Ballistic Protective Clothing: an Overview
Indian Journal of Fibre & Textile Research Vol. 22, December 1997, pp. 274-291
Ballistic protective clothing: An overview
Pushpa 8ajaj & Sri ram Department of Textile Technology, Indian Institute of Technology, New Delhi 110016, India
Various materials have been employed in the past for the protection against the ballistics with only limited satisfaction without realizing the underlying mechanism of ballistic impact. With the advent of high performance materials and computing techniques in the recent years, the mechanism of ballistic penetration has clearly been established, paving a path for the selection of newer materials for better protection and satisfying the requirements of both the wearer and the technologist. This review attempts to coalesce the ideas of exuberant research that has gone into the field of ballistic protection right from material selection to modelling with emphasis on evaluation methods and mechanism.
Keywords: Armour, Ballistics, Ceramics, Kevlar, Penetration mechanism, Projectile velocity, Soft composites, Spider silk, Ultra-high modulus polyethylene fibre
1 Introduction velocity propagation, tensile properties . and the Ballistic protection is a class of protective fine structure. In the recent years, ultra-high clothing which aims at protecting the individuals modulus polyethylene (UHMPE) fibres -produced from the bullets and steel fragments from hand by gel spinning witnessed increased application in l2 15 held weapons and exploding munitions. The use of the ballistic protection systems . due to a host of armour to protect the personnel has a long history properties which make these fibres highly suitable and dates back to time immemorial. As the man for ballistic protection. developed newer weapons with time, he also With the current trend towards eco-friendly and looked for better clothing for protection. The list of natural products for various applications, spider armour materials has included every conceivable silk is gaining importance in ballistic protection. 5 material '· including the traditional materials like The spider silk is obtained from a spider, 'Black aluminium, steel and less traditional ones like Vido', found exclusively in South America with leather and silk. The best known method of some exceptional properties like elongation at protecting the human body in old days from all break up to 270% and strength higher than Kevlar kinds of missiles, was to use a hard rigid material fibre. which resisted the penetration and dissipated the In this review, a brief account of various fibres 6 • • load of impact " With the advent of synthetic used for the designing of ballistic protective textiles, better ballistic protective systems have clothing has been gIven. The penetration been developed. Although the basic idea of mechanism and the modelling of ballistic spreading the load over a large area is still applied, protection has also been discussed. it is better to dissipate the energy of impact by deformation and breaking the protective material. 2 Ballistic Requirements Nylon, which has high work of rupture The basic objective of the body armour research (toughness), was thought to be ideal for ballistic is to manage the conflicting goals of producing low protection. However, it became very clear that eost, light-weight yet comfortable ballistic simple toughness is not the only criterion for protection systems with superior performance. ballistic protection. Then came the use of aramids No armour design is suitable for all the situations8 due to the fact that ballistic protection process is a and the armour system produced for a specific complex phenomenon involving the transverse application should be able to fit in both user's and BAJAl & SRIRAM: BALLISTIC PROTECTIVE CLOTHING 275
). technologist's points of view. For users, the comfOit, generalizations can be made regarding the ballistic 6 7 degree of mobility and maximum protection against efficiency of the system • . injury/trauma are important, while for technologists, the level of protection required, time span for which 3.1 Fibre Selection the protection is required, and energy absorption The bullet-resistant clothing has to stop the bullet characteristics of the material are important. from penetrating and absorb its kinetic energy .In normal warfare, one tries to ensure minimum converting it into work of deformation. Therefore, weight of the protective garment with maximum the primary factors which influence the performance personal mobility and maXImum protection. of bulletproof or protective material are strength, Therefore, in designing the protective garment modulus and elongation at break, deformability of normally the legs and arms are left exposed, as the projectile and the velocity of transverse shock injury on these parts is least likely to cause death wave in the fibre. Some of the important properties and the vital organs like heart, liver, etc. are of various fibres considered for armour applications covered. This provides additional advantage of are given in Table 1. mobility. Since these garments are worn for greater duration, comfort along with protection are given greater importance. On the contrary, for more 3.1.1 Nylon 66 hazardous jobs like bomb disposal, heavy weight Nylon fibres were the material of choice for most garment is employed covering the whole body. of the ballistic applications in the early days. During Here, the overall protection, not the comfort and the second world war, American Anny produced mobility, is important. 'flak jackets' using steel plates with nylon 66 backing8. Nylon usually absorbs twice the amount of Energy absorption characteristics of the body l energy as p-aramids (Fig. 1). armour &-18 are very important. According to Capilli In p-aramids, the transverse wave velocity is and RothulzenlI, five types of eriergy absorption almost 3-4 times higher to that in nylon. Hence, occur to fulfil the basic function of personnel body stress propagation is more efficient with aramids armour. These include: which is clear from Fig. 2. Here, the elongation of • Kinetic energy of the out-of-the plane fabric the yam around the point of impado was measured movement (the pyramid). approximately 10 I.lS after the shot was fired at a . • Strain energy accumulated in the yams of the speed of 400 rnIs. It is evident that the strain in p pyramid. aramid fibre fabric is spread over a much larger area • Kinetic energy of the fabric moving towards the and the elongation is much lower. Moreover, nylon point of initial impact, in the original fabric creeps under such high strain rates at which the plane between the pyramid and the longitudinal wave form. • Strain energy accumulated in the original fabric plane between the pyramid and the longitudinal 20 wave form. • Energy dissipated as heat by friction (fibre/fibre -;;.. 16 & fibre/projectile). z More details regarding energy absorption ~ 12 on on characteristics will be dealt later. ~ v; 8
3 Material Selection 4 No armour design is suitable for all the situations and the performance of the protective system 5 10 15 20
depends on the interaction of its various Strain (01.) components. Hence, it is important to understand the
mechanism of the ballistic protection by which Fig. I-Stress-strain curves of nylon and p-aramid 10 276 INDIAN 1. FIBRE TEXT. RES., DECEMBER 1997
22
VI :l. 0, Entry holt ~ '\ 20 10 / '\-POLYAMIOE66 . ~ !:! Kevlar c o 0 18 -0-.. i ",0 c .c o ,., I 16 w ~ 5 .. \ Distancfl o ~ - 14 .. i __ _ u Z a. '------t~ - , ~ARAMID 01 , 12 III , III c:.I "2 4 ....~ If) 10 Distance from entr y hal. (e m) 8 Fig. 2-Ballistic comparison between nylon 66 and aramid fibre fabrics 22 6 ballistics operate and sometimes melting and fusion 4 at the interlacement points has also been noted IS. 2
3.1.2 Aramids Since its inception in 1972, Kevlar readily Strain ( 0'0) replaced the nylon in the ballistic protective garments due to its good dynamic energy absorption Fig. 3-Stress-strain behaviour of various hi gh perfonnance characteristics, high specific strength and modulus, fibres and steel wire20 and excellent thermal properties. High Tg and thermal stability ensure integrity of 3-4 years, while PE insignificant at 0.5 fraction of the ballistic structure at relatively high temperature normal breaking load (Fig. 4). Hence, Kevlar in the event of ballistic impact. Its highly crystalline outperforms most of the materials. and oriented structure gives rise to high dynamic Kevlar creeps little at low loads due to high modulus which enhances the rate of wave l crystallinity and orientation, but creeps under propagation to 7700 ms· , which is 3-4 times higher relatively high stresses. Creep rate is almost uniform than that for nylon '9, and rapid response to and the amount of creep strain increases linearly longitudinal deformation. This high wave speed with log (time) to break. along with specific modulus is instrumental for its Kevlar has relatively low compressional a':>ility to involve large volume of material which is properties. When subjected to axial compression or a critical factor in ballistic protection. Moreover, severe bending, it will undergo plastic deformation high tenacity and moderate elongation of Kevlar in the form of bands at 55-600 to the fibre axis aramid fibres provide high toughness and results in called as "KINK BANDS" which is due to its fairly effective absorption of longitudinal strain structural anisotropy. The axial compressive energy and transverse kinetic energy of the ballistic strength of Kevlar fibre is about 115 of the tensile impact. strength, while compressive strain at the fibre yield Kevlar has a tenacity of 18-26 gpd (Table 1 & point is 0.5% compared to 2.5% at the tensile break. 20 Fig. 3), twic~ that of nylon or glass fibres , with an Properties of the various commercially available elongation of only 1.5-4.4% and initial modulus of aramid fibres are given in Table 1. 430-1100 gpd. Its specific modulus lie in between those of fibre glass, boron and carbon fibres. 3.1.3 Ultra-High Modulus Polyethylene (UHMPE) Kevlar fibres support large fraction of their The commercial success of these fibres is due to breaking load for longer time than nylon or their outstanding mechanical properties, their polyethylene (PE). Using a safety factor of 2-3, unmatched damage tolerance, fatigue resistance and Kevlar provides a service life of > 100 years, nylon their ability to fail in shear or compression without BAJAJ & SRIRAM: BALLISTIC PROTECTIVE CLOTHING 277
Table I---Propcrties of hi gh perfonnance fibres 20
Fibre type Density Strength Elongation Modulus" Maximum use Wave velocitl gcm-J gpd (G Pa) % gpd (G Pa) temp. ms-I °C
Aramids Kev lar 29 IA3 23 (2.9) 3.6 550 (70) 250 6996 Kevl ar 49 IA5 23 (2.9) 2.8 950(135) 250 9649 Kev lar 11 9 1.44 24 (3. 1) 4A 430 (55) 250 6 180 Kevl ar 129 IA5 26.5 (3.4) 3.3 780 (99) 250 8263 Kevlar 149 IA7 18 (2.3) 1.5 1100 ( 143) 250 9863 Nomex 138 5 (0.6) 22 140 ( 17) 250 3509 Technora 1.39 27 (3.3) 4.3 570 (70) 250 7096 Ekonol IA 31 (3.8) 2.6 1100 ( 13 6) 150 9856 Vectran 1.47 25 (3 .2) 700 (9 1) 150 7868 UHMPE Spectra 900 0.97 30 (2 .6) 3.5 1400 ( 120) 100 11123 Spectra 1000 0.97 35 (3 .0) 2.7 2000 ( 171 ) 100 13277
Carbon fibres Thomel 1.8 10.8 (1.7) 1940 (308) 500 13081 P55 (Med M) Thomel 1.96 10.8 ( 1. 86) 0.38 3300 (5 17) 600 16241 'PIOO (HM) Celion 1. 8 25 (4.0) 1.8 1440 (230) 500 11 304 3000 (HS)
Ceramics Boron 2.5 11 .6 (2.55) 1.0 1800 (400) 2000 12649 SiC 2.8 16(4.0) 0.6 1700 (420) 1300 12247 Alumina 3.25 6.3 ( 1. 8) 1. 2 730 (2 10) 1200 8038 Nextcl 2.5 7.8 (1.72) 2 690(152) 1200 7797 E-glass 2.55 11 .6 (2.6) 3 320 (72) 350 5313 S-glass 2A8 21.9 (4.8) 5.3 390 (85) 300 5854
modulus 'Modulus (GPa}=(gpdxdensity)/ 11 .33; and bWave velocity = density
20r------~ losing great amount of tensile strength. On the weight basis, they are the strongest and almost the 16 stiffest commercially available materials offering ~ 1/1 ~Kevlar the highest percentage of absorbed energy versus ~ 0 ~ total impact energyl3. ..C\I 12 .D WE One of the most interesting features of UHMPE ....0 ... 100 year .!Io is the large modulus increase with increasing rate of 21 E deformation, particularly at elevated temperature • 8 1 year ~ 01 About 40% increase in tensile strength involving 0 -J ballistic impact with deformation rates exceeding 103 min-I (from 4 GPa at room temperature to 5.6 GPa) is because of the unique morphological
o <----I---'__ ..&..---'----' __ -'----'----' __ ..&....::::. structure of UIiMPE. In addition, due to its low o 0.2 0.4 0.6 0.8 1.0 density (0.97 for Dyneema and Spectra compared to Fraction of normal breaking load 1.45 for Kevlar) the velocity of longitudinal stress Fig. 4--Tensile behaviour of various fibres under fixed load wave is very high, reaching a value almost equal to 20 l2 with time that of diamond . All the ~bove factors make the 278 INDIAN J FIBRE TEXT. RES., DECEMBER 1997
UHMPE the most sought material for ballistic as metals (Table 2) they are still suitable for ballistic protection. applications, primarily due to high compressiv~ Fig. 5 shows interesting relationship between the strength and hardness. Since the dynamic stress energy absorption and diameter of the projectile. It limits of ceramics are higherl , sharp point of the can be readily seen that at smaller diameter of projectile is quickly eroded, leaving less effective projectile, aramid structures are advantageous due to blunt cylinder having lower mass. This would high coefficient of friction . In all the other greatly reduce the energy of impact, further conditions, Spectra (UHMPE) outperforms Kevlar. enhancing the effectiveness of ceramics. However, Despite all the advantages of UHMPE like specific stTength, light and chemical resistance, low Table 2- Density and HEL of common ceramics2 specific weight and good dynamic properties, Material Density HEL negligible moisture sensitivityl4, it is the creep, low g/cm) GPa temperature resistance, adhesion and compression Alp) 39[ I 1.2 properties that limits its application. Various Alp) 3.92 9.2 UHMPE fibres are commercially available under the B,C 2.50 15 .0 trade names like Dynecma SK 60 from DSM and Glass 2.48 7.3 BeO 2.84 85 Spectra from Allied Signals. MgO 3.57 8.9
3.1.4 Carbon Fibres HEL: Hugoniot Elastic Limits Carbon fibres have very high modulus and tensile strength. Fig. 6 shows that energy absorption rate 10 \ increases steadily with the modulus, but flattens out \ V=1. 00 ms' 8 \ "' ~ as the fibre modulus exceed 500 gpd, signifying that U\ VI C ~ \ 0 6 --- Q. while high modulus is needed to achieve high rate of .":J U\ \\, ~o r p t , on ra te 1:' wave propagation, but is accomplished only at the '"~ u ~ ..=, 0 4 cost of elongation at break. Thus, increasing the U\ - .::::- 0:'" fibre modulus would generally increase the fibre o - /----- __ " '_o:~::: :" ,,"'"' wo"' eD brittleness and ultimately would reduce its ability 0 for strain energy absorption. 0 100 200 300 400 500 600 Carbon fibres being high-modulus and high F,bre m odulus ( g /de n) tenacity fibres and having very low strain-to-failure Fi g. ~Effect of carbon fibre mudulus on ballistic response would shear readily under compression (Fig. 7) as [Units (Y ax is), 10 = 0.03 gpd fo r strain energy and 900 gpd 21 reported by Tanner22. These factors make carbon for energy absorption rate ] fibres unsuitable for ballistic applications.
3.1.5 Ceramics " :::'" 20 Cor bon Though ceramics have very high density values z u p-Aromid U\ U\ ~ 10 II ::: r----«1---/-/-"_-,~-~-~~-:-u-.~-,--,-"'O: I 0.3 0.2 0 ·1 p-Aromid 0 ·10 0·40 ~-- ~ U/,'--- -- ~ Kev lor ~ c{ 0.20 ----_---0___ -_-----/ Carbon 0.20 O L------~ Fig. 7-Tensile and compressive behaviour of p-aramid and Fig. 5- Encrgy absorption characteri stics of Kcvlar and carbon fibres with tension in NE quadrant and compression in Spectra " SW quadrant22 BAJAJ & SRI RAM: BALLISTIC PROTECTIVE CLOTHING 279 inherent brittleness and lack of flexural strength of ballistic response of fabrics appears to be closely ceramics renders them incapable of being used related to the ballistic response of the single yams. alone as single material for the fabrication of the In woven fabrics, since there is an extensive armour. interaction among the yams due to interlacements, it was found that transverse deflection causes loading 3.2 Influence of Yarn Structure on Ballistic Protection of the crossover yams and up to 50% of the total Friction plays an important role in the ballistic energy absorption may occur in the secondary yams. protection, which, in tum, depends on the method of Therefore, fabric construction particulars have a yam production. Many of the fibres commonly used major role in the final ballistic protection 16.26. For for ballistic protection are highly drawn, hence the instance, if the weave is too tight or the fabric is too fibre surface is smooth which means that in the stiff, deflection will be restricted, causing shear event of an impact by any object there is a tendency failure due to the concentration of stress at the for the yams to slip apart simply because of their impact point. Whereas too loose a weave or a soft low coefficient of friction, e.g. UHMPE is highly fabric having low yam-to-yam friction would allow drawn up to draw ratios of 20-30 with very low the projectile to penetrate easily by pushing the coefficient of friction. Therefore, efforts have been yams aside or giving too much deflection which can put to roughen the fibre surface by chemical or cause serious injuries or trauma to the wearer. mechanical means to overcome this problem. Therefore, it is the balanced force distribution Coating the surface of fibre is another slow and with maximum interlacements in a given unit area painstaking method. However, current practice is to which makes plain weave construction most suitable use corona treatmenf3/plasma etching for surface for the ballistic protection. Various other weaves treatment. like basket, satin, crowfeet, etc., though inferior to Such high friction fibres comprise a minimum in plain weave, are employed depending on specific the total structure and should only be 10-20% by application. 24 weight. H6genboom proposed that filaments with Table 3 shows various weaves and construction high coefficient of friction should be combined with particulars with Kevlar fibres employed for ballistic fibres having high tensile strength and modulus and protection. A plain weave of 1,100 dtex aramid low coefficient of friction. The two types of fibres filament yam with 31 x 31 threadslinch and 280 may even be twisted or core spun together as a yam. g/m2 fabric weight is regarded as the standard It is also suggested that very thick high performance material. Generally, warp with lower modulus and components may be combined with very thin high higher extension than weft with fabric having same friction elements, leading to improvement in the stiffness in both warp and weft directions is 24 performance. preferred . For polyethylene ballistic-resistant fabric, Harpell Xu and Weitsman27 studied the compressive et al. 15 suggested that the yam with a denier of not response of unidirectionally reinforced hexagonal more than 500 and a tensile modulus of atleast 200 arrays of graphitelPEEK composites by finite gpd can be used. element analysis. Unlike the other models which It was al so observed that the fabrics made from assume layered geometries, the results of this study fine aramid yams9 of 220-440 dtex perform better show the three-dimensional features of stress and than those made from the coarser yams. But due to displacement fields and exhibit the presence of high costs of the finer yams, usually 1,100 dtex is significant stress concentrations in these hexagonal used. fibrous array. Weinberg and Schwartz25 reported that a twist level above 1 turn/cm for Kevlar yams lowers the Knitting though offers considerable advantage in modulus and strength. A twist factor of 1.1 is found terms of cost and in the production of final design of 8 20 , to be suitable for Kevlar yams . contoured armour it has never proved to be successful in the armour field due to the degree of 3.3 Fabric Design for Ballistic Protection interlocking of the yams that occur in the knitting Cunnifr6 found that, to a large extent, the process. 280 INDIAN 1. FIBRE TEXT. RES ., DECEMB ER 1997 Table 3--Balli stic fabrics of Kevlar 29 and Kevlar 129 aramid 20 Kevlar Yam size Weave Ends/cmx Picks/em Wt of fabric Denier Decitex g/m2 29 1000 1110 Plain 12.2x I2.2 282 29 1500 1670 Plain 9.4x9.4 326 29 1500 1670 Plain 6.7x6.7 231 49 1420 1580 Plain 6.7x6.7 231 29 1500 1670 2x2 basket 13 .8x 13.4 475 29 1500 1670 8x8 basket 18.9x 18.9 669 29 3000 3330 Plain 6.7x6.7 475 29 3000 3330 4x4 basket 7.9x7.9 550 29 3000 3330 4x4 basket 9.4x9.4 672 129 840 935 Plain 12.2x 12.2 231 129 840 935 Plain 12 .2x I2 .2 231 Needle-punched nonwovens are useful in the finishes to the ballistic protective garment which ballistic applications. For ideal ballistic protection, would increase the cohesion between the warp and the nonwoven felt has to have a high level of the weft yams. entanglements of long staple fibres with only a Lamination of the fabric has been found to minimum degree of needling, as excessive needling increase the anchoring strength of the yam by a can produce too much of fibre alignment which factor of about 10-15. As a consequence, the yams would aid projectile penetration. Considerable care, hit by bullet do not pull from the fabric. Another therefore, should be taken to optimize the feit positive aspect of the coating is that the yams structure. movement IS impeded in their sideward displacement by piercing projectiles. Kevlar coated Felts of lower weights are probably the most with neoprene chlorosulphonated polymer elastomer effective materials known for ballistic protection. The effectiveness decreases as the weight increases is resistant to acids, fire, toxic gases and steam and and the nonwovens are overtaken by woven tex is able to form an impermeable seaj28. 29 tiles and ultimately by ceramics and metals at Temmerman used electroless metallized higher projectile velocities. materials to coat a layer on the fabrics which improved lightning strike protection. 30 3.4 Finishes for Ballistic Protection Ward found that for UHMPE, oxygen plasma It is interesting to note that the performance of treatment reduced the delamination during impact garments for ballistic protection is influenced with a corresponding reduction In energy significantly with the moisture content, weaving absorption. oils, sizes and other lubricants present in addition to Aramid fabrics coated with heat-resistant fluoro the fabric weave and other weaving parameters. polymers showed improved sp'uttering resistance for This is due to the fact that the interfacial behaviour steel mill workers. The film is fonned at an angle of of the yams is altered, so is the yam mobility in the 0-200°. The composite fabric of m-aramid, p-aramid fabric, due to the presence of moisture or weaving and fire-resistant rayon was impregnated with a oils. fluoroolefin-vinyl ether copolymer, squeezed and Ballistic protective gannents are therefore made treated at 110-160 °c for 8 min to give good 9 3 hydrophobic . About 40% reduction in the ballistic sputtering resistance '. protection has been observed when the bulletproof Nonwoven felts manufactured from linear garment is made wet without any change in the polyethylene,2 modified by corona/plasma treatment 8 9 properties of the component yams . . This suggests or with a filler by folding the web of the carded that water acts as a lubricant between the projectile fibres in zigzag direction, calendering, stretching in and the fabric. transverse direction, stitching or hydroentangling, Scouring of the fabrics is therefore necessary to gave high impact resistance than conventional remove the size, oils, etc. in addition to the anti-slip textiles, BAJAJ & SRIRAM: BALLISTIC PROTECTIVE CLOTHING 281 Apart from polyethylene, fabrics comprising together using threads of high modulus (- 200 gpd), p-phenylenediamine-terephthalic acid copolymer with areal densities in the range of 3.5 - 6.2 kg/ml having elongation at break > 4 %, modulus <600 made out of Kevlar or UHMPE. gpd and breaking strength >24 gpd showed good Montgomery36, in an interesting experiment, ballistic properties33 . reve·aled the construction particulars of 10 layers of In addition to all these, armour made from Kevlar Kevlar 49 plies sufficient to prevent complete should have an extra finish which would reduce the penetration of the bullets within a impact velocity l UV light ·absorption, as Kevlar loses strength upon range of 200-400 ms· . exposure to UV light. While for the armours made Yarn linear density, 1490 den; Weave, plain; from PE, a finish is required to reduce the creep. Ends/cm x picks/cm, 9.45 x 9.45; and Fabric 2 weight, 330 g/m . 4 Designing of Ballistic Protective Clothing . For 0.22 calibre, 2. 6 g, RNSP: Vso 334 ms·\ Principally, two classes of materials, viz. fibrous Projectile - I [ materials and ceramics, have emerged having great For 0.38 calibre, 10.2 g, RNSP:Vso 309 ms· potential in the designing of the ballistic protective Study also revealed that the more pointed garments / composites. projectiles are not decelerated as fast as the blunt Depending on the· mode of their application, ones. ballistic protective clothing can be broadly divided Sacks37, in a US patent, disclosed a light weight into soft armour made from textile material, and bulletproof garment consisting of several layers composite laminate armour or hard armour. sandwiched between an outer cover. The layers comprise closely woven fabrics made from aramid 4.1 Soft Armour fibres of high tensile strength and high stretch Soft armour is constructed from multiple layers resistance. of woven fabric without a resin binder, sewn Zutle38 also described the construction of a together with meander or croSSWIse seam. bulletproof garment which includes an outerlayer Depending on the calibre to be stopped and the yarn and inner layer stitched together. Total structure count, the number of fabric layers in making a bullet comprises several layers of different ballistic proof vest varies from 10 to 50 weighing around 3 materials including an outer layer of Kevlar, which kg. Various fabric lay.ers are stacked with parallel serves as fireproof material as well as an impact yarn alignment which harbours the risk that two absorbing surface, and a reinforcing ply. Inside this bullets impacting in succession at not too great a outerlayer are eight plies of spectra shield, ten plies distance from each other will damage the same warp of Kevlar and beneath are the eight plies of spectra or weft yarn in all the layers. The layers are sewn shield and a single ply of Kevlar. A reinforcing together with high tenacity aramid sewing threads panel of 1.5 mm made from Lexan polycarbonate is which seem to perform better if they are close positioned against this last Kevlar ply. There is a together due to the fact that these sewing threads reduction in the back-face deformation of · the themselves participate in the energy absorption. It armour from normally expected distance of 3.2-3.6 has been observed9 that very high stitch frequencies em to only 1.5-1.9 cm. This construction claims of the order of 10 stitches /cm2 can reduce the fabric substantial increase in the resistance to penetration yarn tenacity by up to 40%. of projectile, thereby reducing the possibility of 4 Kunzendorf reported that Kevlar fibres woven injury to the wearer. in plain weave with fabric weight up to 200 g m·2 with careful wash treatment made into 3 quilted 4.2 Hard Armour layers offer protection against the pistol bullets. The Composite laminate armour or hard armour wearer suffers mearly a haematoma from the impact consists of multilayered fabrics combined together of the bullet. with a resin binder. Another class of hard armour Allied Signals35 revealed the construction uses armour plates made out of ceramics and fibre particulars for ballistic protective garments: reinforced plastics (FRP) of about 10 mm thickness. multilayer construction with 20-30 layers, stitched The main fun ction of such a vest is to reduce the 282 INDIAN J. FIBRE TEXT. RES., DECEMBER 1997 shock effect on the body by absorbing the energy of protective layers beneath. These are so arranged that impact partially or completely. The bullet also the· projectile cannot penetrate between the joints or deforms upon impact with reduction in kinetic overlay areas. This type of construction had' a total energy and hence can be easily stopped by weight of 3.5 kg compared to 3.9 kg of matching subsequent layers9. standard of Kevlar fabric vest. This design also Armours made from the resin binder show less offers more comfort and flexibility to the wearer. 40 dependence on weave construction as the matrix is Armellino and Annellino , in an US patent, responsible for the distribution of the energy to revealed a ballistic protection wear comprising at subsequent layers. Therefore, the choice of least 3 plies of ballistic woven aramid fibre fabric construction is often dictated by the particular type each individually impregnated with of projectile. For ex~niple, protection against fine resorcinol/formaldehyde latex, at least nine plies of projectiles requires a close weave and finer denier non-impregnated ballistic woven aramid fibre while large fragments can be effectively stopped fabric, and further a layer of impregnated fabric with a loose weave and coarser denier. from the outer protective side to the inner side. Selection of the resin binder greatly influences In another patent, Lee41 disclosed the design. of a the ballistic protection I. Ductile resins such as vinyl bulletproof protective shield. It comprises layers of esters tend to perform better than more brittle or woven aramid fabric extensively bonded together to thermosetting ones such as epoxy. Resin showing form a stiff region. An antiballistic ceramic tile is increasing ductility absorbs more energy in both bonded to the laminate. An adhesively mounted crack initiation and propagation. It has also been metal plate or an additional stiff support of proved9 that for best ballistic protection, resin adhesively bound aramid or glass fibre layers may content should be 20-25 wt%. Laminates with lesser be disposed between the stiff regions and the tile. resin content show increased ballistic protection, but The .shield may also include a trauma pack. the deformations are severe and unacceptable for Lee41 reported the use of light weight armour most of the applications. For maximum ballistic made out of Dyneema SK 60 or Spectra with efficiency, fibrous armours must be allowed to aramids. Construction involves outer multi layers of deflect and delaminate, hence should never be aramid positioned in front of matrix comprising a combined with rigid components in a way that first layer, a second layer and a mass of fibres restricts deflection. arranged perpendicular to the layers. 39 42 A number of products and patents -48 have Higham Ferrers developed a range of ballistic appeared in the recent past. Owens Corning materials, most of which are based on the use of fibreglass COrp39 reported a rigid ballistic material glass fibres, incorporating Dyneema (UHMPE) or which is fire resistant, smoke resistant, non-toxic, an aramid. These new types of materials not only non-conductive, self supporting and stiff. The have an impact absorbing surface, but also tend to material is made by coating high strength magnesia retain the projectile rather than cause it to skid away. alumino silicate glass fibres with a solution of A combination of UHMPE and fine filament glass partially-condensed low-molecular weight phenol roving is capable of withstanding impact from most formaldehyde resole reaction product, from which of the small projectiles and would also resist the solvent is evaporated. Coated fabrics are then penetration of knives, daggers and even arrows. 43 heated to increase the resole molecular weight and In another patent, ICI disclosed the construction to prepare easy handle prepeg. Plies of the prepeg of composite armour of a polymeric matrix are then moulded by heating under pressure when reinforced with continuous filaments that will resist volatile by-products are escaped and then heating at penetration by sharp objects. The matrix is normally elevated temperatures to get crosslinked resin. a polymerizable liquid such as acrylic or Abott8 reported the construction of the flak methacrylic acids or methyl esters or styrene or jackets to reduce the problem of trauma caused to some such mixtures. A second component will the wearer due to the ballistic impact. Here, the contain at least two acrylate or methacrylate groups protective panels intended for covering the body are such as triethylene glycol acrylate. The polymer equilateral metal triangles or hexagons fixed to the matrix has a tensile modulus of at least 0.5 GPa. BAJAJ & SRIRAM: BALLISTIC PROTECTIVE CLOTHING 283 There should be a strong fibre-matrix interface been reported by Allied Signals2J. The composite which should have a trans-flexural strength . of at consists of network of high strength fibres and least 24 MPa, preferably > 50 MPa. The continuous thermoplastic polyurethCl;ne matrix material derived filament reinforcement should be in the form of from an aliphatic diisocyanate and polyol. collimated filaments extending the length and width DSM47 reported the use of Dyneema SK56, a of the material. These continuous collimated special ballistic yam, for the applications where, filaments may be arranged in superimposed layers; high energy absorption and low weight are the filaments in adjacent layers being arranged criteria. The armour is made by pressing resin transversely to one another. impregnated fabrics into the desired shapes. Lin et In a typical construction, the filament content al. 48 have reported high strength composite for should be 30-70% by weight made out of UHMPE, ballistic protective clothing made from a network of aramids, carbon or glass. The armour developed will high strength fibres and a matrix compound made 2 have an areal density of at least 6-8 kg/m . Another out of a vinyl ester and diallyl phthalate. preferred alternative is based on the use of woven Bulletproof vests are generally so tailored that fabrics with balanced weaves such as plain or twill they protect the body all round, the front of the vest which may then be built up as layers to create quasi may be stronger than the back assuming that a shot isotropic orientation of the filaments in the resultant will generally come from the front. structure. In a US patent, Coppage and Coppage49 disclosed Allied Signals44 reported a light weight, flexible a bulletproof dress shirt that is adjustable to nonwoven composite which is more effective than accommodate wearers of different sizes. It is made the conventional armour fabric. The spectra shield from standard dress shirt fabric and has inner layers composite is based on UHMPE Spectra fibre which on the front and back panels that are made from is coated with a very thin layer of polymer resin, bicomponent materials to draw perspiration away cross-plied and then encapsulated in another thin from the wearer. In the garment there are panels that protective film. This spectra shield is some 2.5 lb ft-2 can be closed and which will hold removable (12.2 kgm-2), lighter than either glass or aramid bulletproof pads such as layers of Kevlar fabrics or a composites, and can withstand almost all the construction of woven Kevlar with layers of ballistics. nonwoven spectra shield. The front of the shirt may In a completely new approach to making a have an additional layer of fabric enclosing as a vital 45 spectra shield , hundreds of fibre bundles are laid area pad to meet the specific need. All pads are side by side in a unidimensional aligned beam, removable so that the front and back of the shirt can which is then drawn through bath of thermoplastic be laundered normally. Typical construction of polymer to produce flexible composite layer, cut garments consists of an outer layer of the shirt and cross-plied (in a 90° orientation) and pressed to which is visible on the wearer which includes produce a single ply. These plies are then stacked buttons, pockets and areas of stitching, and areas of and pressed together for required ballistic shirt which does not open and merely gives the application. appearance of buttoning of the dress shirt. Sumitomo Bakelite Co. revealed46 high strength Finally, it should be borne in mind that no single polyethylene fibre-reinforced plastic composites ballistic protective garment can provide complete with excellent impact resistance. The composite contains nonwoven fabric sandwiched between protection against all types of projectiles. It is a woven fabrics. This composite is made from compromise between various factors like the extent of protection required, cost, weight and comfort. isoprene rubber nonwoven fabric prepeg, sandwiched between bisphenol A epoxy resin. Woven fabric prepeg showed surface density of 6.1 5 Performance Evaluation kg/m2 and thickness of 6.5 mm with no impact For quantitative assessment of the performance of penetration. the ballistic protective clothing, two .fundamental High strength polyurethane composite especially quantities, viz. ballistic protection and weight 50 useful for sails and ballistic-resistant articles has efficiency, are always considered . 284 INDIAN 1. FIBRE TEXT. RES ., DECEMBER 1997 5.1 Ballistic Protection residual velocity; V;, the limit velocity; and P & t, . This gives details about the degree of protection the constants for projectiles and target. provided by the armour against a specified projectile In this process, a thin aluminium plate is placed or a series of projectiles. behind the armour and when the projectile debris Most popular methodS for testing is V50 which is from the armour penetrates the plate, the armour is defined as the approximation of the velocity at considered to be defeated. which 50% of the impact would result in complete penetration and 50% in partial penetration (i .e. 5.2 Weight Efficiency stopped by the armour) or the projectile velocity at This is measured in terms of weight-merit- rating which the probability of penetration is 50%. It is (WMR) , which is a useful method to rank the determined by taking the arithmetic mean of an armours and to ascertain the weight advantage equal number of the highest partial and lowest offered by a new armour when compared to the complete penetrations impact velocities within an standard. It is defined as the ratio of areal density of acceptable velocity range as described by AbottS and the reference materials to that of the new armour. 9 Stein • The reference material for vehicular type armours is 20 Taking V50 as the measure of the efficiency, MIL-A-12560 steel while for persOImel armour it Blumberg lO showed that at low weight per square is cotton duck fabric. metre, soft ballistics are more efficient than hard 2 ballistics, while at weights greater than 10 kg m- , 5.3 Ballistic Protection Standards hard ballistics are more efficient which may be due The ballistic requirements to be met are normally to the difference in the rigidity between the hard and specified by national standards based on calibre soft ballistics (Fig. 8). type, weight of the projectile and velocity. Various A less common but useful protection criterion is protection classes have been established reflecting the limit velocity. It is defined as the velocity below different levels of exposure. Most widely used are which 100% of the projectiles would fail to The National Institute of Justice (N1J), National penetrate through the target or the maximum Bureau of Standards (NBS), Personnel Protection velocity at which the projectile can still be stopped. Armour Association (PP AA)20 and Ballistic This is primarily a theoretical concept because it is Protection Standards for Gemlany. Most commonly l difficult to define from ballistic tests alone . employed ones are the NIl standards as listed in It can be found by fitting the data to form. Table 4. 0, Vs Table 4--N11 classification of armour systerns20 Type Wt of Type of projectile Velocity projectile ms" g 2.6 0.22 calibre lead 320± 12 11 A 10.2 0.357 magnum jacketed 381 ± 15 (0) lead soft point (JSP) 18 9 mm full - metal jacket 332 ± 15 (FMJ) 11 8 0.357 magnum JSP 425 ± 15 9mmFMJ 358 ± 15 IlIA 15.55 0.44 magnum 838 8 9mm FM] 425 III 223 Remington (556 mmV'MJ) 0.3 carbine FMJ 12 gauge rifle slug IV 10.8 0.3 calibre armour 868 (c) piercing bullets (US military APM2) Fig. 9--Failure mechanism of single fibre under ballistic impact2: (a) Propagation of longitudinal compressive waves, (b) Reflection as tensile wave, and (c) Elongation of the fibre instrumental in understanding the mechanism of ballistic protection as discussed later. boundary condition of zero stress at the fibre ends. 6 Failure Mechanism in Fibrous Materials This movement of tensile wave back towards the The basic idea of ballistic protection with fibrous point of impact causes the flow of material in the materials is essentially based on the conversion of same direction. Ultimately, a tensile strain is the kinetic energy· into work of deformation. Hence, experienced by the fibre in contact with the the principle influencing factors would be the tensile projectile, the magnitude of which is dictated by the properties of the protective material, the impact velocity of the projectile. Continued wave deformability of the projectile and the armour. reflections intensifY the tensile strain with energy Fig. 9 shows the mechanism of deformation when absorbed by the fibre being proportional to this a single fibre/yam is subjected to impact by a strain. At the same time, a second and slower wave projectile, with the longitudinal axis of the fibre propagates along the transverse axis, i.e. across the perpendicular to the path of the projectile. The diameter of the fibre, moving parallel to the impact leads to longitudinal wave propagation and projectile and at the same velocity. Because of this deflection 1,2,21. The longitudinal wave propagation transverse wave, the fibre starts moving in the same velocity 'C' is nothing but speed of the sound in the direction as the projectile path. The fibre continues material which can be expressed in terms of density to absorb energy and deflects until the projectile is and modulus as wider : decelerated and stopped or the fibre strains past the C=(E/p) v. dynamic yield point and breaks. If the impact where C = Speed of the sound or longitudinal wave velocity is sufficiently high, the fibre cannot velocity, E = Initial modulus, and p = Density. respond fast enough to exhibit strain and the lowest The wave reaches the end of the fibre and is velocity for which the fibre breaks without reflected back as a tensile wave. This reflection of exhibiting strain is called "Critical Velocity". It is a the tensile wave is necessary so as to satisfY the function of wave speed, which is indirectly related 286 INDIAN J. FIBRE TEXT. RES., DECEMBER 1997 to the fibre modulus and density as mentioned splits along its longitudinal axis, which would not earlier. only absorb energy but acts as an efficient crack arrestor. 6.1 Mechanism of Ballistic Penetration in Woven Fabrics 1/ 2 Situation obviously gets complicated when 2-D and V=C[E(1+E)-{E(1+E)°.5- E} ] woven fabrics with crossover points are considered. where Its importance is with wave propagation in the 6 U = Transverse wave speed, fabric; say with 10 crossover points in a square C = Longitudinal wave propagation speed, meter of the fabric, one can expect redistribution of E = Fibre strain, and the signals at these points I. A fraction of the wave V = Projectile speed. will continue along the yam, part of it will be Thus, if strain represents the strain-at-break, then reflected back (if node is fixed, all the signals will V is the critical velocity of the projectile above be reflected back) and a part will be redistributed which the material would never be able to stop the along the yam which is perpendicular to the former. projectile because the interaction between the yam Transverse deflection of the fabric is responsible for and projectile is minimum, e.g. Kevlar 29 at 4% 50% of the total energy absorbed by the secondary elongation and a modulus of 48.5 N/tex showed the yams/fibres. It is assumed that the transverse wave critical velocityl2 of 900 m S-I. transfers the energy to the adjacent yams by 8 Therefore, the longitudinal wave velocity is the frictional contact ; hence, the type of weave one which determines the amount of material employed in addition to the fibre selected plays a involved in the interaction with the projectile. major role. One can generalise three main regimes that exist in the materials subjected to ballistic impacf as 6.2 Ballistic Penetration Mechanism in Fibrou Composites under: The failure of the composite laminates is far more • Regime-I: At low projectile velocities, the complex process characterized by interactions fracture in brittle materials is well described by occurring in the individual planes and across those linear elastic fracture mechanics. planes in adjacent fabric layers. Because of the • Regime-II: The projectile velocity approaches in l numerous crossover points found these woven sonic velocity (-500 ms· ) and loading history fabrics, energy is quickly dissipated as the wave gets dominated by the sIWck wave propagation. encounters more and more fibre crossovers. This This transition occurs at the strain rates of about dissipation usually limits the impact damage to a 103 sec-I. small area adjacent to the impact point52 . Energy • Regime-III: Transition to this occurs when the transfer to adjacent layers (between layers) is damage is dominated by inertial effects and the l 28 facilitated by the use of resin binder • . • Strain adiabatic response of the target leads to melting. waves are transmitted from the fabric layer to resin In this region, increasing the projectile velocity matrix and thus to adjacent layers; at the same time, would eventually result in the melting and elastic waves are reflected into the fabric matrix at fusion of the material at the point of contact. each interface. Since the transverse wave energy is The mode by which the fibre separates is also transferred to the adjacent yams by frictional important in determining its resistance to contact, more the yams being utilized, the better penetration. Three principle modes have been would be the transfer of energy. Thus, in the textile identified, viz melting, brittle fracture,and plastic structure, resin bonding restricts the yam movement, deformation with longitudinal splitting. 9 which reduces the ballistic performance • Nylon 6 exhibits large deformations before The amplitudes of these waves are determined by breaking, therefore high energy is absorbed by the a quantity called Mechanical Impedence (I), defined fibre. Brittle fracture with low elongation-to-break is as: typical of glass fibres. Kevlar shows some degree of 1= pC, plastic deformation before rupture while UHMPE where p is the density of fabric, and C, the wave BAJAJ & SRIRAM: BALLISTIC PROTECTIVE CLOTHING 287 speed. Adeyefa 17 based on Finite Element Analysis, applied Elastic waves may be reflected or transmitted for cloth configurations of 11 x 11 and 21 x21 according to the following : orthogonal woven fabric, the time sequence of fabric deformation beyond projectile arrest is 2 PI= ( --It)- x Po obtained. These time sequences clearly elucidate the 11+ 10 dynamics of ballistic impact on woven fabrics as shown in Fig 12. Further, it has been demonstrated Pr=[II - Io] x po II + 10 that the triaxial fabrics out perform all other types in where ballistic protection. PI = Stress amplitude of transmitted wave, Po = Stress amplitude of incident wave, 6.3 Ballistic Protection with Ceramics Although ceramics are typically very brittle, their P, = Stress amplitude of reflected wave, mechanical behaviour is dependent on (i) confining II = Impedence of transmitted material, and l pressure and (ii) strain rate . 1'1 = Impedence of incident material. These usually fail into rubbles upon uniaxial It should be noted carefully that the waves created by the ballistic impact are nearly elastic and compression without plasticization. In addition, if transmission of these waves to resin matrix confinement pressure is increased the modulus is increases the energy absorption capability of the unaffected, but the compressive failure strength laminate. Cracks are formed in the matrix and increases significantly. The increase in strength is propagate parallel to the fabric layers, through brought about by the superposition of the compressive failure stresses, which reduce the delamination process 53 . Impact energy is dissipated in the formation and advancement of these cracks opening of the microcracks parallel to the loading and in the deflection caused by the separation of direction. Strength effects become important thus individual fabric layers. Zee et al. 51 used helium air gun, microvelocity sensor and PE and graphite composites with ::; 5 individual fabrics woven in plain, bound together with epoxy resin of 10 layers each, to demonstrate the failure mechanis.m. Fig. 10 shows the behaviour oL-__~ ___L- __~ ___~_~ of PE composites of 10,20 and 30 layers. Velocity o measurements were made at 12 different positions Poaftion tNn) with the help of microvelocity sensor in such a way Fig. IO--Energy profiles for the slowing down of the that initial ballistic contact occurs at the 3rd position projectile in PE composites of different layers during high with three measurements of incident velocity prior velocity ballistic impactS! [( I) 10 layers, (2) 20 layers, and (3) to impact. 30 layers] The energy loss for all the samples (Fig. 10) started from position 3 only signifies that PE composites do not flex but behave as a rigid system, leading to fracture of fibres and matrix. It is interesting to note that the slope of the energy profile is proportional to the energy dissipation density and the rate of energy loss in a 10-layer composite is lesser than that in thicker composites because of the reinforcing effect. °O~--~---~---7---7.--~ However, the graphite composite shows a typical Poaition(",,,, ) brittle failure with the energy loss observed exactly at the point of impact and energy loss peak in less Fig. II- Energy profiles for the slowing down of the projectile in PE and graphite composites during hi gh velocity than 2.5 mm distance (Fig. 11). ballistic impactS! [(I) Graphite, 10 layers, and (2) PE, 10 In a numerical model developed by Leech and layers] 288 INDIAN 1. FIBRE TEXT. RES., DECEMBER 1997 •• Stage 1 St age II •• •• Stage m Stage I'l •• Stage V. Fi g. 13-Failure mechanism of ceramics under ballistic •• impact2 the pressure curve, which is typical of the liquid state. The final penetration depth of the projectile is •• given by the formula: P=L.{P;fp; •• where L =length of the projectile, Pp =density of the projectile, and PI =density of the projectile target material. •• Since the equation doesn't involve the strength term, the mechanism should be different from that l of fibrous materials , as shown in Fig. 13 and TimlZ =0 . 32 ms described below: TimlZ=l.Olms • • Erosion of projectile tip and formation of Fi g. 12-Time sequence of the fabric deformation under spherical shock front, balli stic impact up to and beyond the projectile arrest 17 (Pre • Appearance of the fracture conoid and failure of strain: 10%. Transverse displacement scaled by 2) the ceramic in compression, • Penetration of projectile into ceramic, when impact pressure is below twice the "Hugoniot • Failure of ceramic in tension, and Elastic Limit" (HEL), which is defined as the largest • Removal of ceramic debris. elastic wave that can be transmitted in a material Fracture behaviour of ceramic can be and can be thought of as a measure of dynamic yield summarized as below : stress of a material. Above the HEL, the material • At ·low strain rates (I': < 102 sec· I), fracture is undergoes irreversible shear . Theoretically, controlled by thermal activations of ceramics are proved to flow hydrodynamically from microcracks. the fact that at higher pressure, i.e. higher impact • At higher loading rates, inertial propagation velocities, the Hugoniot curve becomes parallel to provides significant strain rate hardening. BAJAJ & SRIRAM: BALLISTIC PROTECTIVE CLOTHING 289 • At strain rates approaching HEL (>104 sec' I), the impact is approximated as the point impact. failure is by plastic flow with little The model successfully indicated that the microfracture. Hence, HEL represents the point majority of the energy absorbed by the fabric of true plastic flow, and micro fracture is not under ballistic impact was due to the strain and initiated until stresses exceed at least twice the the kinetic energy in the yarns directly in value ofHEL contact with the projectile. This model was further improved by Roylance himself to 7 Modelling of Ballistic Performance remove the point impact approximation and is l Numerous models have been developed 6-18,54-<>4 to one of the most successful model so far. 58 predict the performance of ballistic protection but • Another model , based on finite element till date, no single model has succeeded in analysis, is reported by Allied Signals where the accounting for all the complexities that are involved time-step of the method and element spacing with the penetration mechanism of the ballistic were modified so that the predictions of the protection. The major problem is that the whole model matched complementing experimental process typically lasts only for few microseconds work. This model is successfully employed to and it is very difficult to get the specific information determine the failure criterion of the composite of all the processes that take place and then use materials. them to validate a particular model. Therefore, all • Srirengan and Whitcomb59 developed a finite the models proposed so far are the poised balance element based degradation model for between the performance characteristics and the composites with transverse matrix cracks. This 2 material properties . model was developed by analyzing a three-ply • Vinson and Zukas54 modelled multilayered body representative volume element for predicting the armour as single constrained conical shell under effective homogenised three-dimensional elastic quasi static loading. They used a technique constants of an interior ply with transverse which requires complex experimental work to matrix cracks. Each crack was modelled augment the numerical calculations. . The discretely, and the resulting model was success of this model is impeded mainly due to evaluated first by comparing the homogenized the simplification of assumptions that are made extensional modulus for a cracked laminate with and excessive dependence on the experimental experimental results and then with finite data which incorrectly showed ultimate strain of eiement discrete models. Using the degradation 185% for nylon fabric and dynamic modulus of model, the prediction of load redistribution for a elasticity as only a small fraction of the static cantilevered laminated plate with matrix cracks one. subjected to transverse end load, and in a • Dent and Donovan55 modelled with an cracked plain weave composite subjected to assumption that only the yarns are directly uniaxial stress, was compared with a discrete contacted by the projectile to absorb energy crack model. The degradation model showed without taking the fabric characteristics into good agreement in predicting the homogenized account and hence, concluded incorrectly that elastic constants and load redistribution. the friction was the primary mechanism of • Ko et a/.60 proposed a modified Florence model energy transfer between the projectile and the for evaluating the effectiveness of ceramic fabric. Later, in the refinement of the model, spheres on ballistic impact resistance, using 56 Dent and Donovan proposed cnmp multifunctional armour with Al 20 3 face and interchange as the primary mechanism. Spectra Shield composite backing plate. Since • Roylance and Wang57 model took care of the the Florence model predicts higher ballistic essential physics of the impact mechanism limit with larger size of the damaged area, this along with impact mechanics, still keeping the modified Florence model is of consequence solution reasonably tractable. The main where a relationship between predicted ballistic assumption is that a multilayered fabric can be limit and areal density is established which thought as a single-ply pin-jointed network and could be used to optimize the system by 290 INDIAN J. FIBRE TEXT. RES. , DECEMBER 1997 controlling the thickness ratio of ceramic face drilling residual stress measuring technique was and composite backing plates. The ballistic limit adapted for evaluating the thermal residual evaluation showed that increasing the backing stress. The hybrid composite showed higher composite thickness significantly affected the thermal stresses, especially the thermal residual kinetic energy absorption of the armour. compressive stresses, which exist in the fibre • Costa and Thaumaturg061 analyzed the effects of direction. Fibre/matrix interfacial strength shock waves induced by ballistic impact on the reduced as the hybrid composites have higher impact-energy absorption capacity of woven expansion in Z-direction of the composite, glass fibre/epoxy composites. They used which results in reduced interlaminar strength of computer simulation of shock-wave propagation the hybrid composite. to determine the impact energy in these composites. It was observed that the energy References absorbance capacity changes with the · acting I Brandon D G, Lanford J & Gray W, in Encyclopaedia of failure mechanism and is also rate dependent. . material science & engineering, Supplement Vol. 2, edited 62 by R W Cahn (Pergamon Press, Oxford, UK), 1990, 697- • For ceramics, the work done by Wilkins is 708 . noteworthy. He used HEMP, a large finite 2 Gray W & Zabal P H, in International encyclopaedia of different lagrangian hydrocode, to model the composites, Vol. 6, edited by S M Lee (VCH impact of 0.50 calibre projectile into disks of Publishers Inc, USA), 1991 , 125- 140. 3 Dhingra A K, & Lauterbach H G, in Polymers : alumina backed by various materials. Through Encyclopaedic source book of engineering properties, this work, the role of ceramic compressive edited by J I Kro schwitz (John Wiley & Sons, New York, strength in the initial failure of the projectile tip USA), 1987, 349. was established. Also, the mechanism of the 4 Lankford J & Gray W, in Concise encyclopaedia of tensile failure and fracture and conoid formation composite materials, edited by P J Cornish (Pergamon Press, Oxford, UK), 1989, 52. 5 were predicted and explained on the basis of Combellick W A, in CO t/ cise encyclopaedia of polymer mechanics of stress wave propagation. science & engineering, edited by J I Kroschwitz (Wiley- • Ali et al. 63 studied the effect of the impact of a Interscience Inc, New York, USA), 1990, 180. paraboloidal projectile on human skin 6 Landrock A H, in En cyclopaedia of polymer science & membrane. Here, the tip of the projectile which technology, Vol. 8, edited by N M Bikales (John Wiley & Sons Inc, New York), 1968,789. is made up of lead was considered to be 7 Serry T J, Weiner L I, Hays M B & Sachs N, Manmade paraboloidal and the threshold velocity, i.e. the textile encyclopaedia , edited by J J Press (Textile Book velocity when the skin membrane is about to Publisher, New York, USA), 1959, 339. rupture, has been calculated for human beings of 8 Abott T A, Protective clothing (Shirley Publication, various age groups. 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(1987) 13-14. that the interlaminar shear strength of the 15 Harpell G A, Dalley I, Kavesh S & Prevorsek DC, US Pat 4,737,401, 12 April 1988; World Text Abstr, 20 (1988) laminated. composite is reduced by intTaply m4. hybridization of the reinforced fibre. Intraply 16 Cunniff PM, Text Res J, 62(9) (1992) 495-509. hybrid composites show poor intralaminar shear 17 Leech C M & Adeyefa B A, Comput Struct, 15(4) (1982) strength with higher anisotropy in thermal 423-432, expansion coefficient and elastic modulus which 18 Prosser R A, Text Res J, 58 (2) (1988) 61-85; 58 (3) (1988) 161-165. would cause higher residual . internal thermal 19 Heintze A, ChemiefasernIText-fnd, 36( 12) (1988) 128- stresses after curing of the composite. The hole 133 . BAJAJ & SRIRAM: BALLISTIC PROTECTIVE CLOTHING 291 20 Yang H H, in Kevlar aramid fibre (John Wiley & Sons, Ltd); High Perform Text, 11(9) (1991) 12. Chichester, UK), 1993. 42 Higham Ferrers, High Perform Text, 13(9) (1992) 2. 21 Gabara V & Prevorsek 0 C, in Synthetic fibre materials, 43 G 8 Pat 2,235,929 (to ICI pic); High Perform Text, 12(8) edited by H Brody (John Wiley & Sons Inc, New York, (1991) 8. USA), 1994, 247-273. 44 Allied Signals, Text Month, October (1988) 17; High 22 Tanner 0 , Fitzgerald J A, Riewald P G & Knoff W F, in Perform Text, 12(3) (1991) 8. Handbook offibre science & technology: Fibre chemistry, 45 Bajaj P & Sengupta A K, Text Prog, 22(2/3/4) (1992) 65- Vol. 3, edited by M Lewin & J Preston (Marcel Dekker, 68. 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(1997) 105041 n. 29 Temmerman L 0, J Coated Fab, 21 (1991) 19\. 53 Novina Trudi, High Perform Text, 12(3) (1991) 2. 30 Ward I M, Compost Sci Technol, 44(3) (1992) 197. 54 Vinson J R & Zukas J A, J Appl Mech , 6(1975) 263-268. 3 I Izawa H, Tatsuoka Y & Tanaka M, Jpn Kokai 08,158,262 55 Dent R & Donovan J G, Technical Report, NATlCKlTR - (to Tejin Ltd, Japan), 18 June 1996; Chern Abstr, 125 (16) 861044L (US Army Natick Research, Development and (1996) 198460r. Engineering Centre), 1986. 32 Van der Loo L L H & Van der Burg R C, NL Pat 56 Dent R & Donovan J G, Technical Report, NATlCKlTR- 92,00,625 (to DSM, N.V.), I November 1993; Chern 881080L (US Army Natick Research, Development and Abstr, 120(18)(1994) 220353d. Engineering Centre), 1988. 33 Chiou M J & Yang H H, CN Pat 1,072,225 (E.I. du Pont 57 Roylance 0 & Wang S S, J Po ~ym Engg Sci, 18( 14) de Nemours anp Co.), 19 May 1993; Chern Abstr, 120 (22) (1978) 1068-1072. (1994) 272977e. 58 Prevorsek 0 C, Strain rate effects in spectra composites, paper presented at NASA Fiber-TEX' 89, Green Ville, NC, 34 KunzendorfF, Melliand Textilber, 13(3) (1984) E17\' USA, October 1989. 35 DSM high performance fibres, High Perform Text, 15(9) 59 Srirengan K & Whitcomb J 0 , Chern Abstr, 126(8) (1997) (1994) 9. 105036q. 36 Montgomery T G, Text Res J, 52(7) (1982) 442. 60 Ko F, Yu J Z & Song J W, Chern Abslr, 126(8) (1997) 37 Sacks M, US Pat 4,413,357, 8 Nov. 1983 ; World Text 10504m. Abstr, (1 984) 243 . 61 Marques Da Costa A (Jr) & Thaumaturgo C, Lat Am Appl Res J, 26 (1996) 19-21 ; Chern Abstr, 126(7) (1997) 38 Zufle T, High Perform Text, 14(9) (1993) 6. 90113m. 39 High Perform Text, 15(3) (1994)9. 62 Wilkins M, Lawrence Livermore Laboratory, UCRL- 40 Armellino R A & Armellino S E, US Pat 4,522,871 , 1 I 50349 (1967). June 1985; World Text Abstr, (1 985) 5945. 63 Ali M M, Singh V & Parrey G N, DefSci J, 46(4) (1996) 41 Lee F H, G 8 Pal 2,130,073 (to Personnel Armoured 233-236; Chern Abstr, 126(1 I) (1997) 140644h. Designs Ltd), 31 May 1984; World Text Abslr, (1984) 64 Kim C & Kang T 1, Hanguk Somyu Konghakhoechi, 33(8) 4452; G 8 Pat 2,232,063 (to Personnel Armoured Designs (1996) 736-741 ; Chern Abstr, 126(4) (1997) 48085d.