Test Methodology and Control of Full-Scale Fatigue Tests on Wind Turbine Blades

Engineers at the National Renewable Energy Laboratory's (NREL) Structural Test Facility are conducting full-scale fatigue tests on wind turbine blades. The test system uses a closed-loop servo hydraulic system that applies repeated load cycles to a wind turbine blade. Two different blades have been tested to date: the U.S. Windpower, Inc. (USW) 56-100 blade with a new LS(1) airfoil design, and the first prototype Atlantic Orient Corporation (AOC) 15/50 blade being developed under the U.S. Department of Energy (DOE)/NREL Advanced Wind Turbine Program. The USW 56-100 blade is constructed of fiberglass and was fatigue tested following multiple destructive tests using a quasi-static, eight-point load distribution. The fatigue tests used single-point loading to match the quasi-static bending moments along the inner 40% blade span. This blade test is still under way. The AOC 15/50 blade was fatigued to validate the design calculations and the manufacturing techniques used to build the first prototype. Both blade tests were performed using constant stroke-amplitude block loading. During full-scale blade fatigue testing, it was necessary to consider several issues that are typically ignored during fatigue tests of material coupons. The issues include choosing the proper load introduction point, dynamic effects, test control, data collection, failure monitoring, and relating test results to material data. This paper describes how several of these issues were addressed during the test setup and execution. The experience we gained will be used to help establish better procedures and improve the quality of future tests.