What are Phased Array Ultrasonics?

What are Phased Array Ultrasonics?

Phased array ultrasonics represents one of the most significant advances in ultrasonic inspection technology over the past three decades. Rooted in physics first developed for radar and medical imaging, this technology gives inspectors electronic control over sound beams by steering, focusing, and shaping them without physically moving the probe.

This article explains the physics behind phased array ultrasonics: how transducers are designed, how beam steering works at a wave-mechanics level, and how advanced imaging techniques like Full Matrix Capture (FMC) and Total Focusing Method (TFM) push resolution beyond what conventional ultrasonics can achieve.

What are Phased Array Ultrasonics?

Phased array ultrasonics is an advanced ultrasonic technology that uses multi-element transducers to generate, steer, and focus sound beams electronically. Rather than relying on a single piezoelectric element that produces one fixed beam, a phased array probe contains an array of individually addressable elements (typically between 16 and 256), each fired with precisely controlled time delays.

The concept originated in the 1960s with radar antenna arrays, where controlling the phase of signals across multiple elements allowed engineers to steer beams without physically rotating the antenna. Medical ultrasonics adopted the principle in the 1970s for diagnostic imaging, and industrial NDT followed in the 1980s. The technology has since become central to modern ultrasonic testing.

The fundamental difference from conventional ultrasonics lies in beam control. A conventional transducer produces a single beam at a fixed angle determined by the probe and wedge geometry. A phased array transducer can produce beams at multiple angles, focal depths, and positions, all electronically within milliseconds. This versatility forms the basis of phased array ultrasonic testing (PAUT).

How Phased Array Technology Works

At its core, phased array ultrasonics exploits the wave physics of constructive and destructive interference. By controlling when each element fires, the technology shapes the resulting wavefront to achieve specific beam angles and focal points.

Transducer Design and Element Arrays

The phased array probe is the critical hardware component. Unlike a conventional single-element transducer, it contains multiple small piezoelectric elements arranged in a defined geometric pattern.

Each element is a thin strip of piezoelectric material (commonly PZT (lead zirconate titanate), or newer piezocomposite materials) that converts electrical pulses into sound waves and vice versa. When voltage is applied, the element deforms and emits an ultrasonic pulse; when a returning echo strikes it, it generates a voltage that the instrument digitizes.

Key design parameters define the probe’s performance:

Parameter	Definition	Typical Range
Element count	Number of individual elements	16 to 256
Pitch	Centre-to-centre spacing between elements	0.2 mm to 1.5 mm
Aperture	Total active length of the array	10 mm to 60 mm+
Frequency	Centre frequency of the elements	1 MHz to 15 MHz
Elevation	Height of each element (perpendicular to array axis)	8 mm to 20 mm

Element pitch is particularly critical. It must be less than half the wavelength at the operating frequency to avoid grating lobes, spurious secondary beams that generate false indications. Smaller pitch allows wider steering angles but requires more elements to maintain the same aperture.

Three primary array geometries exist:

• Linear arrays: Elements in a single row. The most common industrial configuration, offering beam steering and focusing in one plane.
• Matrix arrays: Elements in a two-dimensional grid (e.g., 8×8 or 16×16), enabling beam steering in two planes for three-dimensional beam control.
• Annular arrays: Concentric ring-shaped elements providing variable focusing along the beam axis, though they cannot steer the beam off-axis.

Beam Steering and Focusing

Each element generates a spherical wavelet when it fires. According to Huygens’ principle, the overall wavefront is the superposition of all individual wavelets.

When all elements fire simultaneously, the wavelets form a planar wavefront that propagates straight ahead. When elements fire in a staggered sequence with precisely calculated time delays, the wavelets combine at an angle, producing a steered beam.

To steer at a given angle from the normal:

• The outermost element on the steering side fires first.
• Each successive element fires after a delay proportional to the pitch, the sine of the desired angle, and the inverse of the sound velocity.
• Constructive interference reinforces energy along the desired direction; destructive interference suppresses energy elsewhere.

Focusing works on a similar principle. The outer elements fire slightly before the inner elements, causing wavelets to converge at a focal point. Unlike a conventional fixed-focus probe, a phased array system adjusts its focal depth electronically, with no probe change required.

A single set of time delays applied to active elements constitutes a focal law. Advanced phased array flaw detectors like the OmniScan MX2 store up to 256 focal laws, enabling multi-angle inspection in a single pass. The instrument’s PA2 modules (16:64 to 32:128PR) execute these focal laws at digitising frequencies up to 100 MHz.

Display and Imaging Modes

Phased array data can be visualised in several formats, each revealing different information about the inspected volume.

A-Scan, B-Scan, S-Scan, and C-Scan

Display Mode	What It Shows	How It Is Generated
A-Scan	Amplitude vs. time for a single beam	Single focal law, single probe position
B-Scan	Cross-sectional slice showing depth vs. position	Stacking A-scans from successive probe positions
S-Scan	Fan-shaped cross-section covering a range of beam angles	Sweeping the beam through multiple angles using different focal laws
C-Scan	Plan-view map showing reflector position in the X-Y plane	Raster scanning and recording amplitude or depth at each point

The S-scan (sectorial scan) is unique to phased array technology. It provides a real-time cross-sectional image by sweeping the beam through a defined angular range, typically in one-degree increments. A single S-scan can replace multiple fixed-angle conventional UT passes.

Advanced Imaging: Full Matrix Capture (FMC) and Total Focusing Method (TFM)

Full Matrix Capture and the Total Focusing Method represent the latest evolution in phased array ultrasonics, pushing from beam-based to pixel-based imaging.

In conventional phased array, groups of elements fire together per predefined focal laws. FMC takes a different approach: each element transmits individually while all elements receive simultaneously. For an array with n elements, this produces n x n unique transmitter-receiver waveforms, known as the full matrix of data.

TFM is the imaging algorithm applied to FMC data. For every pixel in the image, TFM calculates the optimal focal law by summing time-of-flight contributions from every transmitter-receiver pair. Every pixel is in perfect focus, hence the name.

The result is a significant leap in image quality:

• Resolution approaching the diffraction limit: TFM can detect flaws smaller than 1 mm.
• High signal-to-noise ratio: Coherent summation suppresses noise while reinforcing real reflectors.
• Multiple imaging modes: Different wave paths (direct, half-skip, full-skip, mode-converted) can be reconstructed from the same dataset.

The OmniScan X3 64 from Evident is a leading portable instrument for Total Focusing Method (TFM) and Full Matrix Capture. It delivers 64-channel TFM imaging with 4 TFM modes, an Acoustic Influence Map (AIM) reflectivity simulator for selecting the optimal mode, and an 800% amplitude range for superior dynamic performance. The AIM simulator is particularly valuable for High-Temperature Hydrogen Attack (HTHA) detection.

For more on these capabilities, see our article on high-resolution phased array UT.

Types of Phased Array Probes

Different inspection scenarios demand different probe configurations.

Linear array probes are the most widely used in industrial NDT. Elements sit in a single row, enabling beam steering and focusing in one plane. Typical configurations range from 16 to 128 elements at frequencies between 2 MHz and 10 MHz.

Matrix array probes arrange elements in a two-dimensional grid, enabling beam steering in two planes simultaneously. They are used for complex geometries where single-plane steering is insufficient.

Annular array probes use concentric ring elements for excellent variable-depth focusing along the beam axis. They cannot steer laterally, making them best suited to applications requiring focused beams at different depths along a fixed axis.

Wedge configurations are critical to how phased array probes are deployed. A probe mounted on an angled wedge (typically Rexolite) introduces the beam into the test piece at a refracted angle. Wedge angle, material velocity, and frequency together determine the range of achievable inspection angles. Profiles are available for flat surfaces, curved surfaces (e.g., pipe saddle wedges), and specialised geometries.

Blue Star E&E offers a comprehensive range of probes and wedges through its partnership with Evident. Our automated phased array ultrasonic systems integrate these into fully engineered inspection solutions for pipe mills, steel plants, and manufacturing facilities across India.

Key Standards and Codes

Phased array ultrasonics is governed by a mature framework of international standards covering equipment performance, examination procedures, and personnel qualification.

Standard	Scope
ISO 13588	Ultrasonic testing of welds: use of automated phased array technology
ASME Section V, Article 4	Ultrasonic examination methods, including phased array techniques
ASTM E2491	Evaluating performance characteristics of phased array UT instruments
EN 13588	Ultrasonic testing: use of automated phased array technology (harmonised with ISO 13588)
ISO 9712	Qualification and certification of NDT personnel
ASNT SNT-TC-1A / CP-189	Qualification and certification of NDT personnel
ASME CC 2235	Permits phased array UT as an alternative to radiographic testing

Personnel must be qualified under ISO 9712 or ASNT schemes. Most codes require Level II certification for performing phased array examinations, with Level III oversight for procedure development.

Frequently Asked Questions

How does phased array differ from conventional ultrasonic testing at a physics level?

Conventional UT uses a single piezoelectric element producing one beam at a fixed angle and focal depth. Phased array ultrasonics uses multiple elements fired with controlled time delays, exploiting wave interference to steer and focus the beam electronically. A single phased array probe can replicate the function of many conventional probes.

What is the difference between phased array and TOFD?

Phased array is a beam-forming technology that controls how ultrasonic energy enters the material. TOFD is a specific technique that analyses diffracted signals from flaw tips for accurate sizing. They are complementary and often used together: TOFD for precise through-wall sizing, phased array for imaging and volumetric coverage.

What is the advantage of TFM over conventional phased array imaging?

In conventional phased array, the beam is focused at one depth per focal law, so resolution degrades away from the focal point. TFM applies a unique focal law to every pixel using FMC data, achieving focus everywhere simultaneously. This yields resolution close to the diffraction limit, detecting flaws smaller than 0.1 mm.

What determines the maximum steering angle of a phased array probe?

The maximum steering angle is governed by the ratio of element pitch to wavelength. Smaller pitch relative to wavelength allows wider steering. The pitch should be less than half the wavelength at the operating frequency; exceeding this limit produces grating lobes that degrade image quality.

Can phased array ultrasonics replace radiographic testing?

In many applications, yes. ASME Code Case 2235 permits phased array UT as an alternative to radiographic testing for weld examination. Phased array offers real-time imaging, no radiation hazard, single-sided access, and superior detection of planar defects such as lack of fusion and cracking.

Blue Star Engineering & Electronics is an authorised distributor of Evident (formerly Olympus) phased array technology in India, with over 40 years of NDT expertise and 5,000+ customers nationwide. From portable OmniScan phased array instruments, including the OmniScan X3 64 with TFM/FMC and the OmniScan MX2 with advanced PA2 modules, to fully automated phased array ultrasonic systems for production environments, Blue Star E&E delivers complete turnkey solutions backed by a pan-India service network of 30+ offices. Contact us to discuss your phased array requirements.