Enhanced approach to match damage-equivalent loads in rotor blade fatigue testing

In the design process of current wind turbine blades, certification testing is a critical step to confirm design assumptions and requirements. To demonstrate reliability in fatigue testing, the blade will be loaded in all areas of interest to the load levels, which, at the end of such a test campaign, adequately represent the blade's lifetime. These loads are typically derived from aero-elastic load calculations with a combination of different design load cases in the form of accumulated bending moment distributions. The current practice includes two fatigue test sequences, which are aligned with the first flapwise and lead-lag modes, with the aim of reaching defined target bending moment distributions. However, these two test sequences combined may not cover all areas of interest, and some areas could be insufficiently tested. Also, in some areas, the conventional target bending moment formulation does not correctly represent fatigue damage of the material, as it is not derived from stress- or strain-based damage calculations and does not allow for mean load correction. The aim of this work is to demonstrate these shortcomings on a particular test case and to propose an enhanced method to derive representative target loads, which cover all areas of interest and are strain proportional, allowing for correct material damage accumulation and mean load correction. It is shown for the test case that, compared to conventional methods, the enhanced target loads require 16 % higher test loads at certain positions along the blade within the four main load directions and even more for load directions in between.