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Assessment of Research Needs for Wind Turbine Rotor Materials Technology
Trends of longitudinal tensile fatigue S-N data for unidirectional composites with various fibers.
Source: Mandell (1990).
As can be inferred from Table 3-2, the in situ tensile failure strain of E-glass fiber is as high as 2.5 percent, whereas it is only 1 percent for carbon fiber. However, E-glass fiber has a much lower fatigue ratio than carbon fiber, that is, 0.3 versus 0.75 at 10 million cycles (Figure 3-1). Therefore, both fibers have the same fatigue strain of 0.75% at 10 million cycles. Beyond 10 million cycles, however, the carbon fiber is expected to outlast the glass fiber. The cost ratio to obtain the same long-term fatigue strength is then at least the same as that needed to obtain the same stiffness. An additional benefit is that a hybrid composite blade will have a longer lifetime because of the reduced fatigue load resulting from the weight savings. Thus, a careful study is needed to explore the full benefits of a hybrid composite blade from a life-cycle point of view.
Fibers are used in various forms. The commonly used fiber preforms include unidirectional tow, woven cloth, knitted fabric, continuous strand mat, chopped strand mat, and braid as well as chopped fibers in sheet and bulk molding compounds. Depending on the application and manufacturing process used, one fiber preform may be preferred to others. The various fiber preforms are shown schematically in Figure 3-2.
Where high strength is required, unidirectional bundles of fibers known as tows should be used. Woven cloths and knitted fabrics are easier to use, especially over complex contours. Woven cloths have disadvantages in that