Lead Session Chair:
Stephan Barth, Managing Director, ForWind - Center for Wind Energy Research, Germany
Bent F. Sørensen (1) F P Helmuth Toftegaard (1) Malcolm McGugan (1) Gilmar F. Pereira (1) Kim Branner (1)
(1) Technical University of Denmark, Roskilde, Denmark
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Presenter's biographyBiographies are supplied directly by presenters at OFFSHORE 2015 and are published here unedited
Professor Bent F. Sørensen is head of the Composites and Materials Mechanics section, the Department of Wind Energy at the The Technical University of Denmark (DTU). He obtained his MSc from DTU in 1989 and his PhD in 1993. He obtained a Dr. Techn. degree from DTU in 2010 and became Professor mso in Materials Mechanics and Composite Materials in 2011. His research field focuses on interface mechanics. Experimental work includes the development of new test methods; modelling work covers micromechanical modelling and fracture mechanical models using the J integral, e.g. approaches for the determination of mixed mode cohesive laws.
Very large wind turbine rotor blades require damage tolerance and damage monitoring
A major challenge to design future very large rotor blades so that they can endure minimum 20 years in service in harsh off-shore environments. Rotor blades are made as very large parts using relative low-cost fibre composite materials and low-cost manufacturing methods. It is not possible to manufacture “perfect” blades. Furthermore, blades will also be subjected to different loading histories. Consequently, each blade will undergo its own unique damage evolution. Regular manual inspection is neither an economical nor a technical efficient a solution, since manual inspection of off-shore wind turbines is costly and difficult.
To meet the challenges summarized above, we propose a novel approach that allows blades to contain defects and develop stable damage under operation. The key idea is to use damage tolerant materials and design methods to ensure that defects do not develop into unstable damage leading to blade failure.
Main body of abstract
The approach involves damage monitoring, i.e., the use of built-in sensors that can detect damage in an off-shore wind turbine rotor blade. Such sensors should be built-in during manufacturing. Sensors should be able to detect the location of damage in a blade and send a warning to an on-shore surveillance centre. A maintenance team can then be sent out to inspect the damaged area of the blade using non-destructive inspection techniques to identify the type of damage, it size and depth. From this information it will be possible to use models to predict the residual fatigue life and residual strength of the damaged blade. Based on this, it will be possible to assess the criticality of the detected damage and make a decision about whether the damaged blade can be used as it is, or whether its aero-loads should be reduced (to meet the targeted lifetime), or the blade should be repaired or replaced by another blade.
A key issue is thus to create damage tolerance, meaning that damage from defects must always progress stably, i.e. slowly under increasing load, while at the same time be detectable by sensors. Damage tolerant design can be obtained by structural design optimization and by the use of damage tolerant materials, e.g. materials that possesses increasing fracture resistance with increasing crack extension. Recent research has shown that it is possible to increase the delamination resistance of composite materials significantly by changing manufacturing and processing details.
We think that the development of more damage tolerant structures and materials together with damage monitoring can be the technological opportunity that enables the development of future very large wind turbine rotor blades approaching 100 metres in length.
This new approach enables the service life of each blade to be decided individually on the damage state of each blade. It becomes possible to extend the lifetime of healthy blades beyond their originally planned service life. The proposed approach relies on damage monitoring (by sensors), non-destructive methods (for damage characterisation) and damage and fracture mechanics modelling to predict progressive damage evolution.
To explain how a new approach involving damage tolerant structures and materials and damage monitoring can be the technological opportunity that enables very large wind turbine rotor blades approaching 100 metres in length.