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Polyurea (PU) elastomers have become increasingly popular in recent years due to their exceptional blast mitigation, ballistic protection, and wear resistance properties. However, their applications as protective materials are often limited as a result of their high flammability. In this study, aromatic dianhydrides (ArDAs) were employed to capture the thermal decomposing products of PU during combustion and facilitate the generation of highly heat-resistant aromatic polyimides (i.e., end group capturing effect), thus enhancing the material's char-forming ability and improving its flame retardancy. The ArDAs were found to significantly reduce the peak heat release rate (PHRR), total heat release (THR), total smoke release (TSR), and total CO release (TCO) of PU, and meanwhile increased the amount of char residue. At 20 wt% content of 3,3′,4,4′-benzophenonetetracarboxylic dianhydride (BTDA), PHRR, THR, TSR and TCO of PU were reduced by 70.6%, 32.5%, 44.7% and 58.8%, respectively, the char residue was increased by 405%, and the mechanical properties of PU were well maintained (88.5% tensile strength and 91.0% elongation at break of pure PU). These findings demonstrate that ArDAs could serve as a new kind of halogen/phosphorus-free flame retardant for PU elastomers. [Display omitted] • Novel halogen/phosphorus-free flame retardants were designed for PU elastomers. • The flame retardant mechanism is end group capturing by aromatic dianhydrides (ArDAs). • PHRR, THR, TSR and TCO were simultaneously reduced, and char residue was improved. • Both excellent flame retardant property and high mechanical property were achieved. [ABSTRACT FROM AUTHOR]

The ballistic performance of Glass Fiber Reinforced Polymer (GFRP) composite [0°/90°] s target has been studied against 19 mm diameter conical nosed steel projectiles. The effects of target thicknesses, geometry and boundary conditions have been studied on contact force, residual velocity and induced damage. The ballistic tests have also been reproduced by carrying out three dimensional finite element simulations on AUTODYN hydro code. The nonlinear material behaviour of GFRP has been modeled employing shock effect for studying the different modes of failure in the target. The energy absorbing capacity and ballistic limit velocity of the target have also been discussed with varying target thickness, aspect ratio and boundary conditions. The numerical results show the significant contribution of aspect ratio and boundary condition on the ballistic perforation characteristic of the target. The propagation of pressure wave that causes the generation of various stresses in laminate due to projectile impact has been discussed and the numerical results thus obtained in terms of residual velocities and damage have been found to have close correlation with the experimental results. [ABSTRACT FROM AUTHOR]

This paper concerns the ballistic resistance of hybrid composite shields. The analysis covers three types of two-layer composites. The first shield layer were plates of tungsten carbide (WC), ceramic balls Al 2 O 3 (aluminum oxide 92) or steel balls (100Cr6 steel) embedded in a polymer matrix. The second layer was a laminate consisting of 10 layers of aramid fabric in a matrix of styrene-butadiene-styrene (SBS) copolymer. The study presents the analysis of damage caused by a ballistic impact, further compared to the results of computer simulations conducted using the finite element method (FEM). Samples were overshot along the ballistic track from the Beryl rifle. The 5.56 × 45 mm ammunition with the bullet of SS109 MESCO type was used. [ABSTRACT FROM AUTHOR]

A constitutive model based on the concept of continuum damage mechanics has been proposed to study the progressive damage behavior of composite laminates under ballistic impact. The proposed model is investigated in five steps: First, the quadratic form of damage initiation criteria are presented to predict the initiation of failure in different modes. Second, an exponential form damage evolution law combined with characteristic length based fracture energy approach has been presented. Stiffness degradation is characterized by a variable determined by the equivalent displacement for each failure mode. Third, an experimentally verified strain rate model that considers the rate dependency of the strength and modulus of the composite laminate is considered. Fourth, cohesive elements are inserted at every inter-layer for modeling the delamination evolution. Fifth, the constitutive model has been combined with an element erosion algorithm for the removal of highly distorted elements. Simulations have been performed using reduced integration hexahedra elements (RIHE) and full integration hexahedra elements (FIHE). Implementation of cohesive elements exhibited better delamination progression. Experimentally verified strain rate model enhanced the efficiency of the model showing good correlation between the present simulations and experimental observations, in terms of damage patterns, residual velocity, kinetic energy and ballistic limit. [ABSTRACT FROM AUTHOR]

An investigation on the ballistic impact behaviour of tungsten blunt projectiles on Kevlar ® 29 plain-woven fabrics with an epoxy matrix has been carried out for the purpose of gaining an insight in the phenomena at the level of the composite components. Numerical models have been developed and have then been compared with experimental results. The numerical models have been developed in LS-DYNA adopting two different approaches: a Macro-homogeneous model, in which each layer is modelled as an orthotropic equivalent, and a Meso-heterogeneous one, based on the definition of all the details of the fabric architecture in the area of impact of the projectile's tip (in this case, yarns and matrix have their own properties). In both models, the projectile is considered a rigid body. An in-depth analysis of the macro parameters, such as the residual velocities, their dependency upon the yaw impact angle and delamination has been performed. Also the stress state inside the layers and the morphological features of the damaged plates has been studied and interesting results regarding the behaviour of different layers of Kevlar ® 29 plain-woven fabrics during impact have been obtained. Performances and the pros/cons of the different numerical approaches have also been discussed. [ABSTRACT FROM AUTHOR]

This study evaluates the ballistic impact resistance of soda-lime silicate glass strengthened by ion exchange for application in lightweight and thin bulletproof materials. The maximum flexural strength values of the strengthened glass with thicknesses of 3, 4, 8, and 10 mm were 0.63, 0.68, 0.73, and 0.77 GPa, respectively, values that were 3.5 times higher than that of the parent glass. By laminating polycarbonate and multilayer defense film with the strengthened glass, we achieved a ballistic limit velocity of 973.8 m/s, which was 16% higher than the standard. Also, the transmittance satisfied the standard for bulletproof windows. [ABSTRACT FROM AUTHOR]

This work investigates balsa core sandwich composites with thin carbon fiber skins subjected to single and multi-site sequential ballistic impacts. For the sandwich composite composed of core and facesheet thicknesses of 0.95 cm and 0.25 mm respectively. The ballistic limit was determined to be 96 m s −1 , indicating that it was capable of withstanding impacts from small projectiles such as, secondary debris from blast, hurricane, tornado and foreign object debris from roads and runways. The visual inspection and non-destructive evaluation (NDE) tests showed that the damage was relatively localized. A neat perforation was observed on the strike side facesheet whereas the back facesheet indicated greater spread of damage. The effect of prior damage in terms of residual and absorbed energies became more pronounced as the number of impacts increased. Damage areas were measured in order to quantify the extent of visible failure and/or perforations, while NDE tap testing was implemented to extract the information regarding interfacial damage. [ABSTRACT FROM AUTHOR]

Abstract: Strong, low density fibres have been favoured materials for ballistic protection, but the choice of fibres is limited for making body armour that is both protective and lightweight. In addition to developments of improved fibres, alternative approaches are required for creating more protective and lighter body armour. This paper reports on a study on hybrid fabric panels for ballistic protection. The Finite Element (FE) method was used to predict the response of different layers of fabric in a twelve-layer fabric model upon impact. It was found that the front layers of fabric are more likely to be broken in shear, and the rear layers of fabric tend to fail in tension. This suggested that using shear resistant materials for the front layer and tensile resistant materials for the rear layer may improve the ballistic performance of fabric panels. Two types of structure, ultra-high-molecular-weight polyethylene (UHMWPE) woven and unidirectional (UD) materials, were analyzed for their failure mode and response upon ballistic impact by using both FE and experimental methods. It was found that woven structures exhibit better shear resistance and UD structures gives better tensile resistance and wider transverse deflection upon ballistic impact. Two types of hybrid ballistic panels were designed from the fabrics. The experimental results showed that placing woven fabrics close to the impact face and UD material as the rear layers led to better ballistic performance than the panel constructed in the reverse sequence. It has also been found that the optimum ratio of woven to UD materials in the hybrid ballistic panel was 1:3. The improvement in ballistic protection of the hybrid fabric panels allows less material to be used, leading to lighter weight body armour. [Copyright &y& Elsevier]

Abstract: In this paper, based on LS-Dyna code, a new finite element (FE) simulation of the ballistic perforation of the ceramic/composite targets, which impacted by cylindrical tungsten projectiles, has been presented. Research on this method has been conducted by a few research groups in recent years. The ceramic material, which is the front plate, has been made of Alumina 99.5% and composite back-up plate composed of Twaron fibers. The 2-dimensional (2D), axi-symmetric, dynamic-explicit, Lagrangian model has been considered in this simulation. The Johnson–Cook, Johnson–Holmquist and Composite-Damage materials behaviors have been used for projectile, ceramic and composite materials respectively. The brittle fracture and fragmentation of ceramic conoid, the failure criteria based on fracture of fibers or matrixes of composite materials and erosion or flattening of projectile during perforation have been considered. The residual velocity and perforation time has been obtained and compared with the available analytical models. The results show that when the ceramic is impacted by a projectile, a fragmented ceramic conoid breaks from ceramic tile and the semi-angle of ceramic conoid with increasing initial velocity decreases. Furthermore, the dishing of composite layers at high impact velocities and the delamination of layers near the ballistic limit velocity decrease. [Copyright &y& Elsevier]

Abstract: The ballistic impact damage of a bi-axial warp-knitted (BWK) composite was simulated based on a simplified microstructure model. The microstructure was simplified by transferring the BWK composite into an equivalent laminate. Residual velocities of conically cylindrical steel projectiles (Type 56 in China Military Standard) and impact damage of the BWK composite were calculated with finite element method (FEM) and compared with those in experimental. Good agreements between FEM and experimental were found. The agreements verify the availability of the simplified model for the ballistic performance prediction of the BWK composite. We expect such an effort could be extended to the design of the BWK composites in field of aircrafts, high speed vehicles and ballistic protections. [Copyright &y& Elsevier]

Abstract: During highly dynamic and ballistic loading processes, large inelastic deformation associated with high strain rates leads, for a broad class of heterogeneous materials, to degradation and failure by localized damage and fracture. However, as soon as material failure dominates a deformation process, the material increasingly displays strain softening and the finite element predictions of ballistic response are considerably affected by the mesh size. This gives rise to non-physical description of the ballistic behavior and mesh-dependent ballistic limit velocities that may mislead the design of ballistic resistant materials. This paper is concerned with the development and numerical implementation of a coupled thermo-hypoelasto-viscoplastic and thermo-viscodamage constitutive model within the laws of thermodynamics in which an intrinsic material length scale parameter is incorporated through the nonlocal gradient-dependent damage approach. This model is intended for impact and ballistic penetration and perforation problems of heterogeneous metallic targets such as metal matrix composites with dispersed particles at decreasing microstructural length scales. An evolution equation for the material length scale as a function of the material microstructural features (e.g. mean grain size in polycrystalline materials or particle size and inter-particle spacing in metal matrix composites), course of plastic deformation, strain hardening, strain-rate hardening, and temperature is presented. It is shown through simulating plugging failure in ballistic penetration of high-strength steel targets of different thicknesses by a hard blunt projectile that the length scale parameter plays the role of a localization limiter allowing one to obtain meaningful values for the ballistic limit velocity independent of the finite element mesh density. It is also shown that a local damage model incorporating viscosity and heat conduction as localization limiters, which are known to implicitly introduce length scale parameters, is insufficient in illuminating the mesh sensitivity at impact velocities close to the ballistic limit and that the mesh sensitivity increases as the target thickness increases. Therefore, the proposed nonlocal damage model leads to an improvement in the modeling and numerical simulation of high velocity impact related problems. [Copyright &y& Elsevier]

Abstract: A qualitative analysis of experimental results from small caliber ballistic impact and dynamic indentation on a 3D glass fiber reinforced composite are presented. Microscopic analysis of the damaged specimens revealed that the current 3D weaving scheme creates inherently two weak planes which act as potential sites for delamination in the above experiments. It is concluded that while the z-yarns may be effective in limiting the delamination damage at low loads and at low rates of impact, at high loads and high loading rates delamination continues to be the dominant failure mode in 3D woven composites. It is shown that dynamic indentation can be used to capture the progression of damage during impact of 3D woven composites. [Copyright &y& Elsevier]

Abstract: This paper presents ballistic impact damages of 3-D orthogonal woven composite in finite element analysis (FEA) and experimental. A unit-cell model of the 3-D woven composite was developed to define the material behavior and failure evolution. A user-defined subroutine VUAMT was compiled and connected with commercial available FEA code ABAQUS/Explicit to calculate the ballistic penetration. Ballistic impact tests were conducted to investigate impact damage of 3-D kevlar/glass hybrid woven composite. Residual velocities of conically-cylindrical steel projectiles (Type 56 in China Military Standard) and impact damage of the composite targets after ballistic perforation were compared both in theoretical and experimental. The reasonable agreements between FEA results and experimental results prove the validity of the unit-cell model in ballistic limit prediction of the 3-D woven composite. We believe such an effort could be extended to bulletproof armor design with the 3-D woven composite. [Copyright &y& Elsevier]

Composite structural armor (CSA) is a multi-functional structure that provides ballistic protection, stiffness and strength at minimum weight. It consists of a multi-layered architecture of polymer composites, rubber and ceramic tiles, stacked in a precise manner to obtain optimal ballistic performance. In the present work, the finite element method is used to conduct a detailed analysis of the mechanisms of load transfer and deformation of CSA subjected to bending loads. The results from two modeling approaches (three-dimensional and two-dimensional simulations) are compared to assess the accuracy of the computationally efficient two-dimensional model. The calculated deflections and surfaces strains from both models are found to agree very well with experimental results. The stress transfer between the layers is further analyzed using the two-dimensional model and the resulting through-thickness strain and stress distributions are discussed. It is found that the deformation of this multi-layered construction is complex and dependent upon the mechanism of stress transfer between the outer surface layer and the ceramic tiles. The effect on non-linear behavior of the constituent materials is investigated. The gap filled with polymer that separates adjacent ceramic tiles is shown to significantly influence the stiffness and strength of CSA. It is found that the plastic deformation of the resin corresponds to the onset of non-linear structural response. [Copyright &y& Elsevier]