PAUT Inspection of Composites (CFRP & GFRP): What works, what limits performance, and how to choose a defensible setup

Phased Array Ultrasonic Testing (PAUT) is widely used to inspect composite structures, notably carbon-fiber (CFRP) and glass-fiber (GFRP) reinforced polymers. Compared with many metallic inspections, composite PAUT is more constrained by attenuation, anisotropy, and part geometry. As a result, inspection reliability depends heavily on matching probe frequency, coupling strategy, and scanning approach to the component and the damage mechanism.

Typical composite applications

PAUT is commonly applied to:

• Aerospace CFRP skins, stringers, spars, bonded repairs and assemblies

• Wind turbine blades (skins, webs, bonded joints)

• Honeycomb and sandwich panels used in aerospace and industrial structures

The most common targets are internal defects such as delaminations, disbonds, porosity/voids and impact-related damage.

Which defect types PAUT detects well

Many critical composite flaws create planar or volumetric reflectors that respond well to ultrasound:

• Delaminations between plies

• Skin-to-core disbonds in honeycomb

• Porosity clusters and inclusions

• Barely visible impact damage (BVID) that forms multi-layer delamination networks

PAUT supports efficient mapping of these flaws via A-, S- and C-scan representations, particularly with encoded scanning.

Frequency selection: penetration vs resolution

Composite frequency selection is not a preference; it is a trade-off:

• Lower frequencies are often used when penetration is limiting (thicker or highly attenuative regions)

• Higher frequencies are used when resolving smaller delaminations or porosity is the priority (thin skins, repairs)

Because laminate architecture and thickness vary widely, frequency ranges should be validated on representative reference parts or calibration standards.

Coupling, alignment, and scanning repeatability

In practice, composite PAUT results degrade quickly when coupling is unstable or probe alignment changes across curvature and local features. This is why composite inspections often use:

• Delay lines (solid or water path)

• Soft contact interfaces or roller probes

• Scanning fixtures and encoded scanners

These tools reduce misalignment errors and improve repeatability across large surfaces.

Known limitations and complementary methods

Some defect mechanisms remain difficult for ultrasound alone, especially weak/kissing bonds or localized fiber breakage without strong planar interfaces. In such cases, PAUT is often most effective within a combined strategy, potentially complemented by other NDT methods depending on component design and certification constraints.

Conclusion

PAUT is a reference ultrasonic technique for composites when procedures are tailored to laminate structure, thickness, and defect type. The most defensible results come from qualified setups that control frequency selection, coupling stability and scanning repeatability. For production and maintenance environments, probe consistency and availability also influence execution reliability and project timelines.