Introduction
Within the framework of a European funded research project, entitled OPTIMAT BLADES (ENK6-CT2001-00552), repair techniques are reviewed and adapted in order to enable their suitability in applications for wind turbine blades. The objective of these investigations is to develop repair methodologies applicable to the load carrying laminates of wind turbine blade structures, 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 or for assessing the load-carrying capability of damaged areas. Thus, blades those are damaged or have productions deficiencies in their laminated structural parts are sometimes destroyed even if the damage or deficiencies are local. As the blades become larger more material is wasted due to such localized deficiency.
To this end, the typical repair procedure, namely the damage identification on the structural part, the decision for repair, the removal of the damaged area, the application of repair and the repair quality inspection (Heslehurst 1996), is surveyed step by step and adapted for the special needs of wind turbine blades application.
The location, type and importance of damaged zone are identified and semi-empirical stress calculation procedures are proposed, as selected from literature (e.g. Heslehurst 1996; Hoskin and Baker 1986), for a preliminary repair design.
A minimum target value for the repair efficiency is stated with respect to both strength and stiffness. Repair techniques are surveyed and evaluated on aspects like complexity, suitability to highly stressed, load carrying laminated parts and application on site if possible. The most promising techniques are selected and applied to the repair of flat specimens.
Within an extensive testing campaign these repaired specimens are tested in uniaxial tension and results with respect to both strength and exhibited stiffness are compared with that of flawless specimens, which are tested to form the necessary baseline. 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 80%. Verification of behaviour under fatigue loading is carried out for the selected repair method with similar results.
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INTRODUCTION