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Chapter 2
CHAPTER
2
Ultrasonic Testing
This chapter gives an overview of the basics of ultrasonic testing techniques,
classification of ultrasonic probes and their applications, the main defect that occurs in
the steel industry and the existing ultrasonic testing system and method used to detect
them.
Ultrasonic detection techniques have developed at a tremendous rate to fulfil the
requirement for high quality defect free material in many industrial and research
applications. The basic method is the Conventional Pulse-Echo Method. As dictated by
its name, the Pulse-Echo method use an array of probes in which each probe pulses and
receives sequentially. This method was replaced gradually by some new techniques
based on Time-of-Flight Diffraction (TOfD), because these new methods collect extra
information from diffracted signal which was previously discarded by the pulse-echo
detection.
When Ultrasound is incident at a linear discontinuity such as a crack, diffi-action takes
place at its extremities in addition to the normal reflected wave. This diffracted energy
is emitted over a wide angular range and is assumed to originate at the extremities of the
crack. This is in marked contrast with conventional ultrasonic detection which relies on
the amount of energy reflected by discontinuities.
The TOFD technique, which is an off-line method, uses angled beam probes to pick up
the diffracted signal, while the NPD technique can get the same result with normal
probes by using multi-receivers, thus enabling the method to be implemented in realtime on a production line.
Both the TOFD"aird NPD"ultfasonic~techniqueT-elies~^^^
"diffraction of ultrasonic
energies from 'comers' and 'ends' of internal structures in a component under test. This
is in contrast to the conventional pulse echo method which relies on directly reflected
signals from internal structures and only can test the depth of the defect, while the
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