Soft armor materials constructed with Kevlar fabric and a novel shear thickening fluid

Journal Pre-proof Soft armor materials constructed with Kevlar fabric and a novel shear thickening fluid Jianbin Qin, Borui Guo, Le Zhang, Tianwei Wang, Guangcheng Zhang, Xuetao Shi PII:

S1359-8368(19)35398-3

DOI:

https://doi.org/10.1016/j.compositesb.2019.107686

Reference:

JCOMB 107686

To appear in:

Composites Part B

Received Date: 12 October 2019 Revised Date:

28 November 2019

Accepted Date: 4 December 2019

Please cite this article as: Qin J, Guo B, Zhang L, Wang T, Zhang G, Shi X, Soft armor materials constructed with Kevlar fabric and a novel shear thickening fluid, Composites Part B (2020), doi: https:// doi.org/10.1016/j.compositesb.2019.107686. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

Soft armor materials constructed with Kevlar fabric and a novel shear thickening fluid Jianbin Qin a, Borui Guo a, Le Zhang b, Tianwei Wang a, Guangcheng Zhang a*, Xuetao Shi a* a

Department of Applied Chemistry, School of Natural and Applied Science, Northwestern Polytechnical University, Xi’an 710072, China b

China Academy of Space Technology (Xi’an), Xi’an 710100, China

Abstract: This study presents the dynamic stab resistance and quasi-static mechanical properties of soft armor materials constructed with Kevlar fabric impregnated with a novel shear thickening fluid (STF) composed of silica microsphere and ionic liquids (ILs). Rheological results indicate that this STF presents a unique double continuous shear thickening behavior and good conductivity. Both the stab resistance and mechanical properties of the Kevlar fabric are enhanced significantly due to the presence of the STF. With increasing the addition of STF, the stab resistance of the STF-processed Kevlar fabrics (STKF) is enhanced, and the optimal stab resistance is obtained when the STKF contains 34.89 wt % STF. By comparison with the neat Kevlar fabric, the maximum friction between the yarns in the STKF increases by dozens of times, and the tensile strength of the STKF and yarn increases by approximately 2 times and 1.5 times, respectively. Tensile and pull-out tests demonstrate that both filaments and yarns are restrained for the unique double continuous shear thickening of the STF. Furthermore, the STKF has conductivity and the electrical resistance of it has high sensitivity to the applied deformation. The 1

electromagnetic interference shielding of the STKF was as high as 27 dB. These provide a potential to exploit the novel next generation of soft armor with both excellent protective performance and intelligent wearable property. Keywords: Shear Thickening Fluid, Ionic Liquids, Stab Resistance, Tensile Strength, Electromagnetic Shielding

1. Introduction Shear thickening fluid (STF) is a concentrated suspension with distinct rheological properties. When the shear rate or shear stress increases to a critical value, the viscosity of the STF will increase dramatically, even by several orders of magnitude, with the transformation from liquid to solid-state [1-4]. Moreover, the shear thickening behavior of the STF is reversible. In other words, the STF will transform from a liquid to a solid-like state when the critical shear rate is applied, and then it will turn from a solid into a liquid again when the applied shear is removed [1-4]. The mechanisms of the shear thickening behavior have been proposed and studied, for example, order to disorder theory [2], hydrocluster theory [3-5], and jamming theory [1, 6, 7]. For the unique shear thickening behavior, the STF has been exploited as soft armor materials [8-26], damping composites [27, 28], and other protective composites [29-35]. Conventional body armor is usually constructed with numerous layers of fabric made from special fibers such as p-aramid, ultrahigh molecular weight polyethylene and polybenzobisoxazole. This makes body armor too bulky in practical applications. To improve the flexibility of conventional body armor, Wagnar et al. first introduced 2

STF into Kevlar fabric and exploited liquid body armor, which significantly improved the ballistic performance of the Kevlar fabric [25]. The bulletproof performance of 4 layers of STF-processed Kevlar fabric is nearly equivalent to that of 14 layers of the neat ones, and the flexibility of STF-processed Kevlar fabric is noticeably superior to that of the original Kevlar fabric. Another novel soft body armor was constructed by impregnating aramid fabric into a dual-phase STF composed of silica and calcium carbonate nanoparticles [17]. It has been demonstrated that the impact energy by this soft body armor was reduced by 40 % with the same bulletproof performance as that of traditional body armor. The bulletproof performance of 19 layers of STF-treated Kevlar fabric is nearly equivalent to that of 32 layers of the original Kevlar fabric. The energy absorption and ballistic response of woven Kevlar fabric loaded with different quantities of STF have also been studied under high-velocity impact. Experimental results have revealed that the absorbed energy of the Kevlar fabric impregnated with STF increases with increasing silica nanoparticle loading in the STF. However, the effectiveness of the STF decreases when it contains too much silica nanoparticle. The energy absorption of 35 wt % STF-treated Kevlar fabric is 2.3 times higher than that of the original Kevlar fabric [21]. Decker et al. [8] investigated the STF-processed fabrics and concluded that they presented great improvement compared to that of the neat fabrics. The effects of fumed or submicron silica particles in the STF on the stab resistance properties of STF-treated fabric were also investigated [22]. It was demonstrated that STF-treated fabric containing submicron silica presented greater stab resistance. Li et al. [13] 3

reported that the stab resistance of ultrahigh molecular weight polyethylene (UHMWPE) fabric impregnated with STF was significantly enhanced. Cao et al. [14] demonstrated that the STF can prevent fabric yarns from slipping and enhance the impact resistance of the fabric. Related studies demonstrated that high concentration of nanoparticle and large particle size of silica in STF led to better stab resistance for STF-treated composites [35]. Mawkhlieng and Majumdar investigated and concluded that the shear thickening of the STF played a dominant role in improving the impact resistance of the Kevlar fabrics [24]. Bai et al. [31] concluded that the work of external force on the STF-treated fabrics is equivalent to the dissipated energy of friction between the yarns. All of the experimental results reveal that the STF can primarily reduce the mobility of filaments and yarns. Moreover, sandwich composites filled with STF in honeycomb cores or foams can provide meaningful hypervelocity impact resistance against simulated small micrometeoroid and orbital debris threats [18, 28, 36, 37]. Although the STF can significantly enhance the impact resistance of STF-treated soft armor composites, there are major weaknesses in it. In the literature above, polyethylene glycol (PEG) with low molecular weight is usually selected as the dispersion medium of the STF, and silica particles are often used as the dispersion phase [8, 10, 11, 13-24, 30, 31, 33, 35, 36, 38]. It is easy to precipitate for silica particles in the STF because the density of the silica particles is higher than that of the dispersion medium. This will make the STF and STF-treated composites fail. A few studies have used polymer microspheres as the dispersion phase to avoid precipitation 4

in the STF [9, 39-41]. Compared with the dispersion phase of the silica particles, the improvement in the protective performance for STF-treated composites was weakened due to the low hardness of the polymer microspheres [9]. To obtain composites based on STF with significant enhancement of impact resistance, changing the dispersion medium is an effective way to overcome the precipitation in the STF. Indeed, all suspensions are expected to exhibit shear thickening under correct interactions between the dispersing phase and the medium and can be adjusted by changing the dispersing medium or the dispersing phase with different forms to obtain a variety of STFs [3]. Compared with the traditional organic solvents, ionic liquids (ILs) are composed entirely of ions and liquid with melting points below 373 K (or near room temperature). Their unique physicochemical properties, including highly concentrated ionic atmosphere, low vapor pressure and low flammability, high chemical and thermal stability and sufficiently high ionic transport, are received considerable interest. Based on above unique properties, ILs have been regarded as a green solvent and widely used in many fields, such as chemical and catalytic reactions, separations, and purifications [42-46]. In the meanwhile, some ILs also have been used in the application of ILs as dispersion medium for colloidal nanomaterials [42-50]. A few studies have demonstrated that hydrogen bonds would form between the hydrophilic groups of cations or anoins in ILs and the silanol on the surface of the silica particles [42, 44]. Compared with the dispersion medium of PEG in the STFs, the viscosity of ILs is usually higher than that of PEG, which is beneficial for the preparation of STFs with high strength [36]. The maximum viscosity of the STF was 5

increased by as high as six orders of magnitude with increasing of the viscosity of PEG 200 [35]. The most important is that the precipitate of dispersion phase in the STF can be weakened for the high density of ILs, and the period of validity for the STF is prolonged. Furthermore, the STF will be given high chemical and thermal stability if the dispersion medium of it is ILs. Especially, the shear thickening of the STF will be enhanced with increasing of the sizes of the dispersion phase [22]. However, so far, silica particle with large sizes and ILs have not been used to make the STF. So that it will be an innovation and challenge to construct the STF with silica microsphere and ILs. In addition, Liu et al. have constructed a conductive wearable electronic and protective textile composite by depositing carbon nanotubes (CNT) on the surface of the STF-treated Kevlar fabric [26]. For good conductivity of the ILs, it is a possibility that soft armor with excellent protective performance and intelligent wearable property will be constructed with more simple method. In this work, to enhance the strength and stability of the shear thickening behavior, submicron silica microspheres and ILs are selected as the dispersion phase and medium to construct the STF for the first time, respectively. The rheological and viscoelastic properties of these novel STFs are systematically investigated by the rheometer. Then, the soft armor material, STF-treated Kevlar fabric (STKF), is constructed by impregnating Kevlar fabrics with these novel STFs. Yarn pull-out tests, dynamic stab impact tests and quasi-static tensile strength tests are performed to investigate the dynamic impact resistance and mechanical properties of the soft armor. Furthermore, considering the good conductivity of the ILs, the conductivity, 6

responsiveness of electrical resistance for the STKF during different deformation and the electromagnetic interference shielding effectiveness of the soft armor material are investigated.

2. Materials and methods 2.1 Materials Tetraethoxysilane, absolute ethyl alcohol and aqueous ammonia (28 wt %) were purchased from the Sinopharm Chemical Reagent Co., Ltd. 500 mL absolute ethyl alcohol, 200 mL ultrapure water and 72 mL NH3·H2O were mixed homogenously at room temperature, and then TEOS (40 mL) was quickly added with magnetic stirring for 6 h. Silica microspheres were collected by centrifugation, washing and dried in a vacuum oven. Monodispersed silica microspheres were prepared by sol-gel method, with a diameter of 500 nm, as shown in Fig. 1(a). Four different of ILs, 1-Butyl-3-Methylimidazolium Tetrafluoroborate ([BMIm][BF4]), 1-Butylpyridinium Tetrafluoroborate ([BPy][BF4]), 1-Butyl-3-Methylimidazolium Hexafluorophosphate ([BMIm][PF6]),

1-Ethoxyl-3-Methylimidazolium

Tetrafluoroborate

([EOHMIm][BF4]), as shown in Fig. 1(b) were purchased from Sinopharm Chemical Reagent Co., Ltd. [BMIm][BF4], [BPy][BF4] and [EOHMIm][BF4] are hydrophilic ILs because of the anion of [BF4]— and the hydroxy on the cation of [EOHMIm]+, while the [BMIm]PF6 is a hydrophobic ILs for the anion of [PF6]— [43]. Plain woven Kevlar fabrics with area density of 200 g/m2 were supplied by the Dupont Company.

2.2 Preparation of the STF 7

STFs with different concentrations ranging from 50 wt % to 64 wt % were prepared by dispersing silica microspheres in ILs with ultrasonication. The silica microspheres were added in parts of equal quantity at the same interval time to ensure full dispersion. The STFs were disposed in a vacuum oven at 80

for 24 h to remove

gas and water. 2.3 Rheology behaviors of the STF Both the rheological and viscoelastic properties of the STFs were characterized by steady and oscillatory shear with a stress controlled rheometer (Anton Paar Rheometer Physica MCR 301). The cone and plate system was selected, and the diameter of the cone was 50 mm, the cone angle was 2o, and a gap size of the system was 0.99 mm. To remove the history of stress before every test, the STFs were applied with a shear rate of 10 s-1 for 60 s. All tests were carried out at room temperature. 2.4 Preparation of the STKF Four samples containing 50 g of STF with 64 wt % of silica microspheres in [EOHMIm][BF4] were diluted by adding 200 ml, 150 ml, 100 ml and 50 ml ethyl alcohol respectively. Then, four groups of Kevlar fabric were impregnated into the diluted STFs and disposed by ultrasonic treatment for 30 min. The impregnated Kevlar fabrics were heated with 60 80

for 12 h and then disposed in a vacuum oven at

for no less than 24 h to eliminate the ethyl alcohol. The weight percentage (wt %)

of the STF loading in the STKF was calculated by Equation 1. Different STKFs were prepared and their detailed parameters are shown in Table 1. 8

STF (wt %) =















×100 %

(1)

2.5 Dynamic knife impact tests Specified by the NIJ Standard-0115.00, dynamic stab resistance testing was performed with the apparatus as shown in Fig. 2(a) at room temperature. Targets of neat Kevlar fabric (T-NKF) and targets of STKF (T-STKF) for dynamic stab-resistance testing were assembled as shown in Table 2 and Fig. 2(b), respectively. The multilayer backing of the target consisted of four layers of neoprene sponge with a thickness of 6.0 mm, followed by one layer of 30 mm thick polyethylene foam panel, backed by two layers of about 6.4 mm thick rubber sheet, as shown in Fig. 2(b). The assembled targets were held by four clamps, as shown in Fig. 2(c), and the designed knife for the sab-resistance test is shown in Fig. 2(d). The knife thickness was 4.0 mm, the tip angle was 23±0.5º, and the length of the end V-section was 49.2 mm as shown in Fig. 2(d). The drop mass consisted of a knife and steel holder, and it was controlled by the sensor system. The drop tube was designed to make the drop mass fall under the influence of gravity and strike the target at a predetermined point. The impact energy was calculated by Equation 2 ignoring the air drag and friction on the drop mass. We made a hypothesis that the gravitational potential energy of the drop mass was completely converted into kinetic energy, and the theoretical impact velocity was calculated by Equation 3. As shown in Table 3, the experimental parameters of the knife impact test are set up. The depth of the knife on the back of the target after the stabbing test was defined as the penetration depth.

Impact energy = m × g × h 9

(2)

Velocity of impactor = sqrt(2×g×h)

(3)

where m is the mass of the drop mass, g is the gravitational acceleration, 9.8 m/s2, and h is the vertical distance between the tip of the knife and the top of the target.

2.6 Single yarn pull-out tests To investigate the role of the STF in the STKF, the single yarn pull-out test was conducted on the electronic universal testing machine (CMT 8502, MTS, Measure range: 0~500 N, measurement accuracy within 1 ‰) at a rate of 100 mm/min at room temperature. 2.6 Quasi-static mechanical tests Quasi-static mechanical tests of the STKF, including warp tensile strength of the yarn and fabric, were carried out with the electronic universal testing machine (CMT 7204, MTS, Measure range: 0~20 kN; CMT 8502, MTS, Measure range: 0~500 N, both of measurement accuracy within 1 ‰) reference ASTM D 2256 and ASTM D 5034 at a rate of 100 mm/min at room temperature, respectively. 2.7 Conductivity, responsiveness, and electromagnetic interference shielding tests In addition, the conductivity of the STF was characterized with a digital conductivity meter (DDSJ-318, INESA Instrument Co., Ltd), responsiveness of electrical resistance for the STKF during different deformation were performed on the electrochemical station (VSP, Biologic), and the electromagnetic interference shielding effectiveness (EMI SE) of the neat Kevlar fabrics and STKFs in the high frequency range of 12-18 GHz was characterized by a Agilent vector network 10

analyzer (N5232A, USA) at room temperature.

3. Results and discussion 3.1 Rheological properties of the STFs The rheological properties of different concentrated suspensions with silica microspheres in different ILs are shown in Fig. 3. Suspensions in [BMIm][BF4], [BPy][BF4] and [BMIm][PF6] present shear thinning behavior. In contrast, suspensions in [EOHMIm][BF4] show noticeable shear thickening behavior. For the hydrophobic property of [BMIm][PF6], the silica microspheres will be conglomerated because of the hydrogen bonds between the silanol groups on the surface of the silica particles. Therefore, the suspensions of silica microspheres in [BMIm][PF6] present a solid-like gel, as shown in the inset of Fig. 3. The conglomeration of silica microspheres will be broken progressively under the applied shear stress, which leads to the continuous decrease in the viscosity, displaying shear thinning behavior. The hydrogen bonds may be formed between the F atom of [BMIm][BF4] or [BPy][BF4] and the silanol group on the silica microspheres [42, 44], which is beneficial to the dispersion of the silica microspheres in ILs. However, a network structure of silica particles will still form by the hydrogen bond between the silanol groups [42, 44]. Therefore, suspensions of silica microspheres in [BMIm][BF4] and [BPy][BF4] also displays a solid-like gel, as shown in the inset of Fig. 3, and the viscosity of the suspensions in [BMIm][BF4] and [BPy][BF4] is obviously larger than that of the suspensions in [BMIm][PF6]. The viscosity of the suspensions in [BMIm][BF4] and 11

[BPy][BF4] also decreases continuously because the particle network structure is disrupted with the applied shear stress. However, more hydrogen bonds are formed between the silanol groups and the anion of [BF4] or the hydroxy on the cation of [EOHMIm] in the [EOHMIm][BF4], which makes the silica microspheres adequately disperse in [EOHMIm][BF4]. Although the concentration of silica microspheres in [EOHMIm][BF4] is much higher, it displays a fluid-like materials as shown in the inset of Fig. 3. The viscosity of traditional STFs usually decreases slightly at low shear rates and it increases at high shear rates. However, the STF of the silica microspheres in [EOHMIm][BF4] exhibits a double shear thickening behavior at initial shear rates and high shear rates respectively. These novel shear thickening behaviors are rarely discovered in existing STFs. Moreover, this STF is reversible and it will recover to fluid-like material as soon as the applied shear stress is removed as shown in the inset of Fig. 3. Both the rheological properties and the viscoelastic properties of the STFs with different silica concentrations in [EOHMIm][BF4] are shown in Fig. 4 and Table 4, respectively. The shear thickening behavior of the STF with low concentration (50 wt % and 55 wt %) is weak. Both viscosities and shear stresses exhibits much low values at all shear rates. The shear thickening behavior is enhanced dramatically as the concentration increased. More particle clusters are constructed in the STF with high silica concentration, which may lead to the increase of viscosity [1]. The maximum viscosity reaches approximately 800 Pa·s in the STF with a concentration of 64 wt % silica microspheres as shown in Fig. 4(a), which is much higher than that in traditional 12

STFs composed of polyethylene glycol [22, 24]. The shear stress, storage modulus and loss modulus in the STF will increase significantly as soon as the shear thickening behavior happens, as shown in Fig. 4(b), (c) and (d). It is clear that both storage modulus and loss modulus are nearly constant before shear thickening. Especially for the maximum concentration of the STF, both of them even slightly decline with increasing of strain. It may result from the ordered arrangement of silica microsphere. The critical strain of the STF decreases for both storage modulus and loss modulus with increasing of concentration. Both storage modulus and loss modulus increase to as high as about 1000 Pa and 2000 Pa in the shear thickening region, respectively. This indicates that the STF can store and dissipate a vast amount of energy in the shear thickening region. Moreover, the strong hydrodynamic coupling between the silica microspheres results in the formation of silica microsphere clusters, and the sudden increase in the storage and loss modulus reveals that silica microspheres conglomerate as the shear thickening happened [42]. More clusters will form in the STF with high silica concentrations with the increase of shear rate, resulting in the enhanced shear thickening behavior. Meanwhile, some silica microsphere clusters will be broken as the shear stress increased, leading to the decline in the viscosity at high shear rates.

3.2 Stab resistant performance The stab resistant performance of the neat Kevlar fabrics and STKF against the knife impactor is shown in Fig. 5. The depth of the knife penetration increases as the 13

impact energy increased in most cases. The T-NKF-1 with 10 layers of neat Kevlar fabric is punctured absolutely by the knife, and the penetration depth on the backing is not less than 10 mm even under the minimum impact energy (4.41 J). It is clear that the stab resistance is improved remarkably for the targets composed of STKF. For T-STKF-2 with 8 layers of STKF, although the area density of it is lighter than that of T-NKF-1, it is punctured only under the maximum impact energy (13.23 J), and the penetration depth is less than 12 mm. The optimal stab resistance is obtained when the STKF contains 34.89 wt % STF. In this case, T-STKF-3 with 8 layers of STKF is never punctured, even at the maximum impact energy. Moreover, the area density of T-STKF-3 is nearly equal to that of T-NKF-1. However, the stab resistance of the STKFs declines seriously when they contain too much STF. The T-STKF-5 with 8 layers of STKF containing 98.45 wt % STF is punctured absolutely by the knife and the penetration depth on the backing is higher than that of most of the other targets composed of STKF. The penetration depth of the T-STKF-5 is slightly less than that of the neat Kevlar fabric under low impact energy and it is even higher than that of the neat Kevlar fabric under the highest impact energy. Moreover, the area density of T-STKF-5 is much higher than that of T-NKF-1. Fig. 6 shows the micromorphology and photographs for the damaged areas of the top layers of T-NKF-1 and T-STKF, respectively. In neat Kevlar fabric, smooth surfaces and clean filaments are observed as shown in Fig. 6(g). By comparison, the filament is rough and uniformly coated with the STF in the STKF as shown in Fig. 6(e), (h) and (i). Filaments are elongated and pulled out around the cut for the neat 14

Kevlar fabric as shown in Fig. 6(a). The cutting ends of the composites are messy and scattered as shown in Fig. 6(d). Some fibrils are peeled from the fractured filament as shown in the magnified image in Fig. 6(d). However, the filaments in both STKF-2 and STKF-4 are cut orderly and neatly as shown in Fig. 6(b) and (c), respectively. As shown in Fig. 6(e) and (f), the cutting end of the filament is neat in STKF-2 and STKF-4, and there are no fibrils at the cutting end as shown in the insert of cross section for the cut. This suggests that the STF makes filaments to be bundled and cut at the same time. As shown in Fig. 7, we can make ansatz that the STF disperses homogenously in the Kevlar fabric at original state and the shear thickening behavior of it will be triggered when filaments are compelled to move under impacting of the knife, and therefore some clusters of silica microsphere are produced between filaments or yarns around the impact point of the knife, as shown in the red circle in Fig. 7. Some mechanical interlockings between filaments or yarns will be formed under compression around the impact point, as shown in the yellow circle in Fig. 7, which leads to more filaments or yarns to load the external impact force at the same time. The smooth surface of the filament in the neat Kevlar fabric is easily shifted and fractured randomly under the impact energy. By comparison, most of the filaments sustain the impact energy at the same time in the STKF, which leads to the significant enhancement of stab resistant performance. In addition, the STF will store and dissipate some energy for the shear thickening in the STKF, which also enhances the stab resistance at some level [24]. As shown in Fig. 6(f) and (i), many surplus STF will attach on the surface of the filaments when the Kevlar fabric contains with too 15

much STF. The hardness of the silica microspheres is much higher than that of the Kevlar filaments. Therefore, some silica microspheres may be squeezed into the filaments under the impact energy, resulting in additional damage to the surface of the filaments. This may be why the ends of the damaged filaments are flattened as shown in Fig. 6(f). Therefore, the cut area in STKF-4 is obviously larger than that in STKF-2. This is consistent with the reported research results [21]. Compared with other STF-treated fabrics reported in the literature, the STKF displays much better performance in stab resistance under similar conditions, including the number of layers in the target, area density and quantity of impregnated STF [8, 13, 35].

3.3 Yarn pull-out behavior To deeply investigate the effects of the STF on the stab resistance of the STKF, the pull-out test for single warp yarn was performed as shown in Fig. 8(a). Some weft yarns are removed from the top and bottom of a sample, and approximately 10 cm of unabridged fabric in the middle of the sample is reserved. The holding lead of the pull-out yarn in the middle of the sample is hold by the upper clamp of the tensile testing machine. Except for the free tail of the pull-out yarn, all the yarns at the bottom of the sample are clamped in the under clamp. The gauge length between the two clamps is set at 12 cm. The pull-out force of the yarn is the accumulation of the friction between the warp and weft at the interlacing point, as shown in Fig. 8(b), and it consists of static friction and sliding friction, respectively. The crimping yarn is progressively straightened from the top to the bottom of the fabric in the static friction 16

stage. During this stage, the force on the yarn increases strongly and ascends to the maximum value. The pull-out force oscillates and gradually decreases with the increase of displacement. It is periodically oscillating because crimps on the pull-out yarn slip in and out through the interlacing point. As the displacement increases, the number of interlacing point decreases, and therefore, the pull-out force gradually declines. Compared with the neat Kevlar fabric, the pull-out force and the amplitude of it in different STKFs is significantly high, as shown in Fig. 8(c). As the loading of the STF increasing, the pull-out force increases, and the maximum force is about thirty-five times greater than that in the neat Kevlar fabric, as show in the insert of Fig. 8(c). These results indicate that the friction between the yarns is increased because of the addition of the STF, which has also been demonstrated by previous studies [9, 12, 13, 20-22, 24, 31, 32]. On the one hand, the increased viscosity of the STF gives rise to a higher friction of the pull-out yarn. On the other hand, the harder silica microspheres are able to embed into the softer filaments, leading to more direct mechanical coupling between the filaments and yarns [9], which also enhances the pull-out resistance of the yarn. The pull-out resistance of the STKF is higher than that in other STF-treated Kevlar fabrics in reported studies [9, 19, 21, 22, 24, 31], which may result from the unique double shear thickening behavior of this STF in the whole range of the shear rate. The increased pull-out force between the yarns dissipates extra energy and therefore enhances the impact resistance of the STKF. Therefore, the STKF has excellent stab resistance performance. However, stab resistance may be optimized at some intermediate degree of mobility. Relative movement between yarns 17

is excessively restrained under excessively high friction, which may result in stress concentration, which weakens the stab resistance of STKF-4.

3.4 Quasi-static tensile properties of the yarns The single yarn is separated, as shown in Fig. 9(a), and the quasi-static tensile strength test for the single yarn is shown in Fig. 9(b). The slope of the force-displacement curve for the single yarn in the STKF is slightly higher than that in neat Kevlar fabric, i.e., the tensile modulus of the yarn in the STKF is higher than that of the yarn in the neat Kevlar fabric. This indicates that the yarn in the STKF has a strong non-deformability under loading. In addition, the broken elongation of the yarn in the STKF is longer than that of the yarn in the Kevlar fabric. In particular, the tensile strength of the yarn in the STKF is noticeable higher than that of the yarns in the neat Kevlar fabric as shown in Fig. 9(d) and (e). The UTS is slightly enhanced with increasing the loading of STF. The maximum tensile strength of the yarn in the STKF is about 1.5 times higher than that of the yarn in neat Kevlar fabric. STF treated yarns can consume more energy under tensile loading because of their higher modulus, higher breaking elongation and higher tensile strength, which immensely enhances the impact resistance of the STKF. The transverse component force restrains all the filaments in the yarn under the strong axial tensile force as shown in Fig. 9(c). We can make an ansatz that many shear fields will be formed between the filaments in the yarn of the STKF when adjacent filaments generate relative slip under external work as shown in Fig. 9(c). It 18

is the objective that the STF completely and uniformly fills the interstices between the filaments as shown in Fig. 6(h) and (i). The shear thickening behavior of this STF will be triggered as soon as the shear field is formed. In this case, the pull-out resistance of the filaments increases with the rise in viscosity in the STF around the filaments. In the meanwhile, some silica microspheres will be jammed into a bump in the confined shear field between the filaments for the shear thickening, as circled in blue in Fig. 9(c). Hard silica microspheres in clusters embedded in the filaments are under compression, and they provide direct mechanical coupling between the filaments, which makes more filaments to overcome the tensile loading at the same time. Therefore, the tensile strength of the yarn is enhanced. A higher tensile force is required to overcome the greater interlocking between the yarns when STF loading increases in the STKF.

3.5 Quasi-static tensile properties of fabrics Quasi-static tensile tests of fabrics are carried out as shown in Fig. 10(a). An original specimen of 15 cm by 15 cm was cut at first, and then some yarns along the tensile orientation were taken apart to ensure that the specimen was clamped along the width direction, and a specimen of 15 cm by 10 cm was recommended. Some yarns are taken apart from both sides of the sample according to standard code. The tensile strength of the STKFs is remarkably higher than that of the Kevlar fabric as shown in Fig. 10(d) and (e), and it is enhanced with the addition of the STF increasing in the STKF. The maximum tensile strength of the STKFs is about two times greater than 19

the tensile strength of the untreated Kevlar fabric. The slope of the tensile force-displacement curve of the STKFs is noticeable higher than the slope of the tensile force-displacement curve of the neat Kevlar fabric, as shown in the inset of Fig. 10(d), which indicates that the STKF has a higher tensile modulus. Furthermore, the breaking elongation of the STKFs is nearly equal to that of the neat Kevlar fabric. Therefore, the STKF can consume more energy under tensile loading, which is also beneficial for increasing the stab resistance of the STKF. The tensile strength of the yarn and Kevlar fabric is improved because of the introduction of the STF. However, the response of the fabric to tensile loading is different from that of the yarn. On the one hand, there is strong friction between the warp and weft yarn at the interlacing point due to the addition of STF in the STKF. On the other hand, the crimping warp yarn in Kevlar fabric is straightened under the strong tensile stress, and the transverse component force on the warp yarn will be generated at the interlacing point as shown in Fig. 10(b). Restrained by the weft yarn, the warp yarn is squeezed at the interlacing point as shown in Fig. 10(c). As a result, the mechanical interlocking between the silica microspheres and filaments in the yarn of the STKF is enhanced due to the squeezing force. Therefore, the improved effectiveness of the STF on the tensile strength of Kevlar fabrics is better than that on yarns.

3.6 Properties of conductivity, responsiveness, and EMI SE for the STKF In addition, in contrast to other traditional STFs, the dispersion medium of the 20

STF in this work is ILs, which endows excellent conductivity to these novel STFs. As shown in Fig. 11, the conductivity of [EOHMIm][BF4] reaches approximately 7000 µS/cm, and it declines dramatically with the introduction of too much silica microspheres. The conductivity of the STF decreases with the increase of silica microsphere concentration. The ionic migration of ILs in the STF may be confined to a high concentration of silica microspheres [50]. However, the STF with a concentration of 64 wt % silica microspheres still has a good conductivity of about 2500 µS/cm. Different from the method of depositing carbon nanotubes (CNT) on the surface of the STF-treated Kevlar fabric in [26], and the CNT may be easy to fall off, while the conductive STF-treated Kevlar fabric composite is constructed by using the good conductivity of ILs in ours work. As shown in the inset of Fig. 12, the LED bulb lightens up when the circuit composed of the STKF is connected, which suggests that the STKF possesses conductivity. The electrical circuit is formed in the STKFs because the STF is homogenously filled in the Kevlar fabric as shown in Fig. 6 (e), (f), (h) and (i). The normalized electrical resistance (∆R/RO) single will response instantly and periodically to the repeatedly deformation of the STKF as shown in Fig. 12. ∆R/RO of the STKF is constant at stationary state. It will change in range of - 0.7 % to + 0.6 % as soon as the small-angle bending is applied, and ∆R/RO will increase as soon as the bending angle is increased. These indicate that the electrical resistance of the STKF has a high sensitivity to the applied deformation, and we can monitor the motion of warfighter whose armor composed of the STKF. This provides a chance to 21

exploit novel soft armor with both excellent protective performance and intelligent wearable property. However, the conductivity both of the STKF-1 and STKF-2 is too low to monitor, so that the balance between impact resistance and the conductivity of the STKF should be investigated in future. The sample is composed of 3 layers of fabric stacked to avoid the effect of fabric windowing, and the results of the EMI SE are shown in Fig. 13. Compared with the neat Kevlar fabric, the STKFs containing different STFs present a higher level of EMI SE. Moreover, the EMI SE is enhanced with increasing the addition of the STF. It is possible that a conductive network is constructed in the STKF when the STF is completely coated on the filaments and filled in the gaps between the filaments. For most of polymer EMI shielding composites, the shielding frequency of electromagnetic wave is X-band (8.2–12.4 GHz) [51]. For the structure of fabrics [52], the EMI SE increases significantly around the frequency of 16 GHz, and it reaches as high as 27 dB for STKF-4, which means more than 99 % shielding efficiency.

4. Conclusions Different hydrophilic and hydrophobic ILs were selected as the dispersion medium to make the STF. Rheological experiments reveal that only [EOHMIm][BF4] can be used to prepare STF with monodispersed silica microspheres. The STF of the silica microsphere in [EOHMIm][BF4] exhibits a unique double shear thickening behavior both at low shear rates and high shear rates. This novel behavior has never been discovered in the existing STFs. Furthermore, the shear thickening of this STF is 22

reversible and it is enhanced with concentrations of silica microspheres. Both the stab resistant performance and mechanical properties of Kevlar fabric are improved significantly for the present of this STF. The stab resistance of the target composed of 8 layers of STKF is better than that of the target composed of 10 layers of the Kevlar fabric when the loading of the STF is appropriate. With increasing the addition of the STF, the stab resistance of the STKF is enhanced, and the optimal stab resistance is obtained when the STKF contains 34.89 wt % STF. However, the stab resistance of the STKF declines seriously when the STKF contains too much STF. The maximum friction between the yarns in the STKF is dozens of times higher than that in the neat Kevlar fabric. Compared with the neat Kevlar fabric, the tensile strength of the STKF and its yarn is approximately increased by 2 times and 1.5 times, respectively. Moreover, the elastic modulus of Kevlar fabric is enhanced by the addition of STF. These help to enhance the stab resistance of the STKF. Both filaments and yarns are restrained due to the shear thickening effectiveness and mechanical coupling between the filaments and particles, which is the essential reason for the improvement of the Kevlar fabric. For the unique double shear thickening behavior of this STF, the impact resistance of the STKF is better than that of the STF-treated Kevlar fabric in previous similar studies. In addition, introducing ILs as the dispersion medium endows the STF with good conductivity. The conductivity reaches 2500 mS/cm for the STF with 64 wt % concentration of silica microspheres. The electrical resistance of the STKF has a high sensitivity to the applied deformation, which can be used to monitor the motion of 23

human body. Furthermore, the STKF has a level of EMI SE for the good conductivity of the STF, and the EMI SE even reaches 27 dB around the frequency of 16 GHz. These provide a chance to exploit the novel next generation of soft armor with both excellent protective performance and intelligent wearable property.

Conflicts of interest There is no conflict of interest to declare.

Acknowledgement We acknowledge our colleagues for their instructive help during experimental measurements. We are grateful for the financial supported by the Fundamental Research Funds for the Central Universities (310201911cx015). We thank the Analytical & Testing Center of Northwestern Polytechnical University for supporting of the equipment. We thank the postdoctoral research station of Northwestern Polytechnical University for support in work and life.

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[52] Afzali A, Mottaghitalab V, Seyyed Afghahi SS, Jafarian M, Atassi Y. Electromagnetic properties of absorber fabric coated with BaFe12O19/MWCNTs/PANi nanocomposite in X and Ku bands frequency. Journal of Magnetism and Magnetic Materials, 2017, 442: 224-230.

27

Tables: Table 1 Parameters of STKF Dimension: Length (cm) Label

Areal density STF (Wt %) (g/m2)

by width (cm)

Neat Kevlar fabric

15×15

0.00

240.00

STKF-1

15×15

15.77

277.85

STKF-2

15×15

34.89

323.74

STKF-3

15×15

53.49

368.38

STKF-4

15×15

98.45

476.28

Table 2 Parameters of target for dynamic stab resistant test Dimension: Length Label

Number of

Areal density

layers in target

(g/m2)

STF (Wt %) (cm) by width (cm)

T-NKF-1

15×15

0.00

10

2400.00

T-STKF-2

15×15

15.77

8

2222.78

T-STKF-3

15×15

34.89

8

2589.89

T-STKF-4

15×15

53.49

8

2947.01

T-STKF-5

15×15

98.45

8

3810.24

28

Table 3 Impacting parameters for the stab resistant test Drop mass (kg)

Drop height from the

Theoretical impact

Theoretical impact

tip of the knife (cm)

velocity (m/s)

energy (J)

10

1.40

4.41

15

1.71

6.62

20

1.98

8.82

25

2.21

11.03

30

2.42

13.23

4.50

Table 4 Rheological and viscoelastic results for STF with different concentrations Concentration

Maximum

Maximum Shear

Maximum Storage

Maximum Loss

(wt %)

Viscosity (Pa·s)

Stress (Pa)

Modulus (Pa)

Modulus (Pa)

50

0.58

49.90

0.64

4.86

55

2.89

287.00

4.55

26.20

60

43.30

3105.92

10.30

55.20

62

130.00

6098.40

28.20

122.00

64

818.00

8030.00

748.00

1980.00

29

Figures:

Fig. 1 (a) SEM image of silica microspheres, and (b) Chemical structures of ILs

Fig. 2 (a) Stab resistant testing apparatus, (b) Construction of stabbing target, (c) Target holder, and (d) The knife for the stab resistant test

30

Fig. 3 Shear rate dependencies of viscosity for suspensions with different concentrations of silica microsphere in different ILs

Fig. 4 Shear rate vs. (a) viscosity and (b) shear stress, and strain vs. (c) storage modulus and (d) loss modulus for STFs with different concentration of silica microsphere in [EOHMIm][BF4], respectively 31

Fig. 5 Impact energy dependencies of the penetration depth of the knife on the backing for different targets

32

Fig. 6 Photographs and SEM images of the cuts for top layer in (a), (d) and (g) the T-NKF-1; (b), (e) and (h) the STKF-2; and (c), (f) and (i) the STKF-4 after the dynamic stab test with impact energy of 13.23 J, respectively. Inserts in (d), (e) and (f) are cross section of the cut

Fig. 7 Schematic illustration of the ant-impact mechanism of STKF under impact of the knife 33

Fig. 8 (a) Photograph of single yarn pull-out test, (b) Mechanical schematic illustration of yarn pull-out test, and (c) Pull-out force vs. displacement curve for the single yarn of the STKF and the neat Kevlar fabric, respectively. Insert: the maximum pull-out force for different samples.

34

Fig. 9 (a) Method of single yarn tear out from Kevlar fabric; (b) Tensile test for single yarn; (c) Mechanical schematic illustration of filaments in tensile test of yarn; (d) Force vs. displacement curves and (e) Tensile strength for different yarns

35

Fig. 10 (a) Tensile strength test for fabric; (b), (c) Mechanical schematic illustration of yarn in tensile test of fabric; (d) Tensile force vs. displacement curves, and (e) Tensile strength for the STKF and the neat Kevlar fabric

36

Fig. 11 Conductivity for [EOHMIm][BF4] STFs with different concentrations of silica microsphere

Fig. 12 Normalized electrical resistance versus time for the STKF-3 during different bending circles and static non-deformation, respectively 37

Fig. 13 EMI SE dependencies of frequency for the STKF and the neat Kevlar fabric

38

Author statement

Manuscript title: soft armor materials constructed with Kevlar fabric and a novel shear thickening fluid

Jianbin Qin: Conceptualization, Methodology, Investigation, Data Curation, Writing of Original Draft, Funding acquisition. Borui Guo: Investigation, Data Curation. Le Zhang: Resources. Tianwei Wang: Investigation. Guangcheng Zhang: Conceptualization, Methodology, Supervision, Project administration. Xuetao Shi: Writing of Review & Editing, Visualization.

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: