Effects of moisture on glass fiber-reinforced polymer composites.pdf - JOURNAL OF COMPOSITE M AT E R I A L S Article Effects of moisture on glass

Effects of moisture on glass fiber-reinforced polymer composites.pdf

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JOURNAL OF COMPOSITE MATERIALS Article Effects of moisture on glass fiber-reinforced polymer composites V Alzamora Guzman and P Brøndsted Abstract Glass fiber polymer composites are used in wind turbine blades because of their high-specific strength and stiffness, good fatigue properties, and low cost. The wind industry is moving offshore to satisfy economies of scale with larger turbines. High humidity in this environment degrades mechanical performance of wind turbine blades over their lifetime. Here, environmental moisture conditions were simulated by immersing glass fiber-reinforced polymer specimens in salt water for a period of up to 8 years. The mechanical properties of specimens were analyzed before and after immersion to evaluate the degradation mechanisms. Single-fiber tensile testing was also performed at different moisture conditions. The water-diffusion mechanism was studied to quantify the diffusion coefficients as a function of salt concentration, sample geometry, and fiber direction. Three degradation mechanisms were observed: polymer plasticization, fiber stress corro- sion, and interface degradation, where the latter was found to be the most detrimental for wind-industry applications. Keywords Glass fibers, polymer–matrix composites, environmental degradation, mechanical properties, fiber/matrix bond Introduction Lightweight structures often imply the use of composite materials, which have superior properties to any one material and can be tailored to the needs of a particular application. Composites can have a high-specific strength and stiffness, good fatigue properties, interest- ing chemical resistance, and in some cases, lower cost. One of the large application areas for glass fiber- reinforced polymer (GFRP) composites is the wind industry. The installation of new wind farms is often offshore due to higher available wind speeds and to avoid competition with other forms of land use. As offshore wind installations are very expensive, and it is essential to maximize the energy capture of the wind turbines, which translates to increasing their size. The elastic properties of the turbine blades are a constraining design parameter, because the strain in the blades during operation has to be minimized to with- stand the degradation effects of fatigue during their lifetime. A particular challenge of the offshore environ- ment is the high level of moisture, which may degrade the elastic properties of the composite wind turbine blades, increasing the working strain because the work- ing loads are constant (on average), leading to a reduc- tion in the lifetime of the turbine blades. To predict and design structures capable of with- standing offshore conditions, it is necessary to quantify and understand the effects of moisture on the mechan- ical properties of the composite materials and identify the degradation mechanisms. The parameters of mech- anical performance can be correlated to a specific phys- ical effect. For example, the longitudinal Young’s
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