NDTnet 1998 Aug, Vol.3 No.8

Abstract

We present a novel method, called VIGRAL, to reconstruct, off-line, the reflecting surface of a flaw and characterize it. The method operates on standard B-scans recorded with traditional transducers, to extract only Time-of-Flight (ToF) information which is then back-projected .The VIGRAL approach represents the flaw as an ensemble of virtual point sources. Consequently, the B-scan data, collected by scanning a transducer along the material's surface, relates to the distribution of these virtual sources. Using inverse-problem methods, the VIGRAL algorithm locates a series of point sources that would generate the measured experimental B-scan form. We show here that the distribution of such virtual sources differs for different flaw shape and can be used to characterize and accurately size defects.

To evaluate the VIGRAL method in a standard pulse-echo arrangement we scan a steel testblock with side drilled holes (SDH) and a flat-bottom hole (FBH). Using a upi-50 ultrasonic instrument and a Flexitrak scanner with a 2 MHz, 45° shear-wave contact probe we record, one-line B-scan images for each flaw. The scan pixel size is /4, (0.4 mm), and the A-scan sampling period 10 ns (100 MSPS). The B-scan data is processed off-line in two stages; first the ToF of the echo at each scan-pixel is evaluated. Then the form of the ToF Vs. scan position is back-projected to determine the desired distribution of virtual sources.

Figure 1 compares the virtual source distributions for a FBH [Fig. 1(a)] and a SDH [Fig. 1(b)]. Whereas strong edge-reflections, with an accumulation of many virtual sources at either end characterize the former, the latter displays a center-peaked reflection pattern that is typical of a rounded reflector. The extent of the virtual source distribution relates to the size of the holes. Table 1 compares VIGRAL-based sizing to sizing by the half-amplitude method.

Fig. 1: FBH (a) & SDH (b) distributions

Table 1: VIGRAL and half-amplitude sizing results.


Defect	Measured size
Diameter and depth	Half-amplitude	VIGRAL
SDH, ø 2, h=15 mm	6.8 mm	2.0 mm
SDH, ø 2, h=44 mm	9.2 mm	1.9 mm
FBH, ø 4, h=15 mm	15.2 mm	4.4 mm

We conclude that the VIGRAL algorithm offers accurate sizing of flaws and generates theform of the radiation pattern from the flaw. In this and other experiments we show that theVIGRAL can size the extent of the reflecting surface to within ± /2. It can also be used todifferentiate between flat flaws, such as cracks, and volumetric flaws, such as inclusions.

Abstract Source:
Book of Abstracts, 7th European Conference on Non-Destructive Testing, 26-29 May 1998, ISBN: 87-986898-0-00

Full-Text Source:
Proceedings of the 7th European Conference on Non-Destructive Testing, 26-29 May 1998, ISBN:

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