Soft Body Armor Research Articles

Room-Temperature Self-Healing Polyurea with High Puncture and Impact Resistances

Polyureas are widely used as protective materials because they have good mechanical strength, toughness, and strain-rate-dependent behavior. However, the fast reaction between −NCO and the −NH2 component results in gelation in seconds, limiting their performance regulation and applications. In this study, we report a novel strategy to fabricate one-component polyurea with room-temperature self-healing, puncture resistance, and impact resistance. First, a latent curing agent was synthesized via the Schiff base reaction of an aliphatic diamine with isobutyraldehyde. Then, they were mixed directly with isocyanate. Such a mixture was uncured in storage without moisture, but it was cured once in contact with moisture in the application. So, it was called one-component polyurea. The polyurea exhibits room-temperature self-healing properties with a healing efficiency of up to 98%. It has puncture displacement as high as 15.1 cm by sharp objects. The polyurea also exhibits excellent impact resistance and dissipative energy capacity because it possesses strain-rate-dependent behavior. The polyurea can be integrated into Kevlar fabrics to form a flexible puncture-resistant composite and combined with carbon nanotubes to form flexible and wearable strain sensors. It is expected to find applications in intelligent soft materials, soft body armor, wearable flexible sensors, and other fields.

Impact-Protective Bicontinuous Hydrogel/Ultrahigh-Molecular Weight Polyethylene Fabric Composite with Multiscale Energy Dissipation Structures for Soft Body Armor.

Soft body armor greatly improves the comfort and security of the wearers. Although laminates based on high-performance fabrics have been adopted, it remains an enormous challenge to develop fabric laminates having flexibility, low bulge deformation, and ballistic protection capability simultaneously. Herein, we report a bullet-proof bicontinuous hydrogel (BH)/ultrahigh-molecular weight polyethylene fabric (UPF) composite. The presence of the BH significantly improves the impact resistance performance of the UPF, without compromising its flexibility. In specific, the multiscale energy dissipation structures composed of hydrogen bond associations in the chain scale, bicontinuous phase structures in the nanoscale, and fibers in the microscale are broken to dissipate energy. As a result, the impact energy of the bullet is greatly absorbed and the bulge height of the composites is significantly reduced in contrast to the neat UPF laminates. This study indicates that the flexible BH-UPF composites with multiscale energy dissipation structures have a promising application in soft body armor.

Study on Low-Velocity Impact Behavior of Twaron® Fabric Subjected to Double-Impactor Impact from a Numerical Analysis Perspective

Abstract In the present study, a finite element impact model was developed and analyzed using commercial FEM code ANSYS® and then validated via a drop-weight impact experiment. Moreover, double-impactor impact models were designed and developed with different impact distribution and locations of two impactors to compare impact properties. A total of 18 impact scenarios comprised of asymmetric and symmetric types were performed. The effect of impact location on the impact resistance force and duration time was investigated: the closer the impact point is to the fabric center, the longer the impact duration time. In addition, the effect of impact location on impactor failure deflection was also investigated and it was concluded that regardless of the symmetric or asymmetric impact scenario, the smaller the average distance between the impact location of the two impactors from the fixed boundary, the smaller the overall average failure deflection that occurs. The relevance of impact location and fabric energy absorption capacity was also identified. Furthermore, the effect of impact location on fabric stress distribution and transverse deformation and of the variation of the impact velocity on fabric impact behaviors were investigated. These findings will provide important guidance for engineering soft body armor and composite materials.

Inorganic Ionic Oligomers Induced Organic‐Inorganic Synergistic Toughening Enabling Mechanical Robust and Recyclable Nanocomposite Hydrogels

AbstractMechanical robust hydrogels are ideal for applications in energy, environment, biomedicine, and structural engineering materials fields. However, high strength and high toughness are usually in conflict with each other, and simultaneously achieving both of them within a hydrogel has been challenging. Herein an organic‐inorganic synergistic toughening strategy is reported via in‐situ inorganic ionic polymerization of calcium phosphate oligomers within polymer composite networks composed of polyvinyl alcohol chains and aramid nanofibers. The composite hydrogels are provided with a prestress‐induced hierarchically fibrous structure through the assembly, which resulted in the mechanical strength and toughness up to 24.15 ± 1.12 MPa and 15.68 ± 1.78 MJ m−3, respectively, surpassing most toughened hydrogels. Through lamination and crosslinking, bulk hydrogels with controllable mechanical anisotropy and significant energy absorption/dissipation ability are produced. Moreover, the recycling and regeneration of the hydrogels are easily realized owing to the physically crosslinked network and acid‐induced dissolution of the inorganic units of the hydrogels, which lays a foundation for the sustainable large‐scale production and application of the hydrogels. This study provides an alternative approach for the development of mechanical robust and recyclable nanocomposite hydrogels for various applications including soft body armor, flexible electronics, soft robotics, etc.

Impact Resistant Flax Fiber Fabrics Using Shear Thickening Fluid

Shear thickening fluids (STFs) have been shown to improve the effectiveness of fabrics used in soft body armor applications. They are used to increase the puncture and ballistic impact resistance of Kevlar® fabrics. However, the effect of using STFs with natural fabrics such as flax appears to have never been studied. Similarly, the hybridization of different fabric types impregnated with STF has also only undergone limited study. The rheology of STFs at varying concentrations of nanosilica dispersed in polyethylene glycol (PEG) was studied at different temperatures. It was found that the STFs behave as a non-Newtonian fluid in response to changes in shear rate. In this study the effectiveness on the puncture and ballistic impact resistance of impregnating flax fabric with STF at concentrations of 30%, 50%, and 70% w/w of nanosilica in PEG was investigated. The effect of hybridization of flax and Kevlar® fabrics impregnated with STF was also investigated. The puncture resistance of both flax fabrics treated with STFs and hybrids treated with STFs was found to increase significantly and can be controlled by STF concentration. The ballistic impact resistance was also found to increase in the hybrid samples when STF concentration was at least 50%. The flax treated with STFs showed either a decrease in specific energy absorption per layer for the lower STF concentration, or a very small increase at 70% STF concentration.

Synthesis and properties of SiO2/SiO2@Ag two-phase STFs.

Soft body armor with a strain-sensing function using conductive shear thickening fluids (STFs) has gradually gained research interest. In this study, conductive SiO2@Ag core-shell microspheres were synthesized and the influence of process parameters on their properties was evaluated. Subsequently, SiO2 and SiO2@Ag were used as dispersed phases to prepare two-phase STFs, the effect of the core-shell microspheres' proportion on the rheological properties of the STFs was investigated, and its mechanism was discussed. The results indicated that SiO2@Ag core-shell microspheres were coated with elemental silver and when the concentration of sodium hydroxide and glucose were 0.07 and 0.09 mol L-1, respectively, the coating surface was the most uniform and compact, and the conductivity reached the minimum value of 0.56 Ω cm. The two-phase STFs exhibited good and reversible shear thickening behaviors and the critical shear rate decreased with increasing core-shell microsphere concentration. Additionally, when the mass fraction of SiO2 and SiO2@Ag core-shell microspheres was 45% and 20%, respectively, the thickening rate was 325%, and the resistance of two-phase STFs decreased simultaneously with the emergence of shear thickening that reached the lowest value of 795.16 kΩ. This study provides a novel strategy for synthesizing conductive STFs for strain-sensing flexible stab-resistant composites.

Exploration of disentangled UHMWPE tape as a soft body armour material

Disentangled ultrahigh molecular weight polyethylene (UHMWPE) is a very promising material with extremely high tenacity and modulus. In this work, the effect of drawing on the morphological, physical, and mechanical properties of disentangled UHMWPE tapes was studied. The tapes were characterised by using the density gradient column method , wide-angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC), polarised optical microscopy and scanning electron microscopy (SEM). The density and crystallinity of tape improved after each stage of drawing. The unoriented crystalline structure of the as-prepared UHMWPE tape gradually transformed into an oriented crystalline structure on drawing as confirmed by WAXD patterns. As the draw ratio increased, concomitant increase in birefringence, and tensile properties were also noticed. The tape obtained after the final stage of drawing had tensile properties similar to that of gel spun UHMWPE fibres. Hence, this tape was used to make prototype of soft armour panel (SAP) that stopped all the bullets fired when subjected to ballistic test and yielded very low back face signature (BFS) justifying the potential of disentangled UHMWPE tape as a ballistic material.

Magnetically tunable rheological properties of PVDF doped with superparamagnetic Fe3O4 nanoparticles synthesized by rapid microwave method

We report on the surfactant/additives free synthesis of single domain superparamagnetic Fe3O4 nanoparticles (FNPs) by rapid microwave synthesis method. X-Ray diffraction (XRD) and Scanning Electron Microscopy (SEM) studies reveal that FNPs were crystallized in the magnetite (Fe3O4) lattice and have granular shape with average particles size of ∼14 nm. The band gap of FNPs calculated from UV–Visible absorption spectrum is ∼2.3 eV. FNPs exhibit single domain superparamagnetic behaviour as confirmed from magnetic hysteresis loops. The nanocomposites of FNPs and β-phase of PVDF were prepared via mixed solvent approach using THF/DMF with different concentrations of FNPs. The rheological behaviour of the nanocomposites was studied with and without ageing in an applied magnetic field of 3500 Gauss for 3 h. Samples depicts shear thinning to shear thickening transition, decrease in zero shear viscosity and long frequency range viscoelastic behaviour as function of increasing FNP concentration. Increase in CST region, fluidity and complex modulus are the main consequences of magnetic aging. FNP-PVDF nanocomposites with such properties could find applications in flexible magnetic and mechanical films for soft body armor and memory storage.

Ballistic loading and survivability of optical fiber sensing layers for soft body armor evaluation

The authors previously demonstrated the use of FBG sensors in Kevlar mats behind body armor to measure the transient back face deformation (BFD) during ballistic testing. This paper presents a novel sensor system based on a Fiber Bragg Grating embedded in silicone mats to improve the survivability of the body armor in-situ strain sensing layers. Due to the large amount of deformation, a relative slip between the optical fibers and the supporting structure is needed to maintain the performance of the sensors and determine the relationship between the measured strain and deformation shape. Two silicone materials were tested, Smooth-Sil 950 and Sorta-Clear 40, in both 1 mm and 2 mm thicknesses to evaluate their survivability and impact on BFD. To enhance slipping between the fibers and surrounding silicone a thin layer of petroleum jelly was placed on the fibers prior to being cast in the silicone mats. The 1 mm Sorta-Clear 40 mats performed best in silicone survivability, FBG survivability and minimal impact on the BFD. The new system improves on key deficiencies that were found from inserting the fibers directly into the Kevlar with minimal to no impact on the back face deformation.

The Critical Role of a Backing Material in Assessing the Performance of Soft Ballistic Protection

Penetrating trauma by energised fragments is the most common injury from an explosive event. Fragment penetrations to the truncal region can result in lethal haemorrhage. Personal armour is used to mitigate ballistic threats; it comprises hard armour to protect from high-velocity bullets and soft armour to protect against energised fragments and other ballistic threats (such as from a hand gun) with low impact velocities. Current testing standards for soft armour do not focus on realistic boundary conditions, and a backing material is not always recommended. This study provides a comprehensive set of evidence to support the inclusion of a backing used in testing of soft body armour. Experiments were performed with a gas-gun system using fragment-simulating projectiles (FSPs) of different shapes and sizes to impact on a woven aramid and a knitted high-performance polyethylene ballistic fabric, with and without the ballistic gelatine soft tissue simulant as the backing material. The results showed statistically significant differences in the impact velocities at 50% risk (V50) of fabric perforation across all test configurations when the gelatine backing was used. Furthermore, the backing material enabled the collection of injury-related metrics such as V50 of tissue-simulant penetrations as well as depth of penetration against impact velocity. The normalised energy absorbed by the fabric could also be calculated when the backing material was present. This study confirms that a backing material is essential, particularly when assessing the performance of single layer fabrics against FSPs of low mass. It also demonstrates the additional benefits provided by the backing for predicting injury outcomes.

Intelligent safeguarding Leather with excellent energy absorption via the toughness-flexibility coupling designation

High-performance intelligent body armors with both soft wearability and good energy absorption are increasingly required in modern social life. Leather, a breathable, pliable, and tough natural material has extensive applications in the anti-impact field. In this study, a multifunctional Leather/CNT-SSG/PU sandwich structure with high energy absorption and excellent sensing monitoring properties is designed by integrating tough leather and flexible conductive shear stiffening gel (CNT-SSG). The maximum force dissipation capacity of the Leather/CNT-SSG/PU reaches 84 % in the drop hammer test and the limit penetration velocity up to 142.4 m/s in the ballistic test. Moreover, the Leather/CNT-SSG/PU exhibits excellent stress wave dissipation properties due to the hierarchical structure. By introducing the interdigital electrode array on the non-woven fabric layer, a bulletproof vest (Leather/CNT-SSG/NWF) with intelligent anti-impact performance and in situ monitoring behavior is successfully achieved. This toughness and flexibility coupling designation provides a novel method for next-generation soft body armor.

Novel Kevlar fabric composite for multifunctional soft body armor

Besides resistance to stab and impact, good flexibility and lightweight, soft body armors are also desired to have capabilities of monitoring human physical state, maintaining body temperature, and being nonflammable. In this work, a three-coaxial-layer fabric composite with Kevlar fabric as a core protection layer, reduced graphene oxide (RGO) and nano-SiO2/shear-thickening gel (STG) as functional layers is developed with the above-mentioned features for multifunctional soft body armors. The as-prepared SiO2/STG/RGO@Kevlar (SSG@Kevlar) fabric composite has excellent electro-heating performance under a low work voltage of 5 V, an extraordinary strain sensing ability (gauge factor over 41, outstanding linearity (R2 > 0.99), a fast response (69 ms), high strain resolution (0.05%)) and an effective impact energy sensing ability. Benefited from the introduction of SiO2/STG layer, the SSG@Kevlar fabric composite also exhibits an obviously enhanced anti-impact performance (a peak force reduction of 40%) and a significantly improved knife-stab resistance (a penetration depth reduction of 12 mm), due to the shear thickening effect of STG and the yarn interaction force enhancement caused by SiO2 nanoparticles. Furthermore, the SSG@Kevlar fabric composite offers remarkable fire-resistance, no open flame and glowing observed under continuous ignition due to the dense char formed. Consequently, the SSG@Kevlar fabric composite developed is promising for multifunctional soft body armors for protecting wearers in various aspects.

Multifunctional Polyurethane Composite Foam with Outstanding Anti-impact Capacity for Soft Body Armors.

Herein, a multifunctional polyurethane (PU) composite foam with a hierarchical structure is fabricated by dip-coating a carbon nanotube/shear-thickening gel (CNT/STG) and spray-coating nano-SiO2/STG on PU foam. The prepared nano-SiO2/CNT/STG@PU (SCS@PU) composite foam is lightweight, highly compressive, electrically conductive, superhydrophobic, and impact-energy absorptive. As a result, it possesses an excellent sensing ability to compression with a stable response up to 80% strain, an outstanding linearity of R2 > 0.99, and a wide response frequency of 0.01 to 1 Hz; it can also be used for effectively detecting impact force and sensing various human motions. Moreover, the superhydrophobicity with a water contact angle up to 154° of SCS@PU composite foam endows it with an excellent resistance to hazardous liquids (strong acid and alkali) to ensure its service reliability under harsh circumstances. In particular, the SCS@PU exhibits an outstanding anti-impact capability with an impact force attenuation rate of SCS@PU as high as 81%. Finally, its applications as soft body armors are demonstrated in protecting a wearer wearing a helmet with the SCS@PU as liner and using the SCS@PU as a smart kneecap against impact. On consideration of its excellent strain-sensing ability, superhydrophobicity, and outstanding anti-impact capability, the multifunctional SCS@PU composite foam developed is promising for personal safety protection.

A New Anthropomorphic Mannequin for Efficacy Evaluation of Thoracic Protective Equipment Against Blast Threats.

Exposure to blast is one of the major causes of death and disability in recent military conflicts. Therefore, it is crucial to evaluate the protective capability of the ballistic-proof equipment worn by soldiers against the effects of blast overpressure (i.e., primary blast injuries). A focus will be made on thoracic protective equipment (TPE). An anthropomorphic mannequin, called BOPMAN, and anesthetized swine both wearing soft, hard or no ballistic protection, were subjected to an open-field high-intensity blast. For swine, thoracic wall motion (acceleration and velocity) was recorded during blast exposure and severity of lung injury was evaluated postmortem. Different data were collected from BOPMAN thoracic responses, including reflected and internal pressure, as well as the force at the rear face of the instrumented part. The severity of blast-induced lung injuries (contusion extent, Axelsson Severity Scale) and the thoracic wall motion were decreased in animals protected with thoracic ceramic hard plates as compared to those wearing soft or no protection. There was a clear trend towards greater lung injury in animals protected with the soft body armor used, even when compared to unprotected animals. In line with these experimental data, the measured force as well as the force impulse measured using BOPMAN were also decreased with a ceramic hard plate protection and increased when a soft ballistic pack was used compared to no protection. Comparison of data collected on BOPMAN and swine equipped with the same protection level revealed that those two force parameters were well correlated with the level of blast-induced lung injury (force, R2 = 0.74 and force impulse, R2 = 0.77, p < 0.05). Taken together, our results suggest that the force and the force impulse data from BOPMAN may help estimate the efficiency of existing TPE regarding lung protection under blast exposure and may represent an important tool for development of future TPE.

Polypyrrole coating on aramid fabrics for improved stab resistance and multifunction

As an epoch of intelligent clothing is coming, soft body armor is required to possess not only improved mechanical performance but more functions, such as electronic conductivity, anti-ultraviolet capability, electrical heating property, etc. In this work, polypyrrole (PPy) is coated on Twaron fabrics to enhancing their stabbing resistance and simultaneously endowing them with multifunction. The resultant PPy-coated Twaron fabric exhibits better tensile strength. The strength of the PPy-coated fiber is increased by 12.03% and the maximum friction between the yarns in the PPy-coated fabric increases by 8.96 times. The quasi-static stab resistance of the PPy-coated fabric is also significantly improved in comparison with that of the neat counterpart. Furthermore, PPy-coated fabric exhibits anti-ultraviolet capability, high electrical conductivity, and stable electrical heating properties. The saturation temperature from the electrical heating is tunable by adjusting the applied voltage. These functions ensure that the treated fabrics can be applied in more areas and facilitate relieving the adverse effects from the external environment (rainstorm, blizzard, insolation, etc.) for the soft body armor. This study offers the potential of developing the novel generation of smart soft body armor with both excellent protective performance and intelligent wearable properties.

A numerical study on the low-velocity impact behavior of the Twaron®fabric subjected to oblique impact

AbstractIn this study, a finite element low-velocity impact model of Twaron®plain-woven fabric was created and analyzed using the commercial code ANSYS®-AUTODYN, and then was validated by drop-weight impact experiments. As a bullet or a fragment can strike a protective system from any angle in space, it is necessary to investigate fragment impact behavior response to impact threats from all angles of space. Therefore, in-plane obliquityθ, and spatial obliquityφ, were employed in this study and 17 different simulation test impact scenarios with different impact obliquity values were carried out using a standard hemispherical-head impactor. Results showed that the energy absorption of Twaron®fabric decreases with increasingθ, whereas under the sameθ, the energy absorption increases with increasingφ. This study also evaluated and compared the low-velocity impact performance of Twaron®fabric as a function of impactor shape, such as hemispherical, flat, and ogival heads, with differentθ. The results showed that under the same density, volume, and diameter conditions and at the normal impact scenario of a flat-head impactor, the fracture mechanism of the yarn is the same with all impact scenarios for a hemispherical-head impactor; the contacted yarns of the fabric fractured almost simultaneously. For the other oblique impact scenarios of the flat-head impactor, as well as impact scenarios of the ogival-head impactor, the yarns of the fabric fractured intermittently. Additionally, for the impact scenario with the ogival-head impactor, the effect of impact obliquity on energy absorption of the fabric was completely opposite to that of the hemispherical-head impact scenario. This is because in the hemispherical-head impact scenario, the fabric yarn tends to be damaged by tension, whereas in the ogival-head impact scenario, the fabric tends to be damaged by out-of-plane shear. These findings provide important guidance for the engineering of soft body armor and composite materials.

Criticality of inter-yarn friction in high-performance fabrics for the design of soft body armour

Inter-yarn friction plays a pivotal role in determining the impact resistance of woven fabric based soft armour panels (SAPs). This study presents a comparison of ballistic performance of SAPs made from para-aramid, ultra-high molecular weight polyethylene (UHMWPE) and poly para-phenylene-2,6-benzobisoxazole (PBO) woven fabrics and demonstrates the importance of inter-yarn friction. Plain woven fabrics were prepared using para-aramid, UHMWPE and PBO yarns and SAPs of constant areal density were produced using multiple layers of these fabrics. It is found that despite lower tenacity and modulus of para-aramid fibres, in comparison to those of UHMWPE and PBO, para-aramid SAP exhibits the lowest back face signature (BFS) owing to the highest inter-yarn friction. On comparing UHMWPE based SAPs made from fabrics with yarns of different fineness, fine UHMWPE yarn based fabric exhibits higher inter-yarn friction owing to more number of crossovers per unit length, which further translates into lower BFS of respective SAP. Therefore, while selecting fibres for soft body armour applications, inter-yarn friction should be considered along with the tensile properties.

Influence of Topical Cross-Linking on Mechanical and Ballistic Performance of a Woven Ultra-High-Molecular-Weight Polyethylene Fabric Used in Soft Body Armor

We demonstrate that topical cross-linking of woven ultra-high molecular weight polyethylene (UHMWPE) fabric with a small-molecule bis-diazirine reagent can enhance the fabric’s resistance to tear and perforation and could provide a lead to further increase the protection provided by a soft body armor made from UHMWPE materials. A series of experiments confirms that the enhancement is likely due to cross-links formed within and between the yarns constituting the woven fabric and that cross-linking particularly strengthens the fabric along its bias direction. The treated fabric was incorporated into multilayer composite panels and challenged in ballistic tests. These measurements revealed that as low as a 1 wt % cross-linker can significantly increase the V50 ballistic limit by 18 m/s.

Concerted effect of boron and porosity on shear thickening behavior of hybrid mesoporous silica dispersions

In the present work, the influence of porosity and boron on shear thickening behavior of hybrid mesoporous silica has been studied. Three different levels of boron modification were performed by varying the molar composition of boric acid viz., 1.5 mmol, 2.5 mmol, and 3.5 mmol in a co-condensation approach. The incorporation of boron in mesoporous silica network was confirmed by various techniques such as Fourier transform infra-red (FTIR), and 11B solid- state nuclear magnetic resonance (NMR) spectroscopy. The morphology and particle size were confirmed by using scanning and transmission electron microscopy. To evaluate the effect of boron and porosity on the shear thickening behavior, dispersions were prepared from mesoporous boron- modified silica (MSiB), control mesoporous silica (MSi), non-porous boron-modified silica (SiB), and control non-porous silica (Si) in polyethylene glycol. The shear thickening behavior was studied using steady shear rheology. The dispersion prepared from different loadings of synthesized MSiB containing 1.5 mmol boron showed more than 16 times increase in viscosity (657.7 Pa.s) compared to that of MSi (39.2 Pa.s) at a fairly low volume fraction (φ = 0.15) of silica. It is expected that the highly ordered mesoporous architecture of hybrid silica has improved the interaction between the particle and the dispersing medium through hydrogen bonding. The porous morphology of the hybrid mesoporous silica as well as the incorporation of boron in the silica network favors the formation of a frictional contact network, and a transition from continuous shear thickening (CST) to discontinuous shear thickening (DST) behavior was observed. Therefore, silica prepared via incorporation of boron as well as porosity can be material of interest in variety of applications, for example, soft body armors, sporting goods, and shear thickening electrolytes for high impact resistant batteries.

Failure Investigation of Layered LFT SB1plus Package after Ballistic Tests for Level IIA.

The main objective of this study focuses on designing and testing body protection systems using advanced materials based on aramid fibers, for high impact speeds of up to 420 ± 10 m/s. Ballistic applications of aramid fiber-based composites mostly include soft body armors. The investigation of the failure mechanisms identifies issues of protective fabrics, major challenges and technological problems for efficient development of these systems. The authors present an investigation on the failure processes and destructive stages of a ballistic package made of successive layers of LFT SB1plus, a trade name for a multiaxial fabric by Twaron Laminated Fabric Technology (LFT), taking into account the particular test conditions from NIJ Standard-0101.06 Ballistic Resistance of Body Armor. The main parameter of interest was the backface signature (BFS), but also details of projectile arrest and SEM investigation could offer arguments for using this material for individual protection. For the reported tests, the maximum and minimum values for BFS were 12 mm and 24 mm, the mean value being 18.66 mm and the standard deviation being 3.8 mm.