The failure modes
Leading-edge erosion
Rain, hail and salt wear the leading edge coating and then the laminate, fastest near the tip where the speed is highest. It is a performance problem long before it is a structural one, and it is entirely external. First seen by: external inspection, drone or ground camera. Nothing inside the blade shows it until water gets through.
Trailing-edge bond failure and trailing-edge cracking
The trailing edge is a thin, highly loaded joint. The adhesive cracks, the crack runs along the bond, and eventually the two shells separate and the laminate cracks with them. It starts on the inside and runs for metres before the outside shows a line. First seen by: an internal inspection that reaches the outer half of the blade. By the time a drone sees it, the repair is a large one. The trailing-edge bond is the finding most often missed, because a technician from the root never gets to it.
Shear web debonding and web failure
The web-to-shell bond separates over a length, the section loses its stiffness, and the shells begin to work against each other. Buckling waves in the web and core shear cracks belong here too. Nothing shows outside until the blade's shape changes. First seen by: internal inspection, as a line at the web foot, a step, or a wave in the web. A monitoring system may see the change in stiffness as a change in the blade's vibration signature, without saying where.
Spar cap wrinkles and delamination
A manufacturing wrinkle in the unidirectional laminate concentrates strain, delamination starts around it under fatigue, and the cap loses capacity. The eventual failure is sudden. Outside: nothing. Inside: a ridge or wave in the cap surface, later a blister or a whitened area. First seen by: internal inspection for the wrinkle; a measurement method for the delamination below the surface, which is what NDT for wind turbine blades is about and what Quantra is being built for, with 2027 targeted availability.
Root laminate and insert failure
Cracks between inserts, fretting at the bushings, wrinkles in the root build-up. Rare, and very serious when it happens. First seen by: internal inspection at the root, which is the one part a technician does see well, and by root-connection monitoring sensors where fitted.
Lightning damage
The most common single cause of blade damage. Attachment away from a receptor punctures the shell; a degraded connection arcs and burns the laminate; a loose conductor chafes. First seen by: the lightning detection or strike counter for the event; an electrical continuity test for whether the path is intact; an internal inspection along the conductor for where and what. Lightning damage inside a wind turbine blade goes through it.
Water and ice
Water enters through drainage holes, receptor seals, punctures and cracks, and collects at the tip. It adds mass, unbalances the rotor, freezes, and sits against laminate. First seen by: internal inspection, where standing water and moisture staining are plain; sometimes by the turbine's own vibration and imbalance alarms.
Foreign objects and debris
Manufacturing leftovers, failed balancing weights, pieces of a broken conductor fixing. They migrate toward the tip and abrade the laminate every revolution. First seen by: internal inspection, and by nothing else.
The matrix
| Failure mode | External drone | Internal crawler | Electrical LPS test | Blade monitoring sensors | Sub-surface NDT |
|---|---|---|---|---|---|
| Leading-edge erosion | First | No | No | No | No |
| Trailing-edge bond failure | Late | First | No | Late | Depth |
| Shear web debonding | Very late | First | No | Sometimes, no position | Depth |
| Spar cap wrinkle | No | First | No | No | Extent |
| Delamination below surface | No | Sometimes | No | Sometimes | First |
| Root laminate and inserts | No | Root zone | No | Root sensors | Depth |
| Lightning damage | Attachment point | Conductor route | Path intact or not | Event | No |
| Water and ice | No | First | No | Imbalance | No |
| Foreign objects | No | Only | No | No | No |
"First" means the method that sees it earliest in the failure's life; "Late" means it sees it only once it is large.
Reading the matrix
Three things stand out. Most of the failure modes that end blades are in the "internal crawler: first" column, and those are the ones most fleets inspect least. The methods do not substitute for each other: an electrical test tells you the lightning path is broken and nothing about where; a monitoring sensor tells you something changed and nothing about what. And the methods that measure depth and extent, which are what a repair decision needs, are still mostly factory methods, which is why carrying them inside the blade is the next step.
Turning it into a strategy
A strategy is the matrix with a cadence attached. A reasonable starting point for a fleet:
- External drone inspection on a regular cycle, for erosion, surface cracks and receptors.
- Internal crawler inspection at the points where structure decides money: commissioning as a baseline, before the end of the warranty, after a recorded lightning strike, when a drone finds a trailing-edge or leading-edge crack, and on a fixed cycle for blade types with a known history. Commissioning and end-of-warranty inspections covers the first two.
- Electrical LPS testing on its own cycle and after strikes, read together with the internal recording of the conductor.
- Monitoring sensors where the fleet or the blade type justifies them, as the trigger that sends the crawler to a specific turbine.
- Sub-surface NDT on the findings that need a depth or an extent before a repair or a claim.
The record ties it together. Every method's findings for a blade belong in one place, with a position and a history, so that a drone's crack, the crawler's view of the bond behind it, and the continuity test's result are read as one story. That is what Fenestra is for: one record per finding, across suppliers and methods, in the owner's own SharePoint. From inspection findings to asset decisions sets out why that matters more than any single method.
Frequently asked questions
What are the most common wind turbine blade failure modes?
Leading-edge erosion, trailing-edge bond failure and cracking, shear web debonding, spar cap wrinkles leading to delamination, root laminate and insert failure, lightning damage, water and ice, and damage from loose objects inside the blade. Lightning is the most common single cause of damage; the structural modes are the ones that end blades.
Which blade failures can a drone not detect?
Anything that starts inside: trailing-edge bond cracks, shear web debonding, spar cap wrinkles, delamination, water in the tip, loose objects and damage along the lightning conductor. A drone sees them only once they have grown large enough to change the shell.
Is condition monitoring a substitute for inspection?
No. Monitoring sensors can flag that a blade's behaviour has changed, sometimes months before failure, but they do not say what changed or where. An inspection answers that. The two work best as a sequence: the sensor picks the turbine, the inspection finds the defect and gives it a position.