Abstract
Abstract. Wind turbine rotor blades are designed and certified according to the current IEC (2012) (International Electrotechnical Commission) and DNV GL AS (2015) (Det Norske Veritas Germanischer Lloyd Aksjeselskap) standards, which include the final full-scale experiment. The experiment is used to validate the assumptions made in the design models. In this work the drawbacks of traditional static and fatigue full-scale testing are elaborated, i.e., the replication of realistic loading and structural response. Subcomponent testing is proposed as a potential method to mitigate some of the drawbacks. Compared to the actual loading that a rotor blade is subjected to under field conditions, the full-scale test loading is subjected to the following simplifications and constraints: first, the full-scale fatigue test is conducted as a cyclic test, wherein the load time series obtained from aeroservoelastic simulations are simplified to a damage-equivalent load range. Second, the load directions are typically applied solely in two directions, often pure lead–lag and flapwise directions which are not necessarily the most critical load directions for a particular blade segment. Third, parts of the blade are overloaded by up to 20 % to achieve the target load along the whole span. Fourth, parts of the blade are not tested due to load introduction via load frames. Finally, another downside of a state-of-the-art, uni-axial, resonant, full-scale testing method is that dynamic testing at the eigenfrequencies of today's blades with respect to the first flapwise mode between 0.4 and 1.0 Hz results in long test times. Testing usually takes several months. In contrast, the subcomponent fatigue testing time can be substantially shorter than the full-scale blade test since (a) the load can be introduced with higher frequencies which are not constrained by the blade's eigenfrequency, and (b) the stress ratio between the minimum and the maximum stress exposure to which the structure is subjected can be increased to more realistic values. Furthermore, subcomponent testing could increase the structural reliability by focusing on the critical areas and replicating the design loads more accurately in the most critical directions. In this work, the comparison of the two testing methods is elaborated by way of example on a trailing-edge bond line design.
Highlights
Reliability, i.e., serviceability and structural integrity of rotor blades, is essential to fulfill the requirements placed on a wind turbine in the field
The subcomponent fatigue testing time can be substantially shorter than the full-scale blade test since (a) the load can be introduced with higher frequencies which are not constrained by the blade’s eigenfrequency, and (b) the stress ratio between the minimum and the maximum stress exposure to which the structure is subjected can be increased to more realistic values
Before a rotor blade design goes into operation, a full-scale blade test (FST) is required in the certification process to validate the assumptions made in the design models
Summary
Reliability, i.e., serviceability and structural integrity of rotor blades, is essential to fulfill the requirements placed on a wind turbine in the field. As part of the certification process, static as well as fatigue loads are applied to the rotor blade. Damage-equivalent fatigue test loads are applied to the rotor blade, but they do not necessarily reflect the actual load direction, amplitude and mean during its service life. The random-phase bi-axial resonant excitation has been a focus of research for a number of years (White, 2004; White et al, 2005; Greaves et al, 2012; Snowberg et al, 2014) Another approach is the phase-locked excitation with frequency ratios, e.g., of 1 : 1 between the flapwise and lead– lag mode (White et al, 2011). By taking the target mean loading and the stress ratio between the minimum and the maximum stress exposure at the trailing edge into account, the structural evaluation of the blade can be approached more realistically compared to field simulations. On the basis of calculations the benefits of SCT over static and fatigue FST are highlighted
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