| BS 7706: 1993 Contents | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Committees responsible | Page
| Foreword | 2 | Guide
| 1 Scope | 3 | 2 References | 4 | 3 Definitions | 4 | 4 Principles of the method | 5 | 5 Equipment | 8 | 6 Setting-up procedures for flaw detection | 10 | 7 Interpretation of the data | 17 | 8 Estimation of flaw dimensions after flaw detection | 19 | 9 Setting-up procedures for accurate flaw sizing | 20
| 10 Confirmatory measurements | 22 | 11 Accuracy of measurement | 22 | 12 Training and qualification | 24 | Annexes
| A (informative) Special techniques based on TOFD | 25 | B (informative) Application of TOFD and reporting criteria | 25 | C (informative) Suggested steps towards the characterizstion of flaw echoes in TOFD | 27 | D (informative) Examples of typical scans | 33 | E (informative) Suggested framework for training and qualification | 35
| Figures
| 1 D-scan (probe movement normal to beam) | 3 | 2 B-scan (probe movement in direction of beam) | 3 | 3 Diffraction from a crack-like flaw | 6 | 4 Typical pattern of pulses from an internal flaw | 7 | 5 Generalized two probe testing geometry | 8 | 6 Calibration block for TOFD using V notch or electrodischarge machined slit | 14 | 7 Gain setting using grain noise | 16
| List of references | Inside back cover
|
Annex D | Examples of typical scans Zoom All 90 KByte | Zoom Each
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Far surface crack
| Near surface crack
| Lack of root penetration
| Lacke of side wall fusion
| Lack of inter-run fusion
| Slag
| Porosity
| Concave root / HI-LO
| Toe cracking / tearing
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Images are not copies from the original paper; they are drawings
illustrating the various concepts. If you want to
view the exact images please refer to the order information of BSI below.
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This revision incorporates updated guidance which has resulted from the increased application of the ultrasonic time of flight diffraction technique (TOFD) in industry. The TOFD technique was originally developed at the National Non-destructive Testing Centre at Harwell and has since seen application in a variety of industries. The primary objective of this British Standard is to facilitate the wide implementation of this relatively new non-destructive testing technology in industry. It is envisaged that this British Standard will in due course be superseded by standards that will detail the methods for TOFD technique examination for specific applications.
This British Standard provides guidance on the application of TOFD technique for the detection, location and sizing of flaws in materials. It may be expanded to cover other important applications of TOFD.
It is most important to bear in mind that echo amplitude is not used
quantitatively by this technique for the determination of flaw size. The
conventional amplitude-based flaw sizing associated with ultrasonics is thus
largely invalid when this technique is used although it may be used for further
discrimination.
The echo amplitude is only employed in TOFD measurements in connection with
three tasks.
- Where the pulses from the flaws being sought have to be resolvable against
the local noise background, for instance the ultrasonic grain scatter. To ensure
that this is the case (or to place limits on the validity of a particular
measurement) the known or experimentally determined variation of pulse
amplitude as a function of range, position in the ultrasonic beam, etc. may be
invoked.
- Where the relative amplitudes of echoes from the top and bottom
extremities of flaws may provide important clues about the character of the
flaw.
- Where the amplitude of the pulse from an artificial flaw can be used to confirm that system calibration has not changed during a measurement or from measurement to measurement.
The use of TOFD should be considered in any inspection task requiring rapid and reliable flaw detection coupled with an accurate assessment of flaw size. It can be adapted to many test geometries with little difficulty but special instructions and equipment may be needed for the inspection of complex geometries such as nodes or nozzles. Care should also be taken if the technique is considered for use on coarse-grained or ultrasonically difficult materials.
It is recognized that equipment suitable for carrying out the TOFD technique is not yet widely available and this British Standard, which is written in sufficiently general terms to be applicable to the types of equipment currently known to be employed for TOFD, describes the nature of the equipment required and also its calibration.
It is assumed in the drafting of this British Standard that the execution of its provisions is entrusted to appropriately qualified and experienced non-destructive testing personnel.