Diffraction Flaw Sizing Page



Time Of Flight Diffraction Based Flaw Sizing



Introduction
The TOFD ultrasonic technique relies on the diffraction of ultrasonic energies from 'corners' and 'ends' of internal structures in a component under test. This is in contrast to conventional pulse echo techniques which rely on directly reflected signals from internal structures. The advantage of this method is that the measurement of flaw depth depends only on the sound velocity along the illuminating path and the diffraction path. It is therefore much more accurate at sizing flaws than the more traditional methods. It's disadvantage is that diffraction waves are less in amplitude and sensitivity to flaws (detection) is correspondingly less.

The scans
Each scan is of a 50% complete submerged arc weld between two flat steel plates (80mm thick). The scans have been generated at Babcock Technology Centre, Renfrew, UK, for the EC BRITE-EURAM II project "NDT methods for flaw detection during welding" using a Zipscan system. The defects shown in the scans have been deliberately inserted so that image processing and pattern recognition methods may be developed to provide automatic sentencing.

The content of each scan has been cropped around the lateral (surface) wave and the backwall echo, at the scan top and bottom respectively.

A - D-scan (side elevation) showing inclusion defect.

B - D-scan (side elevation) showing porosity (top) and lack of side-wall fusion (single arc near centre).
These images are the work of
Shaun Lawson
Mechatronic Systems & Robotics Research Group
Dept. of Mechanical Engineering
University of Surrey
Guildford UK
GU2 5XH
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