JEC COMPOSITES MAGAZINE - Issue #46 - January/February 2009 - 58

KNOW-HOW Quality control Advanced ultrasonic NDT techniques for wind turbine blade inspection This paper presents the development of inspection strategies intended to ensure the quality of the wind turbine blade manufacturing process, and in particular adhesive bond lines. It describes an innovative ultrasonic testing approach that combines specially-developed hardware (the Windblast™ inspection system with hand-held scanner and conformable arrays), signal processing, and evaluation tools for the quality control of large and complex GFRP structures. Specificity of wind turbine blade NDT CHRISTOPHE MATTEI, PROJECT MANAGER, AND MAGNUS DAHRÈN, CONSULTANT, BODYCOTE MATERIALS TESTING AB, SWEDEN s wind turbine blade production volumes dramatically increased over the past ten years, the wind energy industry identified a need for NDT techniques adapted to the specificity of the blade manufacturing process. The quality of laminates and the integrity of adhesive bond lines are the industry’s main concerns. A A number of different blade designs and material combinations are used in the industry. They all include adhesive bond lines that are considered critical to blade integrity. Two types of bond lines can be defined. Shear web bond lines ensure mechanical coupling between the pressure and suction sides through a reinforcement beam that is often referred to as the shear web. Leading edge and trailing edge bond lines ensure direct coupling between the skins at the two edges of the blade. The inspection of these bond lines, i.e. laminate and glue integrity, after manufacturing is the main focus of this development work [1], (Fig. 1). The inspection of shear web bond lines requires a technique that can penetrate through the main laminate – typically a 20-60 mm thick GFRP laminate – and detect the presence of a 2-15 mm thick layer of glue. Due to limited accessibility, especially near the tip, a single-sided inspection technique is required. Ultrasonic inspection at frequencies that can penetrate the main laminate is still the only technique that can detect and characterize the bond line. The technique can also be use to provide information on the laminate itself. Scanning the web bond line requires a system that can ensure good contact between the structure and the probe even though the surface is generally not flat. Ultrasonic testing is recognized as a technique that can provide valuable structural information. It can be used to detect and characterize porosity, dry areas and delamination in laminates, as well as to detect a lack of adhesive and measure bond line width and thickness. However, the standard ultrasonic testing equipment and procedures used in the aerospace or nuclear industry are not adapted to the specificities of blade inspection. From a NDT viewpoint, these structures are challenging because thick laminates are difficult to penetrate, large areas tend to be slow to inspect, and the blade shape implies inspection techniques that can handle geometry and thickness variations from root to tip. Another important specificity of the manufacturing process is the relatively large tolerance range in terms of laminate porosity, bond line thickness and bond line position. For these reasons, skilled NDT operators are needed to analyse the NDT data correctly. This paper presents the results of a development project initiated by a major wind turbine manufacturer and its suppliers, and conducted by Bodycote Materials Testing AB. Fig. 1: Important zones for blade structural integrity. 58 jec composites magazine / No46 January-February 2009

JEC COMPOSITES MAGAZINE - Issue #46 - January/February 2009

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