Delay laws and phased array configurations

Delay laws, or focal laws, simply describe the time delays and amplitude for each individual element of a transducer, which together form the function to create a particular beam shape, direction or focus. Conveniently, modern phased array inspection tools calculate these laws based on the transducer parameters, material velocity, frequency, and the desired scan type, but can be manually calculated. Firstly an element is chosen to have

p ≈ λ/2 p ≈ λ/1.8 p ≈ λ/1.6

p ≈ λ/1.3 p ≈ λ

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Thesis – Ultrasonic phased array testing in the power generation industry – Novel wedge development November 2011

zero delay (centre element) and the delay of the other elements calculated relative to that. If the law was designed to create a focus at a given depth and angle then the relative laws can be calculated as seen in Figure 3-26.

Using simple maths the delays can be calculated for each individual element relative to the others to create focussing and beam steering.

There are a number of different configurations when using phased arrays which allow for high productivity and advanced imaging of the components under test.

Figure 3-26 Calculating delay laws within the near field93

The values for d0 and dj can be calculated:

d0 = x / cosα

dj= x / cosα’

3.4.6.1 Sectorial (Azimuthal) scanning

The ability to phase the elements and produce ultrasonic beams of different refraction angles allows a component to be interrogated through many refraction angles almost instantaneously, giving a large field of coverage from a single position (Figure 3-27). If delay laws were produced to scan angles between 35° and 70°, in 1° increments then an A- scan image representing each angle would be recorded. By representing the amplitude of

dj d0 α Focal spot Transducer elements tj = (dj – d0) / v x y y’ α’ p e

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Thesis – Ultrasonic phased array testing in the power generation industry – Novel wedge development November 2011

each A-scan response in a colour map and stacking them sequentially, an image of the covered region can be created. The image is corrected for volume so that each scan is

imaged at its true orientation angle and a sectorial or azimuthal scan image is created. Figure 3-28 shows a volume corrected sector scan with the original component overlaid to show how responses from geometry are imaged. Within the phased array inspection equipment, individual A-scan images can be displayed to aid defect characterisation, plus on screen measuring gates are available to aid defect sizing.

It is also possible to scan the phased array probe with the aid of encoding devices, to feedback relative position, and collect sectorial scans at fixed intervals to build up D-scan or C-scan images. The three types of scan form a full 3 dimensional image of the component

under test and are commonly used for inspection of large components such as welds99.

In addition to producing multiple angled beams in a single sectorial scan it is possible to focus those beams (within the near field) in various ways. The most common focussing schemes can be seen in Figure 3-29, and include true depth focussing where the focus of all laws are set to a particular through wall depth, projection focus where all the laws are focussed along a projected line, half path focussing where the focal distance is fixed at a

given beam path (x2+z2)0.5, and focal plane focussing where all laws are focussed along a

linear plane (z = ax+b). The choice of focussing method is based almost entirely on the

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Thesis – Ultrasonic phased array testing in the power generation industry – Novel wedge development November 2011

Figure 3-27 Ray tracing representation of 35° to 75° azimuthal scan

Figure 3-28 Imaging with azimuthal sector scans

Figure 3-29 Focussing methods for linear phased array probes100

Transducer Section through a fir tree root Beam trajectories for each delay law

High amplitude echoes from geometric features Transducer Delay laws Sectorial scan (Azimuthal scan) Component overlay

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Thesis – Ultrasonic phased array testing in the power generation industry – Novel wedge development November 2011

3.4.6.2 Electronic linear scanning

Another common mode of operation is the use of electronic scanning which, rather than phasing the delay laws to produce multiple angles, produces fixed beam angles which are electronically scanned along the probe aperture. An electronic scan utilises a block of adjacent elements (active aperture) to form a delay law at some angle and focal depth which is multiplexed along the full aperture of the probe. For example, a 64 element probe could be configured with an 8 element active aperture to form a 0° compression wave focused at 20 mm through wall; the active aperture is incrementally scanned through all 64 elements using elements 1 to 8, 2 to 9, 3 to10, 4 to 11, through 57 to 64, see Figure 3-30. The method records the A-scan for each firing which is colour mapped for amplitude and sequentially stacked to form a sectorial scan image. This method allows large regions under the probe to be electronically scanned to form a B-scan image without the need to raster the probe. Typical applications include rapid corrosion mapping, and fixed angle beam weld inspection.

Figure 3-30 Electronic linear scanning

Active aperture (8)

Scan direction 1

64

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Thesis – Ultrasonic phased array testing in the power generation industry – Novel wedge development November 2011