were chosen for start-up or developmental phases by wind turbine blade producers. However, there is a practical size limit imposed by either the risk of human error in a lengthy lay-up operation or just the risk of dealing with thickness and warpage variations that increase with scale. The greater tooling and facility costs of the mechanized winding process are justified when the processing risk is sufficiently great.

Wood/epoxy blades are made by laminating 0.10-inch-thick wood veneers together using an epoxy resin system. Sheets of length 2.4 m (8 feet) are coated with resin and placed in mold halves end on end from the root to tip. The outside plies are scarf jointed, but all inner plies are simply butt jointed in a staggered pattern. A fiberglass surface ply is used on both the inner and outer surfaces of the skin.

Once lay-up is completed, a vacuum bag system is installed and the laminate is formed to the mold by vacuum pressure. Cure is completed at room temperature in 2 to 8 hours depending on the size of the component. The blade skin is made in two halves, which are trimmed at the mating plane after the skins are cured. A root to tip shear web is made from quality plywood, and grooved blocks are bonded to both surfaces of the blade to receive the web on assembly. The two halves and the spar are bonded together with a filled epoxy.

The root end attachment is accomplished by bonding custom-designed studs into spanwise holes drilled in a thickened section of the spar. Bolts thread into these inserts for attachment of the blade to the hub. This process yields a lightweight, reliable quality blade at costs under $50/lb (Stoddard, 1989).

Blade Root End Concepts

The various types of root ends that were reviewed in Chapter 4 will be discussed here with relation to manufacturing considerations.

The Danish Hütter design turns a flange, in fiberglass, normal to the axis of the blade to allow attachment to a steel hub with tension bolts. The bolt holes are wrapped through 180° with roving that is brought from the shank

of the blade. This concept results in voids, resin pockets, and high resin stress. Metal collars trap this flange material as the attachment fasteners are tightened in an attempt to partially relieve the resin stresses, but field experience has shown this to be an unreliable concept.

U.S. designs utilize a metal flange on a tube that fits inside the fiberglass spar with bonding as the attachment method. The tube and spar are bonded together although fasteners may be added for their fail-safe benefits. The wall of the tube is tapered to better distribute load transfer in the bond. The tapered portion may also vary in diameter along the length to mechanically trap the blade should the bond fail. High matrix strains, which can limit blade life, are introduced at the outboard end of the metal taper because the metal cannot practically be tapered to a knife edge.

A third method employs bonded inserts for bolts that are aligned with the axis of the blade and attach directly to the mating flange. This latter method is used on the laminated wood and glass-reinforced plastic (GRP) blades and seems to be fabricable and reliable.

Manufacturing Methods Influence Blade Life

Manufacturing procedures can introduce conditions in the composite that greatly influence fatigue life. These are local resin content variations, local fiber curvature, and local residual stress. Such conditions are variables in all composite manufacturing processes and should be considered in design. A brief description of each of these potential problem areas follows.

Prior to cure, the viscosity of the resin is low enough that it can move around in the assembly due to the forces of gravity or slight differentials in bag pressure or fiber tension. The result is large variations in resin content. An area of low resin content will be damage prone and subject to microcracking under compressive transverse or out-of-plane loadings. Areas of high resin content or resin pockets tend to be brittle and will crack under



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