Benefits of subcomponent over full-scale blade testing elaborated on a trailing-edge bond line design validation

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.

Understanding progressive failure mechanisms of a wind turbine blade trailing edge section through subcomponent tests and nonlinear FE analysis

“…Typical failure modes observed in full-scale blade tests includes adhesive joint debonding, sandwich core failure and composite laminate failure. In recent years, subcomponent testing has been used [6][7][8][9][10] to understand the structural behavior of critical parts of composite blades. The subcomponent testing could be an important complement to the full-scale blade tests that are mandatory for certification of wind turbine blades.…”

“…The subcomponent testing could be an important complement to the full-scale blade tests that are mandatory for certification of wind turbine blades. Regarding the subcomponents of trailing edges, efforts have been made to elaborate benefits of subcomponent over full-scale testing [8] and comparing different subcomponent test concepts [9]. Recent work [11] has assessed the failure of trailing edge subcomponents by addressing structural instability associated with pre-buckling and post-buckling response.…”

“…The aforementioned studies [8][9][10][11] provide valuable knowledge on the structural response of trailing edge sections in the subcomponent tests. Nevertheless, two important questions remain.…”

Trailing edge subcomponent testing for wind turbine blades–Part A: Comparison of concepts

“…The basis for establishing the test loads is the entire envelope of blade design loads. The full‐scale testing does not necessarily cover all critical loading conditions along the blade length as they would occur in the field, for example, in a trailing edge bond line under combined flap‐wise and lead‐lag loading. Moreover, some transient loads such as impact during transportation and installation cannot be simulated in the full‐scale test, but they may cause local damage in the blade.…”

A novel rotor blade fatigue test setup with elliptical biaxial resonant excitation

“…flapwise and lead-lag. A drawback of these unidirectional tests is that the loads introduced into the blade do not necessarily represent the loading the blade will experience under field conditions as Rosemeier et al (2018) have shown.…”