What delamination is, and what it is not
Delamination is the separation of plies within a laminate, or of a laminate from the core in a sandwich panel. It is distinct from a bond line disbond, which is a failure of the adhesive joint between two separately made parts, though the two are often reported together and can occur together. It is also distinct from a crack, which cuts across plies rather than between them, although a delamination that reaches the surface is usually reported as a crack.
The mechanical consequence is the same in each case: a region of the structure has stopped acting as one piece. In a spar cap that means a loss of compressive strength and a site for buckling. In a sandwich panel it means a skin that can no longer stabilise the core. Along a bond line it means a joint that is opening.
What causes delamination in a wind turbine blade
Manufacturing
Most delamination starts in the manufacturing process. Dry fibres and porosity form where the resin front did not fully reach during infusion, and dry plies are not bonded to their neighbours. Contamination on a ply, from release agent, dust or moisture, prevents the resin bonding to it. A laminate wrinkle creates an out-of-plane load in an otherwise in-plane laminate, and the plies separate at the wrinkle under fatigue. Core that was not properly wetted, or gaps between core blocks that filled with resin, delaminates at the skin-to-core interface. All of these are present from the day the blade leaves the mould and all of them are process defects, which means they are usually serial.
Impact
Lightning is the most common impact. When the current does not go down the conductor it goes through the laminate, and the resulting pressure and heat separate plies over an area far larger than the visible burn; lightning damage should always be assumed to include delamination around the attachment point. Handling damage in the yard, in transport and during lifting produces local delamination under a surface mark that may look trivial. Hail and debris strikes on the leading edge do the same on a smaller scale.
Fatigue and environment
Delamination grows under cyclic load from any of the initiators above, and it grows fastest at free edges, ply drops and geometric transitions where the interlaminar stresses are highest. Water accelerates it: once the coating is open, moisture reaches the laminate and freezes, and a small delamination becomes a large one over a few winters. Leading edge erosion that has exposed the laminate is the commonest route by which environment starts a delamination.
How delamination is detected
The methods, from the simplest to the most conclusive:
- Visual, external. A delamination near the surface shows as a blister, a whitened area, a change in surface texture, or a crack where it has broken through. Drone imagery finds these. It finds nothing deeper.
- Visual, internal. A crawler or a technician inside the blade sees the inner skin: ripples where a wrinkle has begun to separate, whitening in the root laminate, core that has moved. This finds delamination that is invisible from outside but still only what shows on the inner surface.
- Tap test. A coin or a small hammer on the shell: sound laminate rings, delaminated laminate gives a dull note. Cheap, fast, operator dependent, and useful for mapping the extent of a delamination once one is suspected.
- Thermography. Infrared imaging after heating or under solar load shows a delaminated region as a temperature difference because the trapped air insulates. It works on thin shells and near the surface, and it needs the right conditions.
- Ultrasonic testing. The conclusive method for delamination inside a thick laminate. A transducer sends a pulse through the thickness and the reflection from a separation shows its depth and extent. Phased array systems map an area. This is the method that confirms a spar cap wrinkle has delaminated, and the one an owner should ask for when a serial defect is suspected.
The gap between the first and the last of these is the reason delamination is under-reported in fleets that rely on drone inspection alone.
When a delamination is structural
The criteria, in order of weight:
- Where it is. A delamination in a spar cap, in the root laminate, or at a bond line is structural. A delamination in a sandwich panel skin away from the caps is usually a durability finding.
- How large it is, in the load direction. A spar cap delamination is measured along the blade, because that is the length over which the cap has lost its compressive stability. Small and stable can be monitored; long or growing cannot.
- Whether it is growing. Two inspections by the same method a year apart answer this. One inspection does not.
- Whether it is wet. An open path from the surface to the delamination changes the timescale, particularly in a freezing climate.
- What caused it. A manufacturing delamination is likely to be present in other blades of the same production; a lightning delamination is not. The cause decides whether this is one blade's problem or a fleet's.
A delamination that is in a spar cap, growing, and of manufacturing origin is a stop-and-assess finding with fleet implications. One in a panel skin, stable and dry, goes on the repair list.
Repair, and what a repair does not fix
Small, shallow delaminations are repaired by removing the affected plies and laminating replacements, scarfed into the surrounding sound laminate. That is standard field work for a competent repair company. Large delaminations in the spar cap or root are not field repairs: they need the blade on the ground or in a factory, engineering of the repair laminate, and documentation that a certification body or a buyer will later accept. Which repairs can be done on the turbine and which cannot is a decision that should be made by someone independent of the repair provider.
A repair does not fix the cause. If the delamination came from a wrinkle the process produced, the same wrinkle is in the other blades, and repairing one changes nothing about the fleet's exposure.
Frequently asked questions
What is the difference between delamination and disbond?
Delamination is a separation between plies within one laminate, or between skin and core in a sandwich panel. A disbond is a failure of the adhesive joint between two separately made parts, such as the shells and the web. They are often found together and both mean the structure has stopped acting as one piece.
Can delamination be seen from outside the blade?
Only when it is near the surface: as a blister, a whitened area or a crack. Delamination inside a thick laminate, which is where it matters most, needs ultrasonic testing to confirm and to size.
Does delamination always need repair?
No. A small, stable, dry delamination in a sandwich panel skin away from the spar cap can be monitored. A delamination in the spar cap, the root or a bond line, or one that is growing, needs an engineering assessment before the turbine continues to operate.
Is delamination a serial defect?
When it comes from the manufacturing process, usually yes: dry fibres, contamination and wrinkles are produced by the process and will be present in other blades from the same production. When it comes from lightning or impact, no.
How Apex Wind can help
We are an independent blade engineering consultancy in Denmark, independent of every turbine and blade manufacturer, and we sell no repairs or inspection services. When delamination is found and the question is whether it is structural, our blade failure investigation establishes the cause and whether it is serial, and our blade design and structural assessment assesses whether a proposed repair restores the structure the design assumed. If you have a delamination finding and need an independent view, contact us.