ecture and J Llorca(Editors) Elsevier Science Ltd. All rights reserved FIBER FRACTURE: AN OVERVIEW K.K. Chawla Department of Materials and Mechanical Engineering, University of Alabama at Birmingham, BEC 254, 1530 3rd Avenue S, Birmingham, AL 35294-4461, USA Polymeric Fibers Environmental Effects on Polymeric Fibers Carbon Fibers 5589 Metallic Fibers Glass and Ceramic Fibers 17 Conclusions References 24 Abstract fracture of fibers during processing or in service is generally an undesirable feature fracture in fibers, as in bulk materials, initiates at some flaw(s), internal or on the surface. In general, because of the high surface to volume ratio of fibers, the incidence of a surface flaw leading to fracture is greater in fibers than in bulk materials. Very frequently, a near-surface fiaw such as a microvoid or an inclusion is responsible for the initiation of fracture of fiber. In polymeric fibers, the fundamental processes leading to failure are chain scission and or chain sliding or a combination thereof. Service environment can be a major determining factor in the failure process of fibers. A striking example of this was in the failure of aramid fiber used in the tether rope in space. Metallic fibers represent a relatively mature technology. The surface condition and segregation of inclus the two factors that limit the strength of metallic filaments. Ceramic and silica-based fibers(including optical glass fiber) also have the same crack-initiating flaws as in polymeric and metallic fibers. One major problem in glass fibers is that of failure due to static fatigue. In this paper, examples of fracture in different types of fibers are provided. Some of the possible ways to prevent catastrophic
Fiber Fracture M. Elices and J. Llorca (Editors) 9 2002 Elsevier Science Ltd. All rights reserved FIBER FRACTURE: AN OVERVIEW K.K. Chawla Department of Materials and Mechanical Engineering, University of Alabama at Birmingham, BEC 254, 1530 3rd Avenue S., Birmingham, AL 35294-4461, USA Introduction ..................................... 5 Polymeric Fibers .................................. 5 Environmental Effects on Polymeric Fibers .................. 8 Carbon Fibers .................................... 9 Metallic Fibers ................................... 13 Glass and Ceramic Fibers .............................. 17 Conclusions ..................................... 24 References ...................................... 24 Abstract Fracture of fibers during processing or in service is generally an undesirable feature. Fracture in fibers, as in bulk materials, initiates at some flaw(s), internal or on the surface. In general, because of the high surface to volume ratio of fibers, the incidence of a surface flaw leading to fracture is greater in fibers than in bulk materials. Very frequently, a near-surface flaw such as a microvoid or an inclusion is responsible for the initiation of fracture of fiber. In polymeric fibers, the fundamental processes leading to failure are chain scission and/or chain sliding or a combination thereof. Service environment can be a major determining factor in the failure process of fibers. A striking example of this was in the failure of aramid fiber used in the tether rope in space. Metallic fibers represent a relatively mature technology. The surface condition and segregation of inclusions are the two factors that limit the strength of metallic filaments. Ceramic and silica-based fibers (including optical glass fiber) also have the same crack-initiating flaws as in polymeric and metallic fibers. One major problem in glass fibers is that of failure due to static fatigue. In this paper, examples of fracture in different types of fibers are provided. Some of the possible ways to prevent catastrophic
K.K. Chawla failure in different fibers are pointed out. A considerable amount of progress has been lade in the last quarter of the twentieth century. Keywords Alumina; Aramid; Carbon; Ceramic; Fibers; Metal; Polyethylene; Polymer; Optical
4 K.K. Chawla failure in different fibers are pointed out. A considerable amount of progress has been made in the last quarter of the twentieth century. Keywords Alumina; Aramid; Carbon; Ceramic; Fibers; Metal; Polyethylene; Polymer; Optical; Steel; Tungsten
FIBER FRACTURE: AN OVERVIEW INTRODUCTION fracture of a fiber is generally an undesirable occurrence. For example, during processing of continuous fibers, frequent breakage of filaments is highly undesirable from a productivity point of view. When this happens in the case of spinning of a polymer, ceramic or a glass fiber, the processing unit must be stopped, the mess of the solution or melt must be cleaned and the process restarted. In the case of a metallic filament, a break means that the starting wire must be pointed again, retreaded, and the process restarted. In service, of course, one would like the individual fibers whether in a fabric or in a composite to last a reasonable time Fracture in fibers, as in bulk materials, initiates at some flaw(s), internal or on the surface. In general, because of the high surface to volume ratio of fibers, the incidence of a surface flaw leading to fracture is greater in fibers than in bulk materials Fractography, the study of the fracture surface, of fibers can be a useful technique for obtaining fracture parameters and for identifying the sources of failure. In general, the mean strength of a fiber decreases as its length of diameter increases. This size effect is commonly analyzed by applying Weibull statistics to the strength data. As the fiber length or diameter increases, the average strength of the fiber decreases. It is easy to understand this because the probability of finding a critical defect responsible for fracture increases with size. This behavior is shown by organic fibers such as cotton, aramid, as well as inorganic fibers such as tungsten, silicon carbide, glass, or alumina In this paper, the salient features of the fracture process in different types of fibe lymeric, metallic, and ceramic are described. Points of commonality and difference POLYMERIC FIBERS A very important characteristic of any polymeric fiber is the degree of molecular hain orientation along the fiber axis. In order to get high strength and stiffness in organic fibers, one must obtain oriented molecular chains with full extension. An important result of this chain alignment along the fiber axis is the marked anisotropy in the characteristics of a polymeric fiber. Rigid-rod polymeric fibers such as aramid fibers show very high strength under axial tension. The failure in tension brings into play the covalent bonding along the axis, which ultimately leads to chain scission and/ or chain sliding or a combination thereof. However, they have poor properties under axial compression, torsion, and in the transverse direction. Fig. 1 shows this in a schematic manner. The compressive trength of ceramic fibers, on the other hand is greater than their tensile strength. The compressive strength of carbon fiber is intermediate to that of polymeric and ceramic fibers. This discrepancy between the tensile and compressive properties has been the subject of investigation by a number of researchers(see Chawla, 1998 for details) An example of kinking under compression in a high-performance polymeric fiber derived from rigid-rod liquid crystal is shown in Fig. 2(Kozey and Kumar, 1994) Note that this is a single fiber with preexisting striations on the surface. High-strengtI
FIBER FRACTURE: AN OVERVIEW INTRODUCTION Fracture of a fiber is generally an undesirable occurrence. For example, during processing of continuous fibers, frequent breakage of filaments is highly undesirable from a productivity point of view. When this happens in the case of spinning of a polymer, ceramic or a glass fiber, the processing unit must be stopped, the mess of the solution or melt must be cleaned and the process restarted. In the case of a metallic filament, a break means that the starting wire must be pointed again, rethreaded, and the process restarted. In service, of course, one would like the individual fibers whether in a fabric or in a composite to last a reasonable time. Fracture in fibers, as in bulk materials, initiates at some flaw(s), internal or on the surface. In general, because of the high surface to volume ratio of fibers, the incidence of a surface flaw leading to fracture is greater in fibers than in bulk materials. Fractography, the study of the fracture surface, of fibers can be a useful technique for obtaining fracture parameters and for identifying the sources of failure. In general, the mean strength of a fiber decreases as its length of diameter increases. This size effect is commonly analyzed by applying Weibull statistics to the strength data. As the fiber length or diameter increases, the average strength of the fiber decreases. It is easy to understand this because the probability of finding a critical defect responsible for fracture increases with size. This behavior is shown by organic fibers such as cotton, aramid, as well as inorganic fibers such as tungsten, silicon carbide, glass, or alumina. In this paper, the salient features of the fracture process in different types of fibers, polymeric, metallic, and ceramic are described. Points of commonality and difference are highlighted. POLYMERIC FIBERS A very important characteristic of any polymeric fiber is the degree of molecular chain orientation along the fiber axis. In order to get high strength and stiffness in organic fibers, one must obtain oriented molecular chains with full extension. An important result of this chain alignment along the fiber axis is the marked anisotropy in the characteristics of a polymeric fiber. Rigid-rod polymeric fibers such as aramid fibers show very high strength under axial tension. The failure in tension brings into play the covalent bonding along the axis, which ultimately leads to chain scission and/or chain sliding or a combination thereof. However, they have poor properties under axial compression, torsion, and in the transverse direction. Fig. 1 shows this in a schematic manner. The compressive strength of ceramic fibers, on the other hand, is greater than their tensile strength. The compressive strength of carbon fiber is intermediate to that of polymeric and ceramic fibers. This discrepancy between the tensile and compressive properties has been the subject of investigation by a number of researchers (see Chawla, 1998 for details). An example of kinking under compression in a high-performance polymeric fiber derived from rigid-rod liquid crystal is shown in Fig. 2 (Kozey and Kumar, 1994). Note that this is a single fiber with preexisting striations on the surface. High-strength
spun(dried) Strain of rigid-rod polymeric fibers in tension and compressio show strength under axial tension but have poor properties under axial compression, torsion, an organic fibers fail in <I%. Microbuckling or shear banding responsible for easy failure in compression. The spider dragline silk fiber seems to be an exception to this. In general, highly oriented fibers such as aramid fail in a fibrillar fashion. The term fibrillar fracture here signifies that the fracture surface is not transverse to the axis but runs along a number of planes of weakness parallel to the fiber axis. As the orientation of chains in a fiber becomes more parallel to its axis, its axial tensile modules(E)increases but the shear modulus(G)decreases, i.e. the ratio E/G increases tremendously. During failure involving compressive stresses, fibrillation occurs, which results in a large degree of new surface area. This fibrillation process results in high-energy absorption during the process of failure, which makes these fibers useful for resistance against ballistic penetration Various models have been proposed to explain this behavior of high-performance ers. Fig 3 shows two compressive failure models:(a)elastic microbuckling of poly meric chains; and(b) misorientation. The microbuckling model involves cooperative in phase buckling of closely spaced chains in a small region of fiber. The misorientation model takes into account structural imperfections or misorientations that are invariabl present in a fiber. In the composites literature it has been reported that regions of
K.K. Chawla | ..... | . . J Iteat-treated _ .Heat-treated As spun (dried) ,[/' ~ulated ..... V ............................. , ...................................... , ....................................... ., ....................................... I ........................ ik ............. , Strain ---~- l Fig. 1. Schematic stress-strain curves of rigid-rod polymeric fibers in tension and compression. Such fibers show very high strength under axial tension but have poor properties under axial compression, torsion, and in the transverse direction. organic fibers fail in compression at strains < 1%. Microbuckling or shear banding is responsible for easy failure in compression. The spider dragline silk fiber seems to be an exception to this. In general, highly oriented fibers such as aramid fail in a fibrillar fashion. The term fibrillar fracture here signifies that the fracture surface is not transverse to the axis but runs along a number of planes of weakness parallel to the fiber axis. As the orientation of chains in a fiber becomes more parallel to its axis, its axial tensile modules (E) increases but the shear modulus (G) decreases, i.e. the ratio E/G increases tremendously. During failure involving compressive stresses, fibrillation occurs, which results in a large degree of new surface area. This fibrillation process results in high-energy absorption during the process of failure, which makes these fibers useful for resistance against ballistic penetration. Various models have been proposed to explain this behavior of high-performance fibers. Fig. 3 shows two compressive failure models: (a) elastic microbuckling of polymeric chains; and (b) misorientation. The microbuckling model involves cooperative inphase buckling of closely spaced chains in a small region of fiber. The misorientation model takes into account structural imperfections or misorientations that are invariably present in a fiber. In the composites literature it has been reported that regions of
FIBER FRACTURE: AN OVERVIEW O um An example of kinking under compression in a high-l ance polymeric fiber derived from quid crystal(courtesy of Kozey and Kumar). Higl organic fibers fail in compression at <l%. Microbuckling or shear banding is responsible for easy failure in compression Band of buckled hains/fibrils Fig. 3. Two compressive failure models: (a)elastic microbuckling of polymeric chains; this model involves cooperative in-phase buckling of closely spaced chains in a small region of fiber;( b) misorientation; this model is based on structural imperfections or misorientations that are invariably present in a fiber misorientation in a unidirectional composite lead to kink formation under compressive loading (Argon, 1972). The model shown in Fig. 3b is based upon the presence of such a local misorientation in the fiber leading to kink formation under compression Failure in compression is commonly associated with the formation and propagation of kinks. These kink bands generally start near the fiber surface and then grow to the center of the fiber. It has also been attributed to the ease of microbuckling in such
FIBER FRACTURE: AN OVERVIEW Fig. 2. An example of kinking under compression in a high-performance polymeric fiber derived from rigid-rod liquid crystal (courtesy of Kozey and Kumar). High-strength organic fibers fail in compression at strains < 1%. Microbuckling or shear banding is responsible for easy failure in compression. Fig. 3. Two compressive failure models: (a) elastic microbuckling of polymeric chains; this model involves cooperative in-phase buckling of closely spaced chains in a small region of fiber; (b) misorientation; this model is based on structural imperfections or misorientations that are invariably present in a fiber. misorientation in a unidirectional composite lead to kink formation under compressive loading (Argon, 1972). The model shown in Fig. 3b is based upon the presence of -such a local misorientation in the fiber leading to kink formation under compression. Failure in compression is commonly associated with the formation and propagation of kinks. These kink bands generally start near the fiber surface and then grow to the center of the fiber. It has also been attributed to the ease of microbuckling in such