Wspólnotowy Serwis Informacyjny Badan i Rozwoju - CORDIS

Suitable repair techniques for FRP rotor blades

Within the scope of the OPTIMAT BLADES project was the development of repair methodologies suitable for application on the load carrying laminates of fiber reinforced wind turbine rotor blades, so as to avoid possible rejection of products both during production and service life. Currently there are no recommendations available for repairing structural parts of blades. Blades that are damaged or have production deficiencies in the thick structural parts are being destroyed, even if the damage or deficiencies are local. As the blades become larger more material is wasted due to such localized deficiency. However, material and costs savings can be made by adequate repair of locally damaged blades or blades with localized production deficiencies.

To this end, as a first approach the location, type and importance of damaged zones was defined. Defects encountered during production, like dry spots and web to skin delaminations will need different repair techniques than those caused by lightning strikes or impact. Nevertheless, it was selected to concentrate on flaws that are found on parts of the primary structure of the wind turbine blades, e.g. the girder part of the blade and not parts that include structural foam. Repair techniques were surveyed and evaluated on aspects like complexity and suitability to large thickness and application on site. From the available repair methods for application to composite material structures in general, two were found suitable for the repair of the load carrying laminates of rotor blades, namely the scarf repair and the plug/patch. Semi-empirical stress calculation procedures are proposed, as selected from literature for a preliminary repair design. A minimum target value for the repair efficiency was stated with respect to both strength and stiffness. The most promising techniques were selected and applied to the repair of flat specimens.

Within an extensive testing campaign these repaired specimens were tested in uniaxial tension and results with respect to both strength and exhibited stiffness were compared with that of flawless specimens, which were tested to form the necessary baseline. More than 100 static tension tests and 60 fatigue tests have been carried out on Glass/Epoxy coupons to investigate the effect of a number of parameters of the repair techniques, such as repair depth, slope of repair (in case of the scarf repair), form of the material of repair (liquid resin as opposed to use of prepregs), etc. Applied non-destructive inspection techniques for the quality assurance of the repaired area were also shortly assessed. Static test data show that the most promising from the selected repair techniques is the scarf repair with a slope of at least 1:50, which leads to a strength restoration of over 90% and scatter comparable to the reference coupons with adequate material use. Regarding the exhibited stiffness, results are within 10% of the reference elasticity. Verification of behaviour under tension fatigue loading was carried out for the selected repair method with similar results.

Investigation on the effect of other conditions on the behaviour of the repair part, such as compression stresses both during static and fatigue loading will be continued in the near future. Other details of repair techniques applicable to composite material blades, such as curing cycles, environmental effects, etc. will also be investigated in order to arrive at a set of recommendations for the manufacturers leading to reliable repairs, thus prolonging the operating life of the blades and acceptance by accreditation bodies.

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