62 COMPOSITE MATERIALS FOR AIRCRAFT STRUCTURES alumino-borosilicate glass,and "S,"a magnesium alumino-silicate glass.E stands for electrical grade,because compared with other standard forms of glass, its electrical resistivity is high and its dielectric constant low.These are by far the most widely exploited in structural applications,particularly in the non-aerospace area,because of their relatively low cost and high strength.A modified (low boron and fluorine)version of E glass fiber,ECR(E glass chemically resistant),is used where improved chemical properties are required.S stands for high-strength grade,although stiffness is also somewhat increased.These fibers can also withstand significantly higher temperatures than E glass fibers.Thus S glass fibers are used in more demanding structural applications.However,this marginal increase in stiffness is obtained at a relatively high cost.Where high specific strength and stiffness are required (with good dielectric properties) aramid fibers,described later,may be more attractive.More recently,a boron- free E glass has been developed that has markedly improved resistance to corrosive environments,but with no loss in mechanical properties. 3.2.4 Glass Fiber Coatings As mentioned earlier,glass fibers are highly sensitive to surface damage. Because the coefficient of friction between glass fibers is around unity, mechanical damage sufficient to cause a significant loss in strength can result from fiber-to-fiber abrasion during the forming process.To prevent contact damage,within milliseconds of solidifying,the fibers are coated with a protective size that also serves to minimize losses in strength due to atmospheric moisture absorption.For example,the tensile strength of as-drawn fibers can be reduced by over 20%after contact with air during drawing under normal ambient conditions. It seems likely that the atmospheric moisture is absorbed into microscopic flaws,reducing fracture energy because time would be too limited chemical attack.In any case,the tensile strength of the glass fibers drops significantly during the manufacturing process,from as high as 5 GPa immediately after drawing to typically around 2-3 GPa postproduction. The size consists of several components.The simplest is a lubricant,such as a light mineral oil for protection and to aid further processing such as weaving, filament winding,and pultrusion.Binders such as starch and polyvinyl alcohol Table 3.2 Chemical Composition of the Two Main Glass Fiber Types Glass type Si Al203 Cao B203 Mgo Na2O K20 E-Electrical 53 14 18 10 5 <1 S-High strength 65 25 1062 COMPOSITE MATERIALS FOR AIRCRAFT STRUCTURES alumino-borosilicate glass, and "S," a magnesium alumino-silicate glass. E stands for electrical grade, because compared with other standard forms of glass, its electrical resistivity is high and its dielectric constant low. These are by far the most widely exploited in structural applications, particularly in the non-aerospace area, because of their relatively low cost and high strength. A modified (low boron and fluorine) version of E glass fiber, ECR (E glass chemically resistant), is used where improved chemical properties are required. S stands for high-strength grade, although stiffness is also somewhat increased. These fibers can also withstand significantly higher temperatures than E glass fibers. Thus S glass fibers are used in more demanding structural applications. However, this marginal increase in stiffness is obtained at a relatively high cost. Where high specific strength and stiffness are required (with good dielectric properties) aramid fibers, described later, may be more attractive. More recently, a boron￾free E glass has been developed that has markedly improved resistance to corrosive environments, but with no loss in mechanical properties. 3.2.4 Glass Fiber Coatings As mentioned earlier, glass fibers are highly sensitive to surface damage. Because the coefficient of friction between glass fibers is around unity, mechanical damage sufficient to cause a significant loss in strength can result from fiber-to-fiber abrasion during the forming process. To prevent contact damage, within milliseconds of solidifying, the fibers are coated with a protective size that also serves to minimize losses in strength due to atmospheric moisture absorption. For example, the tensile strength of as-drawn fibers can be reduced by over 20% after contact with air during drawing under normal ambient conditions. It seems likely that the atmospheric moisture is absorbed into microscopic flaws, reducing fracture energy because time would be too limited chemical attack. In any case, the tensile strength of the glass fibers drops significantly during the manufacturing process, from as high as 5 GPa immediately after drawing to typically around 2-3 GPa postproduction. The size consists of several components. The simplest is a lubricant, such as a light mineral oil for protection and to aid further processing such as weaving, filament winding, and pultrusion. Binders such as starch and polyvinyl alcohol Table 3.2 Chemical Composition of the Two Main Glass Fiber Types Glass type Si A1203 CaO B203 MgO Na20 K20 E-Electrical 5 3 14 18 10 5 < 1 S-High strength 65 25 -- -- 10 --