Back Face Deformation Research Articles

Ballistic Impact Resistance of Bulletproof Vest Inserts Containing Printed Titanium Structures

Finite element modeling of ballistic impact of inserts containing titanium structures were presented in the article. The inserts containing an additional layer made using additive manufacturing technology were analyzed. The layer was created from repetitive elements made without connections (adjacent cells were inseparable). Four variants of printed titanium structures were placed between layers of Twaron CT 750 aramid fabric to create ballistic inserts. In order to assess the ballistic resistance of the inserts, numerical simulations of ballistic impact phenomenon were carried out using LS-Dyna software. In the simulations the inserts were placed on a steel box filled with ballistic clay and were fired at with the 9 × 19 mm full metal jacket (FMJ) Parabellum projectile. The main aim of the work was to check the effectiveness of such solutions in soft ballistic protection applications and to select the most effective variant of 3D printed structure. Results of the numerical analysis showed a high potential for 3D printed structures made of titanium alloys to be used for bulletproof vest inserts. In all analyzed cases the projectile was stopped by the armor. In addition, thanks to the cooperation of adjacent cells, the projectile energy density was distributed over a large area, as evidenced by large volumes of hollows in the ballistic clay. The indentations in the ballistic clay obtained in the simulations were significantly lower than the acceptable value for the back face deformation (BFD) parameter required by international body armor standards.

Dynamic back face deformation measurement with a single optical fibre

A single optical fibre sensor is used to measure the dynamics of an impact. The method consists of sewing the optical fibre onto a woven Kevlar layer and placing it between the shoot pack and backing material. The measurement is accomplished by using the friction between the layer and the optical fibre to relate the optical fibre strain to impact deformation. Tests are done using a backing material of Roma Plastilina No.1 clay, and transparent ballistics gel with independent high-speed imaging. A final calculated BFD average error of 7.75% is presented as well as a timing error of 15.5% between the imaged dynamic BFD and the dynamic BFD determined by the FBG. This method is also tested at the U.S. Army Aberdeen Test Center in Maryland with a final calculated error of 7%.

Assessment of Thorax Finite Element Model Response for Behind Armor Blunt Trauma Impact Loading Using an Epidemiological Database.

Nonperforating ballistic impacts on thoracic armor can cause blunt injuries, known as behind-armor blunt trauma (BABT). To evaluate the potential for this injury, the back face deformation (BFD) imprinted into a clay backing is measured; however, the link between BFD and potential for injury is uncertain. Computational human body models (HBMs) have the potential to provide an improved understanding of BABT injury risk to inform armor design but require assessment with relevant loading scenarios. In this study, a methodology was developed to apply BABT loading to a computational thorax model, enhanced with refined finite element mesh and high-deformation rate mechanical properties. The model was assessed using an epidemiological BABT survivor database. BABT impact boundary conditions for 10 cases from the database were recreated using experimentally measured deformation for specific armor/projectile combinations, and applied to the thorax model using a novel prescribed displacement methodology. The computational thorax model demonstrated numerical stability under BABT impact conditions. The predicted number of rib fractures, the magnitude of pulmonary contusion, and injury rank, increased with armor BFD, back face velocity, and input energy to the thorax. In three of the 10 cases, the model overpredicted the number of rib fractures, attributed to impact location positional sensitivity and limited details from the database. The integration of an HBM with the BABT loading method predicted rib fractures and injury ranks that were in good agreement with available medical records, providing a potential tool for future armor evaluation and injury assessment.

Development of Lightweight and High-Performance Ballistic Helmet Based on Poly(Benzoxazine-co-Urethane) Matrix Reinforced with Aramid Fabric and Multi-Walled Carbon Nanotubes.

This study aims to develop a lightweight ballistic helmet based on nanocomposite with matrix of the copolymer of benzoxazine with an urethane prepolymer [poly(BA-a-co-PU)], at mass ratio 80/20, reinforced with aramid fabric and multi-walled carbon nanotubes (MWCNTs). This has a protection level II according to the National Institute of Justice (NIJ) 0106.01 standard. The effects of MWCNTs mass content in a range of 0 to 2 wt% on tensile, physical and ballistic impact properties of the nanocomposite were investigated. The results revealed that the introduction of MWCNTs enhanced the tensile strength and energy at break of the nanocomposite; the highest values were obtained at 0.25 wt%. In addition, the nanocomposite laminate with 20 plies of aramid fabric showed the lowest back face deformation of 8 mm which was much lower than that specified by the NIJ standard. According to Military Standard (MIL-STD) 662F, the simulation prediction revealed that the ballistic limit of the ballistic helmet nanocomposite was as high as 632 m s−1. The developed laminates made of aramid fabric impregnated with poly(BA-a-co-PU) 80/20 containing 0.25 wt% MWCNTs showed great promise for use as a light weight and high-performance ballistic helmet.

Star topology increases ballistic resistance in thin polymer films

Polymeric films with greater impact and ballistic resistance are highly desired for numerous applications, but molecular configurations that best address this need remain subject to debate. We study the resistance to ballistic impact of thin polymer films using coarse-grained molecular dynamics simulations, investigating melts of linear polymer chains and star polymers with varying number (2≤f≤16) and degree of polymerization (10≤M≤50) of the arms. We show that increasing the number of arms f or the length of the arms M both result in greater specific penetration energy within the parameter ranges studied. Greater interpenetration of chains in stars with larger f allows energy to be dissipated predominantly through rearrangement of the stars internally, rather than chain sliding. During film deformation, stars with large f show higher energy absorption rates soon after contact with the projectile, whereas stars with larger M have a delayed response where dissipation arises primarily from chain sliding, which results in significant back face deformation. Our results suggest that stars may be advantageous for tuning energy dissipation mechanisms of ultra-thin films. These findings set the stage for a topology-based strategy for the design of impact-resistant polymer films.

Dynamic Behaviour of Carbon/Ultra High Molecular Weight Polyethylene (UHMWPE) Hybrid Composite Laminates Under Ballistic Impact

Hybrid composites are versatile materials for armour and aerospace applications as they can confer the advantages of both the fibres to the system. Carbon/UHMWPE hybrid composites were prepared by varying volume % of carbon and UHMWPE laminates using the hot press method. The hybrid composites were each subjected to ballistic impact from a 7.62 × 39 mm mild steel (MS) projectile at an impact velocity of 730 ± 10 m.s− 1. The effect of hybridization ratio on the energy absorption of hybrid composites was investigated. The specific energy density (SED) dissipated by the hybrid composites was determined and it is found to increase with increasing volume % of UHMWPE. The real time dynamic deformation of the hybrid composites under ballistic impact was recorded using a high speed video camera. The back face deformation and bulge growth rate of hybrid composites were determined and the results are presented. Post impact analysis of the composites was carried out to examine the various failure modes.

In-situ strain measurement of ballistic fabrics during impact using fiber Bragg gratings

Abstract In previous experiments, the authors demonstrated that strain values collected from fiber Bragg gratings (FBG) integrated into a single layer of Kevlar fabric, placed between a soft armor test specimen and backing material, could be related to the time dependent back-face deformation (BFD) of the armor sample. In this paper, we investigate the specific fabric deformation and failure mechanisms that cause observed events in the FBG measured strain behavior and the FBG spectral profile throughout the impact event. For these experiments, the standard clay backing material was replaced with a 20% clear ballistic gel to provide visual access to the back-face. The test specimen was impacted by an 8.24 g steel ball bearing travelling at 248.8 m/s, during which strain was calculated from the measured full spectrum response of the FBG using a high-speed optical interrogation system. The strain response was compared to the BFD of the Kevlar sample. The BFD was measured through the clear ballistic gel using two high speed cameras recording at 100,000 fps. The results from these tests can be used for future testing using a non-transparent backing material to obtain a detailed strain–time history, back-face deformation history and an understanding of the time sequence of physical energy dissipation mechanisms in the fabric.

Postmortem Analysis Using Different Sensors and Technologies on Aramid Composites Samples after Ballistic Impact.

This work focuses on the combination of two complementary non-destructive techniques to analyse the final deformation and internal damage induced in aramid composite plates subjected to ballistic impact. The first analysis device, a 3D scanner, allows digitalising the surface of the tested specimen. Comparing with the initial geometry, the permanent residual deformation (PBFD) can be obtained according to the impact characteristics. This is a significant parameter in armours and shielding design. The second inspection technique is based on computed tomography (CT). It allows analysing the internal state of the impacted sample, being able to detect possible delamination and fibre failure through the specimen thickness. The proposed methodology has been validated with two projectile geometries at different impact velocities, being the reaction force history on the specimen determined with piezoelectric sensors. Different loading states and induced damages were observed according to the projectile type and impact velocity. In order to validate the use of the 3D scanner, a correlation between impact velocity and damage induced in terms of permanent back face deformation has been realised for both projectiles studied. In addition, a comparison of the results obtained through this measurement method and those obtained in similar works, has been performed in the same range of impact energy. The results showed that CT is needed to analyse the internal damage of the aramid sample; however, this is a highly expensive and time-consuming method. The use of 3D scanner and piezoelectric sensors is perfectly complementary with CT and could be relevant to develop numerical models or design armours.

Analysis of dynamic back face deformation of a body armor impact by a rifle bullet using 3D-DIC

Body armor that defeats the threat can still cause severe trauma by striking the thorax with its back face deformation (BFD). In this study, the 3D digital image correlation (DIC) technology was adopted to capture the dynamic BFD of a ceramic/UHMWPE body armor impact by a rifle bullet. The shape and size of the BFD time histories, deformation velocity, acceleration and line slices profile on armor back face were obtained. The maximum BFD height can reach to 34.3 mm within about 1.2 ms, and then slowly decreases to the static deformation height of about 24mm for the impact velocity 675.4 m/s. The maximum deformation velocity and acceleration are 129.9 m/s and 1.29e6 m/s^2, respectively.

Study on Behind Helmet Blunt Trauma Caused by High-Speed Bullet.

The mechanism of Behind Helmet Blunt Trauma (BHBT) caused by a high-speed bullet is difficult to understand. At present, there is still a lack of corresponding parameters and test methods to evaluate this damage effectively. The purpose of the current study is therefore to investigate the response of the human skull and brain tissue under the loading of a bullet impacting a bullet-proof helmet, with the effects of impact direction, impact speed, and impactor structure being considered. A human brain finite element model which can accurately reconstruct the anatomical structures of the scalp, skull, brain tissue, etc., and can realistically reflect the biomechanical response of the brain under high impact speed was employed in this study. The responses of Back Face Deformation (BFD), brain displacement, skull stress, and dura mater pressure were extracted from simulations as the parameters reflecting BHBT risk, and the relationships between BHBT and bullet-proof equipment structure and performance were also investigated. The simulation results show that the frontal impact of the skull produces the largest amount of BFD, and when the impact directions are from the side, the skull stress is about twice higher than other directions. As the impact velocity increases, BFD, brain displacement, skull stress, and dura mater pressure increase. The brain damage caused by different structural bullet bodies is different under the condition of the same kinetic energy. The skull stress caused by the handgun bullet is the largest. The findings indicate that when a bullet impacts on the bullet-proof helmet, it has a higher probability of causing brain displacement and intracranial high pressure. The research results can provide a reference value for helmet optimization design and antielasticity evaluation and provide the theoretical basis for protection and rescue.

Dynamic response of sandwich panels with multi-layered aluminum foam/UHMWPE laminate cores under air blast loading

The air blast responses of sandwich panels with multi-layered aluminum foam/UHMWPE laminate cores were analyzed experimentally and numerically. The focus was placed on the effects of foam core gradation and the locations of UHMWPE laminates. Multiple failure modes were exhibited by the panels, including local indentation and global bending deformation of face sheets, crushing and fragmentation of foam core, debonding of adhesive, and delamination and fracture failure of UHMWPE laminate. Experimental results showed that the ascending density foam core was beneficial to decrease the face sheet deformations and to alleviate the shear-induced collapse of foam core. Incorporating the UHMWPE laminates would induce a lower front face deformation and a higher back face deformation. The phenomenon would be more evident with the UHMWPE laminates placed close to the front face. Numerical results demonstrated that deploying the UHMWPE laminate as a first layer core significantly lowered the center velocity of front face. The existence of UHMWPE laminate would change the response modes of the panels. Most of the blast energy was dissipated by the multi-layered cores. The UHMWPE laminates exhibited less efficient in absorbing energy than the aluminum foam. Generally, the findings revealed the undesirable performance of the panels with multi-layered aluminum foam/UHMWPE laminate core under air blast loading.

Development of cost effective personnel armour through structural hybridization

The objective of the present study is to develop cost effective thermoplastic hybrid laminate using Dyneema® HB50 and Tensylon®HSBD 30A through structural hybridization method. Laminates having 20 mm thickness were fabricated and subjected to 7.62 × 39 mm mild steel core projectile with an impact velocity of 730 ± 10 ms−1. Parameters such as energy absorption, back face deformation and rate of back face deformation were measured as a function of hybridization ratio. It was observed that hybrid laminate with 50:50 ratio (w/w) of Tensylon® and Dyneema® with Tensylon® as front face showed 200% more energy absorption when compared to 100% Tensylon® laminate and showed equal energy absorption as that of expensive 100% Dyneema® laminate. Moreover, hybrid laminate with TD50:50 ratio showed 40% lower in terms of final back face deformation than Dyneema® laminate. Rate of back face deformation was also found to be slow for hybrid laminate as compared to Dyneema® laminate. Dynamic mechanical analysis showed that, Tensylon® laminate has got higher stiffness and lower damping factor than Dyneema® and hybrid laminates. The interface between Tensylon® and Dyneema® layers was found to be separating during the penetration process due to the poor interfacial bonding. Failure behaviour of laminates for different hybridization ratios were studied by sectioning the impacted laminates. It was observed that, the Tensylon® laminate has undergone shear cutting of fibers as major failure mode whereas the hybrid laminate showed shear cutting followed by tensile stretching, fiber pull out and delamination. These inputs are highly useful for body armour applications to design cost effective armour with enhanced performance.

Impact resistant hybrid composites reinforced with inorganic nanoparticles and nanotubes of WS2

Impact resistant nanocomposites are recognized as promising in saving lives in the car, skiing and bicycle accidents, preventing the failure of polymer-based prostheses and orthotic devices, as well as for various defense applications. This research reports improved impact and ballistic resistance of composite materials based on p-aramid fabric impregnated with phenolic resin and poly (vinyl butyral) (PVB), reinforced with small amounts of inorganic fullerene-like nanoparticles of tungsten disulfide (IF-WS2) and multiwall inorganic nanotubes (INT-WS2). The evaluation of the tensile testing resulted in significant improvements in the tensile strength (24.7%) and toughness (64.1%) for the composites with the addition of only 0.3 wt% INT-WS2 nanotubes. The impact toughness increased by 35.8%; the back-face deformation depths after shots were reduced by 12%; the absorbed energy of the knife stab 8.5% higher and the deformation depth 40.4% smaller than for the sample without the nanoreinforcement. The reaction of the composite material to shot impact was recorded by high-speed and infrared camera, to observe heating and cooling rates of the sample caused by dissipation of the kinetic energy of the shots.

Ballistic Head Protection in the Light of Injury Criteria in the Case of the Wz.93 Combat Helmet

This paper discusses the general conditions relating to ballistic head protection, analyzing the risks that may occur on contemporary battlefields. A thorough literature review has enabled us to present development trends for helmets used in the largest armies in the world. The authors have focused on impacts to the helmet shell, overloading the entire helmet-protected head–neck system. The main objective of this study is to investigate the protective capability of a helmet shell when subjected to projectile–helmet contact, with contact curvature taken as being an indicator of the impact energy concentration. Blunt head trauma was estimated using backface deformation (BFD). The Wz.93 combat helmet was used for testing. Analytically, dependencies were derived to determine the scope of BFD. A five-parameter model of the helmet piercing process was adopted, thus obtaining the optimal BFD range. Verification of theoretical considerations was carried out on a specially developed research stand. In the ballistic tests, dynamic deflection of the helmet’s body was registered using a speed camera. On the impact testing stand, a fragment of the helmet was pierced, producing results in the low impact velocity range. Data have been presented on the appropriate graph in order to compare them with values specified in the relevant standard and existing literature. Our results correlate well with the norm and literature values.

Experimental investigation on the performance of PVC foam core sandwich panels under air blast loading

The performance of sandwich panels with metallic face-sheets and polyvinyl chloride (PVC) foam ungraded/graded cores under air blast loading was investigated experimentally. In present paper, the performance of interest included the permanent deformation, failure modes and the associated mechanisms underlying the overall response. The majority of the paper was concentrated on the effects of geometric parameters and core gradation on the blast performance of panels. Experimental results showed that regardless of the panel configuration considered here, the panels exhibited a localized dishing deformation of front face and a global dome deformation of back face. The way to achieve a more effective design against air blast is to increase the thickness of the face sheet towards the blast rather than the other sheet. The panel system with a low-density (large thickness) core appeared to be favorable for the mitigation of back face deformation and core cracking failure. Effect of core gradation indicated that placing the low density material at the third core layer and the high density material at the second core layer, thus reducing the momentum transmitted to back face and increasing the crushing deformation of graded core, helped enhance the blast resistance of panel. Furthermore, the comparisons of blast performance between ungraded sandwich panel and graded panels were made. It turned out that overall graded panels did not always outperform the ungraded panel. An optimal core gradation would bring benefits for blast resistance in terms of the face sheet deformation and core cracking failure, but also was required to face the risk of delamination failure.

Soft body armor time-dependent back face deformation (BFD) with ballistics gel backing

This paper presents a method for obtaining time dependent back face deformation (BFD) data for body armor during ballistic impact using a clear ballistics gelatin backing and high-speed cameras to capture the deformation profile. Using this method, baseline fabric characterization data was obtained for samples comprised of varying layers of 467 g/m2 Kevlar K29 fabric impacted with 8.24 g steel ball projectile and backed with NATO standard 20% clear ballistics gelatin. For these tests, deformation depths were seen to increase with increasing impact energy and decreasing total areal density. A limited study of the various test parameters was performed by testing one additional fabric, projectile, and ballistics gelatin. From these comparisons, it was observed that 122 g/m2 Kevlar KM2+ fabric performs better per weight than 467 g/m2 Kevlar K29 fabric in terms of BFD, 9 mm FMJ projectiles produce deeper BFDs than 12.7 mm steel ball projectiles, and backing a sample with FBI standard 10% ballistics gel increases the BFD considerably over NATO standard 20% ballistics gel.

Body armor for stab and spike protection, Part 2: a review of test methods

In many countries, stabbing is still the most common cause of homicide. It is difficult to quantify the magnitude of the force used in a stabbing incident by an alleged assailant. Pathologists describe the stabbing force using relative subjective terms, such as mild, moderate, considerable or severe. More precise objective evaluation of the severity of stabbing is very important for quantitative understanding of stab wounds and for scientific assessment of the effectiveness of stab-resistant armor. In this perspective review, the standard objective methods used for the quantification of the protection provided by the stab and spike or puncture resistant materials are described. In addition, the nonstandard test methods developed by various research organizations have also been discussed. This paper is a follow-up to the previous paper (Body armor for stab and spike protection, Part 1: scientific literature review) published in the Textile Research Journal. Special emphasis has been given to the recent developments in both test standards and experimental methodologies used to evaluate stab and spike protection. This paper has systematically reviewed the standard test methods used for evaluating the stab and spike protection performance of body armor for comparisons. Furthermore, the backing materials, which are used for stab and puncture resistance tests to quantify the back-face deformation, were highlighted. Also, the approach for the wearer’s trial to predict the performance was discussed. This paper will provide guidelines to researchers, manufacturers and defense department working in the area of design and fabrication of body armor systems.

Experimental study of the deformation of a ballistic helmet impacted with pistol ammunition

Currently, ballistic helmets are mostly designed to stop fragments from diverse explosive devices. Nowadays, new requirements have emerged for helmets, such as stopping direct impacts from revolver and pistol threats. The probability of these types of impacts on helmet systems is increasing due to the changes in warfare and military operations. Although, it is possible to stop this kind of projectile, there is a lack of studies regarding the possible injuries suffered by the user due to non-perforating impacts.In this research, a comparison between the results obtained with a clay head form and a head surrogate with force sensors was done to estimate the load force on the skull during an impact. 9 × 19 mm FMJ projectiles were fired against a ballistic helmet to study the indentation and the force generated by the back face deformation against the two different head forms.

Relevant factors in the design of composite ballistic helmets

In this paper, the design methodology of composite ballistic helmets has been enhanced considering biomechanical requirements by means of finite element analysis. Modern combat helmets lead to a new type of non-penetrating injury, the Behind Helmet Blunt Trauma (BHBT), generated by the deformation of the inner face of the helmet, the so-called backface deformation (BFD). Current standard testing methodologies use BFD as the main measure in ballistic testing. Nonetheless, this work discusses the relationship between this mechanical parameter and the head trauma (BHBT) by studying different head injury criteria. A numerical model consisting of a helmet and a human head is developed and validated with experimental data from literature. The consequences of non-penetrating high-speed ballistic impacts upon the human head protected by an aramid combat helmet are analysed, concluding that the existing testing methodologies fail to predict many types of head injuries. The influence of other parameters like bullet velocity or head dimensions is analysed. Usually, a single-sized helmet shell is manufactured and the different sizes are adjusted by varying the foam pad thickness. However, one of the conclusions of this work is that pad thickness is critical to avoid BHBT and must be considered in the design process.

Preliminary Investigation of Skull Fracture Patterns Using an Impactor Representative of Helmet Back-Face Deformation.

Military combat helmets protect the wearer from a variety of battlefield threats, including projectiles. Helmet back-face deformation (BFD) is the result of the helmet defeating a projectile and deforming inward. Back-face deformation can result in localized blunt impacts to the head. A method was developed to investigate skull injury due to BFD behind-armor blunt trauma. A representative impactor was designed from the BFD profiles of modern combat helmets subjected to ballistic impacts. Three post-mortem human subject head specimens were each impacted using the representative impactor at three anatomical regions (frontal bone, right/left temporo-parietal regions) using a pneumatic projectile launcher. Thirty-six impacts were conducted at energy levels between 5 J and 25 J. Fractures were detected in two specimens. Two of the specimens experienced temporo-parietal fractures while the third specimen experienced no fractures. Biomechanical metrics, including impactor acceleration, were obtained for all tests. The work presented herein describes initial research utilizing a test method enabling the collection of dynamic exposure and biomechanical response data for the skull at the BFD-head interface.