Torsional Effects on Wind Turbine Blades and Impact on Field Damages

The purpose of this paper is to present the importance of torsional loads when understanding the significant increase in structural damages on large wind turbine blades. By comparing the stress state of two blades of different length, it is shown that torsional loads grow with an exponent of four (4). Thereby, a function of the length of the blades leading to a new range of challenges that are not met by the current process for design, testing and certification. The paper presents the study of these new challenges in detail, based upon: - An experimental perspective, with results from large-scale cyclic loading tests on two blade cut-outs conducted under controlled laboratory conditions in. Both blades were tested to failure. - A numerical perspective, non-linear geometrical 3D FEM simulations has been performed to study the impact of torsional loads on relevant current structural failure modes on large wind turbine blades. The conclusion is that the larger torsional loads may explain shear-web disbond, as well as sandwich skin debonding in both the transition zone and in the max chord area of wind turbine blades. In view of the findings, the paper includes proposals for remedies that are expected to mitigate some of the significant failures currently seen in the field.