objects. TOFD has proven to be a valuable and economically attractive quality control tool for
new objects. This can be indicated by the following two examples. Figure 3 is taken as fingerprint of a heavy
wall pressure vessel 5 years ago, during manufacture. Figure 4 is taken during maintenance inspection this year.
It can also be observed that the more recent image is smoother and has higher resolution and signal to noise ratio,
due to improvements in equipment and ultrasonic probes.
Fig 3: object five years ago Figure
Fig 4: recent examination of the object|
9. Complex Geometries
A perhaps less well-known but powerful capability of TOFD is the inspection of complex geometries such as T-joints,
nozzle attachment welds and offshore nodes, which may be very difficult or even impossible to inspect by
other NDT techniques. When TOFD is used, the scanning operation even on such geometries is relatively simple,
thanks to the presence of recently developed "smart" scanners, and provided probe configuration and probe
positions have been carefully planned. A powerful software package has been developed by RTD to assist the
TOFD operator and to optimise the reliability of the inspection.
Fig 5:example of a complex weld to inspect with ultrasound|
The Time-of-Flight Diffraction (TOFD) technique is becoming a more used and accepted technique in NDT. The
main developments can be described by the following items:
- TOFD transducer technology, higher frequency and improved resolution probes
- modelling and interpretation tools in the software, which further increase the range of applications and
- inspection of complex geometries, more experience and improved scanners
- codes and standards are now available to meet most inspection requirements
- a qualification scheme for TOFD operators is being implemented
soon TOFD will have gained its place as a standard technique in the NDT toolbox, next to other techniques
considered as conventional such as radiography and pulse-echo UT.
- H.H.J. Huynen, "Advances in Ultrasonic transducers", Röntgen Technische Dienst bv,
- F.H. Dijkstra, J.A. de Raad and T. Bouma, "TOFD & Acceptance criteria: A perfect team",
Röntgen Technische Dienst bv, Rotterdam, The Netherlands, 1998