How do I determine the size of a reflector by an ultrasonic scan?

Estimating the size of a reflector is one of the foundational tasks in ultrasonic testing. Whether using manual techniques or automated systems, the approach relies on careful probe manipulation and analysis of echo amplitude behaviour. This article outlines the standard and advanced scanning methods used to determine equivalent reflector sizes (ERS), providing insights into practical techniques and interpreting results using threshold-based diagrams.

Manual scanning using beam movement

The oldest method of determining the size of a reflector ultrasonically is by scanning it using the sound beam of the probe. By “wandering around” the reflector its contours can be estimated. If this method is used on large flat reflectors (plate testing) then the echo indication, as compared to the maximum indication, decreases by exactly 6 dB if half of the sound beam strikes the reflector and half of it passes by (fig. 44). 

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Threshold-based methods for edge detection

If, by moving the ultrasonic probe, one looks for the 6 dB drop-off point then the axis of the beam points directly to the edge of the reflector (half-value method). The -20 dB method is also used. Here the probe is moved until the indication amplitude has dropped by 20 dB (de- crease in amplitude to a tenth of the maximum value) practically to zero. Then the whole of the sound beam passes by the reflector. If the shift is corrected by the width of the sound beam b' then one obtains the position of the edge of the reflector (fig. 45). 

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Limitations of standard methods and alternatives

Relative movement if the reflector is small in comparison to the diameter of the sound beam, then the half-value method described or the 20 dB method can no longer be used.

Sometimes however, in such cases, even the simple DGS-method leads to unusable results because the distance law of the reflector does not agree to that for a disc. 

For evaluating such reflectors there are special scanning diagrams for different reflector distances (fig. 46, 47). After scanning in two perpendicularly off-set directions the result will be an ellipse using these two dimensions. 

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Scanning a reflector can be done in two ways: 

1. Scanning with relative threshold 

First one looks for the location of the reflector which produces the highest echo and then moves the probes to both sides until the echo drops to half the maximum amplitude. This corresponds to a decrease in the echo amplitude by 6 dB. 

The distance between both these positions is the ‘half-value extension’. It is determined with the -6 dB threshold relative to the maximum echo. Fig. 46 shows the diagram of flaw scanning with the relative threshold.

Normalized sizes are used so that this diagram can be applied in general: The normalized reflector size G, the normalized probe movement B as well as the normalized distance Z.

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d = diameter of the ellipse, ERS in the scanning direction

b = half-value extension
Di = effective diameter of the

transducer

The following example shows how to use the diagram.

Given:

Di = 19 mm (effective diameter of the transducer)

Ni = 61 mm (effective nearfield length)

Measured values:

zr = 60 mm (distance to the reflector) b = 9,5 mm (half value extension)

Normalized measured values:

Zr = zr/Ni = 60/61 ≈ 1
B = b/Di = 9,5/19 ≈ 0,5

Evaluation:

the curve for Zr = 1 (fig. 46) is to be used for B = 0,5 the related value is G = 0,4 thus d = G ∙ Di = 0,4 ∙ 19 mm = 7,6 mm

The scanned reflector has an equivalent reflector size (ERS) of 7.6 mm in the scanning direction. 

Now the reflector is still scanned in a direction perpendicular to the first scanning direction and estimated in the same manner. Then the ellipse of both the determined equivalent reflector sizes is applicable as the equivalent reflector. 

2. Scanning with a fixed threshold 

Automatic ultrasonic testing installations cannot search for the location of a reflector which gives the highest echo. Sometimes the discontinuities are so fissured that the highest echo is difficult to find even when testing manually. In these cases, it is recommended that the evaluation be done using a fixed threshold. 

The probe displacement at which the echo of the reflector remains above this threshold is measured. A monitor gate can take over the control of the echo amplitudes. Fig. 47 is a diagram for flaw scanning using a fixed threshold: Here too the normalized actual reflector size G (equation 42), the normalized probe movement B (equation 43) and the normalized distance Z (equation 40) are used so that the diagram has a general application. 

In order to have a universal scale for all probes it is best to relate the position of the threshold to the gain V(0) of the DGS-diagram (fig. 40, 43). The threshold data in the diagram is be understood as follows: V(0) -10 dB; V(0) -20 dB etc. up to V(0) -50 dB. 

The following table shows how to use the diagram. 

Given:

Di = 19 mm (effective diameter of the transducer)

Ni = 61 mm (effective nearfield length) Threshold: V(0) -30 dB

Measured values:

zr = 120 mm (distance to the reflector) b = 38 mm (extension rel. to -30 dB)

Evaluation:

Zr = zr/Ni = 120/61 ≈ 2 B = b/Di = 38/19 = 2

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Evaluation:
A diagram (Fig. 47) must be used for the reflector distance = Ni with the aid of the curve (-30 dB), B = 2 re- sults in an associated G = 0.35 to
B = 0.5 the value G = 0.4 is therefore d=GDi =0.35∙19mm=5.7mm
The scanned reflector has, in the scanning direction, an equivalent flaw size of 5.7 mm.
If one scans the reflector perpendic- ularly to the first direction and, in the same way, determines the equivalent reflector size, then the equivalent reflector is the ellipse whose diame- ters are the established equivalent reflector sizes.