High-velocity impact onto a high-frictional fabric treated with adhesive spray coating and shear thickening fluid impregnation

https://doi.org/10.1016/j.compositesb.2020.107742Get rights and content

Abstract

The ballistic performance of a bullet-proof fabric can be increased by an increment in the friction between fibres. For enhancement of this performance, numerous studies on the shear thickening fluid (STF)-impregnated fabric have been conducted. The STF as a fluid, however, has inherent shortcomings. Our research aim is to understand and compare experimentally two different bullet-proof fabrics treated with a simple spray coating and STF impregnation. In this study, 71 single yarn pull-out and 90 high-velocity impact experiments were carried out. It was remarkable that the newly proposed Heracron fabric coated with a commercial coating spray increased by more than 90% the energy absorption before penetration, with only less than 15% of add-on weight. It was found that the polymeric anchors created on the fibre produce an exceptionally high level of friction between fibres, according to a microscopic morphological analysis and the single-yarn pull-out experiment. This study revealed the physical explanation of this coating method, showed its feasibility, and considered its effectiveness with excellent results.

Introduction

Bullet-proof fabrics made from advanced fibres of aramid, polyethylene, and polybenzoxazole serve to stop high-velocity projectiles by absorbing their kinetic energy. Under a high-velocity impact, the kinetic energy of the projectile can be dissipated through many mechanisms, for example, deformation of the yarn and fabric structure, and friction between yarns, layers, and the projectile [1]. Owing to the different protection mechanisms, the ballistic performance depends on various conditions: type of fibre and yarn, weave structure, shape, and material, and incident angle of the projectile. As a critical design factor, the friction between yarns has been studied by many researchers. Briscoe and Motamedi conducted experiments with various fabric weaves having three different levels of inter-yarn friction. They found that more projectile kinetic energy is absorbed in a fabric with higher inter-yarn friction [2]. Other researchers conducted numerical studies and concluded that the increment in friction between yarns prevents their slipping near the impact area, leading to more yarns directly involved in the absorbing mechanism [[3], [4], [5], [6], [7]].

Therefore, a higher level of kinetic energy can be absorbed and dissipated with an increment in the friction coefficient. The boundary condition of a ballistic fabric also has a significant influence on the role of friction and the overall performance of the fabric [8]. However, the increment in friction between yarns may not always guarantee the increment in ballistic performance. By conducting numerical simulations, Wang et al. found that the increase in friction between the yarns reduces the level of stress at the edge area of the impact. This implies that the yarns with high friction coefficient need longer time to be fractured than the yarns with low friction coefficient. This prolonged time of fracture of the yarns can increase the kinetic energy absorption. However, they also found that the increase in friction can decrease the longitudinal stress wave velocity. In their model, the kinetic energy absorption decreased when the friction coefficient of the yarns was as high as 0.9 because of the decrement in wave velocity [9]. However, the absorption of kinetic energy in the high friction case is higher than that in the frictionless case. The decrement of wave speed caused by the increment in friction was also reported by Briscoe and Motamedi [2]. Zhou et al. showed that beyond a certain level, a highly frictional fabric can lead to premature yarn failure, resulting in no further increment in its ballistic performance [6]. Although there is an optimum level of friction coefficient for a fabric, a high friction coefficient tends to increase the ballistic performance compared to the frictionless and pristine fabric cases.

Shear thickening fluid (STF) impregnation is the most common method to increase the inter-yarn friction. The STF is a dense colloidal suspension that shows a rapid increment in viscosity as a response to a high shear rate. When a bullet-proof fabric is impregnated with the STF the fabric would absorb more energy. The STF can improve the impact resistance by increased friction between yarns and fibres, shear thickening rheological behaviour, and improved coupling and load transfer [11]. Distinguishing and controlling the mechanisms independently may give an insight. A few works of multi-phase STF impregnation and some simulations suggested which a mechanism contributes to the impact resistance mainly. When impact velocity is comparably low, not only friction increment but also shear thickening effect of impregnation seem to have a substantial influence on the impact resistance. Mawkhlieng and Majumdar conducted low-velocity impact experiments and characterization with fabrics impregnated by monodispersed and bi-dispersed STFs. The critical shear rate increased but the peak viscosity decreased when the fabrics impregnated by the bi-dispersed STF hindering creation of hydro-clustering. However, the yarn pull-out forces increased almost uniformly regardless of the dispersion type. The resistance performance against the low-velocity impact interestingly is superior in case of the bi-dispersed STF impregnated fabric. The results suggested that rheological behaviour of STF significantly contributes to the impact resistance improvement [11]. On the other hand, the study conducted by Gurgen and Kushan showed different outcomes. By adding SiC particles into STF, they achieved high yarn pull-out force of the impregnated fabric but rheological behaviour of the STF was almost same. The stab impact resistance of the impregnated fabric with adding the SiC particles increased remarkably [12]. The results suggest that the impact resistance can be influenced by the shape of impactor in spite of the low-velocity impact condition. Gurgen et al. also conducted high-velocity impact tests with various types of STF which has different rheological behaviour. By adding the SiC particles, the types of STF showed the almost identical rheology however the impregnated fabrics had various yarn pull-out forces depending on the fraction of the SiC particles. It was found that the higher yarn pull-out force of the impregnated fabric, the higher resistance against the high-velocity bullet impacts [28]. In numerical works of Park and Lee, experimental results of high-velocity impact were simulated successfully applying a Coulomb fiction model for simulating inter-yarn friction [[13], [14], [15]]. The high-velocity impact tests and simulations were carried out in a wide range of velocities from 800 to 2000 m/s. It was shown that the increment of friction yields higher impact resistance. Other numerical studies also exhibit similar trends in spite of different projectile and impact velocity [[2], [3], [4], [5], [6], [7]]. As discussed earlier, the friction increment prevents critical opening so called a windowing effect and more yarns can involve in absorbing kinetic energy directly. Duan et al. showed numerically the alleviated windowing effect by increased friction [16]. Fahool et al. insisted that fraction of the STF in treated fabrics is seemed to be too small to contribute to absorbing kinetic energy in case of high-velocity impact [17]. Gurgen also insisted that the contribution of STF under the high-velocity impact is mainly based on the high inter-yarn friction [10]. Therefore, our study was carried out focusing only on the relation between inter-yarn friction and high-velocity impact resistance. In simulation and experimental works, the STF impregnated fabric showed more energy absorption, especially at a 200–300 m/s velocity impact [15]. Impregnated bullet-proof fabrics show better characteristics and potentials in low to hypervelocity impact [4,[11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38]]. It has been reported that the addition of, among other materials, nanoparticles, graphene, and SiC can enhance the ballistic or impact resistance of an STF impregnated fabric [27,28,36]. LaBarre et al. introduced a carbon nano-tube coating on Kevlar to increase yarn friction. The level of friction increased approximately from 10 to 20% depending on the process conditions. However, degradation of the physical properties of yarns was observed [39].

Although the evidence shows the effectiveness of STF impregnation to fabrics, the method has a few critical drawbacks. By nature, the STF has low resistance against environmental factors. Moreover, the performance of the impregnated fabric is highly affected by the temperature condition. The peak viscosity and critical shear rate of the STF can vary enormously with a small change in temperature [4,23,24,31,38]. Generally, when adding STF into a fabric, a significant increment in weight is required to influence the performance. Furthermore, the STF is naturally a viscous liquid, which can flow down for a long period of time. Consequently, the effectiveness and fields of application of the STF have been limited owing to the aforementioned critical shortcomings. Instead, a polymeric coating method can result in high inter-yarn friction with low weight addition and proven environmental resistance. There are a few patents regarding the polymeric coating method to increase friction between yarns [[40], [41], [42]]. Among the patents, the most similar concept was found in a work done by Harpell et al. They invented a flexible fabric fabricating method introducing discontinuous matrix islands binding strong filaments together. However, the invention only intended to give high flexibility comparing to a fully coated fabric structure. The invention does not consider either an alleviation of a windowing effect by increased friction and comparison with fully coating method or pristine fabric. It was reported that coating fabrics with rubber materials can enhance their ballistic performance [5,[43], [44], [45], [46], [47]]. Khodadadi A et al. conducted a comparative research on the Kevlar/rubber and Kevlar/epoxy composites. By conducting low-speed impact tests, the impact resistance of a two-layered fabric was increased dramatically in the case of a rubber matrix with less than 15% of weight increment. The Kevlar/epoxy had less energy absorption than the Kevlar/rubber composite [47].

By coating a fabric with a polymer instead of an STF material, an enhanced bullet-proof fabric with sufficiently high friction can be manufactured because of their almost identical mechanical principle [10]. The fabric with coating is also expected to have higher environmental resistance because a polymer, such as rubber or an adhesive, is basically more stable than an STF, as fluid or gel. However, a comparison study between the polymeric coating and the STF material for enhancement of ballistic fabrics has not been conducted yet. In addition, a study with a simple adhesive coating for the ballistic enhancement of a bullet-proof fabric without a rubber matrix has not been performed. In this study, polymeric adhesive coating and STF impregnation were applied independently to a single layer of Kevlar and Heracron fabrics. The remarkable results from the comparison between the pristine, adhesive coated, and impregnated fabrics show that the coating method with adhesive is very promising for enhancement of bullet-proofing fabrics.

Section snippets

Specimen preparation

Heracron HT840 and Kevlar 720 fabrics with areal density of 200 g/m2 and 261 g/m2, respectively, were used in this study. The detailed information of the fabrics is summarised in Table 1. The filaments of the fabrics have similar mechanical properties. However, the number of yarns per inch and the yarn size is different. The all fabrics were cut as a 120-mm square. After treating the fabrics with STF impregnation and spray coating, the excessive fluid and adhesive polymer were eliminated by a

Results and discussion

The flexibility of treated fabrics and pristine fabric was estimated experimentally by simple bending angle test as shown in Fig. 9. There were only a 4° bending angle decrement after STF impregnation and spray coating onto Heracron fabrics. On the other hand, there was no bending angle change in the case of the STF impregnated Kevlar. The spray coated Kevlar had a 7° bending angle decrement. The bending angle differences imply that the flexibility can be influenced by treatment and fabric

Conclusion

A number of studies to enhance the ballistic performance of fabrics by friction increment through coating and STF impregnation have been conducted, and they have introduced numerical models and various coating methods. STF coating is the best known method, and ballistic improvement by STF has been achieved fairly. However, the use of STF is limited owing to unavoidable shortcomings such as temperature dependency, environmental resistance, and excessive weight add-on because of its nature of

Acknowledgement

The present work was supported by the National Research Foundation of Korea (NRF-2014M1A3A3A02034828). The authors would like to thank the foundation for the financial support received.

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