Why orientation and location matter more than length
A blade is a hollow composite beam. The bending load runs along the blade in the spar caps, the shear is carried by the webs and the shells, and the parts are held together by adhesive bond lines along the leading edge, the trailing edge and the web flanges. A crack matters in proportion to how much load runs across it:
- A crack running across the blade, chordwise, cuts fibres that carry bending. Over a spar cap that is the most serious crack a blade can have.
- A crack running along the blade, spanwise, usually follows a bond line or a panel edge. It is a joint opening, which is serious but progresses differently.
- A crack that stays in the coating cuts nothing structural, but it is the point at which water and ultraviolet reach the laminate.
Length is the last thing to look at, not the first. A 50 mm transverse crack over the spar cap matters more than a metre of gelcoat crazing.
Transverse cracks
Transverse cracks run chordwise across the shell, at right angles to the blade axis. Over the spar cap they indicate that the laminate carrying bending has cracked, usually in the suction-side cap under compression. Contributing causes are a laminate wrinkle that reduced compressive strength, a ply drop that concentrated stress, or a bond line disbond that let the panel buckle. A transverse crack over a spar cap is structural until proven otherwise; it should be treated as a stop-and-assess finding rather than a repair item.
Away from the caps, a transverse crack in a sandwich panel is less critical but still marks a load path that has been interrupted, and it tends to grow towards the cap. The question to ask of any transverse crack is where it sits relative to the spar cap, in millimetres, and whether it has grown since the last inspection.
Longitudinal cracks and bond line openings
Longitudinal cracks run spanwise. On the shell they usually follow the edge of the spar cap or a panel boundary, and they indicate that the panel is moving relative to the cap, which points to a disbond beneath. Along the trailing edge they are the outward sign of the bond line opening, which is covered in detail in trailing edge cracks. Along the leading edge they usually follow erosion that has cut through to the bond.
The mechanism behind most longitudinal cracks is adhesive: a bond that was starved, too thin, poorly cured or voided at manufacture, and that has opened under fatigue. Once open, a bond line admits water, and a wet bond line in a freezing climate opens faster. Longitudinal cracks grow, and the rate of growth between inspections is the most useful number an inspection report can give about them.
Trailing edge and leading edge cracks
Edge cracks are the ones most often reported and most often misgraded. The trailing edge carries edgewise loads through a long adhesive joint near maximum chord, and a trailing edge crack there is a structural finding. Further outboard, where the chord is small, a short trailing edge split is usually a repair item.
At the leading edge, cracking almost always follows leading edge erosion. The sequence is coating loss, pitting, gouging, then exposure and delamination of the laminate, then a crack that reaches the bond line. Up to the point where the laminate is exposed the finding is aerodynamic. After it, water is in the structure and the finding is structural. An inspector who records the erosion stage, not just the crack, makes that call possible.
Root transition cracks
The root transition is where the circular root becomes an aerofoil, and it is a region of thick laminate, changing geometry and high stress. Cracks here are rarer than edge cracks and more serious: they can indicate the early stage of root insert failure, fatigue in the thick root laminate, or moisture that has reached the inserts. A crack in the root transition, or whitening in the root laminate seen from inside, is a finding that justifies an internal inspection and, where the blade type has a root history, a check of bolt tension.
Coating and gelcoat cracks
Gelcoat cracks and crazing are the most numerous finding on an ageing blade and the least structural. They come from ultraviolet, thermal cycling, and the coating's own brittleness, and a network of fine cracks over a large area usually means nothing more than an ageing coating.
Two cases deserve attention. A line of coating cracks that follows the edge of the spar cap, or runs parallel to it, is not a coating problem: it is the coating showing where the laminate beneath is straining, and it should be treated as a possible longitudinal crack. And coating cracks that have opened enough to hold water become the entry point for the leading edge and trailing edge problems above. Cosmetic and irrelevant are not the same word.
How to tell a structural crack from a cosmetic one
From a good inspection record, the questions in order:
- Orientation. Chordwise or spanwise?
- Location. Distance from the root in metres, and position relative to the spar cap, the bond lines and maximum chord.
- Depth. Coating only, into the laminate, or through the shell? A close photograph usually shows fibres if the laminate is cut.
- Growth. Compared with the last inspection, has it lengthened, widened or been joined by others?
- Behaviour under load. Does it open when the blade is loaded, which a crawler or a rope access technician can sometimes see, and a sensor can sometimes hear?
A crack that is chordwise, over the spar cap, into the laminate and growing is structural. A crack that is spanwise along a bond line and growing is a joint failure in progress. A crack in the coating only, not aligned with any structure and not growing, is a maintenance item. Everything in between needs an engineer, and how to read the inspection report that describes it is a skill worth teaching to the people who decide.
When a crack requires repair, and when the blade should be stopped
The decision framework is set out in when does blade damage require repair. For cracks specifically: coating cracks are repaired for durability on the normal maintenance cycle; edge cracks outboard of maximum chord are repaired when found; trailing edge cracks near maximum chord, longitudinal cracks along the cap, and any transverse crack over a spar cap are assessed by an engineer before the turbine runs again. A repair that is carried out on a structural crack without the structure being assessed is a crack with a patch on it, and it will be back.
Frequently asked questions
Are all wind turbine blade cracks structural?
No. Most cracks recorded in inspections are coating cracks, which are cosmetic and a durability concern. Structural cracks are those that cut laminate carrying load or open a bond line: transverse cracks over the spar cap, longitudinal cracks along the cap or the trailing edge, and cracks in the root transition.
What is the difference between a transverse and a longitudinal crack?
A transverse crack runs across the blade, chordwise, and cuts the fibres that carry bending. A longitudinal crack runs along the blade and usually follows a bond line or a panel edge, indicating a joint opening. Transverse cracks over the spar cap are the more urgent.
Can a gelcoat crack become a structural problem?
Not by itself, but it can be the entry point. Coating cracks that hold water lead to erosion and bond line damage at the edges, and a line of coating cracks that follows the spar cap edge is often the laminate beneath showing strain.
Should the turbine be stopped for a blade crack?
For a transverse crack over the spar cap, a growing longitudinal crack along the cap, a trailing edge crack near maximum chord, or a root transition crack, the blade should be assessed by an engineer before further operation. For coating cracks and small outboard edge cracks, 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 a crack is reported and the question is whether it is structural, our blade failure investigation and root cause analysis establishes what the crack is, why it is there and whether other blades of the same type are likely to share it. For inspection, operations and claims teams who read these reports, our blade training teaches how to tell one crack from another and which ones cannot wait. If you have a crack you are not sure about, contact us.