What is Phased Array Ultrasonic Testing (PAUT)?
In this article:
• Phased Array Fundamentals. PAUT uses multiple small transducer elements and electronic delay laws to steer and focus ultrasonic beams, enabling high-resolution flaw detection and imaging.
• Beam Steering & Focusing. Time-delayed excitation allows control over beam angle and focus depth, improving sensitivity and enabling techniques like sector (S-scan) and compound scans.
• Imaging Capabilities. PAUT produces real-time 2D and 3D visualizations (A-, B-, C-, D-scans), offering better flaw characterization, sizing, and positioning than conventional UT.
• Advanced Scanning Techniques. Electronic linear scanning (L-scan), compound scanning, and multigroup setups enable efficient, high-coverage inspection even on complex geometries.
• Position Encoding & Data Analysis. Integration with position encoders allows spatially accurate, repeatable inspections. Stored A-scan data supports retrospective analysis and report generation.
• Discover Waygate Technologies Equipment. Instruments like the Waygate Technologies Krautkrämer USM 36 and USM Go+ are ideal for PAUT. They offer multi-channel phased array capability, sector scanning, real-time imaging, and support for position encoding and data export.
What is Phased Array Ultrasonic Testing (PAUT)?
Phased Array Ultrasonic Testing (PAUT) is a powerful non-destructive testing (NDT) method that uses advanced sound beam manipulation to detect flaws in materials with high precision and flexibility. Unlike conventional ultrasonic testing, which relies on fixed-angle probes, PAUT employs an array of small transducer elements that can be individually controlled to steer, focus, and scan ultrasonic beams electronically. This enables rapid inspection, improved detection capability, and detailed imaging of internal structures—especially valuable for complex components or critical welds.
Basic Principle of Phased Array Technology
By subdividing the piezoelectric transducer into many small transducer elements and using an ultrasonic instrument capable of individually controlling and evaluating each of these elements temporally, it becomes possible to modulate the interference pattern of the sound beam to achieve a desired sound pressure distribution that is optimal for a specific application.
By exciting the elements at different times, the phase of the elementary waves of the individual elements is influenced, thus giving rise to the term “Phased Array.” In German, the term “Gruppenstrahler” (group radiator) is also used, as a group of n transducer elements is always ex- cited with a specific time delay pat- tern, the so called “delay law”. This is not a physical law but rather a sequence of delay times calculated by the instrument, relative to the individual elements of the group, intended to achieve a specific characteristic of the sound beam.
Linear Array Configuration and Component Structure
In this linear array, also called a “1D array” (one-dimensional arrangement of elements) (fig. 60) the direction of element arrangement is referred to as the primary (or active) axis. The perpendicular direction is the secondary (passive) axis. In the simplest case, the transducer elements are arranged linearly (array), thus forming the linear phased array probe (fig. 61).
The narrow transducer elements— approximately 0,3 mm to 2 mm wide—are electrically and acoustically isolated from each other by a gap filled with synthetic resin and are individually wired. Consequently, the connecting cable consists of many coaxial lines, which in most cases are permanently connected to the probe.
Signal Control and Imaging Process
The phased array instrument has n independent channels (fig. 62). All functions are controlled by a computer. The transmit pulser provides the clock signal. The Delay Law Calculator (DLC) calculates the desired time delays for each transmit and receive channel. The n received signals are summed with correct phasing for each shot and, if necessary, rectified. A desired two-dimensional image is generated from a sequence of A-scans. During the scan of the test object, all A-scans are recorded and stored according to their position (raw data). From these data, the instrument ultimately calculates the desired images of the test result.
Phased Array Probe—Characteristics
According to the standard EN ISO 23243, there are new geometric parameters for the phased array transducer (fig. 60), which are fundamentally important for the respective application.
When a single, small transducer element is excited, a sound field with large divergence and a very small near-field length is generated (fig. 63). This means that the sound waves propagate in almost all directions, with the sound pressure (shown on the left in the figure) decreasing as the beam angle deviates from the normal. This is rep- resented qualitatively by the length of the arrows and in the wave train (shown on the right in the figure) by the thickness of the line.
Electronic Beam Steering
When several adjacent elements are excited simultaneously, only the wave trains with the same phase interfere constructively, i. e., along the normal to the surface. The wave- front becomes a straight line parallel to the coupling surface (fig. 64).
If adjacent elements are now excited with a constant time delay ∆t relative to the neighbouring element, only the wave trains that lie on an oblique line to the coupling surface interfere constructively. This means the sound wave now propagates into the workpiece at a specific angle to the normal (fig. 65).
Effective Aperture and Near-Field Considerations
Without the use of a wedge, a straight beam probe effectively becomes an angle beam probe, whose beam angle β can be varied by changing the time delay ∆t. This is illustrated in fig. 65 by the columns above the elements. Because the constructive interference of the individual elementary waves is no longer along the normal to the coupling surface, the resulting amplitude is reduced accordingly, as only the portions of the signals emitted at the corresponding angle are superimposed.
This type of sound field manipulation is referred to as “steering” and naturally only works as long as the pulses still overlap due to their length. In addition to the amplitude, the shape of the sound field also changes during steering. The reason for this is the “effective aperture.” In phased array testing, the aperture refers to the total length of all elements currently in use. With angled insonification, only the portion of the total transducer area that lies in the projection perpendicular to the beam direction can be effective
(fig. 66).
(55) N' ≈ N∙cos2β
Due to this influence, the natural near-field length N of the sound beam is reduced, and the angle of divergence 𝛄 is increased. Consequently, the echo amplitude of a reflector at a distance greater than N also decreases with increasing beam angle. The new near-field length N' can be calculated approximately using eqn. (55).
Grating Lobe
Since the elementary waves in a phased array probe originate from individual transducer elements arranged at a fixed pitch p, constructive interference can occur not only in the desired direction (fig. 67).
The signals at 0° are summed with- out a path difference, i. e., with the same phase, and thus generate the maximum sound pressure here. However, at an angle ⌽ that de- pends on the wavelength λ and the element pitch p, constructive interference occurs again. Here, the signals, each shifted by exactly one wavelength, add up to a real but elongated signal. This undesired sound pressure maximum at angle ⌽ is called a “grating lobe”, analogous to the optical diffraction phenomena at the lattice of a crystal. Grating lobes6 only occur as long as the ratio of the wavelength λ, or a multiple thereof, to the element pitch p is less than 1. The angle of the grating lobe(s) can be calculated using eqn. (56):
When using a PA probe with 4 MHz and a 2 mm pitch for perpendicular insonification in steel, a grating lobe will occur at -48° and +48°, which can lead to undesirable effects
(→ false indications). This problem is solved by choosing a probe with a
1 mm pitch: Eqn. (56) then no longer yields a valid result because the sine value becomes greater than 1.
Steering the sound beam by applying constant time delays ∆t during the excitation of the individual elements has practical limitations due to various influences. The maximum useful steering angle7 βg is calculated approximately using eqn. (57), as long as the value does not exceed 1, i. e. 0,44λ < p:
6Rieder et al.: Handbuch für die Materialprüfung mit Ultraschall-Phased-Array, page 32
7Rieder et al.: Handbuch für die Materialprüfung mit Ultraschall-Phased-Array, page 28
The two examples show that the us- able angular range is greater for longitudinal waves than for shear waves. In applications using shear waves, the array is typically mounted on a wedge (made of plexiglass or polystyrene) to generate the shear wave via mode conversion during refraction into the workpiece, thereby excluding the transmission of the longitudinal wave. The wedge angle is often chosen to result in a natural beam angle β of 55°. Thus, the usable steering range of the shear wave of ±21° covers the commonly used angular range for weld inspection very well (fig. 68).
Electronic Sector Scan—S-Scan
Fig. 68 also illustrates the typical application for beam steering: the sector scan, S-scan. This consists of a sequence of n individual shots with the same aperture, where the beam angle is successively incremented by the angle step ∆β. Thus, the individual sound beams sweep across a fan-shaped cross-sectional area in the test object, which is displayed on the screen of the test instrument after this sequence (salvo). The amplitudes of the individual A-scans are converted into colour values according to a predefined colour scale. In this way, all indications from the scanned cross-sectional area are displayed simultaneously and in real time in a two-dimensional image (fig. 69).
The number n of shots in the sequence (salvo) is calculated using the following formula:
Typically, an image frequency of 30–60 Hz is achieved, whereby—in contrast to conventional testing— the test result of a cross-sectional area is displayed two-dimensionally (→ imaging).
Focusing
Using a different delay law, the focal point of the sound beam can also be changed. This means that a detected discontinuity can be captured with the highest sensitivity and best resolution. This is particularly advantageous for characterizing indications. To focus the sound field, the delay law is calculated so that the individual elementary waves meet at the desired focal depth simultaneously and thus have the same phase (fig.70).
Fig. 71a shows the natural near-field length and angle of divergence without time delays. The near-field length, here for a rectangular trans- ducer8, can be calculated using the approximation eqn. (59):
For the array in fig. 71a, N◻ = 45 mm (without focusing).
• Fig. 71b shows focusing at half the natural near-field length, i. e., with a focus factor of 0,5 or 50%.
• Fig. 71c shows focusing at one-quarter, and
• fig. 71d shows focusing at one- eighth of N◻—a focus factor of 0,125.
The physically possible focus factor is always less than or equal to 1, meaning that focusing is only possible within the natural near-field length. For focusing, the delay law follows a hyperbola, which becomes steeper the smaller the focus factor is set. If a focal depth greater than the natural near-field length is entered into the ultrasonic instrument, this value is ignored, and the instrument does not apply any time de- lays. This results in a sound beam as shown in fig. 71a, i. e., unfocused.
Steering and focusing are also possible in combination (fig. 72).
The delay law is now a superposition of a straight line and a hyperbola.
In this sequence, the same number of elements (group) is always used. This results in a fixed aperture A, which is the active length of the group. The larger the aperture and the number of active elements within the aperture, the more flexibility the sound field can be steered, and the greater the depth range within which focusing can be achieved. However, the maximum number of elements that can be used in a group also depends on the phased array instrument: Most currently used instruments have 16 channels and can therefore only excite 16 elements of a probe simultaneously. Instruments with higher channel counts allow for more flexible use of phased array applications. This allows for more elements to be operated simultaneously, resulting in larger apertures, higher sound pressure, greater focal depths, and the use of multiple array probes simultaneously.
However, a fast electronic switch (→ multiplexer) even in simpler instruments ensures that arrays with more than 16 elements can be con- trolled, albeit in two or more tempo- rally and spatially offset shots.
Linear Electronic Scanning
Another way to modulate a linear array is through electronic linear scanning. Here, too, a sequence of n shots is generated, but this time with the same delay law applied to different element groups.
For example, shot 1 controls elements 1–16 of the array with 64 elements. In shot 2, elements 2–17 generate the sound beam, in shot 3 elements 3–18, and so on, until in the last shot, 49, elements 49–64 are excited (fig. 73).
The amplitudes of the 49 A-scans are converted into colour values along the acoustic axes and then displayed next to each other at intervals of p (pitch), so that the scanned, two-dimensional cross- section of the range appears on the screen with a width of q (scan width).
(60) q ≈ (n–a) p
where:
n = number of elements in the array a = number of elements in the
aperture
p = pitch [mm]
This representation corresponds to a B-scan, which would be acquired with a conventional probe by moving the probe q mm. However, since it is generated here only by the electronic shifting of the sound beam along the array, it is referred to as an L-scan (fig. 74).
If one element is skipped during this scan (element step 2), only 25 shots at intervals of 2p result. This means that the image is built up in half the time, but the lateral resolution is only 2p, so the image appears somewhat coarser. However, twice as many images are generated per unit of time compared to scanning with an element step of 1. The number z of shots per salvo is calculated using eqn. (61):
The sequence shown in fig.75 can even run simultaneously as so-called multigroups and be displayed in different images. A typical application for this is the inspection of a weld from both sides simultaneously using a scanner, where two L-scans or S-scans are acquired and evaluated from the right and left of the weld respectively (fig. 76).
Compound scanning
A linear scan, in which the beam angle is additionally changed in a fixed angle step for each shot, is called compound scan. This allows for further applications such as the compliant testing of the thicker welds while fulfilling the requirement for near-perpendicular insonification of the entire weld flank with an angle of impact between -6o and +6o according to EN ISO 13588 (fig.77).
The geometric simulation shown simultaneously determines the ideal probe position, here 50 mm, relative to the weld centerline, at which the volume of the weld, including the heat-affected zone, is completely covered. A further advantage is the smaller sound path differences for all locations within the weld, resulting in a reduced influence of the distance law on the echo amplitudes of indications from the weld.
Position Encoding
The most important addition to phased array ultrasonic testing is the encoding of the probe position during the scanning of a test object. Knowing the insonification positions in conjunction with the stored A-scans, L-scans, or S-scans makes the test result three-dimensional. The location and extent of discontinuities in the scanned test object can thus be recorded and evaluated.
B-Scan
The stored data is used to generate the B-scan: a vertical cross-section along any path (y-coordinate) in the mechanical scan direction (x-coordinate), in which the depth position (z-coordinate) of indications and/or the wall thickness of the test piece is displayed to scale (fig. 78). A variant is the cumulative B-scan, in which the indications for all recorded y-coordinates are dis- played simultaneously.
C-Scan
A C-scan is a top view of the test object, in which the amplitudes of detected discontinuities are dis- played with their x and y coordinates: the ideal way to determine the extent of an indication (fig. 79). Another variant is the D-scan, in which—as in the C-scan—the indications appear in a top view, but the colour values refer to the depths (z) of the indication(s) and not to the amplitudes. Since all three spatial coordinates of the indications are available, three-dimensional representations of the inspection volume are now possible.
Monitor Gate(s)
Generally, the sound paths of indications are only available numerically if they were captured with the monitor gate in the A-scan. This means that they only appear in the subsequently generated C-scans. It follows immediately that if the monitor gate is not set correctly, existing indications will not be displayed and thus overlooked, or a C-scan will appear entirely in one colour be- cause the monitor gate only captured the backwall echo as the highest echo. Fortunately, such im- properly generated images can be corrected retrospectively (offline) because the A-scans of all shots (→ raw data) have been saved, and thus the gate data can be adjusted to recalculate the image data. When using multiple gates, the images can also be filtered according to selected depth ranges.
Sizing of Indications
As an imaging ultrasonic method, the indications of detected discontinuities are now available to us to scale in the many possible display formats, and their extents can be easily determined using appropriate tools, such as crosshairs (cursors). But do these dimensions correspond to the actual reflector size?
The answer to this question is initially a clear NO, because the two- dimensional image of the indication is the result of all captured amplitudes when scanning the reflector with the sound beams of the probe. Therefore, valid amplitudes are obtained even when the reflector is hit by the peripheral beams of the sound beam. The extent of an indication image is thus determined by the cross-section of the sound beam at the location of the reflector. The indication size determined with the crosshairs in the image is there- fore always larger, in principle, than the actual reflector size. For reflectors whose extent is larger than the sound beam diameter, applying the 6 dB drop method (half value method) can provide a more accurate result, but this does not help us with small reflectors.
To a certain degree, the greater flexibility of phased array testing can nevertheless help us:
If it is possible to focus the sound beam on the detected reflector, the sound beam diameter is minimal at this point, and thus the lateral blur- ring of the image is also minimized. However, the influence of the un- known shape and orientation of a natural discontinuity is not taken into account. But here, too, the possibilities of phased array testing help us again by insonifying the discontinuity with different beam angles to determine the orientation of the reflector or to distinguish between a planar or volumetric discontinuity. As in conventional testing, the maximum amplitude of the indication is also used in phased array testing when determining the acceptance level of an indication; see EN ISO 19285.
Other Array Types (Overview)
Although linear phased array probes are used in most applications, other types have been developed for specific inspections. Their uses are so diverse that they will not be described further here.
Only annular arrays (fig. 80) are capable of generating rotationally symmetric sound fields. For the annular array in fig. 80a, only perpendicular insonification and focusing are possible; with the segmented annular array in fig. 80b, steering is also possible.
In a daisy array, the elements are arranged in a circular pattern (fig. 81). Particularly in combination with a 45° steel cone positioned centrally in front of the probe, this allows
for the circumferential inspection of pipes from the inside or for wall thickness measurements.
For automated pipe and bar inspection, there are 1D arrays that are designed as full or partial circumferential arrays (fig. 82). These allow for simultaneous detection of longitudinal internal and external cracks in addition to wall thickness and ovality measurements.
In all matrix arrays (2D), the elements are arranged in a two-dimensional grid (fig. 83). The elements can be the same or different sizes, and the layout can be square or rectangular. The modulation of the elements and element groups is extremely flexible: steering the sound beam in almost any direction, focusing, and linear electronic scanning in the x and y directions. This flexibility is particularly advantageous for test objects with limited space for probe movement.
Two matrix arrays in dual mode on a wedge are optimal for the demanding inspection of austenitic welds (fig. 84).
Procedure for a Phased Array Inspection
The properties (material, geometry) of the test object, as well as its in- tended use (type of loading, dynamics, temperature range), determine the probe selection, instrument set- tings, and scanning of the workpiece.
- Once the probe(s) to be used are determined, the next step is to define the probe positions on the test object and their scan paths (→ scan plan). Simulation pro- grams are commonly used today to visualize the insonification relative to the test object with the possible electronic scans to scale.
- After implementing the instrument setting(s) from the simulation, they must be calibrated. Appropriate calibration or reference blocks are used for this purpose. During range calibration, the zero points for all shots of the used sequence(s) are determined, and the range and monitor gates are set so that the inspection volume is completely covered (100% coverage).
- The reference block(s) contain/s reference reflectors (drilled holes, notches) that are used for amplitude correction during steering, for setting up depth compensation, and for the prescribed adjustment of the test sensitivity.
- For accurate scan path recording, the position encoder must be con- figured and calibrated.
- The instrument settings created in this way are saved (Setup). This allows for reproducible repeat inspections with the same test conditions.
Now the actual inspection can be- gin. After each scan section, the inspection data is saved and evaluated either immediately or later (offline). All modern phased array instruments include the capability to generate inspection reports from the stored inspection data. Additional PC software is often used today to more intensively evaluate the raw data from the instruments and generate corresponding im- ages and reports.
In phased array ultrasonic testing, the many possibilities for modulating the sound fields of arrays, as well as the imaging capabilities for displaying the test results, represent the most important differences compared to conventional ultra- sonic testing. The physics of sound propagation and sensitivity remain unchanged. However, the greater flexibility of phased array testing, with optimization for the respective test application, leads to a higher POD (Probability Of Detection) and thus to greater inspection reliability.
By simultaneously modulating different electronic scans (groups) within a single probe and using multiple probes in a scanner, the application can also be optimized in terms of time. For example, a complete weld inspection from both sides can be performed with just one scan.
Phased array technology is also predominantly used in automated inspection systems, offering the advantages of higher inspection speed, reduced mechanical complexity (less wear), and convenient imaging of the test results.
In summary
Phased Array Ultrasonic Testing (PAUT) represents a significant advancement over conventional ultrasonic testing by offering adaptable beam control, real-time imaging, and comprehensive data analysis. Its ability to electronically steer and focus sound waves allows for highly sensitive, flexible, and efficient inspections, especially in safety-critical industries such as aerospace, power generation, and oil & gas. As instrumentation and software continue to evolve, PAUT is poised to become the default ultrasonic inspection technique, combining reliability with precision imaging and higher Probability of Detection (POD).
Further reading:
[31] Ed Ginzel: Phased Array Ultra- sonic Technology
Eclipse Scientific Products Inc.
[32] Rieder et al.: Handbuch für die Materialprüfung mit Ultraschall- Phased-Arrays, DGZfP (2020)