Finite Element Modeling of a Dual Axis Resonant Test System for Wind Turbine Blades

Structural testing of wind turbine blades is required for designing reliable, structurally efficient blades. Full-scale blade fatigue testing conducted at the National Renewable Energy Laboratory’s (NREL) National Wind Technology Center (NWTC) provides blade manufacturers quantitative information on design details including design assessment, manufacturing quality, and design durability. Blade tests can be conducted as a single axis test (flapwise or lead-lag) or a dual-axis test (flapwise and lead-lag simultaneously). Dual-axis testing is generally the preferred full-scale test method as it simulates to a greater extent the characteristic loading the blade is subjected to in the field. Historically, wind turbine blade fatigue testing has been performed through forced displacement methods using hydraulic systems which directly apply load to the blade. More efficient methods of fatigue testing are being developed at the NWTC that employ resonant excitation systems to reduce hydraulic supply requirements, increase the test speed, and improve distributed load matching. In the case of a dual-axis resonant test, the blade is excited through multiple actuators at two distinct frequencies corresponding to the flapwise and lead-lag frequencies. A primary objective of a dual-axis test is to test the blade to equivalent damage moments in multiple axes. A code was developed to simulate the performance of the dual-axis resonant test system, comparing the predictions to actual test results. Modeling of this test system was performed using a MATLAB script that integrates the NREL FAST code with a commercial dynamic simulator package ADAMS. This code has the advantage over existing methods to more accurately simulate the coupled response between the flapwise and lead-lag directions. In summary, this paper will provide information on the modeling of wind turbine blade dual-axis resonant test systems.