Effects of architecture on ballistic resistance of textile fabrics: Numerical study

Article

Effects of architecture on ballisticresistance of textile fabrics:Numerical study

P Tran1, T Ngo1, EC Yang1, P Mendis1 and W Humphries2

Abstract

Composite textiles composed of materials such as Kevlar, Dyneema and Zylon are extensively used in

many force/impact protection applications, such as body armor, and automobile and airplane engine

fragment resistant containment. Significant effort has been devoted to ballistic testing of composite fabrics

made from various manufacturing processes and designs. Performing comprehensive ballistic and impact

tests for these composite textiles is a very time-consuming and costly task. Numerical models are pre-

sented in this research, thereby providing predictive capability for the manufacturer and designer to

minimize field testing, as well as shedding light on to the damage mechanisms of composite fabrics

subjected to ballistic impact. Several representative composite fabric architectures (such as plain

weave, basket weave and knitted fabrics) are generated for finite element analysis. Numerical investigation

is conducted on these fabric structures of the same mass per unit area subjected to projectile impacts.

Failure patterns of woven and knitted fabrics obtained from numerical simulations are compared with

those observed experimentally. Performances of the representative textile structures are evaluated based

on the resultant velocity of the projectile, as well as various energy components. The influences of yarn–

yarn and yarn–projectile friction properties on the ballistic performance of various textile structures are

presented. To highlight the effects of projectile geometry and angular rotation on the fracture of woven

and knitted fabrics, a series of simulations are also performed with three distinctive projectiles of the same

mass and impact energy.

Keywords

Ballistic impact, knitted fabric, friction effect, composite fabrics, body armor, textile performance, fabric

architectures

International Journal of Damage

Mechanics

2014, Vol. 23(3) 359–376

! The Author(s) 2013

Reprints and permissions:

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DOI: 10.1177/1056789513495246

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1Department of Infrastructure Engineering, The University of Melbourne, Victoria, Australia2Commonwealth Scientific and Industrial Research Organisation (CSIRO), Victoria, Australia

Corresponding author:

P Tran, Department of Infrastructure Engineering, The University of Melbourne, Victoria 3010, Australia.

Email: phuong.tran@unimelb.edu.au

at The University of Melbourne Libraries on March 20, 2015ijd.sagepub.comDownloaded from

Introduction

The impact resistance of high-strength fibers makes them favorable materials for military and civilprotective applications, such as armor clothing, helmets and plates for soldiers and law enforcementofficers. Many other applications include: protective layers for airplane turbines against fragmentsduring service, composite materials for marine structure hulls against under water blast impulse andmilitary vehicles operating in landmine-risk areas. Widely used fibers for ballistic impact resistanceinclude aramids such as Kevlar (DuPont), Twaron (Teijin), PBO fibers such as Zylon (Toyobo) andultra-heavy molecular weight polyethylene such as Spectra (Allied Signal; Grujicic et al., 2008a).These fibers are characterized by their stiffness and strength-to-weight ratios, and are woventogether into a fabric structure providing a strength and toughness that substantially surpassesthose of individual strands. This impact resistance of the fabrics is generally attributed to variousfactors, including weaving architecture, yarn crimp, and several mechanisms of energy absorptionand dissipation of the fabric (Cheeseman and Bogetti, 2003; Tabiei and Nilakantan, 2008).

Understanding the influence of static and dynamic properties of fibers, yarns and fabric layers onthe ballistic response of composite armor is very critical for soft armor designers. When subjected toimpact, the yarn experiences an instantaneous increase in stress, of which the magnitude is asso-ciated with the impact velocity. Two cases are considered as follows. If the stress level is below thecritical value (yarn strength) then the stress concentration is not enough to rupture the fabric,resulting in stress wave propagation to other sections of the fabric. It is, therefore, important forthe fibers to possess high tensile strength and large failure strain to mitigate the considerableimparted energy. In the other case, with sufficiently high impact velocity, the fabrics will be perfo-rated during the initial stress rise. This is a consequence of limited stress relaxation due to thepropagation of transverse deflection from the impact center. It is clearly important for the materialto have high wave velocity, which is proportional to the elastic modulus and inversely proportionalto the density of the yarn (Tabiei and Nilakantan, 2008). Many studies in literatures, however,indicated a lack of properly measured high-rate properties (Shim et al., 2001) and consideredstatic ones instead. Several commercial software used for simulating the impact on textile materials(such as LS-DYNA or ABAQUS) do not directly support the strain-rate dependent material model.Nevertheless, more rigorous material models taking strain-rate sensitivity into account could beimplemented to these software by using user subroutines, which will be addressed in our futurepublications.

Besides the importance of high strength, high modulus properties of the individual fibers/yarns,and the combination of these filaments into a fabric structure also contributes remarkably to theoverall impact response. For ballistic applications, the woven fabrics are widely used in typical plainand basket weave patterns. The density of the weave, which is determined from the width and pitchof the warp and weft yarns, designate the coverage factor of the fabric (Padaki and Fangueiro,2010). Several research have shown that coverage factors of fabrics ranging from 0.6 to 0.9 areneeded for ballistic purposes; while fabrics with coverage factors less than 0.6 or higher than 0.95 aretoo loose or degraded during weaving process, respectively (Cheeseman and Bogetti, 2003). There isalso a report on the optimum twist angle of filaments with which yarns exhibit their maximumtensile strength. Another fabric structural property that has been noted to affect the ballisticperormance is crimp, which is the undulation of the yarns due to their interlacing in the wovenstructure.

The two-dimensional (2D) and three-dimensional (3D) fabrics that comprise high-strength con-tinuous filaments are common composite structures used in protective applications, especially in softarmors. Various studies have been performed to compare the impact resistance performancebetween 2D and 3D fabrics (De Luycker et al., 2009; Mouritz et al., 1999). The 3D structures of

360 International Journal of Damage Mechanics 23(3)

the textile are fabricated by connecting several 2D fabric layers either by interlacing them withz-tows/yarns undulated in the out-of-plane direction or by stitching these 2D layers at certaincrossovers (Jia et al., 2012; Tan et al., 2012; Tang et al., 2011). In these multilayer garments,while interconnections between layers provide some support for transverse loads (Marcin et al.,2011), fabric laminates still play the most important role in carrying and translating impact energyalong the yarn directions. The 2D fabrics (such as plain weave) possess several distinct advantagesdesigned for personal protective soft armor. During the impact event, due to the crimp and spacesbetween the warp and weft tows, the fabric becomes flexible and shearable before locking at thecrossover points of the warp and weft yarns, which is critical for the fabric to adjust around thehuman body or to protect the object’s boundary while ensuring comfort of the clothing. On anotheraspect, the shearing activity of the yarns during the impact makes use of friction between the yarnsconsiderably, which is a very important energy dissipation mechanism (Duana et al., 2006). Thefabrics can take various design architectures, such as plain, basket, twill or satin weave, as well asvarious knitted textile structures. Similar to homogeneous solid materials, a single fabric layer couldbehave isotropically, orthotropically or anisotropically when subjected to out-of-plane impact load-ing, depending on its weaving or knitting architecture. The influences of this orientation-dependentresponse on the deformation, failure initiation and propagation in the fabric layer are not wellstudied or reported in literature. Rather, the major focus of research in the area is on plainweave or unidirectional fabrics and their interactions in multilayer composite structures. There islimited study on the influences of design architecture on the impact resistance and energy absorptionof the fabrics.

As mentioned earlier, friction plays an important role, both directly and indirectly, on theballistic resistance capability of armor textiles. In principle, friction is one of the ways for thefabrics/textiles to dissipate impact energy (Briscoe and Motamedi, 1992; Rao et al., 2009; Tanet al., 2003). Fiber pull-out at the impact point is an example of a fracture event influenced byfriction between the fiber and the matrix. By, however, restricting the ability of the yarn to movelaterally out of the path of the projectile during the impact (by introducing a small amount ofresin), one can increase the energy absorption of the fabric. In another aspect, increasing thefriction between the projectile and the fabric and yarns will prevent the mobility of the yarns andwill require the projectile to engage and break more yarns, thus dissipating more impact energy.Despite the significant effort in determining the fiber–fiber and fiber–projectile’s friction and theirroles in ballistic events, results are often inconsistent and experimental work becomes extremelytime-consuming to cover various yarn/projectile systems. Numerical models developed in thiswork aim at providing a quick and convenient way to address these above issues through sys-tematic parametric studies.

In this work, three common 2D composite textiles: plain weave, basket weave and knitted fabricare modeled and finite element (FE) analysis is performed to investigate their impact resistance andassociated damages induced by projectile penetration. The plain weave and basket weave, as men-tioned earlier, are generally utilized for woven ballistic fabrics. However, of the two weave struc-tures, the plain weave is believed to give a higher strength but lower air permeability. The airpermeability advantage of a 2/2 basket weave over other weave types has been recently reportedin literature. Moreover, the textile structure of basket weave gives it more shearing capability thanother weave types during impact, allowing some yarn slippage and hence energy dissipation throughfriction (as mentioned earlier). The knitted fabric, which is considered in study, is especially desirablewhen next to skin-tight fitting garments with good drape properties are required.

Experimental works on the ballistic resistance of woven and knitted fabric have been carriedout in parallel with numerical modeling work to provide supportive evidences on the failure

Tran et al. 361

mechanisms of different textile architectures. Schematic and actual experimental setups are illu-strated in Figure 1, including a gas gun, laser-based velocity system, high speed camera and acapture box. The high speed camera is initially used to calibrate the incident velocity of theprojectiles and later on used to capture the deformation of the fabric during the impact eventand the resultant velocity of the projectiles. The fabrics were clamped between two steel platestightened by bolted joins and fixed to a sturdy base. Comprehensive experimental program will bereported in the future work.

In the next section, we will focus on reviewing some efforts in modeling composite textiles,followed by a detailed description of the numerical models developed in this study. In ‘Numericalresults and discussions’ section, simulation results of the three abovementioned fabric structuressubjected to projectile impacts are reported and compared. Parametric studies are also performedand discussed to reveal the importance of friction, as well as the projectile’s geometry and angularrotation on the ballistic performance of the three fabric structures.

Figure 1. Schematic (a) and actual experimental setup and (b) for fabric ballistic resistance testing.

Numerical model

Review of numerical modeling of the textiles

Modeling the impact response of fabrics woven or knitted from continuous filament yarns is quitechallenging due to their complicated multi-scale structures and material relations from fiber tofilament, yarn and fabric levels. Several analytical, numerical and hybrid approaches have beenexplored in literature, focusing on each of the scales. Single scale numerical models include modelingthe whole fabric layer as homogenized membranes, or constructing the yarn level architecture expli-citly, or simulating the finest scale at the fiber level.

A homogenized membrane (Grujicic et al., 2008b, 2009) is the simplest approach to overcomethe computational efficiency issue in capturing the mechanical response of multilayer fabrics withrealistic dimensions. However, this method is unable to study many important and complexphysical interactions between yarns and filaments, which are critical to quantify the ballistic per-formance of composite fabrics. Moreover, it is also not possible to model accurately the deform-ation and failure of individual yarn within the impact region. Due to the nature of impact andballistic events, the failures are usually localized in a narrow impact area. It is, therefore, extre-mely important to capture the various failure and deformation modes that affect the outcomes ofthe impact event.

A more sophisticated approach involves modeling the yarn using woven shell elements of con-stant thicknesses (Barauskas and Abraitiene, 2007; Blankenhorn et al., 2003). This technique obvi-ously helps to reduce computational intensity and is quite effective to simulate the mechanicalresponse of woven fabric in regions far away from the impact location. In these regions, yarnelements essentially experience in-plane tension rather than transverse loading; therefore, the shellelement model is suitable for that purpose. The shell element is, however, not efficient in handlingthickness variation across the yarn cross-section, which leads to errors in estimating contact forcesbetween yarns and, therefore, the frictional sliding interactions. Furthermore, the use of shell elem-ents could also result in inaccuracy in wave speed calculation, which is a critical factor for the fabricto dissipate the imparted kinetic energy.

As multi-scale structures, textiles comprise many filaments that are arranged in bundles called ayarn. In turn, yarns are organized into tows, which are then assembled into fabric in particularpatterns through weaving, braiding or knitting processes. As such, composite textile performance isbelieved to relate closely to not only the tow manufacturing process or fabric architecture, but alsoto the fiber interaction at a micro-scale. Many important details related to fabric micro-structureand filament-level physics, filament spatial paths and fiber-to-fiber interaction were simplified inprevious numerical techniques. To capture the failure mechanism of the fabric at the micro-scale, amodel of a bundle of pin-connected digital-rod-element chains was proposed (Miao et al., 2008;Wang and Sun, 2001a, b; Zhou et al., 2003) to simulate a single yarn comprising fiber filaments.As the element length approaches zero, the chain becomes fully flexible, imitating the physicality ofthe yarns. Contacts and frictions between fibers are modeled by contact elements. However, thecomputational cost associated with this method is very intensive and the method is only suitable forsimple and well-defined problems.

The advantages and limitations of these abovementioned methods are balanced effectively byusing 3D solid elements to discretize the yarns. Hexagonal eight-node brick elements with threedegrees of freedom at each node could be used to effectively capture various physical processesassociated with the impact event, including: yarn bending, uncrimping, eroding and contact andfrictional sliding. Moreover, with the 3D element, the anisotropic mechanical properties of yarn canbe modeled and studied conveniently.

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FE model

FE meshes for three textile structures, a plain weave, 2/2 basket weave and single Jersey knittedfabric, are presented in Figure 2. The three fabrics have approximate in-plane dimensions of50� 100mm. The FE meshes are generated from unit cells, which are shown on the right-handside of each corresponding fabric. All the fabrics are composed of the same materials, high modulusTwaron D2200 manufactured by Teijin Aramid, with a density of 1450 kg/m3. To ensure the threefabrics have a similar mass/unit area, which is about 2.35 kg/m2 in the plain weave, the volumedensity of the yarns in the basket weave and the knitted fabric are slightly modified to 1731 kg/m3

and 1583 kg/m3, respectively. The boundary conditions are set up as follows: the short edgesof the fabrics are fixed in all directions, while the long edges are supported to move freely in theout-of-plane direction.

The cross-sections of the three fabrics are illustrated in Figure 3. Due to manufacturing processesor yarn arrangement in the fabrics, the yarn’s cross-section could vary from circular to elliptical orribbon-like. In this study, the cross-sections of yarn in the plain weave and basket weave are ellip-tical (Figures 3a and b), while that of the knitted fabric is circular (Figure 3c). Fundamental geo-metrical parameters defining plain and basket weave architectures are the yarn’s thickness t, widthw and span s. For the Kevlar fabric provided by the manufacturer in this study, the w, t and s valuesare approximately 2.9, 1.2 and 3.9mm, respectively, for the plain weave. These parameters are set to2.9, 1.2 and 9.5mm, respectively, for the basket weave. The knitted fabric is characterized by thediameter of the yarn and the span, which are approximately 2.0 and 10.0mm, respectively. It shouldbe noted that the cross-sectional dimensions of the yarns in the weave and knitted structures arechosen so that they have a similar section area of about 3.14mm2. Hexagonal solid elementsapproximately 0.4mm in size are chosen to discretize the three fabrics. Segment contact interactionsbetween yarns and between the yarn and spherical projectile (Figure 4) are set up separately toaccount for same material contact and contact of different materials. Yarn–yarn and yarn–projectilefrictions are defined as part of the contact setup. The friction coefficients are calculated by:

�c ¼ FD� ðFS� FDÞe�DC Vrelj j ð1Þ

Figure 2. Textile model of (a) plain weave, (b) basket weave and (c) knitted fabric.

where FD is the dynamic coefficient of friction, FS is the static coefficient of friction, DC is theexponential decay coefficient and Vrel is the relative velocity of the surface. In this study, FD, DC andVrel are set to be zeros; �c then equals FS, which is set to equal 0.23 and 0.18 for yarn–projectile andyarn–yarn frictions, respectively. Rigorous experimental estimation of friction coefficients is anongoing research effort and will be addressed in future work.

One of the important tasks in simulating the fabric is to accurately model the orthotropic mater-ial response of the yarns. The LS-DYNA’s material model MAT_ORTHOTROPIC_SIMPLIFIED_DAMAGE for 3D solid element, which includes an optional simplified damageand optional failure, is employed to capture yarn’s mechanical responses. Figure 4 illustrates theprocedure to generate the FE mesh for later definition of the anisotropy of the yarn’s mechanicalproperties. Node numbering for the element is very critical to establish the local coordinate system.For representative elements as shown in Figure 4, the bottom face (based on nodes 1–4) and thecorresponding top face (based on nodes 5–8) are the most perpendicular to the yarn axis. Local axisx0 is coincided with the vector based on node 1 and 2, while the local axis z0 is the cross-product of x0

and the vector based on node 1 and 4. Finally, the local axis y0 is determined as the cross-product ofz0 and x0. The spherical projectile is made of steel and has a diameter of 20mm. The materialproperties of the projectile are as follows: elastic modulus¼ 200GPa, density¼ 7.85E-3 g/mm3

Figure 4. Determination of local coordinate system for modeling anisotropic properties of the yarn (right-hand side

picture is the spherical projectile).

Figure 3. Cross-sectional views of three fabrics. Geometries of yarns and fabrics are defined by yarn thickness or

diameter, yarn width and span. Geometrical parameters are given by the manufacturer.

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and Poisson’s ratio¼ 0.3. The orthotropic material properties of the high modulus Twaron D2200manufactured by Teijin Aramid are given as: longitudinal modulus (E11)¼ 116GPa and transversemodulus (E22, E33)¼ 10GPa. Ultimate strain for yarn breaking is 0.03. It has been shown by Gasseret al. (2000) an orthotropic elastic continuum could capture the yarn behavior given its negligiblysmall Poisson’s ratios and the shear moduli and transverse elastic moduli are very small with respectto the longitudinal elastic modulus. In this work, the Poisson’s ratios are set to zeros. The shearmodulus (G12, G23, G31) are set to equal 10GPa (about one-tenth of the longitudinal modulus). Thisis a reasonable assumption when modeling a zero-twist or low-twist yarn as a homogenized con-tinuum, in which the predominant material property is the longitudinal modulus (Nilakantan et al.,2009). Although significant effort has been devoted to characterize the mechanical properties offibers and yarns, there could be some variations associated with these values and a probabilisticcomputational approach (Nilakantan et al., 2011) has been proposed to address this issue.

Numerical results and discussions

The three fabrics were subjected to impact from a projectile flying with an initial velocity of 60m/s.The spherical projectile was set up to hit the fabrics at their centers.

The impact resistance of the textiles is assessed through the resultant velocity of the sphere andevolutions of various energy components. FE analysis is performed using LS-DYNA 971 R6.0.

Numerical results

Figures 5 and 6 show the side and bottom views of the three fabrics penetrated by the projectile at atime of 0.4ms. As illustrated in these figures, while the sphere has penetrated through all the fabrics,the failure of the knitted textile is the most severe. The projectile has broken both warp and weftyarns in the cases of the plain weave and basket weave, while the knitted fabric is torn only in thehorizontal direction. Interestingly, damage patterns obtained from simulations for the plain weaveand knitted textile are in good agreement with the corresponding experimental results shown inFigure 6 (d and e). It is clearly visible from the experimental results that the plain weave structurebehaves like an orthotropic material with respect to the weft and warp directions. Meanwhile, theknitted fabric exhibits much weaker ballistic resistance in the wale (horizontal) direction in com-parison with the course (vertical) path. The observed failure patterns from the three simulations

Figure 5. Side view of projectiles penetrating fabrics: (a) plain weave, (b) basket weave and (c) knitted fabric at

time¼ 0.4 ms).

clearly show the importance of fabric architecture and yarn arrangement in the ballistic performanceof the textile. In the weave structures, yarns in all directions around the impact centers were involvedin preventing the penetration, while this was not the case for the knitted fabric. With similar fabricdensity, yarn strength, impact energy and projectile shape, the cracks (once initiated) will propagatemuch faster and further within the knitted architecture as fracture energy is focused in one preferreddirection. While this failure mechanism may not be useful for designing single layer textiles, taking itinto account when assembling multilayer fabrics could help to minimize penetration and maximizeenergy absorption. Among the two woven structures, the 2/2 basket weave evidently shows moreshearing and flexibility compared with the plain weave. It is important to emphasize that the experi-mental results are shown here to mainly highlight their similarities in the deformation and failuremechanisms with those observed numerically. More rigorous experiments will be performed tovalidate the numerical models.

The impact resistance performances of the three fabrics are further illustrated in Figure 7,showing the resultant velocity history of the projectile (Figure 7a), in which the plain weaveshows the best performance by reducing the projectile velocity to about 10m/s at the end ofthe simulation compared to approximately 30m/s for the basket weave and the knitted fabric.

Figure 6. Bottom view of projectiles penetrating the fabrics: (a) plain weave, (b) basket weave and (c) knitted fabric

(at time¼ 0.4 ms). Experimental results of (d) woven and (e) knitted Twaron fabrics subjected to ballistic impacts.

Tran et al. 367

Figure 7(b), on the other hand, shows the evolutions of kinetic energy (K) and strain energy(U) associated with each fabric, indicating how much kinetic energy from the sphere isimparted into the fabric. The plain weave has evidently shown the capability to absorbmore impact energy, while the strain energy levels are quite similar for all three cases.Figure 7(c) plots evolutions of frictional energy, showing a distinct difference between theknitted and woven fabrics. The friction energy dissipated in the knitted fabric is noticeablyless than that of woven one, revealing another reason for the poor performance of the knittedstructure. This difference could be attributed to a larger contact area of yarns in the wovenstructure compared to the knitted one. While friction is the main reason for the difference inresultant velocity between the plain weave and the knitted structure, deformation indicated bythe strain energy evolution is evidently the key factor affecting the performances of the plainweave and basket weave. As clearly shown in Figure 7(c), energy dissipated through the fric-tional process is quite similar for plain and basket weaves, while the corresponding strainenergy evolution curves presented in Figure 7(b) are considerably different. In fact, the 2/2basket weave, which has fewer crossovers and allows more yarn shearing, stores less strainenergy during the impact event. Another important observation from this numerical study isthat only a small amount of imparted energy is channeled into kinetic energy of the fabrics.This physical response could be attributed to the interlacing architecture of the textile layers,simply represented by connections of pores and crossovers, which could effectively minimize thedynamic reactions and wave propagation.

Figure 7. Comparison of ballistic performance of the three fabric architectures in terms of (a) projectile resultant

velocity, (b) kinetic and strain energy of fabrics and (c) total dissipated energy by frictional sliding.

Figure 8 displays detailed energy evolution histories of the projectiles and the fabrics of the threetextile architectures. It is visible in this figure as to how impact energy in the form of the projectile’skinetic energy transforms into various energy components associated with the fabrics.

The kinetic energy of the spherical projectile is decreasing with an increase in strain and totalfrictional energy of the fabrics. In the three cases, after the intense impact event occurring between0.2 and 0.4ms, strain energy reaches a relatively plateau level and starts to decrease slowly oncecaught up by the cumulative friction energy. The intersections of the two energy curves, illustratedby the arrows in Figure 8, divide the impact resistance process of the fabrics into strain-dominant(behind the arrow) and friction-dominant (beyond the arrow) domains. Once again, the kineticenergy of all the fabrics is negligible and, therefore, plays no role in the failure process.

Friction effect

As illustrated in the previous sections, friction has an important effect on the ballistic performance ofthe fabric. The amount of energy dissipated through frictional sliding of yarns could be attributed tothe size of the contact zone, which is directly related to yarn count per unit area and frictionalcoefficient. It has been shown in the previous section that the yarn arrangement (known as sett)contributes to the frictional energy. Measuring the friction coefficient between yarns themselves andthe yarn–projectile systems are challenging tasks with inconsistent results. Therefore, it is quitenecessary to perform sensitivity analysis of frictional parameters with regards to impact velocity,energy distribution and fabric architectures. A simple way to examine the importance of frictional

Figure 8. Evolutions of various energy components including kinetic, strain and total frictional energy of the fabric,

and kinetic energy of the projectile for (a) plain weave, (b) basket weave (c) knitted fabric.

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coefficient is to increase the friction coefficient FS by a factor of two and compare the resultantvelocities and total frictional energies with those of the baseline cases (as shown in Figure 9). Similarto the previous study, the three cases of (a) plain, (b) basket weave and (c) knitted fabric areexamined showing some distinctive features.

In this simulation, the static coefficient of friction FS is set to 0.36 for yarn–yarn and 0.48 foryarn–projectile interaction. Total friction energy recorded for the three fabric structures in thebaseline studies are relatively similar. It is quite interesting to see that the change in friction coef-ficient does not affect the resultant velocity of the projectile for the basket weave (Figure 9-b1) andthe knitted fabric (Figure 9-c1); while some changes are observed in the total frictional energy(Figure 9-b2, c2). For the plain weave fabric (Figure 9-a1, a2), a noticeable increase in total fric-tional energy clearly leads to a distinct decrease in resultant velocity. Comparison between the threecases indicates that the influence of friction on ballistic performance is closely related to theyarn arrangement, in which a tight sett is much more sensitive to a change in friction comparedto a loose sett.

Effect of projectile geometry

Fragment geometry has significant influence on the impact resistance performance of the compositefabric, as it is associated with projectile–yarn friction and stress concentration build-up due to sharpedges or irregular surfaces. It is especially even more detrimental for the impact target, as thefragments (in most cases) translate and spin rapidly simultaneously.

In this study, the three projectiles illustrated in Figure 10 (with initial translational and angularvelocities of 60m/s and 31.4 rad/ms, respectively) are impacted to identical knitted fabrics. Theabove angular velocity of the projectile is equivalent to 3.106 r/min spinning of a bullet fired froma rifled barrel. Figure 11 presents the side and bottom views of the projectiles impacting the fabrics.As clearly seen from Figure 11, the chisel-shaped and spherical fragments induce more damagecompared to the cylindrical one.

Of the three projectiles, the cylindrical one has the largest initial contact area with the fabric,which could help prevent stress concentration as well as help spread the imparted energy throughfriction. Ballistic performances of the fabric with respect to the three projectiles in terms of resultantvelocity and various energy components are comprehensively presented in Figure 12, confirming theinitial observations from Figure 11. Figure 12(a) indicates that the chisel-shaped and sphericalprojectiles penetrate the fabric with a resultant velocity of 34 and 27m/s, respectively; while thecylindrical projectile is almost arrested at the end of the simulation. Figure 12 (b and c) presents theevolution history of strain and kinetic energy associated with the fabric. Chisel-shaped and cylin-drical projectiles induce distinctively higher strain energy trapped in the knitted fabric compared tothe spherical one, while the irregular geometry of the chisel-shaped fragment brings about a slightincrease in kinetic energy. Regardless of some variations related to the projectile’s shape, it can benoticed once again that the strain energy levels, which are associated with overall deformation of thefabric, are substantially higher than the corresponding kinetic energy components.

Movements of the fabric and yarns during the impact event are clearly affected by the projectile–yarn and yarn–yarn frictions, as illustrated in Figure 12 (d and e). Total friction energy, which iscontributed from the yarn–yarn and yarn–projectile frictions, is significantly higher than the strainand kinetic energy channeled into the textile layers in all three cases. It is quite interesting to pointout the higher values of frictional sliding energy associated with projectile–yarn compared to that ofyarn–yarn as a result of the projectile’s rotational movements. Figure 12 (d and e) also highlights theinfluences of friction associated with projectile geometry to the impact response of the fabrics.

It is clearly shown that the chisel-shaped projectile induces the highest yarn–yarn friction due to itsmulti-faceted geometry, and the cylindrical projectile dissipated the most energy though its frictionalcontact with the yarns.

As mentioned earlier, it is necessary to take projectile spinning into account in a ballistic simu-lation study. However, most of the studies in literature on the ballistic behavior of textiles do notconsider this an important effect. Figure 13 summarizes the spinning effect study of two spherical

Figure 9. Frictional sliding effects on impact resistance of (a) plain weave, (b) basket weave and (c) knitted fabric.

Resultant velocities and total frictional energies for the baseline and the increased friction cases are compared to

reveal the importance of friction.

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projectiles, with and without spinning, impacting on the knitted fabric. The projectile is, in one case,initially spinning at 31.4 rad/ms, which is equivalent to 5 rounds/ms. As illustrated in Figure 13(a),the rotating sphere penetrates through the fabric with a resultant velocity of nearly 27m/s, while thenon-rotating projectile achieves a final velocity of about 18m/s. The spinning effect is also shown inFigure 13(b), which presents a comparison of the yarn–yarn and projectile–yarn frictional energy forthe two cases. While the yarn–yarn frictional energy is relatively similar for spinning and non-spinning spheres, the projectile–yarn friction energy shows a distinctive difference. A significantincrease in dissipated friction energy due to spinning enables the projectile to push away the blockingyarns during the penetration, leading to a higher resultant velocity.

Figure 11. Bottom views (left) and side views (right) of the damage patterns at about 0.45 ms of the knitted fabrics

subjected to impact of (a) chisel-shaped, (b) cylindrical and (c) spherical projectiles.

Figure 10. Spherical, cylindrical and chisel-shaped projectiles with equal mass of 16.8 g.

To further highlight the significance of fabric architecture on impact resistance as well as pro-jectile geometry, the woven fabric with equivalent areal mass to the knitted one is subjected tospherical and chisel-shaped projectiles. Figure 14(c) presents the top and side snapshots of thewoven fabric impacted by the chisel-shaped fragment at 0.45ms, illustrating the deformation andfailure patterns.

Figure 14(a and b) again presents a study on the effects of projectile geometry on the ballisticresponse of woven fabrics. Similar to the previous analysis, resultant velocity and frictional energyevolutions are compared between two impact events involving spherical and chisel-shaped project-iles. While damage is observed in Figure 14(c), it is clearly evident that both projectiles are arrestedby the fabric at the same time (Vr¼ 0), and bounce-back right after that with slightly different

Figure 12. Evolutions histories of the projectiles: (a) velocity, (b) strain and (c) kinetic energy for the knitted fabric.

Distinctive evolutions of (d) projectile–yarn and (e) yarn–yarn total frictional energy, indicating a strong influence of

projectile geometry.

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resultant velocities. The chisel-shaped projectile indeed induces noticeably more yarn–yarn andprojectile–yarn friction energy dissipation compared to the spherical one (Figure 14b). This studysuggests that the woven architecture of the fabric seems to be effective in suppressing the differencein the impact response caused by the variation of projectile geometry.

Figure 14. Effect of the chisel-shaped and spherical fragments on the impact resistance of the woven fabric in terms

of (a) resultant velocity and (b) frictional sliding energy. (c) Bottom and side views of a spinned chisel-shaped projectile

impacting on the woven fabric at 0.45 ms.

Figure 13. Effect of spherical projectile spinning on the impact resistance of the knitted fabric in terms of

(a) resultant velocity and (b) evolutions of frictional energy components.

Conclusions

This paper presented a numerical and parametric study on the ballistic performance of three rep-resentative fabric architectures: plain weave, basket weave and knitted textiles. In this work, failuremechanisms of the textiles were investigated in various aspects of an impact event. The studyprovided further insights into the influences of yarn arrangements to the ballistic performance ofthe fabrics. Of the three investigated fabrics, the knitted one showed the worst performance.Numerical simulation and experimental study showed strong agreements on the impact-induceddamage patterns on both plain weave and knitted structures. The consistency of crack propagationalong the ‘course’ direction of the knitted fabric was clearly observed from the experiment andsuccessfully simulated by the numerical model, revealing details of the process of yarn breakingdue to transverse shear loading. Knowledge on the anisotropic failure mechanism of knitted textilescould be very useful for the design of multilayer soft armor structures. The 2/2 basket weaveexhibited similar ballistic resistance compared to the knitted fabrics and provided more shearingand flexibility for fabrics compared with the rigidity of the plain weave. Impact resistance of thethree textile structures were evaluated through comparison of resultant velocities of the projectileand evolutions of various energy components. Two main common impact resistance mechanisms offabric structures were interpreted, including energy absorption through deformation and energydissipation through frictional activities. Surprisingly, kinetic energy components were significantlysmaller compared to strain and friction energy, suggesting negligible involvement of inertia in thefailure process. A parametric study was also conducted to study the effect of frictional coefficient onthe impact resistance of the fabrics, confirming the importance of yarn arrangement. Increasing thefriction coefficient by a factor of two did not change the resultant velocities in the basket weave andknitted fabric impact events, as a result of the flexibilities of the yarns and fabrics in both cases.To highlight the influences of projectile geometry and its angular rotation to ballistic performanceand friction energy, simulations were carried out for three different projectiles. Spherical and chisel-shaped projectiles exhibited relatively similar ballistic capability as they both could push yarns awayduring the penetration process. A sharp increase in projectile–yarn frictional dissipated energyoccurred as a result of the projectile’s spinning, leading to a decrease in the corresponding resultantvelocity. This observation confirmed that the angular velocity of the fragment must be included inthe analysis, which is lacking in most studies in the literature.

Funding

This research was sponsored by the Commonwealth Scientific and Industrial Research Organisation (CSIRO)

and was accomplished through the Future Manufacturing Flagship program (Dr Swee Mak, ProgramDirector).

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Effects of architecture on ballistic resistance of textile fabrics: Numerical study

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Transcript of Effects of architecture on ballistic resistance of textile fabrics: Numerical study

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