Extending the Fatigue Life of a T-joint in a Composite Wind Turbine Blade

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Abstract

Wind turbines are classic examples of structures where their operating lifetime is controlled by the fatigue properties of the material. This is exacerbated by the 2D nature of the composite materials used in blade construction which are typically fabrics in a variety of formats (e.g. non crimp fabrics, uniweave, woven). The formation of internal detailed shapes within the blade, allowing features such as spars, shear webs and other connections, inevitably requires these 2D material configurations to be formed into 3D shapes. This introduces positions within the structure where load transfer occurs across regions with no fibre reinforcement. These weak areas become natural positions for the initiation of damage that can occur well before fatigue damage would be expected in the basic material subject to simple in-plane loading. The aim of this study is to modify and improve the blade structure in order to extend its working life and minimize geometry related fatigue issues. To achieve this goal T-sections have been manufactured as representative element of the blade's spar. T-sections have been made of carbon or glass fabric infused with epoxy resin using a vacuum-assisted resin-transfer moulding technique. The structure has been modified with different toughening techniques to increase its interlaminar fracture resistance (toughness) and hence delay or stop crack propagation. Methods such as the use of veil layers, tufting and 3D weaving techniques have been employed to improve the interlaminar fracture toughness of the T-joint. The changing parameters in samples are, the addition of the veil layer to the composite structure, veil material, tufting stitches and use of different 3D fibre weaving architectures in the fabrication of the composite T-joint. For T-joint testing, there was no standardised specimen shapes and no standard for specimen dimensions; as well as no test fixture designs or test procedures. Consequently, it was required to design a test rig and develop a test procedure for tensile and fatigue tests of T-joints. An additional investigation was performed to establish test specimen geometry suitable for testing in available Instron machines. Manufactured specimens were quasi-static and fatigue tested. Test results were compared and showed that 3D woven and polyester veil T-joints had the best performance among modified structures. However, it has been found that these structural modifications are performing differently in quasi-static and fatigue loading. The 3D woven four layer to layer inter wave sample that showed the best result in a quasi-static test was not the one with the best fatigue results but it was amongst the ones with the highest performance. SEM and optical microscopy were used to investigate fractured specimens in an attempt to establish the mechanisms involved in the fracture process of the T-joint. Finally, based on test and investigations results it has been concluded that the 3D weaving was the most effective modification to improve the static and fatigue properties of the T-joint. The T-joint modified with polyester veil showed the second best performance in both static and fatigue tests but the addition of the polyamide caused had negative effects on these properties.

Keyword(s)

3D weaving; Composites; Fatigue; T-joint; Tufting; Veil; Wind turbine

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