FIFTH CANADIAN CONFERENCE ON NONDESTRUCTIVE ... - IAEA

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orientation) it is possible to compare it against a "tolerable" defect as
determined by fracture mechanics which could safely be left in the pipeline
with minimum risk of failure. This permits a realistic decision to be made
as to repairs and avoids unnecessary rework of welds.
Although ultrasonic inspection is capable of detecting significant defects
(many of which are missed by radiography), defect sizing is still a major
concern. Using standard transducers it is difficult to determine the size
of girth weld defects by standard mapping techniques(3). This arises
because the defect (or characteristic) being sized is comparable to or
smaller than the beam used for mapping. The tendency therefore is to
profile the beam rather than size the defect. Size predictions for a
defect would consequently vary depending on the size of the beam used.
One solution is to use a focused beam. This can be achieved simply and
economically by attaching an acoustic lens to the face of a standard
transducer. A test program was sponsored by the Pipeline Research
Committee of the American Gas Association to investigate the capability of
focused ultrasonic beams in sizing girth weld defects.
APPROACH
The main objective of the program was to investigate the defect sizing
accuracy of an inspection procedure using focused ultrasonic beams with
standard mapping techniques presently in use throughout industry. A
secondary objective was to evaluate the technique as applied to field
usage. Accordingly, the following approach was chosen:
i. Use standard ultrasonic transducers which are readily available
commercially "off-the-shelf".
ii. Use contact inspection (as opposed to immersion) requiring various
lens/wedge/probe combinations.
iii. Use a simple scanning device (e.g. linear potentiometers to monitor
probe position) so as to de-emphasize the equipment and concentrate
on the ultrasonic technique itself. (See Figure 1 for the inspection
equipment used).
Lens Design
Theory has been developed to design acoustic lenses for focused beams in
defect sizing (4,5). The fundamental equations of interest are:
1 - _ak
N
!|k _ 0.635 1-^1+0.2128 l-^l I [1]
(Deviation + 9%)*
* Deviation of analytical approximation from the true relationship