Shear thickening fabric composites for impact protection: a review

Abstract

Fabrics with proper mechanical properties and high structural designability are frequently selected as the reinforcement of flexible composites. Shear thickening materials (STMs), including shear thickening fluid and shear thickening gel, are dense colloidal suspensions exhibiting an abrupt increase in viscosity with increasing shear rate. Shear thickening fabric composites (STFCs) which combined STM with fabrics, have been increasingly in demand for impact protective applications due to their tremendous potential. Herein, the characteristics of STM and the effects of fabric reinforcement on STFC impact behaviors are discussed for optimizing their properties and structures. Subsequently, the impact behaviors of STFCs obtained by multiple testing approaches with different impact threats are reviewed systematically. Furthermore, a comprehensive summary of the existing literature on stab, drop-weight, split Hopkinson pressure bar, and ballistic impact protective mechanisms of STFCs is presented. It is found that based on the proper selection of STM types, thorough mechanistic explanations of STFC impact behaviors are instructive and could facilitate the development of wearable protective equipment. The purpose of this critical review is to provide a framework for comparison of the effective impact resistances of STFCs across many applications and to discuss issues for future research.

Keywords

Shear thickening material fabric reinforcement composite impact behavior protective mechanism

There are constant wars and conflicts in today's world, and accidents occur frequently in people's daily life. External impacts cause harm to the human body to different degrees, such as sports injuries, car crashes, sharp knife or bullet impact, which make the wearable protective equipment of the human body particularly important. Crucially, three main requirements of this equipment are flexibility, light weight, and impact resistance.

Highly designable fabric structures woven by flexible fibers or yarns can realize various functions, so they are ideal materials for wearable applications. Although combining a polymer matrix with fabric has been proved to be a practical approach to enhance the impact resistance and energy absorption capacity of the fabric, these composites are usually heavy and inflexible, which limits body movement of the wearer.^1–3 Given the above, shear thickening materials (STMs), including shear thickening fluid (STF) and shear thickening gel (STG), have attracted great attention due to their flexibility and unique impact responsiveness. STMs are dense colloid suspensions characterized by a remarkable increase in viscosity when the applied shear rate increases.^4–6 Notably, other types of matrices cannot achieve this “liquid-to-solid” behavior. From this, the flexible shear thickening fabric composites (STFCs) fabricated by adding STM to fabrics could intelligently enhance the impact resistance of fabrics. Currently, the performance of STFCs has been extensively investigated by conducting different testing approaches, especially ballistic impact. However, the impact behaviors and protective mechanisms under different impact conditions still lack a complete summary and discussion. Besides, STM as the matrix has always been the research focus, while the importance of the reinforcement (e.g., fabric) on STFC impact behaviors is neglected.

In this paper, rheological properties and shear thickening mechanisms of STM are introduced, and influencing factors on the impact behaviors of STFCs are discussed. Obtained by testing approaches with various impact threats, the impact behaviors and protective mechanisms of STFCs based on previous studies are summarized completely to provide a better understanding of their functions. This paper aimed to promote the research and development of STFCs for body wearable protective equipment to some extent.

Shear thickening material

Initially, the shear thickening phenomenon was defined as a problem in industrial processes such as coating and mixing because of jamming in small openings, and overloading mixers, thereby limiting the process rate.⁷ Later, researchers began to study this effect due to its particularity. STF and STG are two typical types of STM, which differ in their natural state and thickening mechanism, and have been widely applied in engineering fields.

Shear thickening fluid

STF is a special class of field responsive non-Newtonian fluid; its viscosity increases dramatically when the shear stress or shear rate increases to a critical value. The critical shear rate is the value of the shear rate corresponding to the beginning of the viscosity increase. When it is subjected to external impact, STF thus exhibits a transition from a liquid to a solid-like state rapidly and returns to the initial liquid state when the external force disappears.⁸

STF is generally formed by dispersed particles (silicon dioxide, polymethyl methacrylate, calcium carbonate, starch, etc.) and a dispersed medium (ethylene glycol, polyethylene glycol (PEG), water, etc.). The dispersed particles in the medium are stabilized by Brownian motion and charge action. The rheological property of STF is affected by many factors, for example, the volume fraction,^9,10 hardness,¹¹ roughness,¹² modification,¹³ aspect ratio,^7,14 size,¹⁵ and size distribution¹⁶ of the dispersed particles, and the properties of the dispersed medium.¹⁷ Moreover, external factors include temperature¹⁸ and additives.^19,20 For all this, there are still many works on the influencing factors worth studying, such as the influence of the interaction between two or more factors on the rheological properties of STF.

On account of its unique performance, STF has generated many applications in vibration control systems,²¹ body protection equipment,²² damping devices,²³ polishing technology,²⁴ and so on. As practical applications for impact protection, soft body armors²⁵ have been produced by using STF-impregnated high‐performance fabrics. The effectiveness of the thickening behavior of STF strengthened the impact resistance of fabrics; meanwhile, these armors were lightweight. However, some shortcomings still appeared in the application process of STF. The dispersed medium is usually a hygroscopic solution, which results in the weakening of the shear thickening effect because of water absorption after prolonged exposure to the air. Particles also tend to precipitate in STF, leading to stratification. In addition, the fluidic STF is difficult to encapsulate, and will inevitably be lost from its composites without any protection. Consequently, some modifications are necessary for maintaining the stability and thickening effect of STF, which will promote it to play a greater role in practical applications.

Over the past few decades, several theories and models have been put forward to explain the shear thickening mechanism of STF. Hoffman^26,27 established the basis for modeling STF flow instabilities in terms of torque balance and energy balance, and firstly proposed the order–disorder transition (ODT) as the shear thickening mechanism by observing the light diffraction patterns of the system (Figure 1(a)). The particles are of ordered packing at the initial small shear rates on account of intrinsic inter-particle repulsive interaction. When the shear rate is above a critical value, the hydrodynamic forces on the particles destabilize the layered structure and make them concentrate on each other. A transition takes place from a two-dimensional (2D) ordered state to a three-dimensional (3D) disordered state, and a drastic increase appears in the suspension viscosity. Nevertheless, the above mechanism suggested that the shear thickening phenomenon occurs only when the particles have a layered orientation, and the viscosity increases due to disordered particles within the suspension.

Figure 1.

Shear thickening mechanisms of shear thickening fluid. (a) Light scattering patterns before and after shear thickening, which drew out the order–disorder transition theory (Copyright 1972, AIP Publishing).²⁶ (b) Hydro-clustering theory (Copyright 2009, AIP Publishing).⁴ and (c) Contact networks of the contact rheology model along the suspension with particle loading of 50%. Frictionless contacts are drawn in gray segments joining the centers of the two involved particles, while frictional contacts are drawn in red (Copyright 2014, AIP Publishing).³² CST: continuous shear thickening; DST: discontinuous shear thickening. (Color online only.)

Later, the hydro-clustering theory^28–30 (Figure 1(b)) was introduced. Bossis and Brady³⁰ reported that the shear thickening phenomenon is not fully dependent on an ordered orientation, because thickening can take place with particle clusters extending in different directions while hydrodynamic forces dominate the particles. The existence of particle clusters impedes fluid flow and increases the viscosity of the system. Unfortunately, this “clustering” structure is unstable, and the spatial network collapses immediately after the external shear rate is decreased or withdrawn. Eventually, the viscosity reduces rapidly and returns to the initial state.

Recently, the contact rheology model^19,31 (Figure 1(c)) was proposed to explain the shear thickening phenomenon. Mari et al.³² reported that hydro-clustering dominates suspensions at small shear rates on account of non-contact rheology. As the shear rate increases, friction is increasingly incorporated as more contacts form, leading to a transition from a mostly frictionless to a mostly frictional rheology. Contact forces are of high effectiveness at the jamming point at which particles meet each other under large shear rates. A rise above the critical shear rate makes the contact forces form thickening-dominating lattices with inadequate hydrodynamic interaction.

Although the detailed mechanism of shear thickening behavior is still under debate, the hydro-clustering theory is the most accepted mechanism to explain mild shear thickening behavior at the onset of thickening. In general, further studies on key factors are needed to understand the shear thickening mechanism, for instance, the critical shear rate, thickening ratio, and critical value for the continuous–discontinuous shear thickening transition.

Based on shear thickening mechanisms, many constitutive functions of STF have been proposed. The parameters in these constitutive functions were fitted with the viscosity data of STF to quantitatively predict the shear thickening behavior. The “power-law model” was the most commonly used constitutive function of STF, as shown in Equation (1)

η (γ) = {m γ}^{n - 1}

(1)

where η is the apparent viscosity of STF, γ is the shear rate, m is the consistency factor related to the properties of STF, and n is the power-law index. It is important to note that n < 1 corresponds to shear thinning behavior, n = 1 corresponds to linear viscous behavior, and n > 1 corresponds to shear thickening behavior. Specifically, when 1 < n < 2, STF could display continuous shear thickening (CST) behavior (Figure 1(c) top); when n ≥ 2, discontinuous shear thickening (DST) behavior is displayed (Figure 1(c) bottom). Thus, n is closely related to the nonlinear viscous behavior of STF, which has variations in different compositions of STF. This function described the tendency of the instantaneous shear thickening effect to increase exponentially in power, but it was difficult to use to describe the shear thinning process before and after the thickening stage. To address this problem, Galindo-Rosales et al.^33,34 proposed a segmental constitutive function based on the power-law model, as shown in Equation (2)

η (γ)

= \{\begin{array}{l} η_{I} (γ) = η_{c} + \frac{{η_{0} - η}_{c}}{1 + {[λ_{I} (\frac{γ^{2}}{γ_{c} - γ})]}^{n_{I}}} for γ \leq γ_{c} \\ η_{II} (γ) = η_{\max} + \frac{{η_{c} - η}_{\max}}{1 + {[λ_{II} (\frac{γ - γ_{c}}{γ_{\max} - γ}) γ]}^{n_{II}}} for γ_{c} \leq γ \leq γ_{\max} \\ η_{III} (γ) = \frac{η_{\max}}{1 + {[λ_{III} (γ - γ_{\max})]}^{n_{III}}} for γ_{\max} \leq γ \end{array}

(2)

where λ_i and n_i represent the constant parameter and dimensionless constant of each stage of characteristics of STF, respectively, η₀ represents the initial viscosity, η_c and γ_c represent the critical viscosity and critical shear rate, respectively, and η_max and γ_max represent the maximum viscosity and maximum shear rate, respectively. When γ ≤ γ_c, STF showed shear thinning behavior and its viscosity dropped to the lowest value η(γ_c) = η_c; when γ_c ≤ γ ≤ γ_max, STF showed shear thickening behavior until its viscosity reached the maximum value η(γ_max) = η_max; when γ_max ≤ γ, STF again showed shear thinning behavior. This constitutive function predicted the rheological behavior of STF more accurately, but it had 11 independent parameters and the fitting process was operationally complicated. Therefore, for accurate and convenient prediction of the shear thickening effect of STF, the existing constitutive function still needs to be improved.

Shear thickening gel

STG is a viscoelastic material whose mechanical properties, such as the storage modulus, elastic modulus, and yield stress, are enhanced by applying external forces.³⁵ As the strain rate increases, STG also changes from a viscous liquid to a rubbery state and then becomes a glassy state. During this transition, the external energy is absorbed against self-deformation.³⁶

Houston³⁷ studied the micro-scale mechanical properties of a viscoelastic material, one that is often referred to as a “solid liquid.”. Goertz et al.³⁸ investigated the influence of temperature on STG, and they found that the mechanical properties shifted significantly with respect to temperature. Wang et al.³⁹ reported high strain-rate compressing behavior of a magnetically responsive shear-stiffening gel (MSTG). The elastic modulus of the MSTG increased sharply with the excitations, which exhibited the enhancement of properties. Owing to the excellent capacities of energy absorption and resistance, STG exhibited broad potential in aircraft, automotive components, and damping, as well as for military protection measures.^40,41 In particular, D3O products were successfully utilized to introduce STG into polyurethane (PU) for developing commercial products, such as body protective clothing and head protection.⁴²

The behavior of “B-O cross bonds” inside STG is commonly considered the main shear thickening mechanism of STG. Zhang et al.⁴³ reported the shear thickening mechanism of STG fabricated by pyroboric acid and dimethyl siloxane, and “B-O cross bonds” are formed among molecular chains (Figure 2(a)). At a low strain rate, molecular chains in STG have enough time to relax and disassemble the entanglements. The breakage and reformation of transient “B-O cross bonds” allow the network certain internal mobility. At a high strain rate, disordered polymer molecular chains slip more severely and cannot adjust themselves to dynamic loadings. Consequently, large numbers of “B-O cross bonds” are broken to endure the external stimuli and dissipate impact energy, which results in the significantly increased storage modulus of STG.

Figure 2.

Likewise, Tang et al.⁴⁴ explained the bullet-proof mechanism of STG with the “jamming” theory (Figure 2(b)). Since the breaking velocity of “B-O cross bonds” is lower than the deformation velocity under high-velocity bullet impact, it cannot have enough time to break. As a result, the molecular chains are severely entangled to enhance the inter-molecular force and “jamming” effect, which greatly increases the storage modulus and loss modulus of STG. Interestingly, the ballistic impact load acting on the STG presents the distribution law of attenuating from the impact area to the surroundings. According to the response of STG to impact loads, the entanglement of molecular chains also shows a distribution law from dense to sparse.

Compared with STF, STG is considered an ideal protective material with better stability and easier encapsulation, and it thus has a wider application prospect in shear thickening composites. Nevertheless, the study of STG performance is not systematic and deep enough, and its responsiveness in complex impact environments remains to be confirmed. Besides, there is no unified standard for the characterization of STG performance, which interferes with the test results, and further development of high-performance STG is hindered.

With the continuous researches, various STMs with different compositions have emerged. To facilitate the reader's understanding, the qualitative and quantitative compositions of most of the STMs mentioned in this paper are listed in Table 1.

Table 1.

The composition of shear thickening materials (STMs) in previous studies

STM type	Compositions			Reference
STM type	Dispersed particle	Particle concentration	Dispersed medium	Reference
STF	SiO₂ (446 nm)	40 wt%	EG	Lee et al.⁴⁵
	SiO₂ nanospheres,SiO₂ microspheres	56, 60 vol%	PEG 200, EG	He et al.⁴⁶
	SiO₂ (500 nm)	63 wt%	PEG 200	Ghosh et al.⁴⁷
	SiO₂ (100 nm)	65 wt%	PEG 400	Kim et al.⁴⁸
	SiO₂ (40 nm)	20, 25, and 30 wt%	PEG 200	Lu et al.⁴⁹
	SiO₂ (20 nm)	20 wt%	PEG 400	Gürgen⁵⁰
	SiO₂ (500 nm)	65 wt%	PEG 200	Mawkhlieng and Majumdar⁵¹
	SiO₂ (12 nm)	20 wt%	PEG 200	Wei et al.⁵²
	SiO₂ (13–22 nm)	23, 33, 35, 40, and 43 wt%	EG	Caglayan et al.⁵³
	SiO₂ (12 nm)	2 wt%	PEG 200	Sun et al.⁵⁴
	SiO₂ (450 nm)	52 vol%	PEG 200	Decker et al.⁵⁵
	SiO₂ (100, 650 nm)	15, 20, 25, 70, 72, and 75 wt%	PEG 200, PEG 400	Liu et al.⁵⁶
	SiO₂ (10–30 nm)	40 wt%	PEG 200	Shih et al.⁵⁷
	SiO₂ (100 nm)	65 wt%	PEG 200	Majumdar et al.⁵⁸
	SiO₂ (12 nm)	13, 18, 23, and 28 wt%	PEG	Lu et al.⁵⁹
	SiO₂ (600 nm)	65 wt%	PEG 200	Chatterjee et al.⁶⁰
	SiO₂ (100 nm)	65 wt%	PEG 200	Arora et al.⁶¹
	SiO₂ (100 nm)	60 wt%	PEG 200	Laha and Majumdar⁶²
	SiO₂ (45 nm)	65, 68, and 70 wt%	PEG 200	Park et al.^63,64
	SiO₂ (40 nm)	35, 40, and 45 wt%	PEG 400	Fahool and Sabet⁶⁵
	CaCO₃, SiO₂	0, 25, 50, 75, and 100 wt%	PEG	Avila et al.⁶⁶
	SiO₂ (75 nm)	20 wt%	PEG 200	Zhang et al.⁶⁷
	SiO₂ (500 nm)	50–64 wt%	Ionic liquids	Qin et al.⁶⁸
	SiO₂ (500 nm)	38 wt%	EG, PEG	Li et al.⁶⁹
	SiO₂ (370 μm)	57 vol%	PEG 400	SampathKumar et al.⁷⁰
	SiO₂ (300 nm)	70 wt%	PEG 400	Zhang et al.⁷¹
	SiO₂ (100 nm)	67.5 wt%	PEG 400	Asija et al.⁷²
	SiO₂ (12 nm)	30, 40, 50, and 60 wt%	PEG 400	Hasan-nezhad et al.⁷³
	SiO₂ (200–300 nm)	15, 18, and 20 wt%	PPG	Hao et al.⁷⁴
	SiO₂ (12 nm)	20 wt%	PEG 400	Jeddi et al.⁷⁵
	SiO₂ (300 ± 100 nm)	56–65 wt%	PEG 200	Zhang et al.⁷⁶
	Styrene/acrylate (300–320 nm)	58.8 vol%	EG	Cheng et al.⁷⁷
	PSt-EA nanospheres	54, 56, and 59 vol%	EG	Cao et al.⁷⁸
	SiO₂ (343 nm)	52 vol%	PEG 400	Lomakin et al.⁷⁹
	SiO₂ (500 nm)	10, 20, 30, and 40 wt%	PEG	Mahesh et al.⁸⁰
	SiO₂ (500 nm);PMMA (1050 ± 110 nm)	52 vol%; 49 vol%	PEG 200	Kalman et al.¹¹
	Cornstarch (5–20 mm)	55 wt%	Water	Haris et al.⁸¹
STG	Boric acid, dimethyl silicone oil, and ethanol			He et al.⁴⁶
	Boric acid, dimethyl silicone oil, and ethanol			Zhao et al.⁸²
	Boric acid, polydimethylsiloxane			Liu et al.⁸³
	Boric acid, hydroxyl silicone oil			Zhao et al.⁸⁴

STF: shear thickening fluid; STG: shear thickening gel; SiO₂: silicon dioxide; CaCO₃: calcium carbonate; PSt-EA: polystyrene ethyl acrylate; PMMA: polymethyl methacrylate; EG: ethylene glycol; PEG: polyethylene glycol; PPG: poly propylene glycol.

Influencing factors of fabric reinforcement

Since having a certain fluidity, STM is usually applied in composites combined with reinforcement materials. Impact resistance of the composite based on STF rheological properties has been conducted variously so far, for example, microcapsule structure,^85,86 honeycomb skeleton structure,⁸⁷ lattice sandwich structure,^88–90 fabric structure,^68,91 cork structure,^92,93 and PU foam structure.^81,94,95 Hence, the impact behaviors of STFCs are significantly affected by many factors in fabric reinforcement, which deserve more attention and are highlighted in this section.

Fiber property

Fabric reinforcements are usually made of high-performance fibers, such as aramid, ultra-high molecular weight polyethylene (UHMWPE), glass fibers, and so on, which are known to possess high strength, large modulus, low density, and high energy absorption capacity.

Quite a few researchers have employed many aramid-based materials for the impact resistance of STFCs, including Kevlar and Twaron. Lee et al.⁴⁵ firstly combined STF with Kevlar woven fabric to develop soft armor, and they found that STF significantly improved the ballistic performance of Kevlar fabric. Inspired by this study, various in-depth investigations on the STF/Kevlar fabric were carried out. He et al.⁴⁶ fabricated a STF/STG fabric composite with high protective performance by selecting Kevlar fibers. The STF and STG effectively incorporated into the gap of the filaments and increased the friction between yarns. Ghosh et al.⁴⁷ demonstrated that STF-treated Kevlar fabric showed a better capacity of energy absorption. Kim et al.⁴⁸ evaluated the ballistic performance of two different fabrics (Kevlar and Heracron) treated with STF; these fabrics exhibited superior ballistic characteristics and a higher level of friction between fibers. Lu et al.⁴⁹ selected Twaron aramid yarn to evaluate the effectiveness of STF-impregnated fabric panels. It was found that the projectile velocities of perforating fabrics were decreased by STF impregnation due to the total movement constraint of the primary yarns. Gürgen⁵⁰ also indicated that STF treatments could provide an additional energy absorption capacity for Twaron fabric subjected to high-velocity impacts. As aramid fiber performance shows a light weight, satisfactory abrasion, high melt point, flexural strength, and fatigue resistance,⁹⁶ the fabrics have become the most popular reinforcements for STFCs.^97–100 Nevertheless, these fibers also have some shortcomings, such as low compressive strength, large moisture absorbency, and difficulties in processing.

In addition, other high-performance fibers were also used as reinforcements of STFCs. UHMWPE fibers have advantages including a low melting point and no water absorption. Thus, Mawkhlieng and Majumdar⁵¹ designed soft body armor by using STF-impregnated fabrics woven from UHMWPE yarns and very high modulus aromatic polymer (VHMAP) yarns. Wei et al.⁵² investigated the low-velocity impact behavior of a glass fiber-reinforced polymer fabric impregnated with STF, and the results revealed that the maximum resistive force and energy absorption capacity of the glass fiber-reinforced polymer were improved by STF. Caglayan et al.⁵³ utilized four plies of twill carbon fiber fabrics for each side of the face sheets, and STF/PU foams were placed between these fabrics. This composite exhibited a higher energy absorption rate and lower damage width compared to its neat counterpart.

For comparison, some studies have been carried out to compare the impact behaviors of STFCs based on different types of fibers. Sun et al.⁵⁴ evaluated the resistive force and energy dissipation of STFCs, and the fabric was weaved by basalt, carbon, or glass fiber. For basalt and carbon fiber, STF only improved the mechanical performance of their composites, while for glass fiber, STF changed the mechanical mechanism. In addition, Decker et al.⁵⁵ implemented stab resistance tests of STF-treated Kevlar and nylon fabrics, and found that they all exhibited significant improvements over neat fabrics of equivalent areal density. However, nylon fibers were more likely to stretch, so the penetrator was allowed to travel through the fabric, and then fibers reversibly recovered after the penetrator was removed. Compared with Kevlar fabrics, nylon fabrics were also more likely to exhibit yarn fracture due to their lower tenacity. This demonstrated that the beneficial effects of STF addition were not restricted to Kevlar fabrics.

Meanwhile, the selection of high-performance fibers involved a high cost and complex manufacturing process. Considering the economic aspects, fabrics made of the most commonly available synthetic polypropylene fibers were coated with STF for soft armor application.^70,101 In particular, environmental natural fiber, such as jute fiber, has also been used for the development of STFCs.⁸⁰

STFCs are generally applied in impact protection, and thereby the mechanical properties of their reinforcement are critical. As a typical high-performance fiber, aramid fiber has been one of the important raw materials of STFCs. The above studies showed that other high-performance fibers could also play an important role in addition to aramid fiber. Nonetheless, the type of fibers for fabric reinforcement should also be selected according to the cost budget or application requirements of products.

Fabric structure

Fabrics are obtained by knitting, braiding, tufting, stitching, weaving, or non-woven production, which provide variable structures and properties. Textile composites are made by matrices and multiple layers of interlaced or laminated fabrics. Meanwhile, fabrics can transfer their outstanding properties to their composites. In engineering applications, high-strength fiber woven fabrics are extensively employed in structural components subjected to impacts, and their properties are significantly related to the fabric structure.

Two-dimensional woven fabrics are frequently employed in STFCs, especially the plain weave structure (Figure 3(a)). Plain weave fabrics have more interlacing points than twill or satin fabrics of the same surface density. Liu et al.⁵⁶ conducted ballistic impact tests on plain woven fabrics impregnated with different STFs. Compared to neat fabrics with a loose structure, STF/fabrics under impact were more compact and acted as an integrated structure. Owing to the increased inter-yarn friction, less yarn slippage occurred during the impact. The results proved that STF could enhance the anti-impact performance of fabric. He et al.⁴⁶ also reported that the impact resistance of plain woven fabric was improved by impregnating STF, and STG in the fabric composites not only improved the resistance of fabric but also protected the STF. Although plain woven fabric has been widely utilized as a reinforcement of STFCs,^69,98,102 since STMs are colloidal suspensions, they tend to run off when combined with plain woven fabric and then affect the display of impact resistance.

Figure 3.

Various fabric structures of shear thickening fabric composites: (a) plain woven structure (Copyright 2018, Elsevier)⁴⁶; (b) laminating and honeycomb structure (Copyright 2021, Springer Nature)⁵⁷; (c) three-dimensional orthogonal woven structure (Copyright 2017, Elsevier)⁵⁸ and (d) warp-knitted spacer structure (Copyright 2013, Elsevier).⁵⁹ STF: shear thickening fluid; STG: shear thickening gel; WKSF: warp-knitted spacer fabric.

In addition to 2D fabrics, fabrics with 3D structures have also been utilized as reinforcements of STFCs. Shih et al.⁵⁷ fabricated composite specimens with different laminating sequences, which were assembled with Kevlar fabric and STF-filled paper honeycomb structure layers (Figure 3(b)). The ballistic testing results confirmed that the STF structure placed at the rear position could significantly contribute to an increase in impact resistance. However, this laminated structure was easy to delaminate under the impact, which would affect the impact resistance of STFCs. Majumdar et al.⁵⁸ developed a STFC by selecting 3D woven orthogonal structure aramid fabrics and reinforcing them with STF (Figure 3(c)). Although fabric with a multi-layer integral structure seemed to be an ideal choice, the multi-layer structure of fabric reinforcement resulted in the loss of light weight and flexibility of composites, which limited the scope of the product application. In view of this, Lu et al.⁵⁹ reported the compressive behavior of warp-knitted spacer fabrics (WKSFs) impregnated with STF when subjected to quasi-static compression and low-velocity impact loadings (Figure 3(d)). This fabric structure alleviated the loss of STM to some extent, and the fabric composites showed higher energy absorption. Besides, Chatterjee et al.⁶⁰ added STF to a 3D mat Kevlar sandwich composite and concluded the enhancement of energy absorption. Nevertheless, an unsealed spacer structure resulted in the loss of STM; thus, sealing is one of the key factors in the structure design of fabric reinforcement.

Moreover, the structural parameters of fabrics are vital for influencing the impact resistance of STFCs. Arora et al.⁶¹ investigated the yarn pull-out force and impact energy absorption of plain woven UHMWPE fabrics with different fabric setts and yarn linear densities treated with STFs. It was discovered that for all levels of yarn linear density and fabric setts, STF treatments increased the inter-yarn friction considerably but were not always beneficial in terms of impact energy absorption. Laha and Majumdar⁶² discovered that STF treatments improved the impact resistance performance of STF-impregnated p-aramid fabrics based on five different weave structures and varying thread densities. The percentage increase in impact energy absorption after STF treatment was more for infirm weaves (matt and satin) as compared to that for firm weave (plain), and was reduced with the increase in thread density of the fabric. Park et al.^63,64 characterized the effects of the laminating sequence and fabric count on the ballistic performance of STF-treated aramid fabrics. The enhanced ballistic performance of fabric was assumed to be due to the synchronized (or coupled) elongation of facing yarns in the frontal layers and those in the following rear layers during impact. On the other hand, fabric specimens with higher fabric counts dissipated a higher fraction of exerted impact energy through dissipating energy by tension actions.

In general, the role of the fabric structure in impact behavior was complementary to that of STF and must be chosen judiciously to achieve the best impact resistance. More importantly, based on possessing the impact resistance effect, the fabric structure needs to ensure the stability of the STM, as well as light weight and flexibility of the composite, which are points worth considering for the future.

Composite technique

Fabric reinforcement with high areal density and special structure is difficult to directly combine with high-viscosity STM to prepare STFCs. For this reason, the composite technique is another important factor, which influences the STFC impact behavior. In fact, the performance of STMs and STFCs is first significantly affected by the STM manufacturing method. As displayed in Figure 4(a), the preparation of STF involved the mixture of particles in the dispersed medium, stirring to homogenize, and ultrasonic oscillation.¹⁰³ These processes ensured that the STF system was homogeneous, thus promoting its shear thickening effect. More importantly, STF could be uniformly distributed among the fibers of the fabric, ensuring that the impact resistance of the STFC was fully demonstrated. In addition, the STG system has cross-linked structures within it, so the temperature factor is crucial in the preparation process. The mixture of raw materials was reacted at over 150°C for over 2 h, then it was cooled to room temperature to obtain STG.⁸³ The stable cross-linked structure in the STG could provide sufficient enhancement of the mechanical property to the fabric. Therefore, the manufacturing methods of STM are key factors related with the protective performance of STFCs and deserve to be studied in depth.

Figure 4.

Composite techniques of shear thickening material (STM) and fabric reinforcement: (a) impregnation method (Copyright 2018, Elsevier)¹⁰³; (b) padding method (Copyright 2020, Elsevier)⁴⁷ and (c) surface morphology of fabric before and after STM treatment by the spray coating method (Copyright 2020, Elsevier).⁴⁸ STF: shear thickening fluid.

Currently, impregnation, padding, and spray coating methods are generally selected as the composite techniques of STFCs. Impregnation has typically been exploited for STFC fabrication (Figure 4(a)). To facilitate impregnation of STF into fabric, an equal volume of ethanol was added by Lee et al.⁴⁵ into the STF. This diluted STF was observed to spontaneously impregnate fabric. Following impregnation, the fabric composite was heated to remove ethanol, and STF was successfully introduced into Kevlar fabric. In order to help the impregnation of Twaron fabrics in the case of high particle loading, Fahool and Sabet⁶⁵ added an amount of ethanol to STF and removed it after impregnation by evaporation. Kim et al.¹⁰³ impregnated STF into Heracron fabric and the hypervelocity impact experiments results suggested that STF-impregnated fabric could be a promising candidate for the improved rear wall of a space shield. Impregnation has become one of the most commonly used methods for the combination of STF and fabrics.^66,104–106 He et al.⁴⁶ fabricated Kevlar/STF/STG composites by the “dip and dry” and “hand layup” methods, and they confirmed that STG enhanced the protective effect of the Kevlar/STF/STG composite by different preparation methods. Interestingly, the “hand layup” method led to a better knife stab resistance performance. Hence, the influence of impregnation on the properties of STFCs is considered.

Ghosh et al.⁴⁷ used a Mathis lab padder, which consisted of two rubber-coated padding rollers oriented in the horizontal direction to treat STF into Kevlar fabrics (Figure 4(b)). To facilitate effective penetration of STF inside fabrics, the prepared STF was diluted with ethanol and homogenized. This diluted STF was then poured at the nip of the horizontal rollers, which were held in contact, and the fabric was dipped in diluted STF and squeezed through the roller nip. Recently, the padding method has been extensively utilized to prepare STFCs, and the impact resistance results have confirmed that this method was feasible.^57,99

The spray coating method has also been utilized as a composite technique to prepare STFCs,⁴⁸ as shown in Figure 4(c). STM was evenly sprayed onto the surface of fiber reinforcement using a tool application or spray gun. However, STMs coated on the surface of fabric tended to fall off because they were difficult to get inside the fabric.

Although the spray coating and hand layup methods are simple and convenient to treat STM on the fabric reinforcement, they cannot guarantee uniform coating and quantitative control. More importantly, STMs are mostly distributed on the surface of fabric by these two methods, which affected the full display of impact resistance. At present, the impregnation and padding methods are considered the main preparation methods adapted for STFCs. Furthermore, polymerization of a monomer on the surface of fibers or fabrics maybe another method to obtain STFCs with high functional properties. In other words, a suitable composite process needs to be selected or designed according to the structure and properties of the fabric reinforcement.

Impact behaviors of shear thickening fabric composites

In contrast to Newtonian fluids, STMs displayed both the shear resistance of high-viscosity fluid and the easy flow characteristic of low-viscosity fluid, which created areas of low stress in fabric composites and improved their impact resistance performance.

In most cases, yarn pull-out, stab, drop-weight, split Hopkinson pressure bar (SHPB), or ballistic impact testing is conducted to evaluate the impact behaviors of STFCs. In terms of these tests, commonly used experiment instruments are the universal material testing machine, drop-weight impact machine, SHPB, gas gun machine, and so on. Furthermore, the impact behaviors of STFCs have been investigated under quasi-static, low-velocity, medium-velocity, and high-velocity conditions. In order to better develop STFC products, it is necessary to comprehensively investigate the behaviors of STFCs under various impact conditions. The commonly conducted testing approaches for impact behaviors of STFCs are as shown in Figure 5. Only the impact behaviors obtained by experimental analysis are reviewed and illustrated in this paper, while the numerical simulation will be further discussed in future work.

Figure 5.

Overview diagram of various testing approaches for impact behaviors of shear thickening fabric composites (STFCs). SHPB: split Hopkinson pressure bar.

Yarn pull-out

Yarn pull-out testing is often conducted to investigate the effect of STM on the friction between yarns (Figure 6), and it is generally implemented under quasi-static conditions by using a universal material testing machine. In fact, yarn pull-out testing results can reflect the impact resistance of STFCs from one aspect.

Figure 6.

(a) Yarn pull-out testing and (b) pull-out force versus displacement curve for the single yarn of shear thickening fabric composites.⁶⁸ Copyright 2020, Elsevier. STF: shear thickening fluid; STKF: STF-treated Kevlar fabric.

Lu et al.⁴⁹ performed yarn pull-out tests on STF-impregnated fabrics. Prior to testing, transverse yarns were manually removed from the top edge of the fabric to expose 100 mm of longitudinal yarns. The remaining fabric was clamped in a special fabric fixture and the top end of the yarn was loaded via the upper grip of the tester. The yarn pull-out force associated with STF-impregnated fabric was higher than neat fabric, which was because the STF could enhance the inter-yarn friction in woven fabrics. Wang et al.¹⁰⁷ pulled out the yarn of STFCs at the velocities of 50, 100, and 200 mm/min, respectively. The maximum pulled-out load became reliant on the pull-out velocity and was positively correlated with velocity due to the shear thickening effect of STF. He et al.⁴⁶ testified that the single yarn pull-out force was increased when fabric was doped with STF or STG due to the larger friction. The shear thickening effect of STF and STG led to a hindering effect when yarns were pulled from fabrics. In addition, STF and STG additives increased the roughness of the yarn surface, so the friction among yarns and fabrics was increased.

Obviously, yarn pull-out testing as an essential approach has been performed to evaluate the friction among yarns.^{84,100,104,108} In terms of STFCs, the stable and effective inter-yarn friction was an important guarantee to improve anti-impact performance.

Stab

The sharp edge threat in conflict events necessitates the further development of flexible protective materials with additional stab-resistant capabilities, and thus the implementation of stab testing (needle, spike, or knife threats) for STFCs is of great significance. Compared with yarn pull-out testing, stab testing has a relatively wider range of impact velocities, from quasi-static to low-velocity impacts.

Zhang et al.⁶⁷ carried out quasi-static and dynamic low-velocity stab resistance tests of STFCs (Figure 7(a)). The quasi-static stab head was spike-shaped or knife-shaped, and the dynamic stab was a needle or non-needle head. Test results confirmed that the incorporation of STF positively influenced the puncture resistance of the composites. Qin et al.⁶⁸ investigated the anti-stab performance of neat Kevlar fabrics and STFCs against a knife impactor, and it was clear that the stab resistance was improved remarkably for the targets composed of STM. Cwalina et al.^109,110 carried out quasi-static needle puncture testing on STF-intercalated Kevlar fabrics with a hypodermic needle, and an improvement in puncture resistance against hypodermic needle threats was achieved. Li et al.⁶⁹ further investigated the dynamic stab resistance properties of STF/UHMWPE fabrics against knife and spike threats. The results demonstrated that the dynamic stab resistance of the fabrics was significantly enhanced due to the presence of STF. STF effectively decreased yarn mobility and accelerated the transverse response of fabrics (Figure 7(b)). As a matter of fact, the stab resistance of STFCs has been evaluated to develop puncture-proof equipment with great application value.^{78,106,111,112}

Figure 7.

(a) Puncture tests with a spike, a knife, and needle threads (Copyright 2020, Springer Nature).⁶⁷ (b) Morphologies of neat fabric and ultra-high molecular weight polyethylene fabric after knife and spike stabs. (Copyright 2016, Elsevier).⁶⁹ and (c) Effects on different indenter geometry puncture resistance investigated by SampathKumar et al.⁷⁰ STF: shear thickening fluid; PP: polypropylene.

On the other hand, SampathKumar et al.⁷⁰ presented static stab resistance tests with different indenter nose shape geometry (hemispherical, elliptical, flat, and conical) on neat polypropylene fabric and STF-impregnated fabrics (Figure 7(c)). It was observed that the energy absorption capacity was highly dependent on the contact surface area of the indenter’s nose, and the energy absorption was more with the hemispherical indenter and less with the conical indenter. Nonetheless, they were not exploring the effect of the indenter nose shape geometry on stab resistance in depth, and lacked mechanism explanations for various stab impact threats on STFCs.

Drop-weight impact

Owing to the characteristics of STM, STFCs are sensitive to external stimuli and their impact behaviors perform differently, which reflects that the setting of the impact velocity is particularly important. Restricted by the drop height, the velocity range of the drop-weight impact (Figure 8(a)) is generally a low-velocity impact.^82,84 As an important index to characterize the impact behaviors of STFCs, energy absorption is frequently measured by drop-weight impact testing.

Figure 8.

(a) Low-velocity drop-weight impact testing (Copyright 2019, Elsevier),⁸⁴ drop-weight tests on (b), (c) electrospun a non-woven mat/shear thickening fluid (STF) composite (Copyright 2020, American Chemical Society),⁷⁴ and (d), (e) a warp-knitted spacer fabric (WKSF)/STF composite (Copyright 2013, Elsevier).⁵⁹

According to the ASTM D7136 standard, Zhang et al.⁷¹ investigated the damage effect and energy absorption of STF/UHMWPE fabric composites under low-velocity impact via a drop hammer impact tester, and the incident velocity was set at 2.5, 3, and 3.5 m/s. They found that STF/UHMWPE fabric composites exhibited higher impact resistance. By increasing incident velocity, the impact resistance of composites also increased because of the shear thickening phenomenon. Likewise, Asija et al.⁷² conducted low-velocity drop tower testing on STF-treated composite panels, and four sets of tests were conducted by varying the impact velocity. However, as the experimental strain rate caused by the impact velocity was less than the critical shear rate required for the onset of the shear thickening phenomenon, the addition of STF did not have any synergistic effect on the improvement of impact resistance or energy absorption capability of STF-treated composite panels. Therefore, the resistance of STFCs was significantly related to impact velocity. Nonetheless, it is necessary to set the impact velocity range and change gradient reasonably according to the properties of the specimens in the research process, so as to evaluate the impact resistance of STFCs comprehensively and accurately.

Furthermore, drop-weight impact testing has been carried out to assess the impact behaviors of STFCs with novel or unusual structures. Hasan-nezhad et al.⁷³ introduced low-velocity impact tests on a drop-weight impact machine to examine the cushioning behavior of multi-ply 3D fabrics impregnated with pure or treated STF. Test results showed that the applied impact force on treated STF-impregnated fabric was less than that on pure STF-impregnated fabric, and thereby the STF properties could affect the cushioning effects of fabrics. Hao et al.⁷⁴ introduced a new composite consisting of electrospun polyamide ultrafine fiber (UFF) non-woven mats and STF to improve the shape stability of fabrics (Figure 8(b)). Drop impact tests were conducted to characterize the viscoelastic response under a high deformation rate. Consequently, at high deformation rates beyond the shear thickening threshold, both the elasticity and viscosity of the UFF-STF composite increased, and the enhancement effect of STF with highly dispersed particle loading was more significant (Figure 8(c)).

Different from the above structure, in a sandwich panel, the plastic deformation including face-sheet bending and core crushing, usually occurs instead of penetrative damages when subjected to low-velocity impact.¹¹³ Using a drop-weight impact testing machine, Jeddi et al.⁷⁵ investigated the energy absorption of aluminum sandwich panels with STF-filled 3D fabric cores under low-velocity dynamic compression impact. A cylindrical-head impactor (to perform a surface impact) freely fell from a height to impact the sandwich panels. They demonstrated that STF improved the energy absorption and compressive performance of the sandwich panels. Similarly, Lu et al.⁵⁹ also reported the compressive behavior of WKSF impregnated with STF when subjected to low-velocity impact loadings, and the test specimens were placed between two plane plates (Figure 8(d)). The compressive behavior of the STF-impregnated WKSF had a significant strain-rate effect, and it showed a lower peak load than those of the neat WKSF (Figure 8(e)). Zhang et al.⁷⁶ attached 3D fabric onto an Ecoflex shell to support the upper surface of a STF/Ecoflex composite, and rigid plate compression tests were conducted. The shear thickening effects became more obvious with the increase of STF concentration and compression rate, which caused more energy to be lost.

Similar to stab tests, the impact response of fabric composites also showed the effect of different impactor geometries in drop-weight tests.¹¹⁴ Cheng et al.⁷⁷ conducted low-velocity impact tests on STF with six different strikers. Higher energy absorption was recorded for the flat-head strikers, yet with lower maximum penetration depth than the cone-shaped strikers of the same diameter. The energy absorption also increased when the striker diameter increased, but with lower penetration depth. However, there is still a lack of relevant studies on the effect of impactor shapes on STFC performance under drop-weight impact.

Therefore, the impact behaviors of STFCs are affected by the type of impact threats. It is meaningful to reasonably select impact conditions (impact velocity, impactor shape, etc.) according to the structural characteristics of the STFC, and design targeted impact tests on the drop-weight testing machine.

Split Hopkinson pressure bar impact

STFCs are generally developed for applications in body impact protective equipment, but the human body may still be vulnerable to injury despite the protection. Stress waves are generated under high-velocity impacts, where the energy of the stress pulse is quite considerable and even deadly. Hence, it is essential to figure out the dissipation of strain energy and dynamic mechanical properties of STFCs under impact. The SHPB system (Figure 9(a)) is widely used to test the mechanical properties of materials at high strain rates varying from 10² to 10⁴ s⁻¹.^115–117 Recently, SHPB impact testing has been conducted to investigate the stress characteristics or impact responses of STMs^118–121 and STFCs with a sandwich structure,^122,123 as shown in Figure 9(b).

Figure 9.

(a) Split Hopkinson pressure bar (Copyright 2018, Elsevier),⁴⁶ (b) specimen loaded between the incident bar (IB) and the transmission bar (TB) (Copyright 2018, Copyright 2017, Elsevier),^72,120 and (c) stress–strain curves of shear thickening fabric composites under different split Hopkinson pressure bar testing conditions (Copyright 2017, Elsevier).⁷⁸ STF: shear thickening fluid; EG: ethylene glycol.

The incident bar is struck to produce an elastic wave in SHPB impact testing. When the incident wave (ε_i) reaches the specimen–bar interface, part of it is reflected to form a reflected wave (ε_r), and the other part passes through the specimen as a transmitted wave (ε_t). Here, ε_i, ε_r, ε_t are measured by strain gauges that attach to the bars. According to one-dimensional stress wave theory,¹¹⁷ by eliminating the time term of Equations (3) and (4), the stress–strain curves are obtained

σ (t) = E_{b} ε_{r} (t)

(3)

ε (t) = - \frac{2 C_{b}}{I_{s}} \int_{0}^{t} ε_{r} (T) d T

(4)

where E_b is the elastic modulus of the bar, C_b is the elastic wave speed in the bar, l_s is the thickness of the specimen, and t is the pulse length. Equation (3) is based on the assumption that the stress and the strain distribute homogeneously in the specimen along its length.

The high strain-rate dynamic properties of STFCs have also been reported by conducting SHPB tests. Cao et al.⁷⁸ studied the high strain-rate mechanical property of a STF/Kevlar composite by using the SHPB system to investigate the anti-impact mechanism. Both the strain rate and the modulus of the STF/Kevlar composite showed an increasing trend with the increase of impact velocity. Besides, the addition of STF and the increase of the number of fabric layers reduced the energy transfer rate, and thereby the energy absorption of STF/Kevlar increased with the volume fraction of STF (Figure 9(c)). Asija et al.⁷² reported the effect of the STF treatment method on the high strain-rate behavior of ballistic fabric composites, and evaluated the high strain-rate response of STF-treated ballistic fabric composites using the SHPB technique.

In addition, the impact resistance of a STF/fabric composite was also successfully investigated via SHPB tests. Lomakin et al.⁷⁹ carried out experimental studies using the SHPB method to determine the dynamic properties of STF. The effectiveness of STF impregnation for the improvement of ballistic impact characteristics of Kevlar woven fabrics was confirmed. Meanwhile, it was concluded that the role of contact conditions between STF and Kevlar in the process of increasing the energy absorption capacity of the Kevlar–STF barrier was significant. He et al.⁴⁶ evaluated the compressive properties of STF- and STG-impregnated Kevlar fabrics under impact by a modified SHPB system at the bar speed of 7.5 m/s. STG clearly exhibited a higher enhancement than the others when exposed to the same strain. Simultaneously, the compressibility of the multi-layer Kevlar/STG composite was smaller and the modulus was larger.

The above results demonstrated that it was feasible to use SHPB impact testing for investigation. However, compared with other impact testing, the SHPB technique for evaluation of STFC impact behaviors still lacked a testing standard and extensive research, and its systematicness and accuracy were scarce. The reason could be attributed to the unique characteristics of STMs, flexible structures of fabrics, and so on. Therefore, according to the principle of the SHPB technique, it is meaningful to carry out this test for the specific structure and characteristics of the STFC, and to deeply analyze its impact protective mechanism.

Ballistic impact

Among the impact behaviors of STFCs, ballistic performance has been the most widely studied (Figure 10(a)). Numerous researchers combine STM with fabric to develop bullet-proof soft body armor by conducting ballistic impact tests with high velocity. Lee et al.⁴⁵ firstly reported that STF-impregnated Kevlar fabric offered higher energy absorption compared to neat Kevlar fabric of the same areal density when impacted with a projectile at 244 m/s. They observed that STF impregnation increased the friction between yarns, which provided fabric with a bullet-proof effect. Since then, ballistic impact tests have frequently been carried out for the evaluation of STFC impact behaviors.

Figure 10.

(a) Ballistic impact testing instrument, (b) fabric morphologies after high-velocity impact (Copyright 2019, Elsevier),⁹⁷ and (c) ballistic impact process of multifunctional shear thickening fabric composites (Copyright 2022, Elsevier).¹²⁵ STF: shear thickening fluid; EK-TENG: enhanced Kevlar-based triboelectric nanogenerator.

The ballistic performance of STFCs was strongly related to the characteristics of fabric reinforcement. Mishra et al.¹²⁴ conducted ballistic tests in the range of 250–700 m/s, and the energy absorption of unidirectional ultra-high molecular weight polyethylene (UD-UHMWP) fabrics could be improved after STF treatment. In terms of the influence of fabric thickness, enhancement in the energy absorption of treated fabric was more apparent in thicker fabric panels. Mahesh et al.⁸⁰ also reported that STF/jute fabric composites with three and six layers of fabric were subjected to ballistic loading using a gas gun apparatus in the velocity range of 15–90 m/s. The ballistic performance of six layers of either neat fabric or fabric composites was better than that of three layers. In addition, Park et al.^63,64 investigated the effect of laminating sequence, fabric count, and shot location on the ballistic performance of p-aramid fabrics impregnated with STF. Chatterjee et al.⁶⁰ carried out ballistic impact testing on three different 3D-mat–Kevlar sandwich composites, and the impact velocity of bullets ranged from 140 to 170 m/s. It was found that the STF-incorporated sandwich composite panels could absorb 96.3% of the incident energy, which accounted for 67.4% more energy absorbed in comparison to hollow panels. Therefore, the optimized fabric structure was a significant factor in improving the protective performance of STFCs.

In addition to fabric reinforcement, STM factors also influence the ballistic performance of STFCs. Khodadadi et al.⁹⁷ conducted high-velocity impact tests on woven Kevlar fabric impregnated with STF made by dispersing silica nanoparticles in PEG. For fabric composites made by impregnating with STF with 35 and 45 wt.% nanoparticles, no yarn was pulled out, while in neat fabric specimens, some fibers were pulled out of fabric and moved with the bullet (Figure 10(b)).

Furthermore, the ballistic behaviors of multifunctional STFCs could also be evaluated by ballistic impact tests. Wang et al.¹²⁵ integrated STM and graphene on Kevlar fabric to develop an enhanced Kevlar-based triboelectric nanogenerator (EK-TENG) with excellent safeguarding and stable sensing capability in harsh loading environments. The ballistic impact experiments were conducted by using a gas gun to drive a spherical bullet with different initial velocities. The EK-TENG effectively resisted a shooting velocity of 126.6 m/s and dissipated 87.4% of the explosion wave under blast loading (Figure 10(c)). Zhao et al.⁸⁴ introduced a novel carbon black–STG–Kevlar (c-STG/Kevlar) soft safeguarding composite fabricated by impregnating c-STG into Kevlar fabrics. The high-velocity ballistic impact testing indicated that the anti-impact properties of c-STG/Kevlar were reinforced by c-STG. The mechano-electric performance of c-STG/Kevlar was explained by evolutions in the structure of the conductive network under external loading. Hence, these studies indicated that ballistic performance was one of the key points of multifunctional STFCs. Ballistic tests could be used to investigate the impact behaviors of STFCs, and the coupling mechanism of the composites could also be analyzed.

As a common approach to evaluating the impact behaviors of STFCs under high-velocity impact, the technology of ballistic testing is relatively mature. Future ballistic impact research will be developed for the evaluation of STFCs with multifunctional or special structures and properties.

Impact protective mechanisms of shear thickening fabric composites

The impact behaviors of STFCs obtained by experiments could provide preliminary guidance for the development of impact protective products, but it was still necessary to conduct in-depth analysis of the protective mechanism. Srivastava et al.¹²⁶ introduced the mechanisms of improved energy absorption in STF-impregnated fabrics as three modes: energy dissipation due to shear thickening behavior; increased inter-yarn friction (yarn pull-out energy); and better coupling and load transfer between fiber to fiber and yarn to yarn. Nevertheless, the protective mechanisms of STFCs under various impact conditions were not analyzed specifically, and their main mode of protection may be different. Besides, the impact protective mechanisms of STFCs based on STG lacked a complete summary. In this section, when subjected to stabs, drop-weights, the SHPB, and ballistic impacts, the protective mechanisms of STFCs were reviewed by analyzing the previous studies.

Stab

When subjected to a needle or spike stab, the fabric initially performs a windowing effect, which means the fibers are separated in the yarn and pushed to the periphery within the fabric, but the fiber does not break significantly. With the cutting effect of fibers, the stab resistance force mainly depends on the inter-yarn friction. Because STM effectively increase the inter-yarn friction, the STFC stops the needle or spike from parting the fibers, and thus the stab resistance is enhanced. Besides, as illustrated in Figures 11(a) and (b), the friction between the stab head and fabric is a secondary mechanism of stab resistance.^69,111,127

Figure 11.

Stab testing on shear thickening fabric composites: (a) the process of the needle stab; (b) morphologies of neat fabric and shear thickening fluid (STF)/fabric after the spike stab (Copyright 2010, Springer Nature)¹¹¹ and (c) protective mechanism under the knife stab (Copyright 2020, Elsevier).⁶⁸

Fiber cutting is the main failure mode of fabric under knife stabbing. The tip of the knife makes a small hole and the knife cuts the fabric along its length. The main failure mode is different from other stab threats. In terms of STFCs, the shear thickening behavior of STM is triggered when yarns are compelled to move under a knife stab. Some mechanical interlockings between yarns are formed under compression around the impact point, which leads to there being more yarns to load the external impact force. Due to the rough surface of fibers, most of them sustain the impact energy in STM, which leads to the significant enhancement of stab resistance. In addition, the STM will store and dissipate some energy for the shear thickening effect, which also enhances the stab resistance at some level,⁶⁸ as presented in Figure 11(c). Thus, the inter-yarn friction caused by shear thickening behavior is responsible for stab resistance.

Drop-weight impact

The protective mechanisms of STFCs with various fabric structures are different when subjected to drop-weight impact. The nanoparticles aggregate and form particle clusters when the shear velocity achieves the critical shear rate of the STM. The force chain generated by particle contacts provides an energy dissipation network inside the mixture, the yarn at impact point is coupled with the surrounding yarns, and the impact resistance of the fabric is enhanced (Figure 12(a)). Furthermore, nanoparticles produce friction with the impact head and dissipate a small amount of impact energy. As a result, inter-yarn friction is considered the main mechanism of low-velocity impact resistance.⁷¹ However, for a special fabric structure, the mechanism is explained by the three aspects⁵⁹: squeezing the flow of STF; the energy absorption mechanism of STF; and fluid–yarn interaction (Figure 12(b)). Shear thickening occurs in STF to absorb energy in WKSF when the impact velocity rate reaches its critical shear rate. Besides, the fluid–yarn interaction also plays a role in the impact process. Therefore, the shear thickening effect is the main protective mechanism for WKSF with a 3D hollow structure.

Figure 12.

Drop-weight impact testing: (a) impact process (Copyright 2021, Elsevier)⁷¹ and (b) results of shear thickening fluid (STF)-impregnated warp-knitted spacer fabric (WKSF) under impact (Copyright 2013, Elsevier).⁵⁹

SHPB impact

There is relatively little research on the SHPB impact protective mechanism of STFCs. As shown in Figure 13, Kevlar fabrics undergo four processes during the SHPB impact process, namely, elastic compaction of the weaving structure, plastic slip of the fabric yarns, deformation of the fabric yarns, and stress unloading of the fabric specimen. The STF mainly works in the slip and deformation section by enhancing the friction between fabric yarns and preventing the fabric yarns from slipping.⁷⁸

Figure 13.

Ballistic impact

The ballistic mechanism of fabrics and their composites has been studied extensively. Chen et al.¹²⁸ concluded that woven fabrics performed two failure modes under ballistic impact, which were tensile failure and shear failure. Khodadadi et al.⁹⁷ indicated that the three mechanisms of fiber/yarn breakage (fracture), slippage (windowing), and yarn pull-out were the most important factors affecting fabric impact resistance.

In terms of STFCs, Chatterjee et al.⁶⁰ introduced a STFC with a 3D sandwich structure. Under ballistic impact, high-energy dissipation is observed due to the sufficiently high supporting stiffness of STF. This is due to the formation of hydro-clusters triggered in STF at the critical shear rate (Figure 14(a)). Mahesh et al.⁸⁰ proposed the damage mechanism of STF-impregnated fabrics under ballistic impact (Figure 14(b)). In the case of neat fabrics, the distortion of the fibers is greater, since there exists mechanical interlocking. This leads to fiber pull-out and breakage at the early stage, since the resistance offered by neat fabrics will be less compared to STF-impregnated fabrics. In the case of STF-impregnated fabrics, the uniform dispersion of STF leads to the triggering of shear thickening behavior when the fibers tend to move under ballistic impact, which causes enhanced impact resistance. In addition, STF impregnation induces structural integrity in fabrics by involving secondary yarns during ballistic impact (Figure 14(c)). The movements of filaments and yarns create a liquid-to-solid transition of STF. Moreover, slippage of yarns is alleviated and more areas of fabric participate in impact energy absorption.¹²⁹

Figure 14.

Ballistic protective mechanism of shear thickening fabric composites: (a) the formation of hydro-clusters (Copyright 2019, Elsevier)⁶⁰; (b) mechanical interlocking (Copyright 2021, Elsevier)⁸⁰ and (c) involvement of secondary yarns (Copyright 2020, Elsevier).¹²⁹ STF: shear thickening fluid.

For STFCs with multi-layer structures, Mishra et al.¹²⁴ explained its ballistic impact failure mechanisms. Due to enhancement in the friction between layers of STF-UHMWPE fabric and the shear thickening effect, in thinner fabric panels (5 and 10 layers), the impact damage is localized and the layer is perforated through shear plugging near the impact region, while the failure mode of thicker fabric panels (20 and 30 layers) is accompanied by the tensile fracture of high-tenacity fibers.

Currently, the protective mechanism of STFCs based on STG under high-velocity impact has also been proposed. By comparison, during the low-velocity drop-weight testing, the adhesive force between the STG and fabrics increases the inter-yarn friction. Although the shear thickening effect strengthens the anti-impact property, the friction between the fabrics is the main mode to resist the failure (Figure 15(a)). Meanwhile, in high-velocity ballistic testing, on account of the shear thickening performance, STG transforms from a plastic to a solid state to resist the impact due to the high impact rate. Then, the yarns are broken to disperse impact energy again. Finally, STG significantly improves anti-impact performance (Figure 15(b)).⁸²

Figure 15.

Generally speaking, the addition of STM significantly enhances the impact resistance of fabric, but the major contribution is not driven by the shear thickening effect. Rather, STM is responsible for the enhancement in inter-yarn friction along with fabrics and fiber/yarn coupling in the fabric. For quasi-static and low-velocity conditions, inter-yarn friction increased by STM is the main mode of impact protection, while the shear thickening behavior assists in enhancing impact resistance, but only plays an important role in sandwich structures due to the high content of STM in STFCs. The areal density of fabric directly determines the effect of inter-yarn friction enhancement and the distribution of STM between yarns, which is not taken into account in existing studies. The connection between the areal density of fabric and distribution of STM should be explored in the future. On the other hand, under the high-velocity impact, coupled with enhanced inter-yarn friction and the shear thickening effect, the deformation of fabric is considered to be resisted, and thereby the impact resistance is improved. Also, achieving the critical shear rate γ_c is a prerequisite for the shear thickening behavior of STM to be triggered. Hence, the scope of impact testing and application of STFCs needs to be determined based on the test value of γ_c. In short, the uniform characteristic of STM is worthy of attention for the development of impact protective products.

Conclusion and future work

In this paper, the shear thickening characteristics and mechanisms of STMs, including STF and STG, were firstly introduced. The influences of fabric, such as the fiber property, fabric structure, and composite technique, on the impact behaviors of STFCs were analyzed secondly. This paper systematically reviewed the impact behaviors of STFCs when subjected to stab, drop-weight, SHPB, or ballistic impact. Furthermore, the protective mechanisms of STFCs under various impact threats were summarized emphatically to promote and accelerate the exploration of body protective equipment. High-performance STFCs have become increasingly optimal in terms of impact resistance, light weight, and great wearability. However, some challenges that need to be addressed in the development process of STFCs still exist, especially in the development of suitable fabric reinforcement and the interaction between the STM and fabric reinforcement according to the impact protective mechanism. There are several findings and future prospects below.

In spite of being widely studied, the STF inevitably flows away from the surface of STFCs without any protection. Compared with STF, STG has better stability and easier encapsulation, and thereby STFCs based on STG generate higher values in impact protection applications. Besides, as a novel STM, the study of STG performance is not systematic and there is no unified standard for its characterization. Based on the shear thickening mechanism, further work should be carried out in the development of high-performance STG, which could make STG possess excellent responsiveness and greater human protection potential under complex impact environments. In addition, attention should be paid to the adsorption and uniform distribution in the micropores of the fibers of fabrics during the selection of STM components.

The impact behaviors of STFCs are significantly related to the fabric reinforcement. The commonly used high-performance fibers provide STFCs with great impact resistance, while other fibers also can be considered for economic or simple processing reasons. It is necessary to design several special fabric structures, such as the 3D hollow structure, etc., to enhance the stability and properties of STFCs or meet the wearable requirements. According to the fabric structure, a reasonable and effective composite technique is also critical for achieving the ideal impact protection effects.

The behaviors of STFCs show obvious differences when subjected to various impact conditions, and they follow multiple protective mechanisms. From the perspective of application requirements and conditions, STFC products should be evaluated by selecting a suitable impact testing approach for obtaining accurate results. Considering this, in the laboratory research and development stage, it is critical to develop STFCs based on the corresponding impact protective mechanism. Furthermore, through the design of the fabric structure, the protective mechanism of STFCs under various impact threats can be effectively utilized, so as to achieve a better anti-impact performance of the STFCs and exploit them in excellent wearable protective equipment.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the Fundamental Research Funds for the Central Universities (grant no. 2232020G-01).

ORCID iD

Yuhao Tan

References

Koffi

Toubal

Mechanical properties and drop-weight impact performance of injection-molded HDPE/birch fiber composites. Polym Test 2021; 93: 106956.

Daelemans

Cohades

Meireman

, et al. Electrospun nanofibrous interleaves for improved low velocity impact resistance of glass fibre reinforced composite laminates. Mater Des 2018; 141: 170–184.

Hawa

Abdul Majid

Afendi

, et al. Burst strength and impact behaviour of hydrothermally aged glass fibre/epoxy composite pipes. Mater Des 2016; 89: 455–464.

Wagner

Brady

JF.

Shear thickening in colloidal dispersions. Phys Today 2009; 62: 27–32.

White

EEB

Chellamuthu

Rothstein

JP.

Extensional rheology of a shear-thickening cornstarch and water suspension. Rheol Acta 2010; 49: 119–129.

Gurnon

Wagner

NJ.

Microstructure and rheology relationships for shear thickening colloidal dispersions. J Fluid Mech 2015; 769: 242–276.

Barnes

HA.

Shear-thickening (“dilatancy”) in suspensions of nonaggregating solid particles dispersed in Newtonian liquids. J Rheol 1989; 33: 329–366.

Brown

Jaeger

HM.

Shear thickening in concentrated suspensions: phenomenology, mechanisms and relations to jamming. Rep Prog Phys 2014; 77: 046602.

Zheng

Shi

Signatures of shear thinning-thickening transition in steady shear flows of dense non-Brownian yield stress systems. Sci China Chem 2015; 58: 1013–1020.

10.

Negi

Osuji

CO.

New insights on fumed colloidal rheology—shear thickening and vorticity-aligned structures in flocculating dispersions. Rheol Acta 2008; 48: 871–881.

11.

Kalman

Merrill

Wagner

, et al. Effect of particle hardness on the penetration behavior of fabrics intercalated with dry particles and concentrated particle-fluid suspensions. ACS Appl Mater Interfaces 2009; 1: 2602–2612.

12.

Feng

Wang

, et al. Effects of different silica particles on quasi-static stab resistant properties of fabrics impregnated with shear thickening fluids. Mater Des 2014; 64: 456–461.

13.

Cao

Qian

, et al. Shear-thickening behavior of modified silica nanoparticles in polyethylene glycol. J Nanopart Res 2012; 14: 747.

14.

Wetzel

Lee

Egres

, et al. The effect of rheological parameters on the ballistic properties of shear thickening fluid (STF)-Kevlar composites. AIP Conf Proc 2004; 712: 288–293.

15.

Maranzano

Wagner

NJ.

The effects of particle size on reversible shear thickening of concentrated colloidal dispersions. J Chem Phys 2001; 114: 10514.

16.

Alince

Lepoutre

Viscosity, packing density and optical properties of pigment blends. Colloids Surf 1983; 6: 155–165.

17.

Qin

Zhang

, et al. Effects of ionic structures on shear thickening fluids composed of ionic liquids and silica nanoparticles. RSC Adv 2016; 6: 81913–81923.

18.

Warren

Offenberger

Toghiani

, et al. Effect of temperature on the shear-thickening behavior of fumed silica suspensions. ACS Appl Mater Interfaces 2015; 7: 18650–18661.

19.

Gürgen

Kuşhan

MC.

The rheology of shear thickening fluids with various ceramic particle additives. Mater Des 2016; 104: 312–319.

20.

Tan

, et al. Effects of flocks doping on the dynamic mechanical properties of shear thickening gel. Autex Res J 2022; 22: 73–79.

21.

Galindo-Rosales

Martínez-Aranda

Campo-Deaño

CorkSTFμfluidics – a novel concept for the development of eco-friendly light-weight energy absorbing composites. Mater Des 2015; 82: 326–334.

22.

Tan

VBC

Tay

Teo

WK.

Strengthening fabric armour with silica colloidal suspensions. Int J Solids Struct 2005; 42: 1561–1576.

23.

Zhou

Yan

Jiang

, et al. Shear thickening fluid–based energy-free damper: Design and dynamic characteristics. J Intell Mater Syst Struct 2014; 27: 208–220.

24.

Crawford

Williams

SKR

Boldridge

, et al. Shear thickening of chemical mechanical polishing slurries under high shear. Rheol Acta 2012; 51: 637–647.

25.

Mawkhlieng

Majumdar

Designing of hybrid soft body armour using high-performance unidirectional and woven fabrics impregnated with shear thickening fluid. Compos Struct 2020; 253: 112776.

26.

Hoffman

RL.

Discontinuous and dilatant viscosity behavior in concentrated suspensions. I. Observation of a flow instability. Trans Soc Rheol 1972; 16: 155–173.

27.

Hoffman

. Discontinuous and dilatant viscosity behavior in concentrated suspensions. II. Theory and experimental tests. J Colloid Interface Sci 1974; 46: 491–506.

28.

Brady

Bossis

The rheology of concentrated suspensions of spheres in simple shear flow by numerical simulation. J Fluid Mech 1985; 155: 105–129.

29.

Brady

Phillips

Lester

, et al. Dynamic simulation of hydrodynamically interacting suspensions. J Fluid Mech 1988; 195: 257–280.

30.

Bossis

Brady

JF.

The rheology of Brownian suspensions. J Chem Phys 1989; 91: 1866–1874.

31.

Seto

Mari

Morris

, et al. Discontinuous shear thickening of frictional hard-sphere suspensions. Phys Rev Lett 2013; 111: 218301.

32.

Mari

Seto

Morris

, et al. Shear thickening, frictionless and frictional rheologies in non-Brownian suspensions. J Rheol 2014; 58: 1693–1724.

33.

Galindo-Rosales

Rubio-Hernández

Sevilla

An apparent viscosity function for shear thickening fluids. J Non-Newtonian Fluid 2011; 166: 321–325.

34.

Galindo-Rosales

Rubio-Hernández

Sevilla

, et al. How Dr. Malcom M. Cross may have tackled the development of “An apparent viscosity function for shear thickening fluids”. J Non-Newtonian Fluid 2011; 166: 1421–1424.

35.

Tian

Ding

, et al . Study of shear-stiffened elastomers. Smart Mater Struct 2012; 21: 125009.

36.

Jiang

Gong

Wang

, et al . Strain rate-induced phase transitions in an impact-hardening polymer composite. Appl Phys Lett 2014; 104: 121915.

37.

Houston

JE.

A local-probe analysis of the rheology of a “solid liquid”. J Polym Sci, Part B Polym Phys 2005; 43: 2993–2999.

38.

Goertz

Zhu

Houston

JE.

Temperature dependent relaxation of a “solid-liquid”. J Polym Sci Part B Polym Phys 2009; 47: 1285–1290.

39.

Wang

, et al. Dynamic behavior of magnetically responsive shear-stiffening gel under high strain rate. Compos Sci Technol 2016; 127: 169–176.

40.

Hogg

PJ.

Composites in armor. Science 2006; 314: 1100–1101.

41.

Huang

Z-M

Zhang

Kotaki

, et al. A review on polymer nanofibers by electrospinning and their applications in nanocomposites. Compos Sci Technol 2003; 63: 2223–2253.

42.

D3O

home

page, pp. 2021-12-19.

https://www.d3o.com/.

43.

Zhang

Wang

, et al. Conductive shear thickening gel/polyurethane sponge: a flexible human motion detection sensor with excellent safeguarding performance. Compos A Appl Sci Manuf 2018; 112: 197–206.

44.

Tang

Dong

Yang

, et al. Protective performance and dynamic behavior of composite body armor with shear stiffening gel as buffer material under ballistic impact. Compos Sci Technol 2022; 218: 109190.

45.

Lee

Wetzel

Wagner

NJ.

The ballistic impact characteristics of Kevlar woven fabrics impregnated with a colloidal shear thickening fluid. J Mater Sci 2003; 38: 2825–2833.

46.

Cao

Wang

, et al. Impact resistance of shear thickening fluid/Kevlar composite treated with shear-stiffening gel. Compos A Appl Sci Manuf 2018; 106: 82–90.

47.

Ghosh

Majumdar

Butola

BS.

Rheometry of novel shear thickening fluid and its application for improving the impact energy absorption of p-aramid fabric. Thin Walled Struct 2020; 155: 106954.

48.

Kim

Sathish Kumar

Park

, et al. High-velocity impact onto a high-frictional fabric treated with adhesive spray coating and shear thickening fluid impregnation. Compos B Eng 2020; 185: 107742.

49.

Yuan

Chen

, et al . Evaluation of ballistic performance of STF impregnated fabrics under high velocity impact. Compos Struct 2019; 227: 111208.

50.

Gürgen

Numerical modeling of fabrics treated with multi-phase shear thickening fluids under high velocity impacts. Thin Walled Struct 2020; 148: 106573.

51.

Mawkhlieng

Majumdar

Designing of hybrid soft body armour using high-performance unidirectional and woven fabrics impregnated with shear thickening fluid. Compos Struct 2020; 253: 112776.

52.

Wei

Sun

Zhu

Effects of parameters controlling the impact resistance behavior of the GFRP fabric impregnated with a shear thickening fluid. Mater Des 2020; 196: 109078.

53.

Caglayan

Osken

Ataalp

, et al. Impact response of shear thickening fluid filled polyurethane foam core sandwich composites. Compos Struct 2020; 243: 112171.

54.

Sun

Wei

Zhu

Low velocity impact performance of fiber-reinforced polymer impregnated with shear thickening fluid. Polym Test 2021; 96: 107095.

55.

Decker

Halbach

Nam

, et al. Stab resistance of shear thickening fluid (STF)-treated fabrics. Compos Sci Technol 2007; 67: 565–578.

56.

Liu

Yang

Zhao

, et al. The influences of rheological property on the impact performance of Kevlar fabrics impregnated with SiO2/PEG shear thickening fluid. Thin Walled Struct 2020; 151: 106717.

57.

Shih

C-H

Chang

C-P

Liu

Y-M

, et al. Ballistic performance of shear thickening fluids (STFs) filled paper honeycomb panel: effects of laminating sequence and rheological property of STFs. Appl Compos Mater 2021; 28: 201–218.

58.

Majumdar

Laha

Bhattacharjee

, et al. Tuning the structure of 3D woven aramid fabrics reinforced with shear thickening fluid for developing soft body armour. Compos Struct 2017; 178: 415–425.

59.

Jing

Sun

, et al. Compressive behaviors of warp-knitted spacer fabrics impregnated with shear thickening fluid. Compos Sci Technol 2013; 88: 184–189.

60.

Chatterjee

Verma

Bhattacharjee

, et al . Enhancement of energy absorption by incorporation of shear thickening fluids in 3D-mat sandwich composite panels upon ballistic impact. Compos Struct 2019; 225: 111148.

61.

Arora

Majumdar

Butola

BS.

Structure induced effectiveness of shear thickening fluid for modulating impact resistance of UHMWPE fabrics. Compos Struct 2019; 210: 41–48.

62.

Laha

Majumdar

Interactive effects of p-aramid fabric structure and shear thickening fluid on impact resistance performance of soft armor materials. Mater Des 2016; 89: 286–293.

63.

Park

Yoon

Paik

, et al. Ballistic performance of p-aramid fabrics impregnated with shear thickening fluid; Part I – effect of laminating sequence. Text Res J 2011; 82: 527–541.

64.

Park

Yoon

Paik

, et al. Ballistic performance of p-aramid fabrics impregnated with shear thickening fluid; Part II – effect of fabric count and shot location. Text Res J 2012; 82: 542–557.

65.

Fahool

Sabet

AR.

Parametric study of energy absorption mechanism in Twaron fabric impregnated with a shear thickening fluid. Inter J Impact Eng 2016; 90: 61–71.

66.

Ávila

de Oliveira

Leão

, et al. Aramid fabric/nano-size dual phase shear thickening fluid composites response to ballistic impact. Compos A Appl Sci Manuf 2018; 112: 468–474.

67.

Zhang

T-T

Peng

H-K

, et al. Synergistic effects of needle punching and shear-thickening fluid on sandwich-structured composites made of nonwoven and woven fabrics. Fibers Polym 2020; 21: 1515–1522.

68.

Qin

Guo

Zhang

, et al . Soft armor materials constructed with Kevlar fabric and a novel shear thickening fluid. Compos B Eng 2020; 183: 107686.

69.

Xiong

Zhao

, et al. Dynamic stab resistance of ultra-high molecular weight polyethylene fabric impregnated with shear thickening fluid. Mater Des 2016; 102: 162–167.

70.

SampathKumar

Duraisamy

Pattabi

, et al . Static puncture resistance characteristics with various indenter nose shape geometry perforation of shear thickening fluid impregnated polypropylene fabric for soft armour application. Polym Polym Compos 2022; 30: 1–10.

71.

Zhang

Liu

Qin

, et al. Characterization of macroscopic impact-resistant behavior of shear thickening fluid impregnated ultra-high molecular weight polyethylene fiber flexible composites. Compos Commun 2021; 25: 100756.

72.

Asija

Chouhan

Amare Gebremeskel

, et al. Impact response of Shear Thickening Fluid (STF) treated ultra high molecular weight poly ethylene composites – study of the effect of STF treatment method. Thin Walled Struct 2018; 126: 16–25.

73.

Hasan-nezhad

Yazdani

Jeddi

. High- and low-velocity impact experiments on treated STF/3D glass fabrics. Thin Walled Struct 2022; 171: 108720.

74.

Hao

Ding

Rutledge

GC.

Shape-stable composites of electrospun nonwoven mats and shear-thickening fluids. ACS Appl Mater Interfaces 2022; 14: 8373–8383.

75.

Jeddi

Yazdani

Hasan-nezhad

Energy absorption characteristics of aluminum sandwich panels with Shear Thickening Fluid (STF) filled 3D fabric cores under dynamic loading conditions. Thin Walled Struct 2021; 168: 108254.

76.

Zhang

Wang

Deng

, et al . A high anti-impact STF/Ecoflex composite structure with a sensing capacity for wearable design. Compos B Eng 2022; 233: 109656.

77.

Cheng

, et al. Effect of striker shape on impact energy absorption of a shear thickening fluid. Compos Commun 2021; 23: 100560.

78.

Cao

Chen

Wang

, et al. High strain-rate dynamic mechanical properties of Kevlar fabrics impregnated with shear thickening fluid. Compos A Appl Sci Manuf 2017; 100: 161–169.

79.

Lomakin

Mossakovsky

Bragov

, et al. Investigation of impact resistance of multilayered woven composite barrier impregnated with the shear thickening fluid. Arch Appl Mech 2011; 81: 2007–2020.

80.

Mahesh

Harursampath

Mahesh

An experimental study on ballistic impact response of jute reinforced polyethylene glycol and nano silica based shear thickening fluid composite. Def Technol 2022; 18: 401–409.

81.

Haris

Lee

Tan

VBC.

An experimental study on shock wave mitigation capability of polyurea and shear thickening fluid based suspension pads. Def Technol 2018; 14: 12–18.

82.

Zhao

Cao

, et al. Anti-impact behavior of a novel soft body armor based on shear thickening gel (STG) impregnated Kevlar fabrics. Smart Mater Struct 2019; 28: 075036.

83.

Liu

, et al. Shear thickening gel reinforced flexible polyurethane foam and its enhanced properties. Smart Mater Struct 2019; 28: 055017.

84.

Zhao

Wang

Cao

, et al. Conductive shear thickening gel/Kevlar wearable fabrics: a flexible body armor with mechano-electric coupling ballistic performance. Compos Sci Technol 2019; 182: 107782.

85.

Liu

Huo

T-T

, et al . Mechanical properties of a STF capsule filled flexible polyurethane composite foam. Mater Lett 2020; 269: 127580.

86.

Zhang

Wang

, et al. Optimization of shear thickening fluid encapsulation technique and dynamic response of encapsulated capsules and polymeric composite. Compos Sci Technol 2019; 170: 165–173.

87.

Warren

Cole

Offenberger

, et al. Hypervelocity impacts on honeycomb core sandwich panels filled with shear thickening fluid. Inter J Impact Eng 2021; 150: 103803.

88.

Tan

Zuo

, et al. Dynamic response of symmetrical and asymmetrical sandwich plates with shear thickening fluid core subjected to penetration loading. Mater Des 2016; 94: 105–110.

89.

, et al . Dynamic compressive behaviour of sandwich panels with lattice truss core filled by shear thickening fluid. Inter J Impact Eng 2020; 143: 103616.

90.

Xiao

Yin

, et al. Experimental study on dynamic compressive behaviour of sandwich panel with shear thickening fluid filled pyramidal lattice truss core. Inter J Mech Sci 2018; 138–139: 467–475.

91.

Haro

Szpunar

Odeshi

AG.

Ballistic impact response of laminated hybrid materials made of 5086-H32 aluminum alloy, epoxy and Kevlar® fabrics impregnated with shear thickening fluid. Compos A Appl Sci Manuf 2016; 87: 54–65.

92.

Gürgen

Fernandes

FAO

de Sousa

RJA

, et al. Development of eco-friendly shock-absorbing cork composites enhanced by a non-Newtonian fluid. Appl Compos Mater 2021; 28: 165–179.

93.

Sheikhi

Gürgen

Anti-impact design of multi-layer composites enhanced by shear thickening fluid. Compos Struct 2022; 279: 114797.

94.

Yum

Kim

, et al. Force attenuation properties of shear-thickening polymer (STP)-based hip protector in simulated sideways fall. J Mech Sci Technol 2020; 34: 5351–5357.

95.

Liu

Qian

, et al . Energy absorption and low-velocity impact response of shear thickening gel reinforced polyurethane foam. Smart Mater Struct 2020; 29: 045018.

96.

Bilisik

Two-dimensional (2D) fabrics and three-dimensional (3D) preforms for ballistic and stabbing protection: a review. Text Res J 2016; 87: 2275–2304.

97.

Khodadadi

Liaghat

Vahid

, et al. Ballistic performance of Kevlar fabric impregnated with nanosilica/PEG shear thickening fluid. Compos B Eng 2019; 162: 643–652.

98.

Wang

Q-s

Sun

R-j

Yao

, et al. The influence of temperature on inter-yarns fictional properties of shear thickening fluids treated Kevlar fabrics. Compos A Appl Sci Manuf 2019; 116: 46–53.

99.

Mawkhlieng

Majumdar

Deconstructing the role of shear thickening fluid in enhancing the impact resistance of high-performance fabrics. Compos B Eng 2019; 175: 107167.

100.

Qin

Wang

Yun

, et al . Response and adaptability of composites composed of the STF-treated Kevlar fabric to temperature. Compos Struct 2021; 260: 113511.

101.

Deepak

Thirumalaikumarasamy

Thirumal

, et al. Preparation and characterization of shear thickening fluid coated polypropylene fabric for soft armour application. J Tex Inst 2020; 112: 1555–1567.

102.

Santos

CMS

Aquino

, et al. Influence of silane coupling agent on shear thickening fluids (STF) for personal protection. J Mater Res Technol 2019; 8: 4032–4039.

103.

Kim

Park

Cha

, et al. Behavior of Shear Thickening Fluid (STF) impregnated fabric composite rear wall under hypervelocity impact. Compos Struct 2018; 204: 52–62.

104.

Gürgen

Kuşhan

MC.

The stab resistance of fabrics impregnated with shear thickening fluids including various particle size of additives. Compos A Appl Sci Manuf 2017; 94: 50–60.

105.

Ahn

Han

, et al. Shear behavior of a shear thickening fluid-impregnated aramid fabrics at high shear rate. Compos B Eng 2016; 97: 162–175.

106.

Chen

Wang

, et al. Stabbing resistance of body armour panels impregnated with shear thickening fluid. Compos Struct 2017; 163: 465–473.

107.

Wang

Zhang

Bao

, et al. Enhancement of the ballistic performance of aramid fabric with polyurethane and shear thickening fluid. Mater Des 2020; 196: 109015.

108.

Wei

Dong

Sun

, et al. Characterization of the rheological behaviors and mechanical properties of fabrics impregnated by different shear thickening fluids at changing temperatures. Smart Mater Struct 2021; 30: 085009.

109.

Cwalina

Dombrowski

McCutcheon

, et al. MMOD puncture resistance of EVA suits with shear thickening fluid (STF) – armortm absorber layers. Procedia Eng 2015; 103: 97–104.

110.

Cwalina

McCutcheon

Dombrowski

, et al. Engineering enhanced cut and puncture resistance into the thermal micrometeoroid garment (TMG) using shear thickening fluid (STF) – Armor™ absorber layers. Compos Sci Technol 2016; 131: 61–66.

111.

Kang

Hong

Yoo

MR.

Preparation and properties of fumed silica/Kevlar composite fabrics for application of stab resistant material. Fibers Polym 2010; 11: 719–724.

112.

Kang

Kim

Hong

KH.

Rheological behavior of concentrated silica suspension and its application to soft armor. J Appl Polym Sci 2012; 124: 1534–1541.

113.

Zhang

Fei

Zhang

Drop-weight impact behavior of honeycomb sandwich panels under a spherical impactor. Compos Struct 2017; 168: 633–645.

114.

Uyaner

Kara

Experimental study of the impact behavior of laminated composites stricken by sharp impactors. Sci Eng Compos Mater 2012; 19: 307–313.

115.

X-F

Ghoshal

Kartashov

, et al. Experimental characterization of the impact-damage tolerance of a cross-ply graphite-fiber/epoxy laminate. Polym Compos 2008; 29: 534–543.

116.

Woo

S-C

Kim

T-W.

High-strain-rate impact in Kevlar-woven composites and fracture analysis using acoustic emission. Compos B Eng 2014; 60: 125–136.

117.

Liao

Gong

Xuan

Magnetic field-induced compressive property of magnetorheological elastomer under high strain rate. Ind Eng Chem Res 2013; 52: 8445–8453.

118.

Lim

Lopatnikov

Wagner

, et al. Investigating the transient response of a shear thickening fluid using the split Hopkinson pressure bar technique. Rheol Acta 2010; 49: 879–890.

119.

Jiang

Gong

Xuan

, et al . Stress pulse attenuation in shear thickening fluid. Appl Phys Lett 2013; 102: 101901.

120.

Asija

Chouhan

Gebremeskel

, et al. High strain rate characterization of shear thickening fluids using Split Hopkinson Pressure Bar technique. Int J Impact Eng 2017; 110: 365–370.

121.

Tan

Zhou

, et al. The influence of graphene on the dynamic mechanical behaviour of shear thickening fluids. Adv Powder Technol 2019; 30: 2416–2421.

122.

Yin

Huang

, et al. Dynamic energy absorption behavior of lattice material filled with shear thickening fluid. Procedia Eng 2017; 199: 2514–2518.

123.

Zhang

Wang

Kurkin

, et al . Mechanical response of shear thickening fluid filled composite subjected to different strain rates. Inter J Mech Sci 2021; 196: 106304.

124.

Mishra

Singh

, et al . Ballistic impact performance of UHMWP fabric impregnated with shear thickening fluid nanocomposite. Compos Struct 2022; 281: 114991.

125.

Wang

Zhou

Wang

, et al. Enhanced Kevlar-based triboelectric nanogenerator with anti-impact and sensing performance towards wireless alarm system. Nano Energy 2022; 91: 106657.

126.

Srivastava

Majumdar

Butola

BS.

Improving the impact resistance of textile structures by using shear thickening fluids: a review. Cri Rev Solid State Mater Sci 2012; 37: 115–129.

127.

Houghton

Schiffman

Kalman

, et al. Hypodermic needle puncture of shear thickening fluid (STF)-treated fabrics. In: international symposium proceedings of the Society for the Advancement of Material and Process Engineering (SAMPE'07), 3-7 June 2007, Department of Chemical Engineering and Center for Composite Materials, University of Delaware.

128.

Chen

Zhu

Wells

An analytical model for ballistic impact on textile based body armour. Compos B Eng 2013; 45: 1508–1514.

129.

Arora

Majumdar

Butola

BS.

Soft armour design by angular stacking of shear thickening fluid impregnated high-performance fabrics for quasi-isotropic ballistic response. Compos Struct 2020; 233: 111720.