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Figure 3-14 Electronic Scan Patterns
● For linear scans, arrays are multiplexed using the same Focal Law.
● For sectorial scans, the same elements are used, but the Focal Laws are changed.
● For dynamic depth focusing (DDF), the both the transmitter and receiver Focal Laws can be changed to optimise pressure and response at a specific depth.
When combined with a motorised scanner that moves the phased array probe in a specified path the entire volume of the test piece can be interrogated.
3.5 Phased Array Beam Focusing and Steering
This is something of a review as we have already covered the principles in the review of the beam characteristics and associated equations as they related to single element probes.
3.5.1 Beam Focusing
Focusing coefficient (K) is defined as N K F
Where F = focal distance N = Near zone length
Beam dimension (dst) in steering plane at Focal Length is given by
A d
st F
Where A is the active aperture, F is the Focal Length and is the wavelength. (In this case the A is equivalent to the diameter D for a single crystal).
3.5.2 Beam Steering
Important aspects of beam steering for phased array probes include:
The capability to modify the refracted angle of the beam generated by the array probe.
Allows for multiple angle inspections, using a single probe
Applies asymmetrical (e.g. linear) focal laws
Can only be performed in steering plane, when using 1D (linear)-arrays
Can generate both L (compression) and SV (shear vertical) waves, using a single probe
Steering capability is related to the width of an individual element of the array
Steering range can be modified using an angled wedge
Maximum steering angle (see also below) (at –6 dB), is given by:
st e
0.5
Where: is the wavelength and “e” the individual element width The maximum steering angle is limited by the following factors:
1. element pitch 2. frequency 3. element width 4. wedge angle
5. number of active elements 6. focal range
Although a pitch of 1/2is the mathematical limiting factor for beam steering, in practise the other variables will cause noise before the 1/2angle is achieved. In particular total internal; reflection and surface wave generation within the wedge will prevent high steering angles. As a recommendation the maximum steering angle should be restricted to approx 15 degrees either side of the beam nominal angle and the probe should be tested through the programmed range for acceptable noise levels before a procedure is finalised.
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3.5.3 Electronic (Linear) Scanning
Figure 3-15 Electronic Scanning
Electronic scanning is the ability to move the acoustic beam along the axis of the array without any mechanical movement (see Figure 3-15).
The beam movement is performed by time multiplexing of the active elements (repeating the focal law stepping through one element at a time using the next adjacent element from the start element of one focal law as the next start element)
Scanning extent is limited by:
● number of elements in array
● number of “channels” in the acquisition system
3.5.4 Sectorial Scanning
When all aspects of the Focal Law are held fixed except for the delays to alter angles such that a range of angles is covered by the beam, the scan pattern is called a Sectorial or S-scan (sometimes also called an Azimuthal scan). Figure 3-16 illustrates the effect of Sectorial scanning. This pattern can be considered to be similar to the old spinning-head probes used in the production of medical B-scans where a small angular window is left open for the probe to transmit and receive through.
Figure 3-16 Sectorial Scanning
Courtesy Olympus NDT
The block imaged has a series of side drilled holes that are drilled at increasing depths as the beam sweeps from left to right. The side-drilled holes in the block imaged in Figure 3-16 are overlaid with a scaled-down transparency to illustrate the sweep motion.
It will be noted that the strong (red) horizontal signal at the bottom of the scan image does not match the bottom surface when overlain on the block. This is merely a projection distortion and the overlay is for illustrative rather than measurement purposes.
3.5.5 Combined Beam Processing
The phased-array technique allows for almost any combination of processing capabilities:
● focusing + steering
● linear scanning + steering
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Copyright © 2008, TWI Ltd/E.Ginzel
3.5.6 Probe Performance and Selection
Basic considerations for phased array probe selection are identical to those considered when selecting a single element probe. The material tested, its geometry (shape and thickness) and the location and orientation of possible flaws to be detected are the main considerations. E.g.
● For coarse grained materials lower frequencies are used
● For thin materials and high resolution higher frequencies are selected
● For long path lengths to the areas of interest larger element sizes (aperture sizes) are selected
● For specific angles and when required that only the shear mode be generated, a refracting wedge of a specified angle is used
● For vertically oriented flaws a tandem configuration of transmitter and receiver may be appropriate
● For very accurate defect sizing or high sensitivities a larger aperture is required.
But in addition to these basic items that are common to both single-element and phased array probes, the pitch and total number of elements in a probe are also factors to consider. This makes some aspects of phased array probe selection a customised process. The total steering angles required will dictate the pitch of the elements and this may be limited by the frequency used. The total number of elements used to provide an effective focus or penetrating capability may be limited by the instrumentation (more pulsers and receivers are required to address more elements at a single firing of a focal law). Total length of a linear array may be a compromise between pitch and steering capability so as to achieve volume coverage of a weld using angled beams in a linear scan.
These aspects of phased array probe selection will be looked at again in the applications section.