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.



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