KK Chawla fibers as well as to the presence of microvoids and the skin-core structure of these fibers. It should be pointed out that poor properties in shear and compression are however, also observed in other highly oriented polymeric fibers such as polyethylene and poly(p-phenylene benzobisoxazole)or PBO fibers, which are not based on rigid-rod polymers. A correlation between good compressive characteristics and a high glass transition temperature(or melting point) has been suggested(Northolt, 1981; Kozey and Kumar, 1994) Thus, with the glass transition temperature of organic fibers being lower than that of inorganic fibers, the former would be expected to show poorer properties in compression. For aramid and similar fibers, compression results in the formation of kink bands leading to eventual ductile failure. Yielding is observed at about 0.5%o strain. This is thought to correspond to a molecular rotation of the amide carbon-nitrogen bond from the normal extended trans configuration to a kinked configuration Tanner et 1986). This causes a 45 bend in the chain, which propagates across the unit cell, the microfibrils and a kink band results in the fiber Efforts to improve the compressive properties of rigid-rod polymer fibers have involved introduction of cross-linking in the transverse direction. There is a significant effect of intermolecular interaction or intermolecular cross-link strength. a polymeric fiber(PIPD)with a compressive strength of 1.6 GPa has been reported (Jenkins et al., 2001). This high compressive strength is ascribed to bi- directional, intermolecular hydrogen bonding. A high degree of intermolecular covalent cross-linking should result in higher compressive strength, as compared to systems in which only hydrogen bonding is present (Jenkins et al., 2001). However, cross-linking may also result in lower tensile strength and increased brittleness of the fiber. Cross- linking by thermal treatment may result in the development of internal stresses. Other cross-linking methods(e. g. via radiation) should be explored in greater detail. One would expect radiation to result in a different cross-linked structure than that obtained by thermal treatment. Here it is instructive to compare the behavior of some carbon fibers. Highly graphitic, mesophase pitch-based fibers show a fibrillar fracture and poor compressive properties. PAN-based carbon fibers, which have some linking of the graphitic planes in the transverse direction, show better properties in compressic not a very fibrillar fracture. Of course metallic and ceramic fibers show brillation during a tensile or compressive failure Environmental Effects on Polymeric Fibers Environmental factors such as humidity, temperature, pH, ultraviolet radiation, n affect the strength and the fracture process in polymeric fibers. Natural polymeric fibers are more susceptible to environmental degradation than synthetic polymeric fibers. Cellulose is attacked by a variety of bacteria, fungi, and gae Micro-organisms use cellulose as a food source. Natural fibers based on protein such as wool, hair, silk, etc, can also be a food source for micro-organisms, but such fibers are more prone to degradation due to humidity and temperature. Polymeric fibers, natural or synthetic, undergo photo degradation when exposed to light (both visible and ultraviolet). Physically this results in discoloration, but is also accompanied by a
8 K.K. Chawla fibers as well as to the presence of microvoids and the skin-core structure of these fibers. It should be pointed out that poor properties in shear and compression are, however, also observed in other highly oriented polymeric fibers such as polyethylene and poly(p-phenylene benzobisoxazole) or PBO fibers, which are not based on rigid-rod polymers. A correlation between good compressive characteristics and a high glass transition temperature (or melting point) has been suggested (Northolt, 1981; Kozey and Kumar, 1994). Thus, with the glass transition temperature of organic fibers being lower than that of inorganic fibers, the former would be expected to show poorer properties in compression. For aramid and similar fibers, compression results in the formation of kink bands leading to eventual ductile failure. Yielding is observed at about 0.5% strain. This is thought to correspond to a molecular rotation of the amide carbon-nitrogen bond from the normal extended trans configuration to a kinked configuration Tanner et al., 1986). This causes a 45 ~ bend in the chain, which propagates across the unit cell, the microfibrils, and a kink band results in the fiber. Efforts to improve the compressive properties of rigid-rod polymer fibers have involved introduction of cross-linking in the transverse direction. There is a significant effect of intermolecular interaction or intermolecular cross-linking on compressive strength. A polymeric fiber (PIPD) with a compressive strength of 1.6 GPa has been reported (Jenkins et al., 2001). This high compressive strength is ascribed to bidirectional, intermolecular hydrogen bonding. A high degree of intermolecular covalent cross-linking should result in higher compressive strength, as compared to systems in which only hydrogen bonding is present (Jenkins et al., 2001). However, cross-linking may also result in lower tensile strength and increased brittleness of the fiber. Crosslinking by thermal treatment may result in the development of internal stresses. Other cross-linking methods (e.g. via radiation) should be explored in greater detail. One would expect radiation to result in a different cross-linked structure than that obtained by thermal treatment. Here it is instructive to compare the behavior of some carbon fibers. Highly graphitic, mesophase pitch-based fibers show a fibrillar fracture and poor compressive properties. PAN-based carbon fibers, which have some linking of the graphitic planes in the transverse direction, show better properties in compression and not a very fibrillar fracture. Of course metallic and ceramic fibers show little fibrillation during a tensile or compressive failure. Environmental Effects on Polymeric Fibers Environmental factors such as humidity, temperature, pH, ultraviolet radiation, and micro-organisms can affect the strength and the fracture process in polymeric fibers. Natural polymeric fibers are more susceptible to environmental degradation than synthetic polymeric fibers. Cellulose is attacked by a variety of bacteria, fungi, and algae. Micro-organisms use cellulose as a food source. Natural fibers based on protein such as wool, hair, silk, etc., can also be a food source for micro-organisms, but such fibers are more prone to degradation due to humidity and temperature. Polymeric fibers, natural or synthetic, undergo photo degradation when exposed to light (both visible and ultraviolet). Physically this results in discoloration, but is also accompanied by a
FIBER FRACTURE. AN OVERVIEW worsening of mechanical characteristics. A striking example of environmental leading to failure of aramid fiber was the failure of the tether rope for a satellite in space. The rope made of aramid fiber failed because of friction leading to excessive static charge accumulation,which led to premature failure of the tether rope and the loss of an ein a multifilament yarn or in a braided fabric, frictional force in the radial direction holds the fibers together. Such interfiber friction is desirable if we wish to have strong yarns and fabrics. However there are situations where we would like to have a smooth fiber surface. For example, for a yarn passing round a guide, a smooth fiber surface will be desirable. If the yarn surface is rough, then a high tension will be required, which, in turn, can lead to fiber breakage. In general, in textile applications, friction characteristics can affect the handle, feel, wear-resistance, etc. In fibrous composites, the frictional characteristics of fiber can affect the interface strength and toughness CARBON FIBERS Carbon fibers are, in some ways, similar to polymeric fibers while in other ways they are similar to ceramic fibers. A characteristic feature of the structure of all carbon fibers is the high degree of alignment of the basal planes of graphite along the fiber axis The degree of alignment of these graphitic planes can vary depending on the precursor used and the processing, especially the heat treatment temperature used. Transmission electron microscopic studies of carbon fiber show the heterogeneous microstructure of arbon fibers. In particular, there occurs a pronounced irregularity in the packing of graphitic lamellae as one goes from the fiber surface inward to the core or fiber axis The graphitic basal planes are much better aligned in the near-surface region of the fiber, called the sheath. The material inside the sheath can have a radial structure or an irregular layer structure, sometimes termed the onion skin structure. The radial core and well aligned sheath structure is more commonly observed in mesophase-pitch-basec carbon fibers. A variety of arrangement of graphitic layers can be seen in different fibers. In very general terms, the graphitic ribbons are oriented more or less parallel to he fiber axis with random interlinking of layers, longitudinally and laterally (Jain and Abhiraman, 1987; Johnson, 1987; Deurbergue and Oberlin, 1991). Fig. 4 shows a two- dimensional representation of this lamellar structure called turbostratic structure. Note the distorted carbon layers and the rather irregular space filling. The degree of alignment of the basal planes increases with the final heat treatment temperature. Examination of lattice images of the cross-section of carbon fiber shows essentially parallel basal planes in the skin region, but extensive folding of layer planes can be seen in the core region is thought that this extensive interlinking of lattice planes in the longitudinal direction is responsible for better compressive properties of carbon fiber than aramid fibers. In spite of the better alignment of basal planes in the skin region, the surface of carbon fibers can show extremely fine scale roughness. A scanning electron micrograph of pitch-based carbon fibers is shown in Fig. 5. Note the surface striations and the roughness at a microscopic scale
FIBER FRACTURE: AN OVERVIEW worsening of mechanical characteristics. A striking example of environmental leading to failure of aramid fiber was the failure of the tether rope for a satellite in space. The rope made of aramid fiber failed because of friction leading to excessive static charge accumulation, which led to premature failure of the tether rope and the loss of an expensive satellite. In a multifilament yarn or in a braided fabric, frictional force in the radial direction holds the fibers together. Such interfiber friction is desirable if we wish to have strong yarns and fabrics. However, there are situations where we would like to have a smooth fiber surface. For example, for a yarn passing round a guide, a smooth fiber surface will be desirable. If the yarn surface is rough, then a high tension will be required, which, in turn, can lead to fiber breakage. In general, in textile applications, frictional characteristics can affect the handle, feel, wear-resistance, etc. In fibrous composites, the frictional characteristics of fiber can affect the interface strength and toughness characteristics. CARBON FIBERS Carbon fibers are, in some ways, similar to polymeric fibers while in other ways they are similar to ceramic fibers. A characteristic feature of the structure of all carbon fibers is the high degree of alignment of the basal planes of graphite along the fiber axis. The degree of alignment of these graphitic planes can vary depending on the precursor used and the processing, especially the heat treatment temperature used. Transmission electron microscopic studies of carbon fiber show the heterogeneous microstructure of carbon fibers. In particular, there occurs a pronounced irregularity in the packing of graphitic lamellae as one goes from the fiber surface inward to the core or fiber axis. The graphitic basal planes are much better aligned in the near-surface region of the fiber, called the sheath. The material inside the sheath can have a radial structure or an irregular layer structure, sometimes termed the onion skin structure. The radial core and well aligned sheath structure is more commonly observed in mesophase-pitch-based carbon fibers. A variety of arrangement of graphitic layers can be seen in different fibers. In very general terms, the graphitic ribbons are oriented more or less parallel to the fiber axis with random interlinking of layers, longitudinally and laterally (Jain and Abhiraman, 1987; Johnson, 1987; Deurbergue and Oberlin, 1991). Fig. 4 shows a twodimensional representation of this lamellar structure called turbostratic structure. Note the distorted carbon layers and the rather irregular space filling. The degree of alignment of the basal planes increases with the final heat treatment temperature. Examination of lattice images of the cross-section of carbon fiber shows essentially parallel basal planes in the skin region, but extensive folding of layer planes can be seen in the core region. It is thought that this extensive interlinking of lattice planes in the longitudinal direction is responsible for better compressive properties of carbon fiber than aramid fibers. In spite of the better alignment of basal planes in the skin region, the surface of carbon fibers can show extremely fine scale roughness. A scanning electron micrograph of pitch-based carbon fibers is shown in Fig. 5. Note the surface striations and the roughness at a microscopic scale
KK Chawla Fig. 4. A two-dimensional representation of the lamellar structure(or fiber. The cross-section of carbon fiber has essentially parallel basal plane folding of layer planes can be seen in the core region. It is thought that planes in the longitudinal direction is responsible for better compress aramid fibe A carbon fiber with a perfectly graphitic structure will have the theoretical Young modulus of slightly over 1000 GPa. In practice, however, the Young modulus is about 50% of the theoretical value in the case of PAN-based carbon fiber and may reach as much as 80% of the theoretical value for the mesophase-pitch-based carbon fiber. The rength of carbon fiber falls way short of the theoretical value of 180 GPa(Reynolds, 981). The practical strength values of carbon fiber may range from 3 to 20 GPa. The main reason for this is that while the modulus is determined mainly by the graphitic crystal structure, the strength is a very sensitive function of any defects that might be present, for example, voids, impurities, inclusions, etc. The strength of carbon fiber thus depends on the gage length, decreasing with increasing gage length. This is because the probability of finding a defect in the carbon fiber increases with its gage length Understandably, it also depends on the purity of the precursor polymer and the spinning nditions. A filtered polymer dope and a clean spinning atmosphere will result in a higher strength carbon fiber for a given gage length Following Huttinger(1990), we can correlate the modulus and strength of carbon fiber to its diameter. One can use Weibull statistics to analyze the strength distribution in brittle materials such as carbon fiber. As mentioned above, such brittle materials show a size effect, viz the experimental strength decreases with increasing sample size. This is demonstrated in Fig. 6, which shows a log-log plot of Youngs modulus as a function
10 K.K. Chawla Fig. 4. A two-dimensional representation of the lamellar structure (or turbostratic structure) of a carbon fiber. The cross-section of carbon fiber has essentially parallel basal planes in the skin region, but extensive folding of layer planes can be seen in the core region. It is thought that this extensive interlinking of lattice planes in the longitudinal direction is responsible for better compressive properties of carbon fiber than aramid fibers. A carbon fiber with a perfectly graphitic structure will have the theoretical Young modulus of slightly over 1000 GPa. In practice, however, the Young modulus is about 50% of the theoretical value in the case of PAN-based carbon fiber and may reach as much as 80% of the theoretical value for the mesophase-pitch-based carbon fiber. The strength of carbon fiber falls way short of the theoretical value of 180 GPa (Reynolds, 1981). The practical strength values of carbon fiber may range from 3 to 20 GPa. The main reason for this is that while the modulus is determined mainly by the graphitic crystal structure, the strength is a very sensitive function of any defects that might be present, for example, voids, impurities, inclusions, etc. The strength of carbon fiber thus depends on the gage length, decreasing with increasing gage length. This is because the probability of finding a defect in the carbon fiber increases with its gage length. Understandably, it also depends on the purity of the precursor polymer and the spinning conditions. A filtered polymer dope and a clean spinning atmosphere will result in a higher strength carbon fiber for a given gage length. Following Htittinger (1990), we can correlate the modulus and strength of carbon fiber to its diameter. One can use Weibull statistics to analyze the strength distribution in brittle materials such as carbon fiber. As mentioned above, such brittle materials show a size effect, viz., the experimental strength decreases with increasing sample size. This is demonstrated in Fig. 6, which shows a log-log plot of Young's modulus as a function