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Appendix J Fiber-Reinforced Polymer Matrix Composites Polymer matrix composites (PMCs) consist of a poly- composites the deformation across the thickness direction mer resin reinforced with fibers, an example of which is the does not vary with depth, whereas for thick composites it does.2 Ballistic performance initially increases linearly with combat helmet. PMCs can be subdivided into two categories, based on whether the fiber reinforcement is continuous or the increased thickness; however, as the composite becomes discontinuous. PMCs with discontinuous fibers (less than thicker the marginal protective gain incurred by increasing the thickness becomes smaller,3,4 although the rate at which 100 mm long) are made with thermoplastic or thermosetting resins, whereas PMCs with continuous fibers usually employ the weight increases is maintained. thermosetting resins. This appendix primarily addresses PMCs containing continuous fibers. The most common de- DEFORMATION AND FAILURE MECHANISMS sign for PMCs is a laminate structure made of woven fabrics held together by a polymer resin. Fabrics are incorporated in When a PMC is subjected to high-velocity impact, the order to take advantage of their high strength and stiffness kinetic energy is transferred from the projectile to the PMC. and to improve energy absorption and distribute the kinetic The existence of two components, the fabric and the matrix, energy laterally. Owing to their highly engineered structures, and their interface, makes the energy absorption mechanism PMCs are lightweight with high specific strength and high more complex than that of ballistic fabrics. The commonly specific stiffness. recognized energy absorption and failure mechanisms are Commonly used reinforcement materials include car- discussed here. bon, glass, aramid, and polyethylene fibers. PMCs can be manufactured by wet and hand lay-up; molding (compres- Cone Formation on the Back Face sion, injection, and transfer); vacuum bag molding; infusion molding; vacuum-assisted resin transfer molding; prepreg1 As with ballistic fabrics, the mode of impact response molding; and other common fabrication techniques. Unlike known as cone formation has also been observed in PMCs. Guoqi et al.5 observed the formation of a cone-shaped common structural composites, which typically contain σf ( ε, ε , T ) deformation zone in the back surface of Kevlar/ up to about 60 vol percent fibers, ballistic PMCs contain a higher volume fraction of fibers or fabrics (up to about 80 polyester laminates during the ballistic impact of a blunt projectile; using high speed photography, Morye et al. 6 vol percent). The effect of this variation in structure on the ballistic protection properties of PMCs has not been thor- documented the temporal evolution of this response for oughly investigated. the ballistic behavior of nylon fabric preimpregnated with PMCs respond to ballistic impact in ways that depend on their particular structure and thus are different from other 2Naik, N., and A. Doshi. 2008. Ballistic impact behaviour of thick com - posites: Parametric studies. Composite Structures 82(3): 447-464. protective materials. Unlike fabrics, with PMCs only the 3Ibid. material in the neighborhood of the impact position shows 4Faur-Csukat, G. 2006. A study on the ballistic performance of compos - a response; thus the response is completely governed by ites. Macromolecular Symposia 239 (1): 217-226. the local behavior of the material and unaffected by bound- 5Guoqi, Z., W. Goldsmith, and C.K.H. Dharan. 1992. Penetration of lami- ary conditions. Additionally, the penetration mechanism nated Kevlar by projectiles—I. Experimental investigation. International Journal of Solids and Structures 29(4): 399-420. is dependent on the thickness of the composite. For thin 6Morye, S., P. Hine, R. Duckett, D. Carr, and I. Ward. 2000. Modelling of the energy absorption by polymer composites upon ballistic impact. 1Semifinished fiber products preimpregnated with epoxy resin (prepregs). Composites Science and Technology 60(14): 2631-2642. 150
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151 APPENDIX J a matrix of a 50:50 mixture of phenol formaldehyde resin and polyvinyl butyral resin. Figure J-1 shows the scheme of cone formation in two-dimensional woven fabric com- posites during projectile impact. The yarns that the bullet directly contacts are called primary yarns; these yarns resist penetration and undergo deformation due to cone formation. The longitudinal compressive stress wave generated upon impact propagates outward along the yarn direction, forming a quasi-circular shape. The conical portion moves backward and stores kinetic energy by its motion. Deformation of Yarns and Failure When a PMC undergoes ballistic impact, the primary yarns deform and resist projectile penetration. The other yarns (called orthogonal yarns) also deform, but to a lesser extent due to primary yarn deformation; this process stores kinetic energy. During cone formation, strain is highest along the middle primary yarns in each layer of the composite. The FIGURE J-1 Cone formation during ballistic impact on the back highest overall strain is at the point of impact, and the strain face of the composite target. SOURCE: Naik, N.K. 2005. Ballistic falls off along the radial direction. After the cone forms, the impact behaviour of woven fabric composites: Parametric studies. top layers of the PMC are compressed, leading to an increase Materials, Science and Engineering: A 412(1-2): 104-116. in the tensile strain of the yarns there. A linear relation be - tween strain and depth along the thickness direction can be assumed; see Figure J-1. Once the strain is beyond the failure strain, sequential breakage will occur beginning at the top continue until total perforation occurs.8 Research has shown9 layer. This yarn failure absorbs additional kinetic energy. that initiation and propagation of delamination occur more frequently along the warp and fill directions than along other directions. Compared to conventional materials, composite Delamination and Matrix Cracks materials contain numerous interfaces between the matrix During ballistic impact, transverse and longitudinal and the fibers, providing multiple locations for cracking waves are formed. The geometry of the deformation influ- to occur. Energy absorption occurs through a combination ences the terminology used to describe the deformation: of cracking, delamination, and shear banding (the latter is The waves that move out in the lateral direction (having dependent on the plasticity of the matrix and possibly of the both longitudinal and transverse polarization) from the point fibers). Typical shapes of delaminated regions after impact of impact are called transverse, and the waves propagating are shown in Figure J-2;10 the noncircular shape is attributed along the direction of the incident projectile are called lon- to the anisotropic nature of these materials (different paths gitudinal. A cone of deformation, quasi-lemniscate in shape, of the stress waves, hence different distances that the stress is formed due to transverse waves.7 As the longitudinal information must travel). waves propagate along the yarns, attenuation occurs, lead- ing to strain variations radially from the impact site in the Shear Plugs target. The matrix has mechanical properties different from those of the yarns, but it must carry the same deformation During impact experiments on conventional carbon- lest delamination or slippage occur due to weak adhesion fiber-reinforced plastic laminates, it was observed11 that a between the yarn and the matrix; there may be damage if the small area of the laminate was sheared off by the projectile yarn strain is higher than the strain at failure in the matrix. As the material deforms, cracking and delamination will 8Naik, N., and K. Reddy. 2002. Delaminated woven fabric composite plates under transverse quasi-static loading: experimental studies. Journal of Reinforced Plastics and Composites 21(10): 869-877. 9Wu, E., and L.-C. Chang. 1995. Woven glass/epoxy laminates subject to projectile impact. International Journal of Impact Engineering 16(4): 607-619. 10Naik, N. 2006. Ballistic impact behaviour of woven fabric composites: 7Wu, E., and L.-C. Chang. 1995. Woven glass/epoxy laminates subject Formulation. International Journal of Impact Engineering 32(9): 1521-1552. 11Cantwell, W., and J. Morton. 1990. Impact perforation of carbon fibre to projectile impact. International Journal of Impact Engineering 16(4): 607-619. reinforced plastic. Composites Science and Technology 38(2): 119-141.
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152 OPPORTUNITIES IN PROTECTION MATERIALS SCIENCE AND TECHNOLOGY FOR FUTURE ARMY APPLICATIONS FIGURE J-3 Schematic showing plug formation. SOURCE: Naik, N. 2004. Composite structures under ballistic impact. Composite FIGURE J-2 Schematic shape of delaminated regions observed in Structures 66(1-4): 579-590 impact experiments. Region 1: area damage in the first time interval after impact; Region 2: area damaged in the (i + 1) time interval. SOURCE: Naik, N. 2006. Ballistic impact behaviour of woven fabric composites: Formulation. International Journal of Impact The Contribution of Different Types of Energy Absorption Engineering 32(9): 1521-1552. Paths Naik and Shrirao13 analyzed the ballistic impact behav- ior of woven fabric composites under a flat head projectile during impact and that a distinct conical-shaped zone was using wave theory and presented an analytical formulation formed. The schematic is shown in Figure J-3. The shear for each energy absorption mechanism. The calculation plug phenomenon has never been observed in glass-fiber- is based on the material properties at high strain rate, and reinforced composites, which may be due to the much higher analytical prediction shows a good match with experimental failure strain of glass fibers compared to that of carbon fibers results. During the ballistic impact, the moving area of the at high strain rates. cone increases, leading to an increase in the kinetic energy of the cone even though the speed of the projectile is re- Friction and Hole Enlargement duced. Next, as the moving speed decreases significantly, the kinetic energy of the cone decreases and becomes zero In contrast to the complex frictional forces present in when the projectile’s speed reaches zero. The kinetic energy neat fabrics (including friction between yarns, between the of the cone is the major energy absorption factor, followed projectile and the yarn, and between the individual fibers), the by deformation of the orthogonal yarns and tensile breakage only friction present in PMCs during impact occurs between of primary yarns; delamination and cracking provide only a the projectile and the laminate. After the yarns and the fabrics small fraction of the energy absorption. The calculations as- fail, friction between the damaged laminates dissipates some sume a relatively thin and flexible PMC system; for thicker of the kinetic energy from the projectile. Goldsmith et al.12 systems, the variation of deformation as a function of thick- calculated the frictional work by using the friction efficiency ness changes the relevant material behavior and requires a between projectile and laminate measured by the quasi-static consideration of friction. method. They found that the friction resistance depends on the shape of the projectile and that it increases with increas- CURRENT ISSUES AND RELATED STUDIES ing composite thickness. Additionally, they calculated the energy dissipated when the projectile enlarges the hole and As noted above, the ballistic performance of laminated found that this process also contributes to energy dissipation. PMCs depends on the properties of the polymer matrix and Although the energy absorbed due to friction is much larger of the reinforcement material, on the stacking sequence, on than that due to hole enlargement, neither of these modes is the fiber architecture, on the qualities of the interface and the major energy absorption mechanism. the interphase, on the environmental conditions, and on the characteristics of the projectile. To date, however, the experimental studies have only focused on certain types 12Goldsmith, W., C.K.H. Dharan, and H. Chang. 1995. Quasi-static and 13Naik, N., and P. Shrirao. 2004. Composite structures under ballistic ballistic perforation of carbon fiber laminates. International Journal of Solids and Structures 32(1): 89-103. impact. Composite Structures 66(1-4): 579-590.
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153 APPENDIX J fabrics.18 Satin and twill weaves also tended to absorb more of composites and ballistic conditions. Thus, the full map energy than the plain weaves,19 possibly due to a decrease of ballistic performance of this class of composites is still unknown, and more analytical experiments and simulations in the crimp angle. It was also found that the architecture of are needed to improve the understanding of the ballistic the fabric is more important in thicker composites than in performance of PMCs. thinner composites, as the decreased crimp angle decreases stress concentration. Improved ballistic performance can be obtained by Material Properties using three-dimensional woven fabrics instead of two- dimensional woven fabrics.20 Walter et al.21 quantitatively The properties of the fabrics, the surrounding ma- trix, and the interfaces affect the overall performance of analyzed results from three-dimensional woven glass-fiber- laminates. Although no thorough map of the effects of the reinforced composites and observed that delamination along properties of fabrics and polymer matrix has been drawn, an the weak layer is the most severe shortcoming in current examination of the experimental literature allows for some three-dimensional woven composites at high load and high preliminary conclusions. Faur-Csukat14 prepared fabric loading rates. In general, Z-stitching increased the resistance composites with a fabric wt percent of approximately 55 by to damage, and it restricted damage to a smaller total area than that in unstitched samples. However, in one study22 a hand lay-up followed by compression molding. The ballistic performance of carbon-, glass-, aramid-, and polyethylene- decrease in ballistic limit was observed in Z-stitched targets, fabric-reinforced composites showed that the efficacy of although no explanation of this decrease was provided. Cohen et al.23 used Spectra 1000 yarns to reinforce reinforcing fibers was as follows: Glass is better than aramid, which is better than or equal to ultrahigh-molecular-weight a UHMWPE matrix with a fiber content of up to 85 per- polyethylene (UHMWPE), which is better than carbon fibers. cent. The shear strength (20-25 MPa) and tensile strength Among the different PMCs studied, carbon-fiber-reinforced (longitudinal, 1.3-1.5 GPa; transversal, 21-25 MPa) of the composites exhibited the worst ballistic performance owing prepared composite are better than those of composites like to their low strain to failure. Roughly, fibers with high strain UHMWPE fiber/epoxy matrix composites and UHMWPE at high strain rate are better energy absorbers than high- fiber/high-density polyethylene (PE) matrix composites. strength fibers with low strain to failure. This conclusion is Furthermore, the tensile strength of the prepared composite the same as that of Naik.15 The fiber-matrix interface and is similar to that of Kevlar fiber/epoxy matrix composites. interphase also play a critical role in ballistic performance. These improvements are attributed to the good self-adhesion It was observed that weaker interfacial interaction resulted and strong bonding of PE fibers to the PE matrix. The ballis- in higher energy absorption.16,17 In composites, fiber-matrix tic response of PE/PE composites under the impact of bullets debonding, cracks, and friction slippage improve energy (9 mm in diameter, weighing 8 g, velocity approximately 400 absorption; this is different from the behavior of noncom- m/s) shot from an Uzi submachine gun has been investigat- ed.24 High-density PE was used as the matrix, and UHM- posite materials. However, excessively low interaction and interfacial strength will lead to pre-ballistic failure problems. WPE fibers such as Spectra and Dyneema were used as the For a full understanding of the effects of material properties, more analytical experiments as well as further modeling and 18Faur-Csukat, G. 2006. A study on the ballistic performance of compos - ites. Macromolecular Symposia 239 (1): 217-226. simulation are needed. 19Hosur, M., U. Vaidya, C. Ulven, and S. Jeelani. 2004. Performance of stitched/unstitched woven carbon/epoxy composites under high velocity Fabric Structure impact loading. Composite Structures 64(3-4), 455-466. 20Shukla, A., J. Grogan, S. Tekalur, A. Bogdanovich, and R. Coffelt. Weave architecture also influences the ballistic perfor- 2005. Ballistic resistance of 2D & 3D woven sandwich composites. Pp. 625-634 in Sandwich Structures 7: Advancing with Sandwich Structures mance of composites. It was shown that (under the condi- and Materials: Proceedings of the 7th International Conference on Sandwich tions investigated), the performance of basket-weave fabrics Structures, O. Thomsen, E. Bozhevolnaya, and A. Lyckegaard, eds. New was better by about 10 percent than that of plain-weave York, N.Y.: Springer. 21Walter, T., G. Subhash, B. Sankar, and C. Yen. 2009. Damage modes in 14Faur-Csukat, G. 2006. A study on the ballistic performance of compos - 3D glass fiber epoxy woven composites under high rate of impact loading. ites. Macromolecular Symposia 239 (1): 217-226. Composites Part B: Engineering 40(6): 584-589. 15Naik, N. 2004. Composite structures under ballistic impact. Composite 22Hosur, M., U. Vaidya, C. Ulven, and S. Jeelani. 2004. Performance of Structures 66(1-4): 579-590. stitched/unstitched woven carbon/epoxy composites under high velocity 16Park, R., and J. Jang. 1998. A study of the impact properties of com - impact loading. Composite Structures 64(3-4), 455-466. 23Cohen, Y., D. Rein, and L. Vaykhansky. 1997. A novel composite based posites consisting of surface-modified glass fibers in vinyl ester resin. Composites Science and Technology 58(6): 979-985. on ultra-high-molecular-weight polyethylene. Composites Science and 17Tanoglu, M., S. McKnight, G. Palmese, and J. Gillespie Jr. 2001. The Technology 57(8): 1149-1154. 24Harel, H., G. Marom, and S. Kenig. 2002. Delamination controlled bal - effects of glass-fiber sizings on the strength and energy absorption of the fiber/matrix interphase under high loading rates. Composites Science and listic resistance of polyethylene/polyethylene composite materials. Applied Technology 61(2): 205-220. Composite Materials 9 (1): 33-42.
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154 OPPORTUNITIES IN PROTECTION MATERIALS SCIENCE AND TECHNOLOGY FOR FUTURE ARMY APPLICATIONS reinforcement phase. The material was created by winding casting, which may trap a nonequilibrium structure during fibers in a unidirectional pattern on large-diameter mandrels solvent evaporation as the solution goes through its glass transition concentration.30 Thus, further phase separation can which were then flattened into film; these films were stacked on top of one another, with each layer rotated 90° from the occur when the temperature is higher than the glass transition one below it to achieve a 0°/90° laminate. temperatures (PMMA Tg = 100°C; PC Tg = 150°C, depend- ing on the component polymer molecular weights). This fur- ther phase separation results in strong optical scattering from PERSPECTIVE: NEW TYPES OF FIBERS the larger domains and loss of transparency. Component immiscibility causes opaque materials for melt processing of Nanocomposites PMMA and PC blends. Additionally, solvent-induced crys- When incorporated into composite materials,25 nano- tallization of PC decreases optical clarity. Another strategy sized fillers have been shown to provide superior reinforce- for addressing the transparency problem is to produce multi- ment due to their outstanding mechanical properties.26 Thus, nanolayer polymer laminates by co-extrusion of PMMA and the ballistic resistance dynamics and capacity of carbon PC; this results in laminates containing individual layers as nanotubes (CNTs) were simulated, and their potential use in thin as 100 nm and an overall structure that has good opti- armor was discussed.27 Simulations found that CNTs with cal clarity. This method was originally developed at Dow the highest ballistic resistance could be resilient to a projec- Chemical Company in the 1960s and further refined at the tile at speeds of 200 m/s to 1,400 m/s if one end is fixed.28 3M Company and at Case Western Reserve University.31 A Additionally, CNT hybrid composites and CNT-reinforced system with two extruders and a co-extrusion block is used fibers all have potential for improving ballistic performance. to extrude two layers that are first sliced vertically, then spread horizontally, and finally recombined. This step can be repeated n times and generate 2(n+1) polymer layers while Polymer Laminates the thickness of the layers is decreased in proportion to their In matrix composites, the reinforcing fibers have me- increased number. The thickness of the PMMA layers plays chanical properties that are much higher than those of the a critical role in the ballistic performance of PC/PMMA polymer laminates.32 The adhesion between PMMA and PC matrix. Because this mismatch can cause delamination and is strong enough to overcome delamination.33 In this case, cracking, which do not absorb as much kinetic energy as other modes of failure, or for other reasons relevant to the the mode of failure depends strongly on the thickness of the intended use of the product, polymer laminates that contain individual component layers. For laminates containing the thickest layers (greater than 0.5 m thick), the composite film two or more kinds of polymer have also been investigated. For example, polycarbonate (PC) is widely used in transpar- is brittle, and the laminate fails in brittle mode. For interme- diate layer thicknesses (between 150 nm and 0.5 m), several ent ballistic applications, but its susceptibility to chemicals, scratching, and other possible service conditions limit ap- different failure mechanisms are present, with microcrack- plications. Two possible solutions have been investigated: ing in the PMMA layers appearing to be the dominant one. (1) blending a second polymer with PC and (2) applying a Materials with layer thicknesses less than 150 nm behave in hard surface coating to the PC. Blending another transparent, a ductile manner and fail with a large amount of plastic flow, chemically resistant polymer such as polymethyl methacry- resulting in increased ballistic impact energy. The ballistic late (PMMA) with PC can improve the chemical sensitivity, performance of polymer laminates of PC with PMMA as but it can also reduce ballistic performance. Similarly, a hard well as with poly(styrene-co-acrylonitrile) (SAN) processed with varying layer thicknesses has also been reported.34 The coating may provide abrasion and chemical protection, but it also reduces impact resistance.29 PC and PMMA are not nor- adhesion between PC and PMMA is 10 times higher than that mally miscible, so blending can only be achieved by solvent 30Kyu, T., and J. Saldanha. 1998. Miscible blends of polycarbonate and 25Njuguna, J., K. Pielichowski, and S. Desai. 2008. Nanofiller-reinforced polymethyl methacrylate. Journal of Polymer Science Part C: Polymer polymer nanocomposites. Polymers for Advanced Technologies 19(8): Letters 26(1): 33-40. 31Mueller, C., S. Nazarenko, T. Ebeling, T. Schuman, A. Hiltner, and E. 947-959. 26Koziol, K., J. Vilatela, A. Moisala, M. Motta, P. Cunniff, M. Sennett, Baer. 1997. Novel structures by microlayer coextrusion–talc-filled PP, PC/ and A. Windle. 2007. High-performance carbon nanotube fiber. Science SAN, and HDPE/LLDPE. Polymer Engineering & Science 37(2): 355-362. 32Hsieh, A., and J. Song. 2001. Measurements of ballistic impact response 318(5858): 1892-1895. 27Mylvaganam, K., and L. Zhang. 2007. Ballistic resistance capacity of of novel coextruded PC/PMMA multilayered-composites. Journal of Rein- carbon nanotubes. Nanotechnology 18(47). forced Plastics and Composites 20(3): 239-254. 28Mylvaganam, K., and L. Zhang. 2006. Energy absorption capacity of 33Ibid. 34Kerns, J., A. Hsieh, A. Hiltner, and E. Baer. 2000. Comparison of ir- carbon nanotubes under ballistic impact. Applied Physics Letters 89(12). 29Hsieh, A., and J. Song. 2001. Measurements of ballistic impact response reversible deformation and yielding in microlayers of polycarbonate with of novel coextruded PC/PMMA multilayered-composites. Journal of Rein- poly(methylmethacrylate) and poly(styrene-co-acrylonitrile). Journal of forced Plastics and Composites 20(3): 239-254. Applied Polymer Science 77(7): 1545-1557.
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155 APPENDIX J between PC and SAN as measured by the T-peel method.35 components, and both PC/SAN and PC/PMMA laminates However, the difference in adhesion has almost no effect on with thin layers exhibited superior ballistic performance to the deformation mechanisms. The ductility of thin layers of that of laminates with thicker layers. Further decreases in the laminates was attributed to the cooperative yielding of both thickness of the PMMA layer should produce better ballistic performance. 35The T-peel method is a way to measure the peel resistance of adhesives.
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