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Laser Shearography of Wind Turbine Blades

Electric power generation from wind turbines is the fastest growing source of electric power in the U.S. and worldwide. According to the American Wind Energy Association (AWEA) U.S. Wind Industry Report (AWEA, 2009) for year ending 2009, over 10 000 megawatts (MW) of wind was installed in 2009 for a total wind power capacity of 35000 MW in the U.S. While the wind industry’s current focus is on new generation installation and startup, future emphasis will be on long-term reliability and maintainability of wind energy assets. Failures of critical wind turbine components are occurring earlier than expected, resulting in increasing maintenance costs, unplanned outages and decreased capacity factors. Today’s wind turbines of the 1.5 MW output class have blades that span approximately 50 to 70 m and up to 6 m in chord, the distance from the leading to trailing edges. Due to the large, complex surface area and access considerations, examination of these structures is challenging. Visual testing (VT) is the primary examination method used by original equipment manufacturers (OEMs) and maintenance service providers to examine wind turbine blades. While VT is limited to surface testing discontinuity detection, other nondestructive testing (NDT) methods are available for blade inspections, which may provide better information about the structural integrity of a blade. One NDT method that is capable of testing large component surfaces is laser shearography. Laser shearography has seen extensive use in aerospace and marine applications for the inspection of composite materials for detection of nonvisible surface and near-surface discontinuities. This paper will discuss the theory behind laser shearography and detail the application of laser shearography NDT techniques for the inspection of wind turbine blades.

References
American Wind Energy Association (AWEA), “U.S. Wind Industry Annual Market Report,” Washington, DC, AWEA, 2009. American Society for Testing and Materials (ASTM), E2581-07, Standard Practice for Shearography of Polymer Matrix Composites, Sandwich Core Materials and Filament-Wound Pressure Vessels in Aerospace Applications, West Conshohocken, PA, 2007.
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