JEC COMPOSITES MAGAZINE - Issue #49 - June 2009 - 44

FEATURE g…/… Turbine blade bonding “Wind Energy” (not per mass) of an adhesive, but also the rheology (flow characteristics) of the product. These factors may significantly affect the shape and size of the adhesive bondline and the amount of "squeeze out" or material wasted by inadvertent flow away from the desired position. Choosing the right adhesive in terms of both density and flow can reduce the total weight of adhesive in a typical blade by as much as 50-100 kg. As blade makers push larger stretched blades onto existing unchanged turbine platforms to pick up lighter winds, keeping blade weight low in this way can be critical. The overall mould cycle time in blade component manufacture has been significantly improved. Assembly of the basic components – such as sparcaps, shear webs, shells and roots – has been the main focus of most development work to reduce these cycle times and achieve higher output. The heating and cooling processes used to speed the curing cycle can lead to severe thermal variation in the adhesive joints between the components, resulting in built-in stresses which may be large, especially in the often thick bondlines used in rotor blade assembly. These stresses can affect lifetime fatigue performance. Rotor blade manufacturers, quite rightly, have a long wish-list of expectations for their adhesive. The combination of required properties is a constant trade-off between cure progression for strength, shrinkage stresses caused by heat and exotherm, and the ability to achieve correct thermal resistance which is often expressed as a glass transition temperature (Tg). Such fast heating and cooling also requires a tough adhesive to prevent thermal cracking, a property that usually reduces Tg. This becomes particularly important in large blade production since dimensional variations in the moulding of 30-50m long composite components that are supposed to fit together can require gluelines to make up the gaps which may be considerably thicker than necessary. Careful consideration is given to these thicker regions that will show even higher cooling shrinkage stresses. Size matters Blade economics is increasingly dominated by transportation costs as blades get bigger. So it is inevitable that sophisticated modular designs with mid-length in-field joints be employed before long, allowing blades to be delivered in transportable pieces. While mechanical joints have many practical advantages for site assembly, the management of stress concentration at these joints has helped design larger areas of adhesive bonded joint. Anyone having burst a button on a shirt will appreciate the need to avoid point loads in a fatigue situation! Even mechanical fittings will have to be bonded to the composite at some point, as is the case for many blade root studs today. The challenge of making this field-practical also involves trade-offs in adhesive design, though it will be financially rewarding if solved by blade producers and adhesive engineers working together at design stage. Design Even before an adhesive is applied to a manufactured component, the assembly joint design is a critical (and often overlooked) factor to success, and blade design is no exception. Everyone knows adhesives work best if peel stresses are kept away from the joint. Nonetheless, this is not a simple task and localised joint design for leading edge, spar cap and other joints can have a dramatic effect. It is often an area where most adhesive suppliers expect the customer’s designers and manufacturing engineers to provide their own solutions. With its raceboat and wind blade engineering pedigree, Gurit is lucky to be one of the few adhesive manufacturers with a large structural engineering service capable of recommending adhesive joint profile designs to optimise component structure whilst integrating production gains – providing the integrated solution. Through careful design of the structure and its joint elements and by giving consideration to the production process, overengineering can be avoided and material consumption, time and energy can be employed in blade production as efficiently as possible to avoid waste. A green approach, if you like, to producing an ecological product. Contact: peter.george@gurit.com More information: www.gurit.com The cross section of a blade. For many blade manufacturers who have had to avoid brittle adhesive cracking, the preferred option has been to address the cracking risk by lengthening cure cycles to obtain lower thermal stresses. This unnecessarily reduces the productive turnaround speed of the mould. Choosing toughened adhesives like Gurit’s Spabond 340 has minimised this need for longer cycles, thus restoring production capacity and opening up the possibility to review further cycle improvements. Keeping an eye on weight problems The high volumes of adhesive used in turbine blade assembly raise another issue in adhesive choice, namely consumption. It is vitally important to consider not only the cost per volume 44 jec composites magazine / No49 June 2009

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JEC COMPOSITES MAGAZINE - Issue #49 - June 2009

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #49 - June 2009