Characterisation of lightning strike induced damage in CFRP laminates and components for wind turbine blades

Abstract

To meet worldwide increases in energy demand Wind Turbine (WT) manufacturers are producing longer blades to generate more energy. These blades contain Carbon Fibre Reinforced Polymers (CFRP) in the load carrying structures to lightweight the blade. The introduction of the CFRP composites has presented new challenges in protecting the structure from lightning. The semiconductive nature of CFRP leads to an additional path to ground for the current in the structure and the anisotropic nature of the material’s thermal and electrical properties leads to large amounts of resistive heating especially in the through-thickness direction where the electrical conductivity is the lowest. The aim of this PhD is to devise a new means of assessing the damage and resulting structural behaviour caused by a lightning strike. A modelling framework is developed and validated against high fidelity experimental data that can be used by design engineers to understand the consequences of various lightning damage scenarios and the effectiveness of lightning protection methods. The framework is validated against a representative scale WT sparcap test component in the form of a large panel subjected to compression.

The novel damage model is a thermal-electrical Joule heating model which simulates the resistive heating in a UD laminate with electric field dependent material properties to account for electric breakdown. The damage prediction is then exported into a structural Finite Element Model (FEM) by assuming the damaged elements have different material properties. The structural behaviour under compression loading is the main design driver for long slender WT blades. Therefore, the structural model simulates the behaviour of a damaged laminate in a non-linear post-buckling FEM. To validate and inform the damage model and the FEM two different types of tests were conducted.

The first type of test simulated the lightning strikes and comprised of direct strike and conducted current tests. The effect of conducting current along the fibre direction showed a deleterious effect on the compressive and shear properties of the material. Initial direct strike tests were used to vary the typical lightning parameters to determine the largest influence on damage among peak current, specific energy, or charge. The last direct strike test is conducted on a representative WT sparcap panel. All damaged panels were evaluated using visual inspection, a new thermography technique, and X-ray computed tomography (CT). The newly developed damage model was validated using the experimental observations with the damage area predictions within 15% of the visual observation and the damage depth within 5% of the CT scans. Hence, the electric field dependency was successfully implemented in the model.

The second test type was a structural test that incorporated the development of a new testing methodology named the compression after lightning strike (CALS) test. Large representative sparcap panel specimens, with and without lightning damage were tested to failure in the CALS rig and Digital Image Correlation (DIC) was used to determine the resulting surface displacements and strains. The structural model closely predicted the compressive behaviour and failure loads identified by the DIC. The resulting structural model calculated the first ply failure stresses from the LaRC failure criteria which were within 8% of experimental values, which provided a successful validation of the modelling framework.

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Identifiers

ORCID for Timothy, Michael Harrell: orcid.org/0000-0002-0783-533X

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