Title: MULTI-SAFT: A FLEXIBLE METHOD FOR DEFECT CHARACTERIZATION
Abstract
An ultrasonic method, based on the Synthetic Aperture Focussing Technique (SAFT), has
been developed for the characterization of inhomogeneities in steel. Such a characterization
implies the determination of the location, size and shape of a detected inhomogeneity.
Multi-SAFT is a flexible imaging technique to be applied in a wide variety of practical
situations. E.g. defect characterization in thin steel plates (flat or cylindrical), but also defect
imaging in complex geometries are within the potentialities.
The main features that distinguish Multi-SAFT from conventional SAFT are:
- Separate source and receiver are used, where the source is fixed and the receiver is
scanned individually. This has the advantage that the position of the source can be optimized
to achieve optimal insonification of the defect.
- Due to the applied split angle configuration, the scanning aperture of the receiver can be
increased.
- The focussing algorithm has the possibility of taking into account the multiple internal
reflections in the object under investigation.
The imaging and characterization performance of Multi-SAFT has been experimentally
evaluated on real weld defects in steel plates. Both planar and cylindrical plates were
inspected, varying in thickness from 6 to 40 mm. Images were obtained from lack of fusion,
lack of penetration, slag inclusion and several types of cracking. These experiments have
shown that the image quality is sufficiently high to allow accurate defect
characterization.
In addition to these experiments, a comparison study of Multi-SAFT versus conventional
techniques, e.g. x-ray and manual ultrasonic, was performed. Thin steel plates (6 - 12 mm)
with defects (types: lack of fusion and slag inclusion) served as inspection objects.
Multi-SAFT showed its strength with respect to defect sizing and localisation. Moreover, it
appeared to be a very promising technique for discriminating between flat and volumetric
defects.
Author
Machteld de Kroon, Peter-Paul van 't Veen, Hugo Vos
TNO Institute of applied physics, Inspection Technology Department
P.O. Box 155, NL-2600 AD Delft, The Netherlands
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Rolf Diederichs 1. Jan 1997 info@ndt.net