NDT for wind turbine blades: which methods work inside the blade

Why blades need NDT at all

A visual inspection sees the surface of the laminate. Many of the defects that decide a blade's life are below it: delamination between plies, a wrinkle buried under the outer layers, a dry region in the middle of a thick spar cap, a void deep in an adhesive joint. A visual record tells you that something is there and where. An NDT measurement tells you how large and how deep, and a second measurement tells you whether it has grown. That is the difference between a finding and a decision. Wind turbine blade defects that start on the inside lists the defects; this article is about measuring them.

The methods

Visual testing

The baseline method, and still the one that finds most defects. Its value depends on light, angle and coverage. A hairline crack in a glossy adhesive fillet is invisible with the light behind the camera and obvious with the light to the side. Inside a blade, visual testing done well means recording every surface, from every angle, under controlled light, with a position reference. That is what Detectra does with 8K 360° video and four individually switched LED panels. It cannot see below the surface, which is where the methods below come in.

Tap testing

The oldest composite method: tap the surface and listen. A debonded skin or a delaminated area sounds dull. It is cheap, fast and surprisingly reliable for sandwich panels, and it is entirely dependent on the person doing it. Instrumented versions measure the impact response and remove the ear from the loop.

Ultrasonic testing

Sound pulses sent into the laminate reflect from the back wall and from any interface in between. A delamination, a void or a debond returns an echo before the back wall. Phased-array ultrasonics scans a strip in one pass and produces a cross-section image. Ultrasonics is the method of choice for thick laminates and bond lines in the factory.

In service, inside a blade, it has three demands: contact with the surface through a couplant, a surface clean enough to make that contact, and time. A crawler carrying an ultrasonic probe needs to press it against the laminate, keep it there, and know exactly where it was. Those are solvable problems and they are why a sensor platform with a known position matters. The Detectra research crawler exists for groups working on exactly this.

Thermography

Active thermography heats the surface briefly with a lamp or flash and films how the heat flows away. A defect below the surface blocks or slows the flow and shows as a warm or cool patch. It is non-contact and covers an area at once, which suits it to blades. Its depth is limited, roughly to the outer few millimetres of a laminate, so it finds skin debonding and shallow delamination well and misses deep defects. Inside a blade, the power needed for the heat pulse is the constraint.

Shearography

A laser interferometry method that measures very small surface deformations while the part is loaded slightly, by heat or by a pressure change. A hidden defect deforms differently from the sound material around it and shows up as a fringe pattern. Shearography detects wrinkles, delamination and cracked core in sandwich panels, and it is non-contact. It needs a stable platform and a way to load the surface, which is why it has stayed a factory and laboratory method so far.

Laser scanning and geometry

A different question from the ones above: not what is below the surface, but what shape the surface is. A laser profile or structured-light scan records the geometry of the inner surface to a fraction of a millimetre. That measures a wrinkle's height and wavelength, the step at a debonded web foot, the width of a crack, and the sag of a buckled panel. Scanned again a year later, it measures growth. Geometry is what a repair decision and a warranty claim can be built on, and it is the first NDT capability planned for Quantra, with micrometre precision for surface geometry.

Spectral methods

Different materials and different states of the same material reflect light differently across wavelengths. Scanning a surface across wavelengths, rather than in three colour channels, can separate resin-rich from resin-starved areas, moisture from dry laminate, and heat-affected from sound resin, without contact. This is the second class of add-on on the Quantra roadmap.

Acoustic emission and other monitoring methods

Acoustic emission listens for the sound a crack makes as it grows, using sensors fixed to the blade. It is a monitoring method rather than an inspection method: it needs the blade under load and sensors installed permanently or for a test. It is mentioned here because it answers the question the inspection methods cannot, which is whether a defect is growing right now.

What in-service NDT inside a blade actually needs

Reading the list above, the pattern is clear. Every method that goes beyond the surface needs some combination of contact, power, a stable platform and a known position, and every one of those is scarce inside a blade in the field. The engineering problem is not the sensor. It is carrying the sensor to the right place, holding it there, and knowing where that place was.

That is the order LUKAN has taken. First, a crawler that reaches up to 80 % of the blade and records where it has been: Detectra. Then a platform that carries someone else's sensor with position and power: the research crawler. Then a crawler with its own measurement add-ons, laser scanning and wavelength scanning first, that scores what it finds against industry severity categories and builds a 3D model of the interior: Quantra, targeted for 2027.

Frequently asked questions

What NDT methods are used on wind turbine blades?

Visual testing, tap testing, ultrasonic testing including phased array, active thermography, shearography, laser and structured-light geometry scanning, spectral imaging, and acoustic emission monitoring. Ultrasonics and shearography dominate in the factory; visual testing and tap testing dominate in the field.

Can NDT be done inside a blade in service?

Visual testing can, and with a crawler it can cover most of the blade. Methods that measure below the surface need contact, power, a stable platform or a position reference, which is why they have mostly stayed in the factory. Crawler platforms that provide position and power are what makes them practical inside the blade.

What is the difference between detecting a defect and quantifying it?

Detecting tells you something is there and where. Quantifying tells you how large and how deep, and a second measurement tells you whether it has grown. Repair decisions and warranty claims rest on quantities, which is why measurement inside the blade is the next step after visual inspection.