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Avoiding Catastrophic Failure in Wind Turbine Blades

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A Study of Delamination Under Various Loading Conditions

ghasemnejadWind turbine blades are typically manufactured from fibre-reinforced polymer (FRP) composites, and delamination failure can be an important issue in these structures. In extreme conditions, such as ice impacting, multiple delamination with a triangular shape is found in different parts of a blade. This delamination introduces local damage, which can then cause catastrophic failure under various loading conditions including buckling. Buckling and post-buckling are two loading conditions that can occur in large wind turbine blades due to gravitational, aerodynamic and centrifugal forces. In the work reported here, experimental studies of buckling and post-buckling failure in multi-delaminated composite beams were carried out. Laminated composite beams were pinned through their thickness using natural flax yarns to control delamination failure during the post-buckling process. Multi-delaminated composite beams were manufactured with laminate designs of [C90/G90]4 and [C0/G0]4 and tested to find the critical buckling load and post-buckling failure mode.

By Dr Hessam Ghasemnejad, School of Aerospace and Aircraft Engineering, Kingston University London, UK

{access view=!registered}Only logged in users can view the full text of the article.{/access}{access view=registered}Fibre-reinforced polymer (FRP) composite materials are widely used because of their high strength-to-weight and stiffness-to-weight ratios compared with many traditional materials [refs 1,2]. One of the most serious failure modes in laminated composite materials is delamination; this can occur in several situations including during the manufacturing process, during maintenance and due to the impact of a foreign body [ref 3]. The area of the delaminations increases through the thickness away from the impact surface as the shock expands away from the point of impact [ref 4], so delaminations due to low velocity impact often appear at several interfaces of the laminated composite material (multiple delamination).

Natural Fibre Composites

Two important concerns for composite manufacturers are manufacturing costs and recycling problems. Natural fibres have recently been introduced to help overcome these difficulties. Natural fibres can come from many different sources including cotton, bark, wood, pulp, nut shells, bagasse, corn cobs, bamboo, cereal straw, flax, jute, hemp, sisal and ramie [ref 5]. These fibres consist mainly of cellulose, hemicelluloses, lignin and pectins, with a small quantity of extractives. The proportions of the fibre constituents vary depending on their origin. Natural fibres have many advantages over conventional inorganic fillers such as glass and carbon fibres. Incorporating tough and lightweight natural fibres into polymer (thermo-plastic and thermo-set) matrices produces composites with a high specific stiffness and strength [refs 6–8].

Specimen Manufacturing
The experimentally delaminated composite beams of [C0/G0]4 and [C90/G90]4 were manufactured from unidirectional GFRP (glass fibre reinforced polymer) and CFRP (carbon fibre reinforced polymer) composites. The thickness of all the laminated beams used in this experiment was 2mm. A Teflon film of 13µm was placed at particular positions to model the delaminations. The structure of a composite beam with multiple delamination is shown in Figure 1. Prior to testing, each specimen was labelled with the specimen lay-up and specimen number. The laminates were then machined to 160 × 20mm2 beams.

Post-Buckling Without Z-Pins
As is shown in Figure 2 for [C90/G90]4, the first part of graph (I) covers the buckling process, which concludes with a critical buckling load of 54N. The jagged part (II) of the graph shows the sudden changes of mechanical strength experienced due to the opening mechanisms of multi-delaminations. After the opening of the delaminations, bending dominates the deformation until (III) the ultimate load is reached. Then the mechanical resistance decreases until (IV) the failure load is reached and catastrophic collapse occurs. For the laminate design of [C0/G0]4 with a critical buckling load of 52N, Figure 3 shows (I) the first phase of delamination opening (II), followed by bending up to the maximum load (III), the resistance offered is then reduced due to the partial opening of delaminations (basically the longest ones played the main role). The strength offered then grew again and global bending led to collapse without any appreciable growth of cracks (IV).

Post-Buckling with Flax Z-Pins
To improve their post-buckling resistance, composite beam specimens were pinned through their thicknesses using natural flax yarns in order to limit crack propagation during post-buckling failure. These pinned specimens were laid-up with the same laminate designs of [C0/G0]4 and [C90/G90]4 and were tested until they collapsed. Figure 4 shows that, for laminate design [C90/G90]4, collapse occurred before crack propagation, so the z-pins did not enhance the post-buckling behaviour. As can be seen in Figure 4, no cracks, even ones with large openings, reached as far as the flax yarn z-pins, because failure occurred under global bending. For the laminate with [C0/G0]4 design, crack propagation was observed during the post-buckling process. In this case, before global collapse, the biggest crack propagation occurs in section (III) of Figure 5 after the maximum load, where the flax yarn z-pins enhance the mechanical properties by intercepting the crack propagations (see Figure 5). The longest delamination (crack) was completely stopped without further propagation or damage to the z-pinning cross-section. Beams that were z-pinned achieved higher ultimate loads and a secondary resistance as a partial propagation of cracks was limited by the z-pins.

Discussion and Conclusion
Experiments showed that this composite configuration offered favourable conditions for achieving the best results with a critical load of 63N for the laminate design [C90/G90]4, and of 53N for the laminate design [C0/G0]4. In this regard, natural flax yarns which are pinned through the thickness of multi delaminated specimens can significantly arrest the crack propagation in the specimens. In this case there is more resistance within the specimens, which results in a higher post-buckling resistance. Recommendations for further studies include repeating the tests with hybrid composite materials that have greater bending resistance, so that the collapse will not be dominated by global bending and cracks can propagate. Then z-pinning, for example with flax yarn, should then be effective, as we have shown in the present studies.

Biography of the Author
Dr Hessam Ghasemnejad holds a full-time lectureship in Engineering Design at the School of Aerospace and Aircraft Engineering, Kingston University, London. He received his PhD in the failure of composite structures in 2009 from Kingston University. His research areas are focused on the interlaminar and intralaminar failure modes in laminated composite materials and structures under dynamic loading conditions such as impact and blast.{/access}
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