Materials For Body Armour Construction Biology Essay

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In this chapter, a literature review relating to the present work was carried out. It was found that it was necessary to introduce some background information on the study, including; materials used in body armour construction, ballistic resistance and failure mechanisms, factors affecting energy absorption of fabric armours, including stitching effect, ballistic testing standards, models developed by other researchers; and works on energy absorption of fabric armours.

Materials For Body Armour Construction

The materials used in body armours depend upon the threat to be defeated and the part of the body to be protected. Similar materials are used to defeat fragmentation and low-energy bullets. According to Tabie & Nilakantan (2008), depending on the class which they belong to, different fibres have different structural properties, leading to different responses to ballistic impact when woven into a fabric. Some examples are aramids, such as Kevlar (DuPont) and Twaron (Teijin); polyphenylene benzobizoxazole (PBO) fibres, such as Zylon (Toyobo); and ultra heavy molecular weight polyethylenes (UHMWPE), such as Spectra (Allied Signal); and PIPD fibres, such as M5. Other examples of commercial brands are Vectran (Hoechst Calaneses), Technora (Teijin), and Nextel (3M Ceramic Fibre Products). These fibres, when woven together into a fabric, possess a strength that is much larger than the sum total of the individual strands and a strength to weight ratio that is higher than steel.

Woven fabric can also be encapsulated within a resin matrix and be pressed into a rigid composite structure. It can be used for protection from higher-energy bullets. Composites used for high-energy bullet protection are usually combined with ceramic strike faces, particularly if the projectile threat contains a steel core, rather than only lead.

Soft amours are made from a variety of natural and synthetic fibres. Soft armours are constructed from multiple layers of fabric. Depending on the bullet calibre to be stopped and the yarn count, the number of fabric layers used in making a bullet proof vest varies from 10 to 50 and weights around 5 kg or less. Most of the fibres used are the synthetic. Wambua et al. (2007) reported that in recent years, researchers have started to venture into the possibility of integrating natural and synthetic fibres into ballistic studies.

Fabric Based Body Armour

In recent years, ballistic resistant materials formed from high tensile strength fibres, such as aramid or polyethylene fabrics, have gone into common use (Neal et al. 2004). These fabrics have been used to form flexible fabric-based bullet defeating vests. These ballistic resistant materials typically have the advantages of greater tensile strength and less weight per unit area than other materials. Multiple layers of aramid fabrics are used in single bullet proof vests, because a single layer is insufficient to stop a ballistic projectile (Neal, 2004). Attaching multiple plies of fabric still leaves the vest lighter and more flexible compared to metal.

Aramid is a family of high-strength nylons that includes fibres known under trade names such as Kevlar, Twaron and Nomex. Aramid is a generic term applied to aromatic polyamides (Chawla, 1998). A summary of aromatic high-strength fibres is shown in Figure 2.1. It shows the poly-para-phenylene terephtalamide (PPTA) repeat unit, which in series builds polymer chains and forms highly ordered crystallite structures. Such crystallites are the building blocks of fibrils that, finally, form aramid fibres. Kevlar and Twaron are mostly used in body armour applications, while Nomex is used in fire retardant products. While both of Kevlar and Twaron are made up of the same polymer PPTA, they are produced by different agencies and characterised by different initial tensile properties (Jain & Vijayan, 2003).

Figure 2.1. Chemical structure of para-aramid fibres

Source: Vilkner, 2003

The Kevlar brand fibre was developed by DuPont in 1965 for the modern generation of concealable body armours. Kevlar is a man-made organic fibre, with a combination of properties allowing for high strength with low weight, and high chemical and cut.

Kevlar fabric was widely used in body armour research conducted by Fugicia (1981), Cunnif (1992, 1996), Billon & Robionson (2001), Carr (1999), Shim et al. (2001), Porwal & Phoenix (2005), Ahmad et al. (2008), and Karahan (2008, 2009). There are also innovative works recorded in international patents, such as works by Armenillo et al. (1985), Mazelsky (1996), Bachner (2000) and Price et al.(1999), using Kevlar for fabric armour applications.

There are many forms of Kevlar (eg. Kevlar KM2, 29, 49, 129 etc.). Each one of them is composed of the same PPTA monomer. Raftenberg & Mulkern (2002) reported that these forms differ on the degree of crystallinity, which reflects the degree of molecular alignment and hydrogen bonding between neighbouring molecules. They studied the quasi-static uniaxial tension characteristics of single ply Kevlar KM2 fabrics, as shown in Figure 2.2. The strength (maximum stress achieved) was found to be at a mean of 2.33 GPa and 2.67 GPa for warp and weft directions respectively. The data was compared to other types of Kevlar as shown in Table 2.1, where Ey is yarn stiffness, σy,fail is yarn strength and ey,fail is the strain corresponding to maximum stress.

Table 2.2 shows the properties of Kevlar 129 (style number 802), which is used in this study.

Figure 2.2: A plain-weave structure consisting of mutually orthogonal families of warp and fill (weft) yarns

Source: Raftenberg & Mulkern, 2002

Table 2.1. Tensile properties of Kevlar yarns

Yarn type

Ey

(GPa)

σy,fail

(GPa)

ey,fail

Kevlar 29

70

2.9

0.036

Kevlar 49

135

2.9

0.028

Kevlar 68

99

3.1

0.033

Kevlar 119

55

3.1

0.044

Kevlar 129

99

3.4

0.033

Kevlar 149

143

2.3

0.015

KM2

63

3.3

0.040

Source: Raftenberg & Mulkern, 2002

Table 2.2: Properties of Kevlar 129 fibres

Weave type

Fibre diameter

(µ m)

Fibre density

(g/cm3)

Yarn count

(tex)

Tensile strength

(GPa)

Tensile Modulus

(GPa)

Elongation at rupture

(%)

Melting Point

(oC)

Plain

12

1.45

111.50

3.4

99

3.3

530

Source: Bilisik & Turhan, 2009

Price et al. (1994) produced a body armour vest using Spectra shield sandwiched between Spectra woven fabric. It is claimed to be a more adaptable vest with increased projectile resistance, reduced weight and improved wearability. The packages of the combined inserts are sewn together with Spectra fibre thread in a box stitch pattern.

Zylon, or P-phenylene benzobisoxzole (PBO), manufactured by a Japanese company, Toyobo, has outstanding thermal properties and almost twice the tensile strength of conventional para-aramid fibres. However, in 2003, the performance of Zylon was investigated on the possibilities of degradation after the critical wounding of a police officer in Forest Hills, Pennsylvania, in the summer of 2003 (NIJ Special Report, 2004). According to Grujicic et al. (2008), the failure of Zylon is due to loss in its mechanical performance under high-temperatures or high humidity conditions, resulting in Zylon fibres no longer being approved for applications in bullet proof vests.

Combination of Materials

Lyons (2003) constructed lightweight ballistic resistance fabric armour consisting of multiple layers of high performance fibre woven fabric arranged in quasi-isotropic orientation. Zylon, aramid fibres (Kevlar, KM2 and Twaron) and ultra-high molecular weight polyethylenes (Spectra and Dyneema) were used alternately for this design. To achieve the quasi-isotropic orientation, the high performance fibre is woven into a balanced, plain weave fabric. Multiple layers of fabric are combined by stitching using high performance thread to create the ballistic material for a vest. According to the inventor, during impact, the kinetic energy from a projectile is transferred to the ballistic filler fabric. The quasi-isotropic orientation provides a widespread dissipation of energy and greatly reduces blunt trauma. The stitching further reduces blunt trauma as the fibres within the fabric are pulled.

Learmont (2007) developed a soft armour, which is claimed to be substantially imperforabile by rifle fire, using Kevlar and Zylon materials. The soft armour consists of 3 sections; 10 layers of Zylon-530, 6 layers of Kevlar 704 and 8 layers of Kevlar 726. Three types of stitching patterns were applied, which are radial stitch lines, perimeter stitch lines and spider-web pattern.

Song (2006) reported on the tensile properties of various types of armour grade fibres, as shown in Table 2.3. Tsu-Yin (2008) reported in his work about the general characteristics of each fibre used in armour applications (Table 2.4).

Table 2.3. Tensile properties of typical armour-grade fibres

Fibre Type

Density

Tensile Strength

Tensile Strain

at Break

Initial Tensile Modulus

(g/cm3)

(g/d)

(MPa)

%

(g/d)

(GPa)

Nylon-66

1.14

10

1006

18.2

45

5

Kevlar-29

1.44

22

2794

3.5

525

67

Kevlar-129

1.44

27

3429

3.3

755

96

Spectra-900

0.97

31

3250

3.6

920

79

Spectra-1000

0.97

38

3510

2.9

1320

113

Zylon-AS

1.54

43

5800

3.5

1325

180

Zylon-HM

1.55

42

5800

2.5

1962

270

Source: Song, 2006

Table 2.4. Ballistic fibre names, manufacturers and properties

Name

Manufacturer

Properties

Kevlar

DuPont

First material used in modern body armour

High strength

Lightweight

High chemical/ cut/ flame resistance

Unaffected in water

Spectra

Honeywell

High strength

Water penetration resistance

High chemical/ cut resistance

Lightweight

GoldFlex

Honeywell

Lightweight

Thin

Good protection against blunt trauma

Twaron

Twaron Products

High energy absorption

Quicker impact dispersal

Dyneema

Netherlands

Very lightweight

High energy absorption

Zylon

Toyobo

High thermal properties

Better tensile strength

Lightweight

Source: Tsun-Yin, 2008

2.4. Ballistic Resistance Mechanism of Fabric Armours

The behaviour of fibre and fabric systems when they are subjected to ballistic impacts has been investigated experimentally and theoretically by various researchers (Roylance et al. 1973, Shim et al. 1995, Cunniff 1996, Simons et al. 2001, Tan et al. 2003, Lim et al. 2003). The best way to understand the impact behaviour of a fabric armour is to study the impact behaviour of the structure's construction units, such as single layer fabric and single yarn. Numerous experimental and theoretical works have been conducted to understand the transverse impact behaviour of single yarns and single layer fabrics.

Smith et al. (1960) and Roylance (1977) studied the response of yarns to high speed transverse impact, while Wilde et al. (1973), Briscoe & Motamedi (1992), and Shim et al. (1995) investigated the response of single layer fabrics. Shim et al. reported that low velocity perforation produced a much larger region of creasing and stretching. Only yarns in the vicinity of impact are broken and holes formed in specimens are always smaller than the projectile diameter. This indicates that the projectile perforated the fabric by breaking a few yarns and slipping through the small opening created. It also noted that the ballistic resistance mechanism differs significantly from low to high velocity penetration.

Woven fabric defeats the projectile through a combination of mechanisms, including yarn uncrimping, yarn stretching, yarn breakage, and yarn pull-out from the fabric (Carr 1999, Jacob & Van Dingenen 2001). According to Kirkwood et al. (2004a), modification to the frictional properties of the fabric can greatly alter, and often enhance, the ballistic performance of the fabric.

According to Cooper & Gotts (2005), the technical approaches for stopping penetration and mitigating non-penetrating impacts are different. The underlying principles of minimising the effect of energy transfer from a projectile are; (1) absorbing the energy in the armour by applying it on the armour materials before it gains access to the body-breaking material (e.g. stretching or compressing the materials, or extending the time over which it is applied to the body); (2) redistributing the energy so that other materials or the body wall are more able to withstand the total energy.

2.4.1 Impact Behaviour of a Single Fibre

A high velocity ballistic impact causes the local target materials to behave like fluids, resulting in wave propagation in the structure. When a single yarn is struck transversely; two waves, which are longitudinal and transverse, propagate from the point of impact as shown in Figure 2.3.

Figure 2.3 Projectile impact on body armour

Source: Cheeseman & Bogetti, 2003

The longitudinal tensile wave travels through the material at the speed of sound in the material in the fibre axis. During the longitudinal wave propagation, the material behind the longitudinal wave front flows toward the impact point, which has been deflected in the direction of motion of the impacting projectile. This transverse movement of the fibre is the transverse wave, which propagates at a velocity lower than the longitudinal wave (Cheeseman & Bogetti 2003).

2.4.2 Impact Behaviour of a Single Ply of Fabric

The response of the transverse impact of a single ply of fabric shows similarities with a single fibre. Cunniff (1992) noted that when a projectile impacts on the fabrics, it produces a transverse deflection in the yarns which are in direct contact with the projectile (principal yarn) and generates longitudinal strain waves that propagate at the sound of speed through the material down the axis of the yarns.

Yarns that intersect the principal yarns (orthogonal yarns) are then pulled out of the original fabric plane by the transverse deflection of the principal yarns as shown on Figure 2.4. These orthogonal yarns undergo a deformation and develop waves similar to those observed in the principal yarns. These yarns then drive yarns with which they intersect. These yarn-yarn interactions, which are a function of the friction between them, produce bowing (the misalignment of orthogonal yarns) towards the impact point. The transverse deflection proceeds until the strain at the impact point reaches a breaking strain. According to Cheesemen & Bogetti (2003), when a projectile strikes a fibre, two waves, which are longitudinal and transverse, propagate from the point of impact down the fibre axis at the sound speed of the material. As the tensile wave propagates away from the impact point, the material behind the wave front flows toward the impact point, which has been deflected in the direction of motion of the impacting projectile. This transverse movement of the fibre is the transverse wave, which is propagated at a velocity lower than that of the material.

Figure 2.4. Transverse impact on a single ply of fabric: (a) Side view. (b) Top view of z displacement contours. (c) Bottom view showing principal yarns under high stress

Source: Cheesemen & Bogetti, 2003

Fabric armours, such as those used for fragmentation or low-energy bullet protection, usually are woven. The yarns used in the fabric have a high specific strength and a high modulus. These properties mean that the fibres are particularly difficult to break. The high modulus allows the energy to be dissipated as a longitudinal stress wave, that is, along the yarn.

Fabric armours work as a web of strong fibres when a bullet strikes. These fibres absorb and disperse the impact energy that is transmitted to the vest from the bullet, causing the bullet to deform in typical "mushroom" shape. Additional energy is absorbed by each successive layer of fabric, until such time as the bullet has been stopped.

According to Karahan et al. (2008), the energy absorption and propagation ability of fabric layers are dependent on the tensile modulus of fibres and yarns forming the fabrics. The tensile modulus and strength of the yarns are the main parameters affecting the ballistic performance of fabric armours.

2.4.3 Impact Behaviour of a Multi-Ply Fabric

As a projectile impacts a certain point on one of the yarns, the energy imparted will travel along the yarn (Cooper & Gotts 2005). When it meets the cross-over, as shown in Figure 2.5, it divides via a number of possible mechanisms. It can continue along the yarn (transmission), it can be reflected back along the yarn, or it can travel along the crossing yarn (diversion). This is a single cross-over on a single layer. For example, in a one centimetre by one centimetre square, there could be well over 100 of these cross-overs. It is assumed that no body armour will consist of a single layer; most will use more than 15 layers, and some even more than 40 layers. Not all the energy is dissipated in the first layer, and hence the impact mechanism on single ply fabric continues through the consecutive layers until the projectile does not have sufficient energy to continue.

Figure 2.5: Distribution of a projectile's energy in textile fibres

Source: Cooper & Gotts 2005

Most of the literature available today deal with ballistic impacts, which focus on experimental and theoretical work of a single fabric (Tabie & Nilakantan 2008). Literature that deal with the ballistic impact of armours composed of multiple identical layers of fabric are; Hearle et al. (1981), Parga-Landa & Hernandez-Olivares. (1995), Vandeurzen et al. (1996), Navarro (1998), Billon & Robinson (2001), Tan et al. (2003), Lim et al. (2002), Zohdi (2002), Blankenhorn et al. (2003), and Porwal & Phoenix (2005).

Material properties, fabric structure, projectile geometry, impact velocity, multiple ply interaction, and far field boundary conditions and friction all play a role in the impact behaviour of armours. Many of the individual mechanisms have been reported in a coupled manner (i.e. multiple ply ballistic panels impacted by different geometry projectiles at varying velocities) because it is difficult to isolate each mechanism.

2.5 Ballistic Impact Failure Mechanism of Fabric Armour

In order to analyse the factors that affect the ballistic properties of fabric armour, the basic failure mechanism of the fabric must be known. There are five basic failure mechanisms during fabric perforation, which are; rupturing of yarns, fibrillation, friction and bowing (Tan et al. (2003).

2.5.1 Rupturing of Yarns

The ballistic impact on fabric will rupture the yarns. It causes the breaking of primary or covalent bonds of these molecular chains. Local yarn rupture occurs when all of a yarn's fibres break apart at the same location, usually at the sharpest point of contact between the penetrator and the yarn. There are two causes of rupture; stretching of the yarns along the length and shearing across the thickness (Lim et al. 2002). The fibres in the yarn will fail when the induced strain crosses the failure strain. Remote yarn failure occurs when the fibre within the yarn breaks at different points along the yarn's length and not necessarily at the point of impact.

2.5.2 Fibrillation

Fibrillation is a damage mode that occurs due to abrasive action across the fibre's length. Martinez et al. (1993) stated that plucking or shagging is involved during a fabric-fabric abrasion and the severity depends on contact pressure between the plies. Fibrillation of fibres is more severe for projectiles that have angled nose profiles, such as that of conical and flat-head projectiles (Lim et al. 2002). Upon the impact of a high-speed projectile, aramids fail by fibrillation. This fibrillation also helps them to dissipate energy at a faster pace thus stopping the projectile. The fibrillation offers higher surface area, and thus, dissipates more energy. This might be true in a non-woven fabric made of aramids or woven fabric. But in composites, since the fibres are stacked parallel to each other and cross-plied, fewer fibres may be in contact with each other slowing down the energy dissipation. This vest may be heavier because it uses high volumes of aramid, which are of higher density compared to HDPE and PBO. Figure 2.6 shows fibrillated fibres in a specimen impacted by an ogival head projectile.

Figure 2.6: SEM scan of a fibrillated fibre from a specimen impacted by an ogival head projectile

Source: Tan et al. 2003

2.5.3 Friction

There is strong proof of effects of friction from abrasion during the impact and penetration of the fabric. Small patches of fibre breakage were seen at the yarn crossovers near the impact region. Experimental impact tests depict that the frictional effects are most domineering at low velocities but decrease at higher velocities. At higher velocities, material at the impact point is broken on contact and more yarns are severed, hence, friction from projectiles squeezing through the perforation is less significant. This phenomenon, however, is less clear with the sharper projectiles. Figure 2.7 obtained from scanning electron microscopy (SEM) illustrates that the fabric failure mechanisms associated with friction include flattening, fibrillation and rupturing of the fibres.

a

b

c

d

Figure 2.7: Failure of fibres by friction; a) Hemispherical head impacted. b) Flat-head

impacted. c) Ogival head impacted. d) Conical head impacted

Source: Tan et al. 2003

2.5.4 Flattening

Flattening of the fibres happens when projectiles press directly onto the fibres. Flattening is most prominent for hemispherical projectiles but least severe for conical projectiles. Fibrillation is more pronounced for conical projectiles because the angled edge adjoining the cone to the shank of the projectiles scrapes against the fibres. Flat-head projectiles also possess sharp edges; however, flattening of the fibres is as prominent as fibrillation for such projectiles, since the flat surface presses directly onto the fibres upon impact.

2.5.5 Bowing

Bowing refers to the phenomenon where the warp yarns become non-orthogonal to the weft yarns (misalignment). Bowing can be caused by two mechanisms. The first mechanism is the pushing aside of yarns during a projectile's passage through the fabric, while the second is stress wave propagation away from the impact point which causes the strained yarns to displace (Tan et al. 2003). According to Lim et al. (2002), bowing is more dominant in the back plies of a multiply system where the projectile tries to penetrate through a wedge-through approach after having been considerably slowed down by initial plies .

2.6 Factors Affecting Energy Absorption of Fabric Armours

The ballistic properties of a fabric armour depend on energy absorption and the propagation ability of the fibres and yarns forming the fabric armour. Factors such as fibres and yarn properties, fabric construction, fabric weight per unit area, and ply number have been studied actively by a number of researchers (Freeston and Clause 1973, Roylance et al. (1973), Roylance & Wang (1979), Fugicia (1981), Cunniff 1992, Briscoe & Motamedi 1992, Chitraganad (1993), Shim et al. (1995), Chocron-Benloulo et al. 1997, Shim et al. 2001, Lim et al. 2002, Larsson et al. 2002, Tan et al. 2003, Shockey at el. 2004, Duan et al. 2005, Zeng et al. 2005, Karahan 2008, Bilisik & Turhan 2009).

2.6.1 Material

The ballistic properties of different materials have been investigated in different studies (Shim et al. 2001, Chocron-Benloulo et al. 1997, Lim et al. 2002). According to the theory proposed by Smith et al. (1960), the speed at which the longitudinal stress wave propagates through a yarn is given by Equation 2.1:

c = [E/ ρ] 1/2 (2.1)

where c is the stress wave speed, and E and ρ are Young's modulus and density of the yarn material respectively. The longitudinal wave velocity in a warp or weft yarn in a plane-woven fabric is c/√2 since the linear density of the yarn along which the wave propagation is effectively doubled in a plain-woven fabric. The higher the modulus of elasticity and the lower the density, the faster the stress wave will propagate through the yarns and the corresponding fabric through the yarn crossover points.

The model developed by Roylance & Wang (1979) has shown that material possessing high modulus, E, and low density, ρ, disperse the strain wave rapidly away from the impact point, which distributes the energy over a wider area and prevents large strains from developing at the impact point. Materials having high-wave velocities were advantageous since the stress and strains could propagate more quickly to neighbouring fibres and layers, thus involving more material in the ballistic event.

Fugicia (1980) reported that the kinetic energy absorbed at v50 velocity for a large number of test panels increases in direct proportional to the areal density of the panels. Figure 2.8 shows that energy absorption of a Kevlar fabric panel is proportional to areal density.

213 m/s

274 m/s

366 m/s

Energy absorbed (J)

Areal density (kg/m2)

Figure 2.8. Energy absorption of a 170 g/m2 satin weave Kevlar fabric

Source: Fugicia, 1980

Another aspect of material is the use of hybrid fabric and hybrid panel. Chitraganad (1993) has suggested the use of hybrid fabrics, in which higher tenacity yarns are used as weft yarns. This kind of fabric delays the fabric deformation, and increases fabric strength and energy absorption of the panels. Hybrid panels consisting of carbon, polyethylene and PBO was studied by Larsson et al. (2002). Karahan et al. (2008) investigated the ballistic performance of hybrid panels formed by combining para-aramid woven and unidirectional non-woven fabrics. The results show an improvement in ballistic performance.

2.6.2 Fabric Structure

The fabric structure is also an important factor in determining ballistic performance. The effect of fabric structure was discussed by many researchers (Freeston & Clause, 1973, Cunniff 1992, Chirtangad 1993, Shockey et al. 2002, Shockey et al. 2004). Cunniff (1992) found that loosely woven fabrics and fabrics with unbalanced weaves resulted in inferior ballistic performances. Research on single-ply impact on Zylon fabrics by Shockey et al. (2004) found that the energy absorbed was roughly proportional to the fabric areal density. Ballistic effectiveness did not appear to be the strong function of mesh density or weave tightness. Chitrangad (1993) suggested that the cover factor (the density of weave over width and pitch of yarn) must lie between 0.6 and 0.95 when used in ballistic applications. Cover factors more than 0.95 imply yarn degradation due to the weaving process, and those lower than 0.6 imply that the fabric is too loose. The crimps of the yarns (yarn undulation) in fabric also contribute to the ballistic properties (Chirtangad 1993, Tan et al. 2005). In a plane weave fabric, the degree of crimp is unbalanced as the warp yarns are usually more crimp than weft yarns. Chitrangad (1993) proposed the use of weft yarn that had a larger elongation at the break than the warp yarn, so both yarns would fail at the same moment, reducing the effect of yarn crimp.

Fabric structures used in ballistic protection are woven, unidirectional (UD), non-woven and fibre-reinforced composite plates. Karahan (2008) investigated the ballistic performances of woven and UD fabric panels with different number of layers. It was found that the unidirectional fabric panel absorbed more energy than the woven fabric panel for the same unit panel weight. Roylance et al. (1973) concluded that, in addition to fibre properties, fabric geometry also had a significant effect on ballistic properties.

Cunnif (1992) studied the effect of fibre properties, fabric structure, number of fabric layers, areal density, projectile parameters and impact parameters on fabric based armour systems of Spectra, Kevlar 29 and nylons with various different yarn denier and weave types. It was found that a decreasing trend in specific energy absorption or loss in total ballistic efficiency occurred with the use of higher denier panels as well as increased ply counts.

Cheeseman & Bogetti (2003) carried out studies on factors that influence ballistic performances of fabric and compliant composite laminates. The factors studied were the material properties of the yarn, fabric structure, projectile geometry and velocity, far field boundary condition, multiple plies, and friction. They concluded that factors of material properties, projectile geometry, impact velocity and multiple plies have a profound influence compared to the other factors.

Fugicia (1981) reported that the superior performance of the satin weave is attributed to the lateral mobility potential inherent in the satin construction because it provides greater yarn mobility and transverse deformation than the more tightly constructed plain and basket weave, which results in higher energy absorption.

2.6.3 Boundary Condition

The target boundary conditions play a significant role in ballistic effectiveness. Shockey et al. (2004) and Zeng et al. (2005) observed that fabric gripped on two edges absorbs significantly more energy than identical fabric gripped on four edges. This was attributed to the transferring of load from impacted to adjacent non-impacted yarns and the triggering of remote yarn failure. Zeng et al. (2005), who studied the ballistic resistance of high strength fabric for personnel protection systems, concluded that orientation of yarn at 45o to clamped edges increases energy absorption as it allows energy dissipation of the entire fabric.

Duan et al. (2005) examined the role of friction during ballistic impact of high-strength fabric structures using two types of boundary conditions of fabric; four edges clamped and two opposite edges clamped. Modelling results showed that friction contributed to delaying of fabric failure and increasing impact load, which allowed the fabric to absorb more energy. Results from the modelling effort also indicate that fabric boundary condition is a factor which influences the effect of friction. The fabric more effectively reduced the projectile residual velocity when only two edges were clamped.

2.6.4 Interface Friction

Another factor which affects ballistic properties is the friction between yarns in fabrics. The effect of friction was investigated by various researchers (Briscoe & Motamedi 1992, Campos et al. 2003, Bazhenov 1997, Duan et al. 2005, Tan et al. 2005). According to Briscoe & Motamedi (1992), fabric with high friction and the lowest effective moduli dissipated the largest amount of energy relative to fabrics with lower friction. Modification on fabric physical structure was done via oiling level (Briscoe & Motamedi 1992), fabric structure loosening (Cunniff 1992), fabric wetting (Bazhenov 1997), and surface treatment by silica colloidal (Tan et al. 2005).

2.6.5 Impact Velocity

The impact velocity of a projectile will affect the performance of fabrics. Shim et al. (1995) and Tan et al. (2003) described the differences observed between low and high velocity impacts. During low-impact velocities, the yarn does not fail during the initial stress rise, and therefore the transverse deflection of the fabric has time to propagate to the edge of the panel, which allows the fabric to absorb more energy. With a high-velocity impact, the damage is localised and the yarn fails before significant transverse deflection can develop. At high-velocity impacts of fabric targets, yarn failure occurs preferentially through primary bond failure as the yarn stiffens according to its viscoelastic behaviour. Damage becomes localised to the immediate area around the point of impact and the transverse deflection of fabric is minimal.

2.6.6 Multiple Plies

Some of experiments have been performed in the effect of multiple plies of ballistic textiles. Cunniff (1992) and Lim et al. (2002) have investigated the ballistic impact of multi-ply systems to characterise the reinforcement effects of multiple layers. Cunniff (1992) impacted panels of Kevlar, Spectra and Nylon with chisel-pointed fragment simulating projectile (FSP), and found that the energy absorbed by spaced single plies was greater than that absorbed by layered systems. Lim et al. (2002) impacted panels of Twaron with various projectile geometries and found that the absorbed energy for layered systems was greater than that of spaced systems for certain projectile shapes.

2.7 Stitching Effect on Ballistic Performances of Fabric Armours

The effect of multi-ply fabric stitching is still not well investigated (Ahmad et al., 2008). There are some research works have been conducted to investigate the contribution of stitching in ballistic performances of fabric armours. These include works by Karahan et al. (2008), Ahmad et al. (2008), Lyons (2000), and Bilisik & Turhan (2009). The factor of stitching is the main focus in this study. Stitching is used in almost all the soft body armour construction designs. Thus, the contribution of stitching becomes an interesting topic for further investigation.

As shown in Figure 2.9, the ballistic protective packages, consisting of Spectra shield sandwiched between Spectra woven fabric (proposed by Price et al. (1994)) are secured together in a quilted arrangement. According to the inventors, quilting reduces the bunching effect and improves the ballistic performances of the woven Spectra fibre fabric packages against multiple bullet strikes.

Figure 2.9: Ballistic protective packages secured in a quilted arrangement

Source: Price et al., 1994

The invention of Armellino Jr & Armellino (1985) consists of multiplicity of fabric plies formed of woven aramid fibres. Criss-crossed rows of stitches are sewn through the fabric plies in a square pattern. The stitching thread used the same aramid fibre used in weaving the cloth plies. Figure 2.10 shows the stitched fabric plies formed of woven aramid fibres.

Mazelsky (1996) constructed armour with breakaway sewing threads as shown in Figure 2.11. According to the inventor, multiple plies of energy absorbing cloth made of polyaramid fibres with stitching threads of relatively lower tensile strength in a pattern such that, upon impact, the stitching threads break at multiple locations in response to stresses from fibres stretched by the impact. The energy absorption action is spread beyond the impacted defined area into other contiguous areas of the panel because of the use of relatively weak breakaway threads for stitching.

Figure 2.10. Stitched fabric plies formed of woven aramid fibres

Source: Armellino Jr & Armellino, 1985

Bullet

Breakaway thread

Breakaway thread

Figure 2.11: Bullet impact on multiply fabric panel

Source: Mazelsky, 1996

The lightweight ballistic resistant fabric armour developed by Lyons (2000) by arranging multiple layers of high performance fibre woven fabric in quasi-isotropic orientation is claimed to be more effective in dispersing the impact energy at the minimal areal density in comparison to the prior art methods that simply stack fabric plies. The quasi-isotropic orientation was achieved by combining the multiple layers of fabrics at alternate angles. Multiple layers of fabric are quilted through stitching (1-2 in diamond stitching). The test result shows that a lower areal density of this armour design can produce the same backface signature as higher areal densities of other previous armour designs.

Czetto (2001) employed multiple layers of ballistic resistant fabric and polyethylene terephalate (PET) in alternating fashion to achieve the desired level of penetration resistance. The multiple layer vests include a number of sets of multilayer arrangement of various materials. The first set, which is the strike side of the vest, comprises of multiple layers of penetration resistant woven fabric made of para-aramid fibres. The preferred number of woven fabric plies in this set is 4-8. The first set of woven fabric plies is secured together at the peripheral edge by sewing, which enables the first set to act as a unit during the ballistic impact. This enables the blunting of the shape of a projectile striking the layers.

According to Griba et al. (2006), the geometry and size of sewing needle can damage to the structure of the fabric. The needle can penetrate at any point in the fabric and it can, therefore, deform the fabric loops or cause the fabric damage. The structure of the fabric can be deformed beyond its elastic limit or can literally be destroyed. The choice of the right needle (size, point) is fundamental to ensure the fabric assembly quality. Fabric characteristics (tightness, weight, number of layer) and sewing thread properties (fibre type, fineness) are two crucial factors affecting the selection of the right needle.

Ahmad et al. (2008) investigated the effect of different stitching patterns on the ballistic impact resistance of a multiple layer fabric system. The fabric system was stitched with 1-in field diamond, 2-in field diamond, diagonal, and perimeter stitching patterns. They found that the 2-in field diamond stitched system gave the highest ballistic limit in comparison with the other stitching patterns and unstitched fabric systems.

Karahan et al. (2008) investigated the ballistic performance of different fabric ply numbers, which are joint using three types of stitching to form the panels. The panels were subjected to ballistic tests according to NIJ standards. They found that the fabric ply numbers and stitching types have significant effects on the ballistic properties of fabrics.

Bilisik & Turhan (2009) developed multiaxially stitched and unstitched layered ballistic structures. The ballistic test results on Kevlar 29 and Kevlar 129 show that there were no significant energy absorption differences between multiaxis stitched and unstitched structures. However, conical depths upon impact on the multiaxis stitched structures were small compared to those of the unstitched structures.

2.8 Models Developed for Soft Body Armours

Over the past few decades, many different techniques have been used to derive the constitutive relations that model the overall fabric behaviour for use in ballistic impact applications (Tabiei & Nilakantan, 2008). The complexity of the analysis increases with the number of fabric layers because of the inclusion of new energy dissipation mechanisms and interactions. Only limited amount of literature deal with the ballistic impact of armour composed of multiple identical layers of fabric (Billon & Robinson 2001, Parga-Landa & Hernandez-Olivares 1995, Chocron-Benloulo et al. 1997, Navarro 1998, Tan et al. 2003, Lim et al. 2002, Zohdi 2002, Blankenhorn et al. 2003, Porwal & Phoenix 2005).

Parga-Landa & Hernandez-Olivares (1995) developed an analytical model for predicting the impact behaviour of soft armours which can be used to calculate the armour ballistic curve as well as the impact force, the tension in each layer, the displacement and velocity of the layers and projectile, the yarn stresses and strains, and the damaged area.

2.9 Energy Absorption of Fabric Armours

Fugicia et al. (1980) studied the physical interaction between flexible fabrics and penetrating projectiles. The energy from a ballistic projectile incident on a fabric target is dissipated through two distinct modes; energy absorbed within the fabric plane and energy exerted in displacing the fabric transversely. The energy absorbed within the fabric plane, Ep, can be estimated as:

Ep = 2 t c n Ey (2.2)

where;

Ep=

energy in fabric plane absorption (J)

t =

penetration time

c =

strain wave velocity in fabric (m/s)

n =

number of yarn under strain

=

Yarn energy potential (J/m)

The transverse energy component, ET, can be estimated from pyramid dimensions and projectile/fabric velocities as:

(2.3)

where

Transverse energy (J)

A = Area of pyramid base (m2)

W = Fabric weight (kg/m2)

= Average velocity of projectile during penetration (m/s)

Maximum ballistic resistance is achieved when the projectile energy dissipated within the fabric plane is absorbed through tensile resistance of the yarns. When this response mechanism is active, the time for projectile penetration is maximised, resulting in increased material involvement and increased energy absorption through both modes.

Karahan (2008) investigated and compared the performance of ballistic protection panels formed with 100% woven and 100% unidirectional nonwoven para-aramid fabrics at different fabric ply numbers. The ballistic performance of the test samples was determined by measuring the energy absorbed by the fabric layers and the energy transmitted to the back of the fabric layers, which were calculated from trauma depth and diameter values.

Jacob & Van Dingenen (2001) developed a simple model that can be used to calculate the performance of material based on its capability to absorb energy locally, and its capability to spread it out fast and efficiently. A simple model has been developed that can be used to calculated the performance of Dyneema armours against deformable bullets and non-deformable FSPs. For calculating the performance of Dyneema UD-based armour, two material related parameters and four projectile related parameters are needed. For non-deformable projectiles, the number reduces to a single material related parameter and two projectile related parameters.

Cunniff (1996) developed a parametric model to predict the performance of body armour systems. The model partitions the energy absorbed by the body armour system into strain and kinetic energy.

Zohdi (2002) developed a simulation technique to estimate the number of ballistic fabric sheets needed to stop an incoming projectile. Such sheets are free of any fortification by a resin. Zohdi (2009) developed an analytical expression which accounts for the dominant mechanisms that strip energy away from a projectile as it penetrates a multilayered ballistic fabric shield. With this expression, one can estimate the number of fabric layers needed to stop the projectile, based on the projectile mass and initial velocity, and microscale fabric properties such as microfibril stiffness and stretch to failure.

According to Novotny et al. (2007), as more plies are added, multi-ply systems absorb more energy, and at the greater rate. However, the increase in the rate of energy absorption is not a whole number multiple. For example, a 2-ply system is less than twice as effective as a single ply, while a 4-ply system is less than four times as effective.

2.10        Summary 

 

The main aim of this chapter is to provide a concise review of fabric armour related researches that have been conducted, and to discuss further information in respect to parameters affecting ballistic performances. The fundamentals of typical failure mechanisms have also been reviewed. Based on the literature review conducted, it is observed that, despite extensive research on impact and penetration into fabric armour, the effect of stitching on fabric armours is still unknown or not fully understood. There is a need for a consistent analysis which can address the contribution of stitching factors.  

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