How the trailing edge is built and loaded
A blade shell is moulded in two halves. The halves are bonded together along the leading edge and the trailing edge, and to the shear webs in between. At the trailing edge, the two shells come together at a shallow angle into a thin wedge, and the gap between them is filled with structural adhesive.
Two things follow from that geometry. The adhesive bond is thick, because the gap has to absorb the tolerance of two large moulded parts, and thick adhesive is weaker per unit area than thin adhesive and shrinks more on cure. And the joint sits at the point furthest from the blade's edgewise neutral axis, so it sees the largest strain from edgewise bending.
Edgewise loading is driven by gravity and turns over at once per revolution, whatever the wind is doing. Over twenty years that is of the order of a hundred million cycles. The trailing edge bond is the part of a blade that is asked to survive the most load reversals, in the geometry least suited to it.
What causes trailing edge cracks
Bond line quality. A void, a resin-starved area, contamination or an unprepared bonding surface leaves a section of joint that carries nothing. Load transfers around it into the adjacent adhesive, which then sees more than it was designed for. Most trailing edge cracks start at a defect of this kind rather than in sound adhesive.
Bond line thickness. Too thick and the adhesive cracks from cure shrinkage and carries load poorly. Too thin, usually at a local high spot, and it becomes a stress concentration. The variation along the length is a manufacturing control issue and the reason the same blade type cracks at the same station.
Geometry at maximum chord. Edgewise strain is highest around the maximum chord region, which is why cracks concentrate there. It is also where the panel is widest and least supported, so the shells can breathe against each other and work the bond.
Panel buckling and shell flexing. Wide unsupported sandwich panels near the trailing edge can deform under load, particularly if core thickness or the core-to-laminate bond is not as designed. The flexing works the bond line along the edge.
Prior damage and repair. Transport and handling damage to the trailing edge is extremely common, and often repaired at the yard or on site. A repair that does not restore the load path leaves a discontinuity, and cracks restart at repair boundaries.
Trailing edge inserts and add-ons. Serrations, flaps and vortex generators bonded near the edge change the local stiffness and introduce their own bond lines. A retrofitted add-on is a new stress concentration on an existing structure.
How a trailing edge crack is diagnosed
Diagnosis is a sequence, and it is often stopped too early.
From the outside. A drone or ground-based image shows the crack as a line along the edge, frequently with a slight opening. The items to record are the crack length, both tip positions by radius, whether the opening is continuous or intermittent, and whether light passes through. Two measurements a year apart give the growth rate, which is the number that actually drives the decision.
By tapping. A coin or hammer tap along the edge distinguishes sound bond from void by the change in tone. It is crude, it works, and it maps the extent of a debond beyond the visible crack, which is usually larger than the crack itself.
From the inside. An internal inspection, by crawler or by a technician where the cavity allows, shows the bond from the other face: adhesive coverage, voids, and whether the crack has gone through the bond or is confined to the outer fillet. Many trailing edge cracks that look alarming from outside are cracks in the external fillet, and many that look minor have a long internal debond behind them.
By thermography or ultrasonics, where the extent behind an intact surface has to be established.
From the fracture faces, where a section can be taken. An adhesive that has separated cleanly from one substrate points to a bonding process failure: contamination, poor surface preparation, or adhesive that did not wet. Arrest marks along the growth direction indicate cyclic growth over time rather than a single overload event. This is the evidence that decides whether the crack is a manufacturing defect or ordinary fatigue, and it is destroyed by the repair.
When a trailing edge crack becomes structural
A crack in the external adhesive fillet, short, not growing, and with sound bond behind it on a tap test, is a repair item for the next campaign.
The finding changes character when any of these appear:
- the crack passes through the bond rather than the fillet, so the shells are no longer joined along that length
- the debond mapped by tapping is substantially longer than the visible crack
- the crack has turned out of the bond line and into the shell laminate, which makes it a laminate crack with a different growth behaviour
- measured growth between campaigns, particularly acceleration
- the opening admits water, which then sits in the cavity, freezes and works the joint further
- the crack is at or near maximum chord and is longer than a metre, where the remaining bond has to carry load the design assigned to the cracked length
- the same crack, at the same station, appears across many blades of the same type and batch
The last item is the expensive one. A trailing edge crack that repeats at the same radius on blades from one plant and production window is a serial defect, and the fleet exposure is the finding, not the individual crack.
What the repair has to restore
A trailing edge repair is not a cosmetic seal. It has to remove the damaged and debonded adhesive back to sound material, restore a bond of the right thickness with a properly prepared surface, and rebuild the laminate where the crack has entered the shell. A repair that fills the visible crack and leaves the debond behind it will reappear, usually at the ends of the repair, within a few campaigns.
Where the crack may be evidence in a warranty or insurance position, the sequence matters: record and where possible sample before the repair, because once the adhesive is ground out, the question of why it failed cannot be answered.
Frequently asked questions
Can a turbine keep operating with a trailing edge crack?
Frequently yes, with a category, a defined re-inspection interval and a documented assessment, and this is the normal outcome for short, stable cracks in the fillet. A crack that has gone through the bond, is growing, or sits at maximum chord needs an engineering assessment against the design basis before it is left to run.
Why do trailing edge cracks come back after repair?
Usually because the repair addressed the visible crack rather than the debonded length behind it, or because the underlying cause, a bond line thickness problem or a panel that flexes, is still there. A repair that starts and stops inside a debonded region creates new stress concentrations at both ends.
Is a trailing edge crack a manufacturing defect or wear?
Both exist, and the fracture surfaces separate them. Clean adhesive separation from a substrate points to the bonding process. Cyclic growth through sound adhesive from a geometric stress concentration points to design or load. Repetition across a batch points to process.
How long can a trailing edge crack be before it matters?
Length alone is a weak criterion, which is why it is a poor place to set a threshold. A 200 mm crack through the bond at maximum chord is more serious than a metre of fillet cracking outboard. Position, depth through the joint and growth rate decide it.
How Apex Wind can help
We are an independent blade engineering consultancy in Denmark. We assess trailing edge findings against the design basis and advise on fitness to operate through blade design and structural assessment, and we establish cause, including whether a crack population is a serial defect, through blade failure investigation and root cause analysis. We carry out no repairs and take no referral fees. If trailing edge cracks are accumulating across a fleet and nobody has yet established why, contact us.