Author: Bladena

A crack along the trailing edge rarely looks like much: a thin line on the back of the blade, quick to note in an inspection report and easy to leave for the next visit. That instinct to log it and move on is one of the costliest habits in blade maintenance.

Here is the uncomfortable part: by the time that thin line is visible at all, the story is already well underway. A failing trailing edge bondline starts from the inside. While peeling forces work away at the adhesive, there is nothing for a drone, a rope team or even an internal inspection to find. When the damage finally shows itself as a hairline crack on the trailing edge, it has already reached a critical state, and that is the moment to act, either by repairing the bondline or by proactively removing the root cause that created the failure in the first place.

It is worth acting on, because it is one of the most common failure modes we see in Bladena’s damage data, and it is not reserved for the newest giants: blades well under 60 metres show exactly the same damage.

In short: trailing edge bondline failure is caused by the blade panels repeatedly moving away from and towards each other under load, a behaviour known as breathing, which transfers peeling forces directly into the adhesive holding the trailing edge together until it cracks and, in time, opens.

What the trailing edge actually does

A blade is two moulded shells (a pressure side and a suction side) bonded together. They meet along the trailing edge, the thin rear edge that runs almost the full length of the blade. That joint does real structural work: it holds the two shells of the blade together, acting as a single stiff structure under load.

Why the bondline starts to fail

As the blade rotates, gravity pulls on it in alternating directions depending on which side of the blade is facing the ground, introducing a sideways, edgewise motion again and again. In the widest part of the blade, the max chord region, that sideways fatigue drives the two shell panels away from and towards one another in a breathing motion. Breathing is the principal driver behind most trailing edge bondline failures.

Every breathing cycle transfers peeling stress into the adhesive, and those cycles accumulate relentlessly over a blade’s operational life. Over time the bond degrades, a crack forms between the adhesive and the blade panel, and once that crack has worked its way to the trailing edge, the edge can split open. At that point the blade needs to come out of service until it is repaired. How long that final stage takes varies from blade to blade, and it is neither easy to predict nor possible to control.

Bigger blades, higher stakes

Blades keep getting longer, and the loads on them are growing faster still. As Mathias Reding, Managing Director at Bladena (RES), puts it: “While the frequency of blade damage may have only slightly increased, the severity of damage has risen sharply with blade length. This leads to higher costs related to downtime, repair and replacement.” (How do we make wind farm blade failures a problem of the past?) The failure mode has not changed. The stakes have.

What inspection sees and when

This is where the hidden start of the failure matters most. While peeling damage progresses inside the bondline, there is nothing on the surface for a drone or a rope team to find. The damage only becomes detectable at a late stage, when the first hairline crack appears and the bondline is already in a critical state. So when an inspection confirms a crack and roughly how long it is, it is telling you the story late and it still cannot tell you why. Two blades can present near-identical cracks in a photograph and be on completely different trajectories: one stable for years, one months from splitting open.

Timing adds a second challenge. Because the damage develops out of sight and only becomes visible late, a routine inspection interval can miss the window entirely: the crack appears after one visit and the trailing edge has split before the next. Where repeat damage is suspected, targeted inspections are strongly recommended, focused on the max chord trailing edge and timed to the risk.

Severity, in other words, is a structural question, not a visual one. Without knowing the driver behind a crack, an operator ends up repairing the symptom while the cause keeps working. That is how sites end up paying to fix the same trailing edge twice or more.

Regular inspection is still essential: it is how cracks are found at all. But on its own it tells you where the damage is, not why it is there or how fast it will grow.

What trailing edge repair actually costs

Caught early, this is a manageable job: a bonded, up-tower repair carried out while the blade keeps producing, at modest cost. Left to run, the economics change fast. The crack propagates, the trailing edge opens, and the options narrow to a major section repair or a full blade replacement: a crane, a blade out of service, and lost production for the duration. Offshore, add vessels and weather windows to the bill.

The figure an asset manager should notice is not the cost of one early repair; it is the multiple of that cost paid when a small crack is logged, left for the next visit, and grows into a section repair.

D-String®: removing the root cause of the failure

Bondline failures in the trailing edge have a structural root cause, and that means it can be addressed structurally. Bladena’s D-String® retrofit was developed to do exactly that: it removes, or strongly reduces, the breathing motion between the trailing edge panels, taking away the peeling stresses that drive the bondline to failure. Installed proactively, it stops the damage from forming; installed after a crack has appeared, it stops the cause from continuing to work while the repair holds.

Because a targeted intervention only has to be paid for once, it avoids the cost of repairing the same trailing edge again and again. As a welcome side effect, it also mitigates transverse cracks in the large max chord panels. But that is a story for a later blog in this series.

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FAQs

What is trailing edge bondline failure in wind turbine blades?

Damage to the adhesive bondline that joins the two shells of a blade along its trailing edge. It begins as peeling inside the bondline, invisible from outside, develops into hairline cracks and, over time, lets the trailing edge split open. It is one of the most frequent structural failures in the field, including on blades under 60 metres.

What is the max chord region?

The widest part of the blade, where the distance from leading edge to trailing edge is greatest, sitting between the root transition and mid-span. Because the shell panels here are large, curved and relatively unsupported (in many designs the distance between the trailing edge and the nearest shear web is large), it is the region most prone to the breathing motion that drives trailing edge bondline failure.

What is breathing between the blade panels? 

The in-and-out movement of the blade’s two shell panels relative to each other as the blade flexes in edgewise fatigue under operation. It is most pronounced in the max chord region, and each cycle puts peeling stress on the adhesive bondline, which is why breathing is the main driver behind trailing edge bondline failure.

How serious is trailing edge bondline failure?

More serious than it looks. The bondline holds the two shells of the blade together, so once the trailing edge starts to open, the blade loses stiffness. Left unaddressed, a minor crack can escalate into a section repair or a full blade replacement. 

Can trailing edge bondline failure be repaired? 

Yes – and early repair is far cheaper. Caught early, it can often be fixed up-tower while the blade stays in service. The key is treating the cause, not just resealing the crack, or the same failure returns.

Can trailing edge bondline failure be prevented? 

Yes. The D-String^® retrofit solution is designed to mitigate the root cause, breathing, before it damages the bondline. Installed proactively, it prevents the damage from forming, or from progressing further if the damage has already started. 

Is this failure mode due to poor blade design and/or a phenomenon seen only for a few manufacturers? 

Wind turbine blades take a huge amount of skill and expertise to design then manufacture consistently to match complex drawings, work instructions and standards, within moulds that can now be longer than the wingspan of the biggest aircraft. Aerodynamic efficiency, structural strength, resilience and cost are just some of the design factors, often at odds with each other, that manufacturers must consider. So, there is no surprise that some blades will need some additional support throughout their lifetime, particularly when you consider the varying environmental conditions they are exposed to once in operation. And, the OEM can be a great source of information and expertise when it comes to operational repairs. 

Resources

• 
  Why blades fail: A trailing edge split starts long before you can see itOctober 19, 2026
• 
  Silent Blade Risk: What’s already hitting your balance sheetOctober 16, 2026
• 
  Reducing uncertainty in wind insurance claims through root cause insightApril 10, 2026

With wind turbines growing larger and offshore capacity expanding fast into sites that are harder to reach and more expensive to insure, Mathias Reding, Managing Director of Bladena, a RES company, explains why blade failure, one of the most common and least understood causes of wind turbine insurance claims, points to a data gap the industry can no longer afford.

Onshore fleets are heading in the same direction: bigger rotors, longer service lives and less patience for unexplained downtime. At Bladena, we see this gap play out repeatedly: a defect that could have been identified, with the cost recovered from the manufacturer, instead becomes a slow, contested insurance case months or years later.

For a marine market increasingly underwriting offshore wind alongside its traditional hull, cargo and liability book, that data gap sits on unfamiliar ground: the same asset now carries loss patterns the wider marine book wasn’t built to price.

The moment the risk changes hands

The clearest example is what happens at the end of a turbine’s warranty period. Overnight, responsibility for undetected blade defects moves from the manufacturer to the owner. The warranty period is, in effect, a window to identify and remedy manufacturing-related issues while the OEM is still contractually responsible; once it closes, the same issue becomes the owner’s problem and often ends up on the insurance book instead.

In our experience, this window is still treated as a formality rather than a commercial opportunity. Inspections in the run-up to warranty expiry are frequently no more thorough than routine annual checks, when they should be looking specifically for the kind of latent defect that only shows up years later.

Catching a defect while the OEM is responsible is materially cheaper than arguing, after the event, that a later failure was caused by something that predates the policy, a case that only gets harder to make once an asset gets older, sits further offshore and out of easy reach.

Anatomy of a blade claim

When a blade claim does reach an insurer, it rarely stands alone. The clearest illustration comes from AXIS Capital’s recent review of its offshore wind claims portfolio, which found that turbines account for 57% of all claims by count and 37% of incurred losses, with blade damage making up 16% of claim count at an average cost of around $1.5 million. The pattern of turbines, and blades specifically, as a leading loss driver is consistent with what we see across the wider fleet, onshore included.

Underneath the physical repair sits business interruption, frequently the larger part of a blade claim. Establishing why a blade failed, and whether that cause was a sudden external event or a slower, pre-existing issue, is where most disputes between owners, insurers and maintenance contractors play out. Insurers respond to a sudden, external triggering event, not to the material tolerances and manufacturing variability inherent in any complex composite structure; the practical effect is that a borderline defect only becomes insurable once something external, most often a lightning strike, turns it into damage, and untangling that sequence is rarely straightforward.

The lightning protection blind spot

The least understood part of blade risk is the lightning protection system itself. Even a system designed to current international standards is not expected to intercept every strike: in Bladena’s engineering assessments, a well-specified lightning protection system can still allow damage in a small proportion of events, commonly cited at around 2%. Owners who have paid for a top-tier system rarely expect that, and the gap between the standard and the expectation is where a lot of confusion, and cost, sits.

Two further developments compound this. The first is that changes in blade materials and construction mean lightning damage can be unpredictable. The second is a monitoring gap. Most standard lightning monitoring setups only record whether a strike occurred and its peak current, in kiloamps. They do not capture voltage, which is often the more useful indicator of whether the system actually functioned as designed. Two turbines can register an identical, modest current reading and end up with entirely different outcomes: one undamaged, one with a shattered blade, and the standard record cannot tell you why.

None of this is a reason to distrust lightning protection technology. It is a reason to invest in better instrumentation once an asset is large enough to justify it, onshore or offshore.

Better data, not fewer disputes

The industry does not need fewer disputed blade claims. It needs better shared data before those
disputes happen: more rigorous end-of-warranty inspection, monitoring that captures voltage as well as current, and a shift in mindset from treating blade risk as an unavoidable cost to something that can be actively managed down. That case only strengthens as offshore capacity keeps expanding, where access is harder, weather windows are shorter, and every extra day of downtime is worth more.

Two questions matter most at renewal: whether end-of-warranty inspection records exist, rather than the annual maintenance log alone, and whether lightning monitoring on site captures voltage as well as peak current. Together, they address most of the risk described in this article.

RES has been part of this conversation for over a decade, including contributing technical insight to one of the first industry-wide studies of blade failure, GCube’s 2014 “Breaking Blades” report. Within that history, Bladena now offers the specialist engineering view insurers and owners need as the asset base scales up, onshore and offshore alike.

If this is relevant to a portfolio you are reviewing, we would welcome the conversation.

First published in the October 2026 edition, Issue 25, of The Marine Insurer titled ‘Global Conflict Dictating Maritime Trade’.

By Andrei Buliga, Digital Blades Project Director at RES, supporting Bladena

When a wind turbine blade fails, insurers need more than a description of the visible damage. They need to understand what happened, why it happened, who may be responsible, and whether the same issue could affect other assets.

That clarity is not always easy to reach. Early information can be limited, conflicting or shaped by the interests of different parties. And where uncertainty remains, insurers can face greater exposure to dispute, misattribution, repeat failures and long-term cost.

Bladena provides independent root cause analysis for wind turbine blade failures, helping insurers make evidence-based claims decisions with greater confidence.

Speak to our balde failure experts

When the cause is unclear, claims decisions become harder

Blade failure claims can quickly become complex.
Multiple stakeholders may be involved, each with their own interpretation of the incident. When there is no clear, independent technical evidence to rely on, it can be difficult to separate fact from assumption, or to determine whether a proposed repair addresses the real cause of failure.

For insurers, this creates uncertainty around some of the most important questions in the claim:

• What was the actual failure mechanism?
• What factors contributed to the failure?
• Is the failure isolated, or could similar assets be exposed?
• Are proposed repairs technically appropriate?
• Is there enough evidence to support a clear liability position?
• Could the same issue happen again

Visible damage does not always explain the true failure

Conventional approaches often focus on where a blade has failed, what repair may be required and what can be seen on the surface. But visible damage is often the outcome of a deeper issue, not the explanation in itself.

Wind turbine blade failures are rarely driven by a single factor. Their behaviour can be shaped by design characteristics, manufacturing conditions, transport stresses and operational loading. In many cases, the true cause of failure only becomes clear when these factors are assessed together.
That’s why root cause analysis is so important.

Bladena’s RCA process is designed to reconstruct the sequence of events leading to failure. By combining specialist blade structural assessment, operational evidence, modelling and simulation, Bladena helps distinguish between the visible symptoms of damage and the true origin of failure.

[Comparison table]

Conventional approaches may focus on…	Independent RCA helps reveal…
What damage is visible	Why the failure occurred
What repair may be required	Whether the repair addresses the underlying cause
The individual incident	Whether there may be wider fleet-level exposure
Differing stakeholder interpretations	Evidence-based understanding of causation
The visible blade condition	A defensible basis for claim evaluation

Evidence-led insight for clearer claims decisions

Bladena’s independent root cause analysis gives insurers a more robust technical basis for claim evaluation, liability discussions and future risk assessment.

With RCA and specialist structural analysis, insurers can:

• Establish a clear, unbiased understanding of what happened
• Assess liability using objective technical evidence
• Distinguish between failure symptoms and failure origins
• Validate whether proposed repairs address the true underlying cause
• Evaluate the risk of recurrence across similar assets
• Support negotiations with transparent, defensible documentation
• Make more informed decisions across claims, underwriting and risk evaluation

This can be particularly valuable in multi-party claims, where differing perspectives and commercial interests can make resolution more difficult.

Independent RCA in action: disputed blade failure claim

Bladena combines specialist structural insight with practical understanding of turbine behaviour in the field. As part of RES Group, Bladena’s insight is strengthened by real-world operational experience across a global renewable energy portfolio.

If you’re working through a disputed claim, liability question or blade failure scenario, Bladena can provide independent technical insight to support claim evaluation, liability discussions and repair decisions.

Background

Following a blade break incident, an insurer faced a disputed claim involving multiple stakeholders and differing interpretations of the technical cause.

The insurer needed an independent assessment to clarify the failure mechanism, understand whether the event was isolated or indicative of wider fleet risk, and establish a defensible position for claim resolution.

The challenge

Different parties had presented different views of causation, leaving the insurer without the clarity needed to assess liability or move the claim forward. Bladena was brought in to provide an independent investigation that could go beyond surface-level damage interpretation and determine the underlying failure mechanism.

Bladena’s role

Bladena carried out an independent root cause analysis, delivered without commercial ties to any of the parties involved.
The investigation combined specialist blade structural assessment with operational evidence to determine the true cause of failure.

The analysis included:
• failure mechanism identification
• load-damage correlation
• design envelope assessment
• manufacturing indicator review
• evidence-based hypothesis elimination

This approach allowed Bladena to test different explanations, rule out unsupported assumptions and build a clear technical causation narrative.

The impact

The RCA gave the insurer a clear, unbiased and technically substantiated basis for claim evaluation, liability discussions and stakeholder alignment.

The findings supported:
• a clearer causation narrative
• improved liability clarity
• reduced uncertainty in claims decisions
• faster claim resolution
• reduced dispute complexity
• a stronger negotiation position
• better visibility of potential fleet-level risk

For the insurer, the value was not only in understanding a single failure. It was in gaining the evidence needed to make a confident decision under scrutiny.

Want to find out more?

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