140 OPPORTUNITIES IN PROTECTION MATERIALS SCIENCE AND TECHNOLOGY FOR FUTURE ARMY APPLICATIONS
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 trans- verse 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 pro- jectile phenomenon in loosely woven fabric than in tightly
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&identifi er=ADA392533. Accessed April 5, 2011.
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 thermo- plastic polymer fibers such as PE and nylons than for aro- matic 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
8Montgomery, T., P Grady, and C. Tomasino. 1982. The effects of pro- jectile 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 im- pact. Journal of Materials Science Letters 18(7): 585-588.
11Kozey,V. H. Jiang, V. Mehta,and S. Kumar. 1995. Compressive be- havior 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 im- pact. 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 Jour- nal of Impact Engineering 27(6): 577-591.
16Carr, D. 1999. Failure mechanisms of yarns subjected to ballistic im- pact. Journal of Materials Science Letters 18(7): 585-588.
APPENDIX G 141
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, com- ing 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
17Prosser, R., S. Cohen, and S. Cohen. 2000. Heat as a factor in the pen- etration of cloth ballistic panels by 0.22 caliber projectiles. Textile Research Journal 70(8): 709-722.
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 mag- nesium 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, poly- mers such as poly(paraphenylene terephthalamide), poly(benzobisoxazole), poly(diimidazo pyridinylene [dihy- droxy]phenylene), ultrahigh-molecular-weight PE, polyure- thane, 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 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 Sponsored by the Magnesium Committee of the Light Metals Division of TMS, 2010. Warrendale, Pa.: Minerals, Metals, and Materials Society.