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Appendix G Failure Mechanisms of Ballistic Fabrics and Concepts for Improvement FAILURE MECHANISMS rate dependence of strength of perfectly ordered polyethyl- ene (PE) and found that the maximum strength may increase Breakage of Fiber Bonds and Yarns from 1.5 GPa to 21 GPa for PE with a molecular weight of 2.2 × 104 g/mol when strain rate increases from 10–1 min–1 As in all materials, when a force is applied to the fiber to 105 min–1. Also, at low strain rate, before bond breakage, or yarn or fabric, a set of competing deformation processes molecular slippage occurs and plastic deformation is ob- can take place, depending on the loading rate, stress state, served. By comparison, at the higher strain rates observed in temperature, and other factors. Polymer fibers are normally ballistic impact, bond breakage and molecular slippage may highly crystalline and highly anisotropic due to the high occur simultaneously, or the primary bond breakage may molecular orientation and the covalent bonds along the even become predominant.2 Although the tensile properties fiber axis versus van der Waals or hydrogen bonding in the of fibers such as aramid and carbon fibers are relatively less transverse directions. However, glass and ceramic fibers can sensitive to the strain rate, fibers such as Spectra are sensitive be essentially isotropic due to their multidirectional ionic- to strain rate, and their failure strain and mechanism at high covalent bonds. The assembly of fibers into yarns and yarns strain rate may be distinctly different from that at low strain into a fabric with a given architecture or geometry leads to rate. There are relatively few studies of the strain-rate de- different overall symmetries for the actual armor. pendence of tensile behavior, and more efforts are needed to When a molecular bond is excited beyond its activa- fully characterize the strain-rate dependence. Gu3 observed tion energy, bond breakage occurs. The activation energies that strength/modulus increased from 2.4 GPa and 62 GPa for shear and interchain slip are lower than for covalent to 2.75 GPa and 72 GPa for Twaron [poly(paraphenylene bond rupture and are strongly affected by ambient tempera- terephthalamide)] and from 1.19 GPa and 20.3 GPa to 1.85 ture, pressure, and the polymer’s intrinsic glass transition GPa and 51.2 GPa for Kuralon (a polyvinyl alcohol), when temperature. When a projectile hits the fabric, the fiber is the strain rate increased from 10–2 s–1 to 103 s–1. Wang and stretched along the axial direction owing to the longitudinal Xia4 tested Kevlar in the strain-rate range from 10–4 s–1 to stress wave. Also, penetration of the projectile leads to shear- 103 s–1 and observed that the strength of Kevlar 49 increased ing across the direction of the fiber thickness. Normally in from 2.34 GPa to 3.08 GPa and its modulus from 97 GPa the contact area of projectile and fabrics, if induced strain to 125 GPa. Zhou et al.5 studied the strain-rate dependence is larger than the failure strain of the fibers, the fiber will of mechanical properties of T-300 and M40J carbon fibers break. For polymer regions that are in a rubbery state (the in the range 10–3 s–1 to 1.3 × 103 s–1 and observed that these noncrystalline component of which may be above its Tg), shear yielding is expected to occur before fracture. However, 2Shim, V., C. Lim, and K. Foo. 2001. Dynamic mechanical properties under a very high strain rate, as is the case for ballistic im- of fabric armour. International Journal of Impact Engineering 25(1): 1-15. pact, the time interval that a stressed bond spends at a certain 3Gu, B. 2003. Analytical modeling for the ballistic perforation of planar stress level is shortened and there is a lower probability for plain-woven fabric target by projectile. Composites Part B: Engineering 34(4): 361-371. bond breaking at that level; thus, strength increases with the 4Wang, Y., and Y. Xia. 1998. The effects of strain rate on the mechanical increase of strain rate. Termonia et al.1 calculated the strain- behaviour of kevlar fibre bundles: an experimental and theoretical study. Composites Part A: Applied Science and Manufacturing 29(11): 1411-1415. 1Termonia, Y., P. Meakin, and P. Smith. 1986. Theoretical study of the 5Zhou, Y., D. Jiang, and Y. Xia. 2001. Tensile mechanical behavior of influence of strain rate and temperature on the maximum strength of per- T300 and M40J fiber bundles at different strain rate. Journal of Materials fectly ordered and oriented polyethylene. Macromolecules 19(1): 154-159. Science 36(4): 919-922. 139
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140 OPPORTUNITIES IN PROTECTION MATERIALS SCIENCE AND TECHNOLOGY FOR FUTURE ARMY APPLICATIONS fibers were strain-rate-insensitive materials. Wang and Xia6 woven fabric, as has been observed by many researchers.8,9 observed that for Kevlar 49 fiber, at a fixed strain rate, the The wedge-through phenomenon is affected by projectile initial tensile modulus decreased and elongation at break geometry, fabric structure, and mobility of yarns, which is increased with the increase in test temperature. correlated to frictional behavior of the yarns. Yarn Pullout Fibrillation If yarn is not well gripped at its ends, the ends may be Anisotropic fibers are subject to splitting along their axial direction.10 High-strength fibers with highly oriented pulled out from the fabric mesh. In this case, yarn pullout may occur and none of the fibers inside this portion of the and extended polymer chains may fail in compression at yarn break. The pullout force is dependent on interyarn very low strains, normally less than 1 percent; kinking and microbuckling are major failure responses.11 When polymer friction and pre-tension. The interyarn friction is related to friction efficiency and interyarn contact area. Yarn pullout chains are highly aligned in a fiber, the tensile modulus may be the major energy dissipation path only when fabric along the fiber axis is very high, whereas the shear modulus is ungripped or not well gripped. is relatively low. Fibrillation can occur during compression and results in high energy absorption during failure, which will be useful for the ballistic performance.12 Fibrillation Remote Yarn Failure was found in para-aramid fibers13 after ballistic impact, Yarn failure may happen away from the impact area and its level was found to increase at low impact energy as but between the impact point and the gripping boundary. compared to high impact energy. Fibrillation is caused by the Shockey et al.7 observed remote yarn failure during Zylon abrasion of a projectile with yarns in the lateral direction to tensile testing. The remote yarn failure occurs in tests of the fiber axis. Flat head projectiles with less possibility of penetration do not promote much fibrillation.14,15 both transverse load (perpendicular to the yarn direction) and cylindrical load (along the yarn direction). The remote yarn failure may be hard to detect, as broken fibers may be buried Other Damage Forms inside the fabric mesh. Remote yarn failure will not affect the load on the projectile until friction force on the yarns During impact, the friction between projectile, fabric, decreases to a value that cannot sustain additional remote yarns, and filaments may cause heat generation and lead to yarn failure. Since remote yarn failure involves yarns in a temperature increase. This is more of an issue for thermo- large area of fabric target, it may significantly increase the plastic polymer fibers such as PE and nylons than for aro- energy absorbance. Remote yarn failure has been observed matic heterocyclic backbone fibers such as Kevlar due to the vastly higher melting points of the latter type of fiber. Carr16 in penetration by a blunt projectile in both two-edge-gripped and four-edge-gripped fabric targets. observed the melting of fibers after the high energy impact Wedge-Through Phenomenon 8Montgomery, T., P Grady, and C. Tomasino. 1982. The effects of pro- The wedge-through phenomenon occurs when the jectile geometry on the performance of ballistic fabrics. Textile Research Journal 52(7): 442-450. formed hole is smaller than the diameter of the projectile. 9Kirkland, K., T. Tam, and G. Weedon. 1991. New third-generation The phenomenon is more predominant in the back side of a protective clothing from high-performance polyethylene fiber: From knives multi-ply system. When a projectile hits the fabric, the trans- to bullets. Pp. 214-237 in High-Tech Fibrous Materials, ACS Symposium verse movement of the yarns locally expands the mesh and Series. American Chemical Society. increases the space between woven yarns. For a projectile 10Carr, D. 1999. Failure mechanisms of yarns subjected to ballistic im - pact. Journal of Materials Science Letters 18(7): 585-588. with a small cross-section and a fabric with only a few layers, 11Kozey,V. H. Jiang, V. Mehta,and S. Kumar. 1995. Compressive be - the projectile may push the yarns aside and slip through the havior of materials: Part 2. high-performance fibers. Journal of Materials hole. There is a greater possibility of a wedge-through pro- Research 10)4): 1044-1061. jectile phenomenon in loosely woven fabric than in tightly 12Chawla, K. 2002. Fiber fracture: An introduction. Pp. 3-26 in Fiber Fracture. M. Elices and J. Llorca, eds. Oxford, U.K.: Elsevier Science. 13Carr, D. 1999. Failure mechanisms of yarns subjected to ballistic im - pact. Journal of Materials Science Letters 18(7): 585-588. 6Wang, 14Tan, V., C. Lim, and C. Cheong. 2003. Perforation of high-strength Y., and Y. Xia. 1999. Experimental and theoretical study on the strain rate and temperature dependence of mechanical behaviour of Kevlar fabric by projectiles of different geometry. International Journal of Impact fibre. Composites Part A: Applied Science and Manufacturing 30(11): Engineering 28(2): 207-222. 15Lim, C., V. Tan, and C. Cheong. 2002. Perforation of high-strength 1251-1257. 7Shockey, D., J. Simons, and D. Elrich. 2001. Improved barriers to double-ply fabric system by varying shaped projectiles. International Jour- turbine engine fragments: interim report III. May, 2001. Available online nal of Impact Engineering 27(6): 577-591. 16Carr, D. 1999. Failure mechanisms of yarns subjected to ballistic im - http://oai.dtic.mil/oai/oai?verb=getRecord&metadataPrefix=html&identifi er=ADA392533. Accessed April 5, 2011. pact. Journal of Materials Science Letters 18(7): 585-588.
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141 APPENDIX G of Spectra fabrics. Prosser et al.17 observed a temperature MPa.18 Alumina fiber, with a tensile strength of 1.7 GPa, is increase on the back surface of a ballistic panel containing 40 the high-performance fiber with the lowest tensile strength. layers of nylon fabrics to as high as 76.6°C after perforation Thus the development of even 1 GPa tensile strength mag- by a .22 caliber projectile. nesium fiber that could be used to replace bulk magnesium alloy in helmets with a magnesium alloy and Spectra fiber construction could be significant. CONCEPTS FOR ENHANCING BALLISTIC In carbon-nanotube-reinforced composites, poly - PERFORMANCE OF FABRICS mers such as poly(paraphenylene terephthalamide), There is an opportunity to develop new fibers, com- poly(benzobisoxazole), poly(diimidazo pyridinylene [dihy- ing up with entirely new methods of processing fibers that droxy]phenylene), ultrahigh-molecular-weight PE, polyure- eliminate defects, and to make fibers from other desirable thane, and so on can be used as a matrix system, with the materials. Magnesium, with a density of only 1.7 g/cm3, is carbon nanotube as the reinforcing entity. Similarly, carbon- an example of such a desirable material. The tensile strength nanotube-reinforced fibers can also be made from metals, of most magnesium alloys is in the range 200 MPa to 400 ceramics, and glasses, wherein during high-temperature processing there exists the probability of compound forma- tion and new types of interfacial bonds. 18Mathaudhu, S., and E. Nyberg. 2010. Magnesium alloys in army ap - plications: Past, current and future solutions in magnesium technology. Pp. 27-33 in Magnesium Technology 2010: Proceedings of a Symposium 17Prosser, R., S. Cohen, and S. Cohen. 2000. Heat as a factor in the pen- Sponsored by the Magnesium Committee of the Light Metals Division etration of cloth ballistic panels by 0.22 caliber projectiles. Textile Research of TMS, 2010. Warrendale, Pa.: Minerals, Metals, and Materials Society. Journal 70(8): 709-722.