High fidelity testing of wind turbine blade substructures

Abstract

The overarching aim of the PhD is to develop a high-fidelity (HF) testing and analysis methodology for wind turbine blade (WTB) substructures, which may be used to inform WTB design and certification procedures. Such a methodology will help to bridge the gap between WTB design and test certification tiers defined in the “testing pyramid”, where material properties determined at the bottom (coupon scale) tier are used in the top (full scale) tier with applied knock-down factors, which may cause inefficient design. To demonstrate the methodology, it was necessary to identify a relevant substructure and design a local substructure test with complex multiaxial operational/design load conditions. In consultation with the project sponsor Siemens Gamesa Renewable Energy (SGRE), the joint that connects the spar cap to the shear web that runs radially along the length of the blade was down selected from a variety of potential components. It is structurally complex and manufactured from unidirectional and biaxial glass fibre reinforced polymer (GFRP) composite layups with wooden core regions. The joint is a critical load carrying structure within the WTB and therefore would benefit from testing by the proposed method, e.g., to explore design configurations and to benchmark and validate high fidelity finite element analysis (FEA) predictions of load response and damage/failure behaviour. The methodology requires HF validation of numerical models and verification of experimental load and boundary conditions; therefore, a quantitative full-field comparison and fusion method “Full-field Data Fusion” (FFDF) was developed. To apply realistic multiaxial loading, it was necessary to use multiple actuators in a specially designed support frame mounted on a strong floor. The Large Structures Testing Laboratory (LSTL) at the University of Southampton National Infrastructure Laboratory (NIL) was selected, and multi-purpose load frames were developed in conjunction with another project (Structures 2025: EP/R008787/1). The facility was under construction for most of the PhD duration, so the FFDF methodology was developed on a simpler 2D ‘Tjoint’ section that was cut from an out of service WTB and loaded under a uniaxial configuration, with the larger multiaxial tests to be carried out in the future. Experimental measurements were taken from the specimens using Lock-in Digital Image Correlation (LIDIC) and Thermoelastic Stress Analysis (TSA) simultaneously under cyclic loading, and numerical predictions were made using HF FEA with a quasi-static load condition. The FFDF method incorporates spatial matching so that every data point of each data set can be compared against one another and is packaged in a MATLAB application with a graphical user interface. The first part of the thesis covers the development of the FFDF method and its demonstration on the 2D T-joint. The later sections cover the design of a complex test fixture to load the 3D representative WTB section supplied by SGRE. The load cases were established by examining the global WTB FEA model ultimate design limit scenarios for substructure conditions (also provided by SGRE), and by recreating key stress components within the constraints of the Structures 2025 support frames and available actuators. Detailed modelling of each load case and their interactions with the rig and strong floor connections was carried out to establish a mechanically and kinematically feasible test matrix. To apply the load cases to the WTB section, it was necessary to design a specialist test fixture, which entailed a detailed clamp design and FEA. The fixture ensures that parasitic effects from the clamping were minimised and establish a ‘gauge section’ in the WTB section where realistic load and deformation components are concentrated. The bespoke multiaxial load rig can apply loads to the 3D specimens with high relevance to WTB design load components, and therefore can be used to establish design allowables at a substructural (mid-tier) scale. Using FFDF, preliminary 2D T-joint test results and predictions were evaluated and compared, which highlighted errors in the manufacture provided material data and asymmetric loading in the test setup, that was used to improve the model and correct the test setup. The FFDF method allows for a quantitative and statistical approach to be used for the validation of numerical models and the assessment of experimental conditions using HF full-field data from various sources. The developed methods may be used for future WTB or other (e.g., aero) composite substructure analyses, and the load rigs may be used for further T-joint testing.

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Identifiers

ORCID for Jack Steven Callaghan: orcid.org/0000-0001-5531-3141

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