PART I PRINCIPLES OF CONSTRUCTION 2003 by CRC Press LLC
PART I PRINCIPLES OF CONSTRUCTION TX846_Frame_C01 Page 1 Monday, November 18, 2002 10:34 AM © 2003 by CRC Press LLC
COMPOSITE MATERIALS, INTEREST,AND PROPERTIES 1.1 WHAT IS COMPOSITE MATERIAL? As the term indicates,composite material reveals a material that is different from common heterogeneous materials.Currently composite materials refers to materials having strong fibers-continuous or noncontinuous-surrounded by a weaker matrix material.The matrix serves to distribute the fibers and also to transmit the load to the fibers. Notes:Composite materials are not new.They have been used since antiquity. Wood and cob have been everyday composites.Composites have also been used to optimize the performance of some conventional weapons.For example: In the Mongolian arcs,the compressed parts are made of corn,and the stretched parts are made of wood and cow tendons glued together. Japanese swords or sabers have their blades made of steel and soft iron: the steel part is stratified like a sheet of paste,with orientation of defects and impurities in the long direction'(see Figure 1.1),then formed into a U shape into which the soft iron is placed.The sword then has good resistance for flexure and impact. One can see in this period the beginning of the distinction between the common composites used universally and the high performance composites. The composite material as obtained is ■Very heterogeneous. Very "anisotropic."This notion of "anisotropy"will be illustrated later in Section 3.1 and also in Chapter 9.Simply put this means that the mechanical properties of the material depend on the direction. In folding a sheet of steel over itself 15 times,one obtains 2=32.768 layers. 2003 by CRC Press LLC
1 COMPOSITE MATERIALS, INTEREST, AND PROPERTIES 1.1 WHAT IS COMPOSITE MATERIAL? As the term indicates, composite material reveals a material that is different from common heterogeneous materials. Currently composite materials refers to materials having strong fibers—continuous or noncontinuous—surrounded by a weaker matrix material. The matrix serves to distribute the fibers and also to transmit the load to the fibers. Notes: Composite materials are not new. They have been used since antiquity. Wood and cob have been everyday composites. Composites have also been used to optimize the performance of some conventional weapons. For example: In the Mongolian arcs, the compressed parts are made of corn, and the stretched parts are made of wood and cow tendons glued together. Japanese swords or sabers have their blades made of steel and soft iron: the steel part is stratified like a sheet of paste, with orientation of defects and impurities in the long direction1 (see Figure 1.1), then formed into a U shape into which the soft iron is placed. The sword then has good resistance for flexure and impact. One can see in this period the beginning of the distinction between the common composites used universally and the high performance composites. The composite material as obtained is Very heterogeneous. Very “anisotropic.” This notion of “anisotropy” will be illustrated later in Section 3.1 and also in Chapter 9. Simply put this means that the mechanical properties of the material depend on the direction. 1 In folding a sheet of steel over itself 15 times, one obtains 215 = 32,768 layers. TX846_Frame_C01 Page 3 Monday, November 18, 2002 10:34 AM © 2003 by CRC Press LLC
Stress Concentration Random Defects Oriented Defects Poor tensile resistance Good tensile resistance Figure 1.1 Effect of the Orientation of Impurities 1.2 FIBERS AND MATRIX The bonding between fibers and matrix is created during the manufacturing phase of the composite material.This has fundamental influence on the mechanical properties of the composite material. 1.2.1 Fibers Fibers consist of thousands of filaments,each filament having a diameter of bet- ween 5 and 15 micrometers,allowing them to be producible using textile machines;for example,in the case of glass fiber,one can obtain two semi- products as shown in Figure 1.2.These fibers are sold in the following forms: Short fibers,with lengths of a few centimeters or fractions of millimeters are felts,mats,and short fibers used in injection molding. Long fibers,which are cut during time of fabrication of the composite material,are used as is or woven. Principal fiber materials are ■Glass Aramid or Kevlar(very light) Carbon (high modulus or high strength) Boron (high modulus or high strength) Silicon carbide (high temperature resistant) In forming fiber reinforcement,the assembly of fibers to make fiber forms for the fabrication of composite material can take the following forms: One wants to have fibers as thin as possible because their rupture strength decreases as their diameter increases,and very small fiber diameters allow for effective radius of curvature in fiber bending to be on the order of half a millimeter.However,exception is made for boron fibers (diameter in the order of 100 microns).which are formed around a tungsten filament (diameter 12 microns).Their minimum radius of curvature is 4 mm.Then,except for particular cases,weaving is not possible. 2003 by CRC Press LLC
1.2 FIBERS AND MATRIX The bonding between fibers and matrix is created during the manufacturing phase of the composite material. This has fundamental influence on the mechanical properties of the composite material. 1.2.1 Fibers Fibers consist of thousands of filaments, each filament having a diameter of between 5 and 15 micrometers, allowing them to be producible using textile machines;2 for example, in the case of glass fiber, one can obtain two semiproducts as shown in Figure 1.2. These fibers are sold in the following forms: Short fibers, with lengths of a few centimeters or fractions of millimeters are felts, mats, and short fibers used in injection molding. Long fibers, which are cut during time of fabrication of the composite material, are used as is or woven. Principal fiber materials are Glass Aramid or Kevlar‚ (very light) Carbon (high modulus or high strength) Boron (high modulus or high strength) Silicon carbide (high temperature resistant) In forming fiber reinforcement, the assembly of fibers to make fiber forms for the fabrication of composite material can take the following forms: Figure 1.1 Effect of the Orientation of Impurities 2 One wants to have fibers as thin as possible because their rupture strength decreases as their diameter increases, and very small fiber diameters allow for effective radius of curvature in fiber bending to be on the order of half a millimeter. However, exception is made for boron fibers (diameter in the order of 100 microns), which are formed around a tungsten filament (diameter = 12 microns). Their minimum radius of curvature is 4 mm. Then, except for particular cases, weaving is not possible. TX846_Frame_C01 Page 4 Monday, November 18, 2002 10:34 AM © 2003 by CRC Press LLC
Filaments Continuous Discontinuous Fiber Fiber Glass Fibers Staple for Fiber Weaving Textile Filament Roving or Strand Figure 1.2 Different Fiber Forms Unidimensional:unidirectional tows,yarns,or tapes ■ Bidimensional:woven or nonwoven fabrics (felts or mats) Tridimensional:fabrics (sometimes called multidimensional fabrics)with fibers oriented along many directions (>2) Before the formation of the reinforcements,the fibers are subjected to a surface treatment to Decrease the abrasion action of fibers when passing through the forming machines. Improve the adhesion with the matrix material. Other types of reinforcements,full or empty spheres(microspheres)or powders (see Section 3.5.3),are also used. 1.2.1.1 Relative Importance of Different Fibers in Applications Figure 1.3 allows one to judge the relative importance in terms of the amount of fibers used in the fabrication of composites.One can immediately notice the industrial importance of fiber glass (produced in large quantities).Carbon and Kevlar fibers are reserved for high performance components. Following are a few notes on the fibers: Glass fiber:The filaments are obtained by pulling the glass(silicon+sodium carbonate and calcium carbonate:T>1000C)through the small orifices of a plate made of platinum alloy. Kevlar fiber:This is an aramid fiber,yellowish color,made by DuPont de Nemours (USA).These are aromatic polyamides obtained by synthesis at -10C,then fibrillated and drawn to obtain high modulus of elasticity. Carbon fiber:Filaments of polyacrylonitrile or pitch(obtained from residues of the petroleum products)are oxidized at high temperatures(300C).then heated further to 1500C in a nitrogen atmosphere.Then only the hexagonal 2003 by CRC Press LLC
Unidimensional: unidirectional tows, yarns, or tapes Bidimensional: woven or nonwoven fabrics (felts or mats) Tridimensional: fabrics (sometimes called multidimensional fabrics) with fibers oriented along many directions (>2) Before the formation of the reinforcements, the fibers are subjected to a surface treatment to Decrease the abrasion action of fibers when passing through the forming machines. Improve the adhesion with the matrix material. Other types of reinforcements, full or empty spheres (microspheres) or powders (see Section 3.5.3), are also used. 1.2.1.1 Relative Importance of Different Fibers in Applications Figure 1.3 allows one to judge the relative importance in terms of the amount of fibers used in the fabrication of composites. One can immediately notice the industrial importance of fiber glass (produced in large quantities). Carbon and Kevlar fibers are reserved for high performance components. Following are a few notes on the fibers: Glass fiber: The filaments are obtained by pulling the glass (silicon + sodium carbonate and calcium carbonate; T > 1000∞C) through the small orifices of a plate made of platinum alloy. Kevlar fiber: This is an aramid fiber, yellowish color, made by DuPont de Nemours (USA). These are aromatic polyamides obtained by synthesis at -10∞C, then fibrillated and drawn to obtain high modulus of elasticity. Carbon fiber: Filaments of polyacrylonitrile or pitch (obtained from residues of the petroleum products) are oxidized at high temperatures (300∞C), then heated further to 1500∞C in a nitrogen atmosphere. Then only the hexagonal Figure 1.2 Different Fiber Forms TX846_Frame_C01 Page 5 Monday, November 18, 2002 10:34 AM © 2003 by CRC Press LLC
Mass (tons) 1.800,000 Glass 1,100,000 7000 Carbon;Kevlar 2000 3500 1984 1987 1990 1993 Figure 1.3 Relative Sale Volume of Different Fibers Carbon fiber Figure 1.4 Structure of Carbon Fiber carbon chains,as shown in Figure 1.4,remain.Black and bright filaments are obtained.High modulus of elasticity is obtained by drawing at high temperature. ■ Boron fiber:Tungsten filament (diameter 12 um)serves to catalyze the reaction between boron chloride and hydrogen at 1200C.The boron fibers obtained have a diameter of about 100 um (the growth speed is about 1 micron per second). Silicon carbide:The principle of fabrication is analogous to that of boron fiber:chemical vapor deposition (1200C)of methyl trichlorosilane mixed with hydrogen. The principal physical-mechanical properties of the fibers are indicated in Table 1.3.Note the very significant disparity of the prices per unit weight. 2003 by CRC Press LLC
carbon chains, as shown in Figure 1.4, remain. Black and bright filaments are obtained. High modulus of elasticity is obtained by drawing at high temperature. Boron fiber: Tungsten filament (diameter 12 mm) serves to catalyze the reaction between boron chloride and hydrogen at 1200∞C. The boron fibers obtained have a diameter of about 100 mm (the growth speed is about 1 micron per second). Silicon carbide: The principle of fabrication is analogous to that of boron fiber: chemical vapor deposition (1200∞C) of methyl trichlorosilane mixed with hydrogen. The principal physical–mechanical properties of the fibers are indicated in Table 1.3. Note the very significant disparity of the prices per unit weight. Figure 1.3 Relative Sale Volume of Different Fibers Figure 1.4 Structure of Carbon Fiber TX846_Frame_C01 Page 6 Monday, November 18, 2002 10:34 AM © 2003 by CRC Press LLC