Wind energy is one of the fastest-growing renewable energy technologies. Over the past decades, global installed wind energy generation capacity increased from 7.5GW in 1997 to 564GW by 2018.

In 2019, 60GW of capacity was added. In 2020, the wind sector installed 93GW, despite global COVID-19 pandemic-induced lockdowns. While in the same year, global installed offshore wind capacity reached ~5% of the total wind capacity and a large expansion of wind energy is planned for the next decades.

However, maintenance of wind turbines remains an important cost factor that influences both the energy price and the competitiveness of renewable energy. For example, onshore wind farm operations spent around $15 billion on operations and maintenance services in 2019, 57% of that unplanned.

The number of wind turbines reaching 20 years of operations in, for instance, Spain and Germany, will reach around 800 annually in the next decade.

An ageing fleet requires increased repairs – with an estimated 700,000 blades in operation around the world, there are on average 3,800 incidents of blade failure every year. The total repair budgets in Europe increased from 2019 to 2020 from $4.7m to $8.6m. Unplanned repairs also increased slightly in Europe (2%) and by 10% in America.

Failure events of different degrees take place relatively often.

Wind turbine blades are subject to complex environmental and mechanical loading during their service time. This includes cyclic deformation, rain, sand and contaminants causing erosion, icing, high moisture and temperature variations.

But it also includes extraordinary events such as transportation damage, lighting strikes and bird impacts.

To reduce and optimise maintenance costs, a detailed understanding of the degradation and failure mechanisms of wind turbines is required. This is important for a reliable prediction of failure events, the planning of maintenance activities and the mitigation of the degradation process.

For wind turbine blades, the lifetime extension is one of the best strategies for their use after 25 years of service time.

In his paper Root Causes and Mechanisms of Failure of Wind Turbine Blades: Overview, Leon Mishnaevsky Jr of the Technical University of Denmark reviews the mechanisms of degradation and failure of wind turbines under service conditions as well as the role of manufacturing defects and possible solutions.

The methods of analysis of failure mechanisms of blades include: • a post-mortem analysis of failed blades (visual, microscopy, testing); • full-scale testing of blades in laboratories, with video-observation, structural health monitoring, analysis of databases and collections of incident reports; • direct monitoring of blade deformation and degradation during service using attached or embedded sensors; • computational modelling of blade deformation and damage; and • combinations of all these methods.

With a view on specific damage mechanisms in specific regions: • testing of subcomponent (eg beam), • reproducing parts or elements of blades (eg joints or sandwiches), or • specific physical process testing (eg rain erosion testing) are employed.

Different methods of investigation allow for a better understanding of the various factors which influence blade damage mechanisms. Analyses of the incident reports lead to a better understanding of the effects of real service conditions, location, weather and manufacturing conditions on the damage mechanisms.

However, the analysis has an apparent time gap – the observations made on 10, 15 or even 20-year-old wind turbines are not always applicable to wind turbines that are manufactured today. Materials, quality control, technologies and sizes can be different.

Full-scale blade testing in laboratories is expensive, applicable only to relatively small wind turbines; does not reflect real service conditions; and uses laboratory-manufactured materials and structures, which might have fewer or different manufacturing defects than those exploited in the field.

Computational models can be quite efficient and have a wide application range. However, their application requires preliminary knowledge of the expected damage mechanisms.

Thus, a combination of all or several of the listed approaches would be best to obtain a better understanding of the real blade damage mechanisms and the possibilities of their mitigation.

The often-observed damage mechanisms include: • leading edge erosion, • adhesive joint degradation, • trailing edge failure, • buckling, and • blade collapse phenomena.

Critical areas of wind turbine blades include: • the outstanding and high-velocity region (blade tip, leading edge), • transitional and tapered areas (ply drops, root region), and • interface regions (adhesive joins in spar/ shell, trailing edge).

Methods of investigation of failure mechanisms of wind turbine blades include the full-scale testing of blades in laboratories, a post-mortem analysis of failed blades, analysis of incident reports and computational modelling.

It turns out the most endangered regions of the blades include the outstanding and high-velocity region (blade tip, leading edge), transitional and tapered areas (ply drops, root), interface and bonded regions (interlaminar layers, adhesive joints in trailing edge, cap/ shell).

Apart from leading edge erosion (which is a multi-physical, mainly microscale process), the failure mechanisms of wind turbine blades are often triggered by interface debonding, delamination and thin layer (interply, adhesive layer) degradation, which interact with composite buckling processes.

Given that the buckling of composites is strongly influenced by interface effects, the role of strong, tough interfaces in blade composites and structures cannot be overestimated. The critical roles of interfaces mean that the development of new technologies of bonding, surface preparation and bond quality control is an important element for blade life extension.

The role of manufacturing defects in the failure mechanisms of wind turbine blades was discussed. The flaws can be classified as “design flaws” (eg ply drops) and manufacturing flaws.

The manufacturing flaws include porosity, debonding, delamination, fibre misalignment and waviness, ply wrinkling, bonding defects, foreign inclusions, incompletely cured matrix, thickness variability and voids in adhesive bonds, and heterogeneities (resin-rich regions and dry spots).

These manufacturing defects can be formed due to imperfect curing conditions, air trapping in polymers, misplacement of fibres or layers, not enough adhesives, and so on. Under loading, these defects act as notches or microcracks, creating local stress concentrations and serving as initiation points for growing cracks.

The critical role of manufacturing defects means that several critical directions for the development of high-quality blades – namely, the automatisation of manufacturing and quality control of blades and improved manufacturing technologies – are especially important for the future.

A number of technical solutions do exist to reduce different damage mechanisms, which can be realised on a structural level (deformation limiting wire solutions and stiffeners eg by Bladena, split spar caps, transverse cap stiffeners or third shear web), or at the material level (vertically aligned carbon nanotube forests, reinforcing the interlaminar region).

It is concluded that the strength and durability of wind turbine blades are controlled to a large degree by the strength of adhesive joints, interfaces, and thin layers (interlaminar layers, adhesives) in the blade. ESI

Root Causes and Mechanisms of Failure of Wind Turbine Blades: Overview was written by Leon Mishnaevsky Jr of the Department of Wind Energy at the Technical University of Denmark. It is an open-access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) licence.

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