Bond-line defects

The two shells of a blade are bonded to each other along the leading and trailing edges, and to the shear webs along the web feet. These adhesive joints are the most common place for a structural defect to start.

Missing adhesive and voids. The adhesive is applied as a paste before the shells are closed, and the closure has to squeeze it into full contact. Where it did not, the joint has a gap. From the inside, a void shows as a hollow in the fillet or a place where the adhesive has not reached the laminate. Small voids are common and mostly harmless; long runs of missing adhesive are not.

Adhesive cracks. A crack along the web-to-shell bond or in the trailing-edge joint, often following the edge of the adhesive fillet. It can be hairline and only visible under grazing light, which is why the direction and control of the light matters more inside a blade than the camera resolution.

Debonding. The adhesive has separated from the laminate on one side, so the joint transmits no load across the gap. On a shear web this changes the stiffness of the section, and the two shells begin to move relative to each other under load. A debonded web foot often shows as a fine dark line where the fillet meets the shell, sometimes with a visible step.

Transverse cracks in the trailing edge. Short cracks across the trailing-edge adhesive, repeated at intervals. On their own each one is small; a row of them along the outer third of the blade is a pattern to track between inspections.

Laminate defects

Wrinkles. Out-of-plane waves in the unidirectional fibres of the spar cap or the root laminate, left over from manufacturing when plies did not lie flat before infusion. A wrinkle concentrates strain in the fibres that bend around it and is a known starting point for delamination under fatigue. From the inside, a wrinkle in the spar cap shows as a ridge or a wave in the surface, sometimes only as a change in how the light reflects.

Delamination. Separation between plies of the laminate. It is the defect most often named in blade failure investigations, and it is the hardest to see from the surface. Inside the blade, a delamination that has reached the inner surface shows as a blister, a whitened area or a raised edge. One that has not reached the surface needs a measurement method, which is the gap Quantra is being built for.

Dry fibres and resin-starved areas. Regions where the infusion did not wet the fibres fully. They look pale and matt against the surrounding laminate. They are weak points for both strength and moisture.

Cracks in the trailing-edge laminate. The trailing edge is a thin, highly loaded panel. Cracks through the laminate itself, as opposed to the adhesive, are more serious and often accompany a bond failure behind them.

Sandwich and core failures

The aft shell panels and the shear webs are sandwich structures: a foam or balsa core between thin skins. Two failures are worth knowing:

Core shear cracks. A crack through the core, parallel to the skins, that lets the skins slide against each other. From the inside, a shear-failed panel looks intact until it is loaded; the clue is often a wave or a soft-looking region in the skin.

Skin-to-core debonding. The skin has lifted from the core. It appears as a bulge or a region where the skin drums instead of lying flat. Panels near the maximum chord are the usual place.

Buckling waves in the web. A shear web that has buckled shows visible waves across its height. This is a load-history problem as much as a manufacturing one, and it belongs in the record even when no crack is visible yet.

Root zone defects

The root carries the whole blade into the pitch bearing through bolted inserts or T-bolts in a very thick laminate. Defects here are rare and serious:

  • Cracks in the laminate between inserts, or radiating from an insert.
  • Movement at the inserts, visible as fretting marks, rust staining or debris around the bushing.
  • Wrinkles in the root build-up, where the thickness changes fastest.

The root is also the only part most technician inspections ever see, so it is often the best documented part of the blade. The problem is what lies beyond it.

Lightning protection system

The down conductor runs the length of the blade from the tip receptors to the root. Damage shows as burn or arc marks on the conductor or its connections, loose or corroded joints, and delamination or charring in the laminate around a receptor. After a recorded strike, the conductor route is the first thing to inspect from the inside. Lightning damage inside a wind turbine blade goes through it step by step.

Water, debris and leftovers

Blades take on water through drainage holes, receptor seals and cracks, and it collects at the lowest point when the blade points down. Standing water at the tip adds mass and freezes. Moisture staining along a bond line is a sign of a path through the shell. Loose objects, from manufacturing leftovers to failed balancing weights, abrade the inner surface as the blade turns and can block drainage.

None of these are structural defects in themselves. All of them are early warnings, and all of them are visible only from the inside.

What matters is the record, not the single finding

Almost every defect above has a benign version and a serious one, and the difference is whether it grows. A single inspection cannot tell the two apart. Two inspections of the same blade, with each finding positioned along the blade and photographed from the same angle, can. That is why an internal inspection record needs three things: the whole blade, not just the root; a position for every finding; and a viewer that lets the next inspector look at the same place again. Detectra provides the first two through a distance-tagged 8K 360° recording of up to 80 % of the blade length; Fenestra provides the third.

Frequently asked questions

Which blade defect is the most serious?

A debonded shear web or a cracked trailing-edge bond over a long length, because both change the load path and grow under fatigue. Delamination in the spar cap is equally serious but far less common. Small adhesive voids and minor moisture staining are common and, on their own, rarely a concern.

Can internal defects be seen from the outside?

Not until they are large. Bond-line cracks, web debonding and wrinkles leave no mark on the shell while they are small. By the time an external inspection sees a line on the trailing edge, the bond behind it has often failed over a much greater length.

How is a defect tracked between inspections?

By recording its position along the blade and its appearance, then inspecting the same position again. A distance-tagged recording of the whole blade interior makes this repeatable; a handful of photographs from the root does not.