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