Composite NDT: Inspecting CFRP and GFRP with Ultrasonic Testing

Composite materials are now found in almost every high-performance industry — from the fuselage of a Boeing 787 to the blades of an offshore wind turbine. Their exceptional strength-to-weight ratio has made them indispensable. But that same complex internal structure that makes composites so capable also makes them genuinely difficult to inspect.

This article explains why ultrasonic testing of composites presents unique challenges, what defects engineers need to detect, how different ultrasonic methods compare, and why Matrix Array Ultrasonic Testing (MAUT) has become the method of choice for both CFRP and GFRP inspection. Do you want something? 

What makes composites difficult to inspect ultrasonically?

Unlike metals, which are homogeneous and isotropic, composite materials are anisotropic — their properties vary depending on direction. They are built up from layers of fibres embedded in a resin matrix, and it is this layered, multi-phase structure that creates the core challenge for ultrasonic NDT.

When a sound beam enters a composite, the different material phases — fibres, resin, any voids or inclusions — absorb, scatter, and deviate that beam. The further the sound travels through the material, the more it is attenuated and distorted. This means that detecting small features deep in a thick laminate requires a careful balance between frequency and penetration: higher frequencies give better resolution but are absorbed more rapidly, while lower frequencies penetrate more reliably but reduce sensitivity to small defects.

CFRP and GFRP present slightly different challenges from one another:

CFRP is typically used in thinner, high-precision aerospace and automotive structures. The carbon fibres provide excellent stiffness but create strong acoustic anisotropy. Inspection frequencies of 3.5–5 MHz are commonly used, offering a good balance of penetration and resolution for the 2–16mm thicknesses typical in CFRP panels.

GFRP is used extensively in wind turbine blades, marine structures, and industrial pressure vessels — often at much greater thicknesses, sometimes exceeding 20mm. Glass fibre is more acoustically attenuative than carbon fibre per unit thickness, and the coarser fabric architecture of many GFRP components further scatters the sound beam. Lower frequencies of 1.5–2.5 MHz are typically required.

What defects are we looking for?

Understanding the defect types is essential to selecting the right inspection approach. In both CFRP and GFRP, the most safety-critical defects are those that separate plies or reduce the effective cross-section of the laminate.

Delamination

Delaminations are planar separations between adjacent plies, caused by impact, manufacturing errors, or in-service fatigue. They are among the most dangerous defects in composites because they dramatically reduce through-thickness strength and can propagate under compressive loading. Critically, delaminations frequently produce no visible surface indication — the outer skin may look perfect while significant internal damage has occurred.

Ultrasonic C-scan imaging detects delaminations by identifying areas where the back-wall echo is lost or significantly reduced, indicating that sound energy is being reflected back from an internal interface rather than passing through the full material thickness.

Impact damage and BVID

Barely Visible Impact Damage (BVID) is a particular concern in aerospace. Aircraft composite structures are routinely struck by ground equipment, tools, or foreign objects during maintenance. At low energy levels, such impacts can cause extensive internal delamination while leaving the outer surface cosmetically intact or nearly so. The decision of whether to ground an aircraft or release it for flight depends on a rapid, quantitative assessment of how deep that damage extends and how many plies are affected.

The dolphicam2 was used to inspect Boeing 787 fuselage skin samples with induced BVID, successfully identifying the damaged area and its depth — in a fraction of the time required by traditional guided A-scan inspection. Scan data was immediately shareable with maintenance engineers and the aircraft manufacturer for post-processing and deeper analysis. Read the full case study

Porosity

Porosity — the presence of small voids distributed through the laminate — occurs when air is trapped during manufacturing or when incorrect cure cycles leave resin-rich regions with micro-bubbles. High porosity levels reduce the interlaminar shear strength of the composite. Unlike delamination, porosity produces a diffuse reduction in back-wall amplitude rather than a discrete reflection, requiring sensitive amplitude C-scan imaging to detect and quantify.

Wrinkles and fibre waviness

As wind turbine blades grow beyond 100 metres in length, consistent quality over vast, complex lay-ups becomes increasingly difficult. Glass fabrics must be laid precisely into the mould and remain in place throughout the resin infusion process. When this goes wrong, out-of-plane wrinkles form within the bulk laminate — fibre deviation that is completely invisible from the surface but which locally reduces the strength of the blade.

Dolphitech inspected a curved wind turbine blade section known to contain natural wrinkles, using a dolphicam2+ with a 1.5 MHz TRM. The out-of-plane wrinkles were clearly detected and characterised in both amplitude and time-of-flight (ToF) C-scan views, allowing the width, depth, and extent of each wrinkle to be measured with point and line markers. Without the ability to detect and quantify such wrinkles, manufacturers must apply conservative knockdown safety factors that add weight and reduce blade efficiency. Read the full case study

Disbonds and kissing bonds

In bonded composite structures — repair patches, bonded assemblies, sandwich panels — the integrity of the adhesive bond line is critical. Disbonds are areas where adhesion has failed entirely. Kissing bonds are subtler: the surfaces appear to be in contact but carry no load, because contamination or insufficient cure has prevented proper adhesion from forming. Both are challenging to detect, and kissing bonds in particular require sensitive amplitude-based methods rather than simple time-of-flight imaging.

Why MAUT outperforms conventional UT and PAUT for composite inspection

Traditional single-element ultrasonic testing requires a skilled technician to manually raster-scan the probe across the surface, interpreting A-scan waveforms in real time. This is slow, highly operator-dependent, and produces no spatial image. It is the method that the impact damage case study customer was moving away from.

Phased Array Ultrasonic Testing (PAUT) improved on this by allowing electronic beam steering through a 1D array of elements, producing cross-sectional B-scan images. But phased array still operates with a linear aperture — it steers the beam in one plane but has no spatial awareness in the perpendicular direction without mechanical scanning.

Matrix Array Ultrasonic Testing (MAUT), Dolphitech’s core technology, uses a 2D grid of elements. This generates a live, two-dimensional C-scan image — a top-down map of the material — in real time, without the need for mechanical encoding. The entire aperture is interrogated simultaneously, and both amplitude and time-of-flight images are available instantly.

For composite inspection, this matters in several concrete ways:

• Speed. A single placement of the MAUT TRM captures a two-dimensional image of the material beneath it. The dolphicam2 is ready to use in under 60 seconds, and large panels can be mapped in minutes rather than hours. For hazardous environments such as aircraft servicing pans or busy industrial environments, speed reduces the exposure to the inspector. As an added benefit, asset downtime is reduced.
• No operator interpretation of A-scans. The colour-coded C-scan images are immediately intuitive. Operators do not need to mentally construct a spatial picture from sequential waveforms — the image presents the defect directly in its correct location and depth.
• Simultaneous amplitude and ToF imaging. For composites, this dual-view capability is essential. Time-of-flight images reveal depth and thickness information with colour coding; amplitude images catch defects — such as porosity or shallow wrinkles — that do not produce a clean reflective interface. Both are available simultaneously and in real time.
• Curved and complex geometry. The Aqualene delay line used on Dolphitech’s TRMs is a rubbery, conformable material that maintains acoustic coupling on slightly curved surfaces without requiring custom fixtures. This makes it practical for inspecting wing skins, pressure vessels, and turbine blade sections in situ.
• Data storage and retrospective analysis. Full waveform datasets are saved, allowing gate settings to be changed retrospectively. In the wind turbine blade study, this enabled both near-surface wrinkles and back-face delaminations to be characterised from a single scan pass — something that would have required multiple setups with conventional equipment.

CFRP inspection in practice

Monolithic CFRP: depth penetration and resolution

An Airbus-supplied CFRP step block — containing back-surface steps of different heights and flat-bottomed holes of different diameters — was used to benchmark the dolphicam2’s performance across the full thickness range of typical aerospace CFRP.

Using the 3.5 MHz TRM (the recommended frequency for CFRP aerospace applications, approved for both aerospace and automotive inspection), the dolphicam2 successfully penetrated the full 16mm thickness of the block, resolving back-wall echoes at all depth steps. Flat-bottomed holes of 4–5mm diameter were clearly resolved in the ToF C-scan with colour-coded depth information; 2–3mm holes, where the flat surface is insufficient to produce a strong ToF reflection, remained detectable in the amplitude C-scan. The real-time availability of both views means operators can use each to reinforce the other. Read the full case study

Curved CFRP repair panels

Aircraft repair introduces additional complexity: repair patches may be circular, applied from the inside of a panel, and the inspection must be performed from the curved outer surface. A 300mm × 300mm CFRP aerospace repair panel — curved with a diameter of ~400mm, average thickness 2mm, containing four repair areas — was inspected using the Rapid Mapper MK II scanner paired with a dolphicam2+ and a BFx 2.5 MHz TRM.

The Rapid Mapper’s pivoting head bracket allowed the TRM to follow the contour of the curved surface across both X and Y axes. The full panel was scanned in under 10 minutes. Each repair area was isolated with a statistical analysis box in the dolphicam2 software, returning amplitude, width, height, and deviation measurements. Thickness data was exported via DolphiDepth to CSV at 5mm grid resolution for records.

This approach is directly applicable to Formula 1 composite bodywork, fast jet inspections, and repair quality assurance in aerospace MRO. Read the full case study

GFRP inspection in practice

Industrial pressure vessels with polypropylene lining

GFRP is widely used in chemical processing plants for pressure vessels and pipe work, often with a polypropylene (PP) inner lining that provides chemical resistance. A 19mm sample — 16mm of structural GRP and 3mm of polypropylene — was inspected to de-risk a planned site inspection campaign on behalf of a Scottish NDT service provider.

The TRM-EC-1.5 MHz was selected for its low-frequency penetration capability and its Aqualene delay line, which maintains coupling on the slightly curved surfaces of vessels with diameters exceeding 600mm. The panel was inspected from both faces (representing outside and inside access). From both surfaces, reliable back-wall echoes were confirmed, internal delamination checks were performed, and the GFRP-to-polypropylene interface was resolved — giving the service provider confidence in the inspection method before mobilising to site. Read the full case study

Wind turbine blade skins: wrinkle detection

As described above, out-of-plane wrinkles in GFRP wind turbine blade laminates are a significant manufacturing quality issue that is invisible to visual inspection. The dolphicam2+ with a TRM-EC-1.5 MHz and a single-probe wheel encoder successfully mapped a 56cm × 15cm section of blade skin, detecting three distinct wrinkles, measuring their width (typically 10–15mm), and resolving their depth within the 20mm laminate. The full waveform save functionality allowed wrinkle analysis and back-face delamination analysis to be conducted from a single encoded scan. Read the full case study →

Selecting the right TRM for your composite application

Dolphitech’s modular transducer system means the same dolphicam2 instrument can be configured for the full range of composite inspection scenarios simply by changing the TRM:

What the composites page covers

For a full overview of the defect types detectable across all composite material and construction types — including CFRP, GFRP, GFRP to balsa sandwich, and thick marine CFRP spars — see Dolphitech’s Composites technology page, which includes a capability reference table covering thickness measurement, delamination, debonding, porosity, voids, impact damage, lightning strike damage, and Foreign Object Debris (FOD) detection.

Summary

CFRP and GFRP share the same fundamental inspection challenge: a heterogeneous, anisotropic structure that attenuates and scatters ultrasound in ways that single-element UT and even phased array equipment struggle to manage efficiently in the field.

MAUT addresses this directly. The 2D matrix array generates a real-time, spatially accurate C-scan image without mechanical raster scanning. The simultaneous availability of amplitude and ToF imaging catches both reflective defects and diffuse conditions like porosity. The Aqualene delay line handles curved surfaces without fixtures or immersion tanks. And the dolphicam2’s full waveform dataset storage means a single scan pass can answer multiple inspection questions retrospectively.

Whether the application is detecting BVID in a Boeing 787 skin, mapping repair patches in a curved CFRP aerospace panel, delamination-checking polypropylene-lined GFRP pressure vessels in a chemical plant, or identifying wrinkles in a wind turbine blade laminate — the dolphicam2 brings the same rapid, clear, quantitative imaging to all of them.

Ready to see what MAUT can detect in your composite materials? Send us a sample for a no-obligation feasibility inspection 

Or explore our MAUT Core Units and MAUT Transducers (TRMs) to find the right configuration for your application.