smaller the fiber diameter, greater is its fracture strength. In the case of continuous fibers, a critical minimum aspect ratio of the fiber is needed to transfer the applied load from the weaker matrix to the stronger fiber. Furthermore, a small diameter allows a stiff fiber to be bent for shaping a preform that is used as a precursor in composite fabrication. Many commercial fibers are flexible, and permit filament winding and weaving techniques to be used for making a pre- form ers are, however, shaped into preforms by using a fugitive binder materia For example, an organic compound that cements the fibers in the desired preform shape may be used. The binder decomposes and is eliminated when the matrix material is combined with the preform to provide it support and rigidity. Selected examples of fibers used in composite matrices are briefly described below. For more details, the reader is referred to the book by Chawla referenced at the end of the chapte Glass. Glass is a generic name for a family of ceramic fibers containing 50-60% silica(a glass former)in a solid solution that contains several other oxides such as Al2O3, CaO, MgO, K2O Na2O, and B2O3, etc. Commercial glass fibers are classified as E-glass (for high electrical resistivity), S-glass(for high silica that imparts excellent high-temperature stability), and C-glass (for corosion resistance Glass fiber is manufactured by melting the oxide ingredients in a furnace and then transferring the molten glass into a hot platinum crucible with a few hundred fine holes at its base. Molten glass flows through these holes and on cooling forms fine continuous filaments. The final fiber diameter is a function of the hole diameter in the platinum crucible, the viscosity of molten glass, and the liquid head in the crucible. The filaments are gathered into a strand, and a sizing is applied before the strand is wound on a drum. Glass is a brittle solid, and its strength is lowered by minute surface defects. The sizing protects the surface of glass filaments and also binds them into a strand. A common type of sizing contains polyvinyl acetate and a coupling agent that makes the strand compatible with various polymer matrices. The final fiber diameter is a function of the hole diameter in the platinum crucible, the viscosity of molten glass, and the liquid head in the reservoir. Another method to grow glass fibers makes use of a sol-gel-type chemical precipitation process. A sol containing fine colloidal particles is used as the precursor: due to their fine size, the particles remain suspended in the liquid vehicle and are stabilized against flocculation through ionic charge adsorption on the surface. The sol is gelled via pH adjustments, i. e, the liquid vehicle in the gel behaves as a highly viscous liquid, thus the physical characteristics of a solid. The gelling action occurs at room temperature. The gel is then drawn into fibers at high temperatures, that are lower than the temperatures used in onventional manufacture of glass fiber by melting. The Nextel fiber manufactured by the 3M company is a sol-gel-derived silica-based fiber Moisture decreases the strength of glass fibers. They are also prone to static fatigue; that is, they cannot withstand loads for long periods of time. Glass fiber-reinforced plastics(GRPs)are widely used in the construction industry Boron. Boron fibers are produced by vapor depositing boron on a fine filament, usually made from tugsten, carbon, or carbon-coated glass fiber. In one type of vapor deposition process a boron hydride compound is thermally decomposed, and the boron vapor heterogeneously ucleates on the filament, thus forming a film, Such fibers are, however, not very strong or dense, owing to trapped vapor or gas that causes porosity and weakens the fiber. In an improved chemical vapor deposition( CVD)process, a halogen compound of boron is reduced by hydrogen gas at high temperatures, via the reaction 2BX3+3H2- 2B+6HX(X=Cl, Br, or I).Because of the high depe temperatures involved, the precursor filament is usually tungsten. Fibers of Composite Materials 399smaller the fiber diameter, greater is its fracture strength. In the case of continuous fibers, a critical minimum aspect ratio of the fiber is needed to transfer the applied load from the weaker matrix to the stronger fiber. Furthermore, a small diameter allows a stiff fiber to be bent for shaping a preform that is used as a precursor in composite fabrication. Many commercial fibers are flexible, and permit filament winding and weaving techniques to be used for making a pre￾form. Very stiff fibers are, however, shaped into preforms by using a fugitive binder material. For example, an organic compound that cements the fibers in the desired preform shape may be used. The binder decomposes and is eliminated when the matrix material is combined with the preform to provide it support and rigidity. Selected examples of fibers used in composite matrices are briefly described below. For more details, the reader is referred to the book by Chawla referenced at the end of the chapter. Glass. Glass is a generic name for a family of ceramic fibers containing 50-60% silica (a glass former) in a solid solution that contains several other oxides such as A1203, CaO, MgO, K20. Na20, and B203, etc. Commercial glass fibers are classified as E-glass (for high electrical resistivity), S-glass (for high silica that imparts excellent high-temperature stability), and C-glass (for corrosion resistance). Glass fiber is manufactured by melting the oxide ingredients in a furnace and then transferring the molten glass into a hot platinum crucible with a few hundred fine holes at its base. Molten glass flows through these holes and on cooling forms fine continuous filaments. The final fiber diameter is a function of the hole diameter in the platinum crucible, the viscosity of molten glass, and the liquid head in the crucible. The filaments are gathered into a strand, and a sizing is applied before the strand is wound on a drum. Glass is a brittle solid, and its strength is lowered by minute surface defects. The sizing protects the surface of glass filaments and also binds them into a strand. A common type of sizing contains polyvinyl acetate and a coupling agent that makes the strand compatible with various polymer matrices. The final fiber diameter is a function of the hole diameter in the platinum crucible, the viscosity of molten glass, and the liquid head in the reservoir. Another method to grow glass fibers makes use of a sol-gel-type chemical precipitation process. A sol containing fine colloidal particles is used as the precursor; due to their fine size, the particles remain suspended in the liquid vehicle and are stabilized against flocculation through ionic charge adsorption on the surface. The sol is gelled via pH adjustments, i.e., the liquid vehicle in the gel behaves as a highly viscous liquid, thus acquiring the physical characteristics of a solid. The gelling action occurs at room temperature. The gel is then drawn into fibers at high temperatures, that are lower than the temperatures used in conventional manufacture of glass fiber by melting. The Nextel fiber manufactured by the 3M company is a sol-gel-derived silica-based fiber. Moisture decreases the strength of glass fibers. They are also prone to static fatigue; that is, they cannot withstand loads for long periods of time. Glass fiber-reinforced plastics (GRPs) are widely used in the construction industry. Boron. Boron fibers are produced by vapor depositing boron on a fine filament, usually made from tugsten, carbon, or carbon-coated glass fiber. In one type of vapor deposition process, a boron hydride compound is thermally decomposed, and the boron vapor heterogeneously nucleates on the filament, thus forming a film. Such fibers are, however, not very strong or dense, owing to trapped vapor or gas that causes porosity and weakens the fiber. In an improved chemical vapor deposition (CVD) process, a halogen compound of boron is reduced by hydrogen gas at high temperatures, via the reaction 2BX3 + 3H2 ~ 2B + 6HX (X = C1, Br, or I). Because of the high deposition temperatures involved, the precursor filament is usually tungsten. Fibers of Composite Materials 399