The difficulty associated with fiber impregnation has led to the development of commingled yarns and fabrics as well as powder-coated yarns and fabrics. A commingled yarn consists of both reinforcing fibers and thermoplastic fibers. During processing the thermoplastic fibers melt and impregnate the reinforcing fibers (Lynch, 1989). Thermoplastic powder is smaller in diameter than the corresponding thermoplastic fiber and hence is more evenly distributed in the yarn, which facilitates a more uniform fiber impregnation (Hartnes, 1988).

E-glass/plastic composites, commonly called glass-reinforced plastics (GRPs), have been widely used in the manufacture of blades of various sizes. Typical resins used in GRPs are polyester, vinyl ester, and epoxy. Although polyester resins can be cured in the shortest time, they show large shrinkage and hence may not be appropriate for use in a hot, dry environment such as California, where moisture-induced swelling would be minimal (Windpower Monthly, 1987). Vinyl ester resins have good environmental stability and are widely used in marine applications. Epoxy resins have good mechanical properties and dimensional stability. Their drawback is longer cure time and higher cost; however, new epoxy resins are now available for pultrusion and resin transfer molding, which require fast curing.

GRP is the main structural material for a number of blades of large machines and is also used as cladding over steel load-bearing frames in others (Phillips et al., 1987). GRP is the most popular blade material used in medium-size machines in Denmark and The Netherlands. As of January 1987, approximately 81 percent of 15,059 wind turbines in the California wind farms had fiberglass rotor blades (Stoddard, 1989; Modern Power Systems, 1986).

Typical properties of a unidirectional E-glass/epoxy composite are compared with those of other unidirectional composites in Table 3-2. For all the composites in the table, the stress-strain behavior is usually quite linear except in transverse compression and in-plane shear. Another exception is thearamid/epoxy composite, which is quite nonlinear in longitudinal compression also. Elastic properties of multidirectional laminates can be calculated from unidirectional properties using laminated plate theory (Tsai, 1989; Jones, 1975).

Tensile stress-strain relationships of several representative E-glass/epoxy laminates are shown in Figure 3-3. When a multidirectional