What is phased array ultrasonic testing (PAUT) in composite NDT

Phased Array Ultrasonic Testing (PAUT) is an ultrasonic NDT method using a multi-element transducer in which each element is fired with a precise time delay (the phase) so the beam can be steered, swept, and focused electronically. The output is a real-time A-, B-, and C-scan image generated without physically moving the probe.

PAUT offers fast coverage and a high probability of detection for planar defects on composite structures: delaminations, voids, and bond defects. It can also measure thickness. The trade-offs are couplant (or an immersion / water path), careful calibration, and skilled interpretation.

How PAUT works#

A phased array transducer contains many small piezoelectric elements (commonly 16, 32, 64, or 128) arranged in a line or a 2-D matrix. The instrument fires each element with a programmed delay relative to its neighbors. By varying the delay pattern, the beam can be:

• Steered. Tilted to a specified angle without moving the probe.
• Swept. Stepped through a range of angles in a single acquisition (the sectorial scan).
• Focused. Constructively combined at a specified depth, sharpening the lateral resolution at that depth.

The instrument repeats the cycle continuously, and the receive electronics digitize and combine the returned echoes from each element to produce an A-scan (amplitude versus time at a single beam angle), a B-scan (cross-sectional image at one probe position), and a C-scan (top-down map of the part as the probe is moved over it).

Composite-specific scan parameters#

PAUT on carbon composites differs from PAUT on metals or welds in several ways that follow from the anisotropic ply structure of the laminate.

Frequency. Primary frequencies on composite skins run 2 to 2.5 MHz, or a general low range of 2 to 5 MHz, on laminates up to about 10 mm (tape) or 20 mm (fabric) thick. Lower frequencies penetrate the laminate without excessive scatter off the ply interfaces. Higher frequencies (5 to 10 MHz), routine on metals, attenuate heavily on composites and lose back-wall signal.

Beam steering range. Typically capped at plus or minus 15 to 30 degrees on composites. Steering further introduces refraction noise at the ply boundaries and mode conversion between longitudinal and shear waves, which clutters the image.

Time-Corrected Gain (TCG). Normalizes amplitude across thickness so a defect signal at the back wall reads at the same amplitude as an equivalent defect near the surface. Without TCG, the natural attenuation of the laminate would make deeper defects look smaller than they are.

Dual-axis or cross-polar scanning. Wrinkles and delaminations oriented parallel to a single beam angle reflect very little energy back to the probe. Scanning at two orthogonal axes (or rotating the probe 90 degrees) ensures that defects of arbitrary in-plane orientation produce a detectable indication.

Couplant. PAUT requires acoustic coupling between the probe and the part. Water, ultrasonic gel, an immersion tank, or a local water column maintained by a wedge accomplishes this. Dry-coupled scanning is not viable for the resolution PAUT is run for.

What PAUT finds on composites#

The defect families PAUT is well suited to:

• Delaminations. The air gap at the interlaminar plane reflects the ultrasonic wave early and attenuates the back-wall echo. PAUT C-scans show delaminations as bright patches against the intact-laminate background.
• Void content clusters and porosity. Distributed scatter from voids attenuates the back-wall signal. PAUT cannot resolve individual sub-millimeter voids but flags zones of elevated porosity.
• Bond defects in honeycomb-core and adhesive joints. Disbonds present similarly to delaminations: early reflection from the air gap, attenuated through-thickness signal.
• BVID. The internal delamination zone under a subtle surface mark is the diagnostic target. PAUT C-scans typically show the BVID damage zone as several times larger than the visible surface mark.
• Laminate thickness. Time-of-flight to the back wall gives a thickness map that flags grind-throughs, ply drops, and excess buildup.

PAUT is less well suited to translaminar (in-plane, through-fiber) cracking, which presents as a small reflector at an awkward angle to the typical inspection geometry, and to deep voids in very thick composites, where the back-wall signal is lost to attenuation before the defect is reached.

Confusion points#

PAUT versus conventional UT. Conventional ultrasonic testing uses a single-element transducer at a fixed beam angle. PAUT uses a multi-element transducer with electronic beam steering, sweeping, and focusing. The acquisition coverage per setup is far higher and the inspection time per part is far lower.

PAUT versus FMC/TFM. Full Matrix Capture is an acquisition strategy in which every array element fires in turn while all elements receive, capturing the complete element-to-element dataset. The Total Focusing Method is the post-processing algorithm that synthetically focuses at every pixel in a region of interest. FMC/TFM delivers optimal lateral resolution at all depths but is slower to acquire and more compute-heavy to reconstruct than beam-steered PAUT. FMC/TFM is more common in high-precision sizing and research than in routine field scans.

PAUT versus pulsed thermography. Both are common composite NDT modalities. PAUT is contact (or immersion) and depth-resolved through the laminate thickness. Pulsed thermography is non-contact and surface-emissivity-based, with depth reach limited by the thermal-wave attenuation. The methods are complementary, not interchangeable.

PAUT versus X-ray CT. PAUT is fast and portable; X-ray CT is slow, expensive, fixed-installation, and requires radiation safety. PAUT is appropriate for production scanning and field inspection; CT is appropriate for failure analysis and high-resolution forensics.

A C-scan is not a 3-D model. A PAUT C-scan is a top-down 2-D map of an attenuation or amplitude metric across the scanned area. Depth comes from time-of-flight at each scan point, not from a 3-D reconstruction. CT delivers a true 3-D model; PAUT delivers a depth-aware 2-D map.

Related terms#

• Delamination: the primary structural defect that PAUT is run to detect on composite skins.
• BVID (barely visible impact damage): the composite damage class for which PAUT is the workhorse imaging modality.
• Void content: the manufacturing-anomaly metric that PAUT attenuation maps flag.
• Non-destructive testing (NDT): the broader practice that PAUT is one specific implementation of.