Failure Investigation of Layered LFT Sb1plus Package After Ballistic Tests for Level IIA

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

Cătălin Pîrvu 1,2,* and Lorena Deleanu 2,*

1 National Institute of Aerospace Research “Elie Carafoli” INCAS, 061126 Bucharest, Romania 2 Faculty of Engineering, “Dunărea de Jos” University of Galati, 800008 Galati, Romania * Correspondence: [email protected] (C.P.); [email protected] (L.D.)

Abstract: 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. Keywords: aramid fiber; ballistic test; failure mechanism Citation: Pîrvu, C.; Deleanu, L. Failure Investigation of Layered LFT SB1plus Package after Ballistic Tests for Level IIA. Polymers 2021, 13, 2912. 1. Introduction https://doi.org/10.3390/ For hundreds of years, metallic materials have been used not only for body armor, but polym13172912 also for the protection of larger objects, such as vehicles, which is called “heavy protection”. However, just a few decades ago, at the end of World War II, lighter solutions emerged, Academic Editors: Beata especially for military personnel, in the form of nylon ballistic vests. However, those did Podko´scielnaand Andrzej Puszka not come close to the current level of ballistic protection offered by aramid fibers, yarns and fabrics, included in personal armors. Another advantage of ultra-fine polymer filaments Received: 8 August 2021 (not only aramid [1]) is that they offer extraordinarily flexible fabrics, which favors a high Accepted: 26 August 2021 Published: 29 August 2021 level of comfort for the wearer. Ballistic applications of aramid fiber-based composites mostly include soft body

Publisher’s Note: MDPI stays neutral armors. The mechanical properties of the aramids and the ballistic effects on its fabrics and with regard to jurisdictional claims in composites have been investigated in several studies [2–5] that involved both experimental published maps and institutional affil- testing and FEM (finite element method) [6–9] and established the effectiveness of the iations. ballistic protecting system and the protection level of bulletproof vests. Recent reviews on ballistic protection [10,11] point out interesting comments on failure mechanisms and very particular solutions in combining different materials for facing very different threats. Research has involved aramid fibers with different architectures, from simple or treated woven fabrics [12] to 3D fabrics [13] and unidirectional or multiaxial non-crimp fabrics, Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. each solution being simulated with particular model conditions and tested for a specific This article is an open access article threat, the conclusion of the documentation conducted by the authors being that each distributed under the terms and solution has to be experimentally investigated and the failure mechanisms understood in conditions of the Creative Commons order to offer a design with very strong statistical reliability before use in combat. Attribution (CC BY) license (https:// Bajya et al. [7] recently presented an experimental report on ballistic protection and creativecommons.org/licenses/by/ failure mechanisms of soft armor packages made of different structures, including woven 4.0/). and unidirectional fabrics. Packages made of para-aramid woven fabrics and unidirectional

Polymers 2021, 13, 2912. https://doi.org/10.3390/polym13172912 https://www.mdpi.com/journal/polymers Polymers 2021, 13, 2912 2 of 17

laminates and hybrid ones were developed and subjected to ballistic impact against 9 mm lead core bullets, at an impact velocity of 430 m/s. The ballistic performance of each structure was evaluated in terms of backface signature (BFS), perforation ratio (PR) and the expansion of the bullet. The unidirectional aramid packages failed mainly due to fiber break, including fibrillation, debonding and delamination. The fiber orientation was only (0◦/90◦) × 2 and the surface density of this type of fabric was 230 g/m2, with this inducing enough flexibility to allow for a too high BFS as compared to other solutions offered by these authors. In general, ballistic threats are either from bullets or fragments. Bullets can be defined as projectiles of different shapes and consistencies, fired from weapons such as pistols, revolvers and rifles. Fragments, on the other hand, may also result from explosions (for example, detonations of a grenade) or from the target the projectiles had already hit. The various kinetic characteristics and deformation behaviors of such a wide range of bullets and fragments must require a customized study for each of this multitude of different projectiles and for the design required to face an impact process, in order to have an effective ballistic protection. Even if, nowadays, many models and simulations have been reported, from mi- cro [14,15], meso [13,16], to macro [17,18] scales, the tests remain the ultimate and restrictive step in adopting a certain type of individual armor, generally based on aramid fabrics. An active field of research concerning the impact resistance of aramid fabrics involves the study of ballistic performance, from destructive investigation based on standard structures of packages [19] or structures involving new entries such as aerogels [20] and methodologies accepted by the interested parties. The standard structure of a ballistic package involves a certain number of layers of material or semi-finished product (as fabrics) for ballistic protection, which can vary within fairly wide limits between 8 and 40 layers (or folds), depending on its architecture and the material characteristics for the ballistic protection. The aim of this study is to evaluate a particular multiaxial fabric, LSF SB1plus, produced by Teijin [21], for designing body armor for the threat level IIA as classified in Ballistic Resistance of Personal Body Armor NIJ Standard–0101.04 [22], to identify failure mechanisms and to measure a well-accepted characteristic, backface signature (BFS), for the designed ballistic protection package. The authors measured BFS and investigated failure mechanisms of this particular package in order to evaluate it as a future application in body armor.

2. Material and Testing Campaign Even though fabrics based on aramid fibers are lightweight and offer better protection, the designer will be forced to reduce the number of fabric layers required without compromising the effectiveness of the final protecting armor to reduce the cost. In general, aramid fibers are 43% lighter than glass fibers, as strong as E-Glass, ten times stronger than aluminum and are close in strength to carbon fibers (in tensile tests). They also have good dimensional stability with a slightly negative coefficient of thermal expansion (−2.4 × 10−6 1/◦C) and could resist chemicals with the exception of a few strong acids and alkalis. Aramid fibers exhibit excellent stability over a wide range of temperatures for prolonged periods with no strength loss at temperatures as low as −196 ◦C and do not melt but will start to carbonize at approximately 427 ◦C[10]. The layers are available in the form of fabrics in multidirectional yarns (0◦, 90◦, −45◦, 45◦) (Figure1), namely LTF SB1plus (a trade name for a multiaxial fabric by Twaron Laminated Fabric Technology) (Arnhem production facility, Arnhem, Netherlands), and they are laminated together with a very thin foil of resin [21]. These fabrics can be combined into various structures and composites, each type having unique protection performance, forming ballistic packages with different uses and providing protection for a wide range of threats. Polymers 2021, 13, 2912 3 of 18

A recent tendency of research in trying to improve the response of aramid fabrics to ballistic impact is 3D architecture [2], but fabrics with unidirectional layers with different angle orientations in sublayers are also intensively used for body armor, due to the fact PolymersPolymers2021 2021, 13, 13, 2912, 2912 that the strength of the yarns is not “consumed” in bending them in a 2D or 3D architec-3 ofof 1718

ture. Of course, technology is obliged to find the adequate film and yarns for maintaining these multiaxial orientations in a single layer. A recent tendency of research in trying to improve the response of aramid fabrics to ballistic impact is 3D architecture [2], but fabrics with unidirectional layers with different angle orientations in sublayers are also intensively used for body armor, due to the fact that the strength of the yarns is not “consumed” in bending them in a 2D or 3D architecture. Of course, technology is obliged to find the adequate film and yarns for maintaining these multiaxial orientations in a single layer.

(a) (b) (c) Figure 1. SEM images of the LFT SB1plus layers after being cut by the help of a power guillotine. (a) Magnification ×300; Figure 1. SEM images of the LFT SB1plus layers after being cut by the help of a power guillotine. (a) Magnification ×300; (b) magnification ×1200; (c) magnification ×2500. (b) magnification ×1200; (c) magnification ×2500.

TheA recent architecture tendency of ofthe research ballistic in package trying tois improveusually made the response of the same of aramid type of fabrics layer or to combinationsballistic impact of istwo 3D or architecture more materials, [2], but the fabrics designer with being unidirectional interested layersin comparing with different ballis- (a) ticangle packages orientations with different in sublayers numbers(b are) also of intensivelylayers in their used structure for body [6]. armor, (Forc) dueindividual to the fact armor, that fabrics could be used as delivered, without adding adhesive among layers. No influence Figure 1. SEM images of thethe LFT strength SB1plus of layers the yarns after isbeing not “consumed”cut by the help in of bending a power themguillotine. in a 2D(a) Magnification or 3D architecture. ×300; Of (b) magnification ×1200; (cofcourse,) magnification stitching technology the ×2500. layers is on obliged the ballistic to find resistan the adequatece properties film and of the yarns package for maintaining has been noticed. these multiaxialTaking orientations into account in the a single stated layer. criteria, the development of a type of soft armor, from the sameTheThe aramid architecture architecture textile of of product, the the ballistic ballistic is quite package package numerous is is usually usually and made technologicallymade of of the the same same advantageous. type type of of layer layer or or combinationscombinationsDesigning of ofa two newtwo or orindividual more more materials, materials, protective the the designer armordesigner requires being being interested interestedadequate in layersin comparing comparing and yarn ballistic ballis- ar- chitecture,packagestic packages withtaking with different into different account numbers numbers the particular of layersof layers inresponse in their their structure ofstructure the system [6 ].[6]. For Forto individualparticular individual threats armor, armor, andfabricsfabrics the could protectioncould be be used used degree, as as delivered, delivered, including without withouthere the adding adding limitative adhesive adhesive request among among as an layers. acceptablelayers. No No influence valueinfluence of backfacestitchingof stitching signature the the layers layers (BFS). on on the the ballistic ballistic resistance resistan propertiesce properties of theof the package package has has been been noticed. noticed. TheTakingTaking packages into into account account were obtained the the stated stated as criteria, criteria,successive the the development layersdevelopment of Twaron of of a a type LFTtype of SB1plus,of soft soft armor, armor, as deliv- from from eredthethe same samein rolls aramid aramid by the textile textile producer, product, product, Teijin is is quite Aramidquite numerous numerous [21]. Twaron and and technologically technologically LFT SB1plus is advantageous. advantageous. a fabric of four layersDesigning Designingof unidirectional aa newnew individualTwaron yarns, protective protective with orientationsarmor armor requires requires (0°, adequate 90°, adequate +45°, layers − layers45°), and laminated and yarn yarn ar- togetherarchitecture,chitecture, with taking taking a very into into small account account amount the the particularof polyme r response [21]. It has ofof thethe a surface systemsystem density toto particularparticular of 430 threats threats g/m2 andandand the the the theoretical protection protection mass degree, degree, of includingthe including 12-layer here here package the the limitative li mitativeof 500 mm request request × 500 as mmas an an is acceptable acceptable 1290 g. The value value man- of of ufacturingbackfacebackface signature signature of the packages (BFS). (BFS). includes the cutting and coupling of the fabrics (in order to have TheaThe narrow packages packages variation were were obtainedin obtained their weight), as as successive successive sewing layers and layers control. of Twaronof Twaron The LFT package LFT SB1plus, SB1plus, has as the delivered asdimen- deliv- sionsinered rolls 500 in byrolls mm the by× 500 producer, the mm, producer, with Teijin an Teijin area Aramid Aramidof 0.25 [21 m]. [21].2, Twaronbetween Twaron LFT recommended LFT SB1plus SB1plus is valuesis a fabrica fabric by ofNIJ-C- of four four ◦ ◦ ◦ ◦ 4layers layers(0.23 ofm of2 unidirectional) unidirectionaland NIJ-C-5 (0.3 Twaron Twaron m2) for yarns, yarns, high with and with orientationsvery orientations high surfaces (0 (0°,, 90 90°,(Figure, +45 +45°,, 2).− 45− 45°),), laminated laminated 2 togethertogether with with a a very very small small amount amount of of polymer polymer [ 21[21].]. It It has has a a surface surface density density of of 430 430 g/m g/m2 andand thethe theoreticaltheoretical mass of the the 12-layer 12-layer package package of of 500 500 mm mm × 500× 500 mm mm is 1290 is 1290 g. The g. man- The manufacturingufacturing of the of thepackages packages includes includes the thecutti cuttingng and and coupling coupling of ofthe the fabrics fabrics (in (in order order to tohave have a narrow a narrow variation variation in their in their weight), weight), sewing sewing and control. and control. The package The package has the has dimen- the dimensions 500 mm × 500 mm, with an area of 0.25 m2, between recommended values by sions 500 mm × 500 mm, with an area of 0.25 m2, between recommended values by NIJ-C- NIJ-C-4 (0.23 m2) and NIJ-C-5 (0.3 m2) for high and very high surfaces (Figure2). 4 (0.23 m2) and NIJ-C-5 (0.3 m2) for high and very high surfaces (Figure 2).

Figure 2. Cutting process of the layers for the ballistic package.

FigureFigure 2. 2.Cutting Cutting process process of of the the layers layers for for the the ballistic ballistic package. package.

Polymers 2021, 13, 2912 4 of 18

Polymers 2021, 13, 2912 4 of 17 After being cut, 12 layers formed a package, this number being selected by the authors after preliminary tests on 8 layers that gave both results as total and partial perforation. In previous works [6,17], macro simulation on equivalent layers gave similar results to the testedAfterbeing package cut, with 12 layers the same formed number a package, of layers this (bullet number was being stopped selected onby the the last authors bro- kenafter layer preliminary for a virtual tests package on 8 layers of 8 thatlayers gave and both simulation results as on total a 12-layer and partial package perforation. gave sat- In isfactoryprevious results; works thus, [6,17 ],this macro number simulation of layers on (12) equivalent was used layers in forming gave similar a protective results pack- to the age).tested package with the same number of layers (bullet was stopped on the last broken layer forThe a virtual projectile package was ofan 8FMJ layers (Full and Metal simulation Jacket) 9 on mm a 12-layer bullet, as package required gave for protection satisfactory levelresults; IIA thus,as given this in number NIJ Standard-0101.06/2008 of layers (12) was used [23]. in forming The layers a protective were laterally package). fixed by sewingThe on two projectile sides, wason a an central FMJ (Fulllocation Metal forJacket) a length 9 mmof approx. bullet, 90 as mm, required in order for protectionto main- tainlevel its IIAintegrity as given and in the NIJ layers’ Standard-0101.06/2008 order. The packages [23 were]. The tested layers in werea controlled laterally environ- fixed by mentsewing enclosure on two (temperature sides, on a central kept in location the range for of a length20 ± 5 of°C approx. and relative 90 mm, humidity in order of to 50…65%,maintain atmospheric its integrity pressure and the of layers’ 760 ± order.15 mm The Hg), packages using a ballistic were tested smooth in abarrel, controlled the ◦ measuredenvironment velocity enclosure of the (temperatureprojectiles being kept 420 in the± 10 range m/s. ofThere 20 ± were5 C used and relative a chronograph humidity Oehlerof 50 ... 43 65%,(Oehler atmospheric Research, pressure Inc., Austin, of 760 Texas,± 15 mmUSA) Hg), for using measuring a ballistic the smoothimpact barrel,velocity the andmeasured a firing velocitytable with of compensated the projectiles rebond. being 420 ± 10 m/s. There were used a chronograph Oehler 43 (Oehler Research, Inc., Austin, Texas, USA) for measuring the impact velocity 3.and Ballistic a firing Impact table Testing with compensated rebond. 3. BallisticIn practice, Impact in order Testing to assess the impact resistance of a protection system, there are reference standards that propose methods of assessment, such as NIJ Standard- In practice, in order to assess the impact resistance of a protection system, there are refer- 0101.06/2008 [23] and NIJ 0101.04/2004 [22], the results giving the possibility to include ence standards that propose methods of assessment, such as NIJ Standard-0101.06/2008 [23] the system in a level of protection [10]. and NIJ 0101.04/2004 [22], the results giving the possibility to include the system in a level of The assessment of the ballistic resistance of the studied protection packages was per- protection [10]. formed according to NIJ 0101.04/2004 (Figure 3) [22], by firing a 9 mm caliber bullet, with The assessment of the ballistic resistance of the studied protection packages was an initial speed of 420 ± 10 m/s, from a distance of 3 m (normal conditions), with shots performed according to NIJ 0101.04/2004 (Figure3)[ 22], by firing a 9 mm caliber bullet, firedwith in an a laboratory initial speed facility. of 420 ± 10 m/s, from a distance of 3 m (normal conditions), with shots fired in a laboratory facility.

FigureFigure 3. 3.FiringFiring arrangement arrangement equipment, equipment, Reprinted Reprinted with with permission permission from from ref. ref. [23], [23 ],Copyright Copyright Year Year Copyright Owner’s Name. * For rife rounds the length may be further adjusted to minimize yaw at Copyright Owner’s Name. * For rife rounds the length may be further adjusted to minimize yaw at impact; however, in such cases the yaw at impact must be experimentally shown to be less than 5° impact; however, in such cases the yaw at impact must be experimentally shown to be less than 5◦ † ° and reasonably close to minimal. Tolerance† for 0° ◦shots. For 30° and◦ 45 shots◦ the tolerance shall beand +25 reasonablymm/−190 mm close (+1.0 to minimal.in/−7.5 in). Tolerance for 0 shots. For 30 and 45 shots the tolerance shall be +25 mm/−190 mm (+1.0 in/−7.5 in). The testing of the realized protection packages in order to evaluate their resistance to The testing of the realized protection packages in order to evaluate their resistance theto 9 the mm 9 mm FMJ FMJ bullet bullet was was discussed, discussed, taking taking into into account account the the resistance resistance to to penetration penetration throughthrough the the depth depth of of the the trace trace left left in in the the support support material material (ballistic (ballistic clay), clay), namely namely backface backface signaturesignature (BFS). (BFS). The The assessment assessment of of the the total total penetration penetration of of a apackage package is isin in many many cases cases simple, when it is found that there is a hole with a diameter at least equal to the caliber of the bullet and that the entire bullet passes through it. When testing personal ballistic protection equipment, the trauma on the human body is assessed by the depth of the BFS imprint that is formed in the ballistic clay on which Polymers 2021, 13, 2912 5 of 18

simple, when it is found that there is a hole with a diameter at least equal to the caliber of the bullet and that the entire bullet passes through it. Polymers 2021, 13, 2912 5 of 17 When testing personal ballistic protection equipment, the trauma on the human body is assessed by the depth of the BFS imprint that is formed in the ballistic clay on which the sample is fixed. The standard admits materials and protection systems as satisfactory thewhen sample they is produce fixed. The an standardimprint in admits the suppo materialsrt clay and that protection does not exceed systems 44 as mm satisfactory [23]. when theyThe following produce an steps imprint are performed in the support when clay testing that doespersonal not exceedarmors 44or mmparts [23 of]. it: • TheThe following test equipment steps are is performedpositioned whenin the testingclamping personal support, armors at the or distance parts of it:imposed • Thefor testeach equipment piece of equipment is positioned from in the the mout clampingh of the support, barrel; atthe the types distance of weapons imposed and forammunition each piece ofrequired equipment for levels from II the and mouth IIA are of verified the barrel; [23]; the types of weapons and • ammunitionThe bullet speed required measurement for levels II system and IIA is are posi verifiedtioned, [ 23starting]; with the distance of 2 • Them from bullet the speed mouth measurement of the barrel, system so that is the positioned, frames of starting the system with are the in distance planes per- of 2pendicular m from the to mouth the firing of the direction; barrel, the so thatdistance the framesbetween of the the frames system of are the in system planes is perpendicular0.5 m and the to distances the firing are direction; measured the with distance an accuracy between of the 1 mm; frames of the system is • 0.5Firing m and is executed the distances on the are test measured package. with an accuracy of 1 mm; • Firing is executed on the test package. 4. Results and Discussion of Failure Mechanisms 4. ResultsFactors and influencing Discussion ballistic of Failure protection Mechanisms against a specified threat include those char- acterizingFactors the influencing target material(s), ballistic protection in terms of against fiber and a specified yarn properties, threat include weave those architecture, charac- terizingsurface thedensity target and material(s), the number in terms of plies, of fiber boundary and yarn conditions properties, and weavethose characterizing architecture, surfacethe impact density (impact and the velocity, number impact of plies, angle, boundary projectile conditions shape and and materials, those characterizing etc.) [10]. the impactHow (impact do we velocity, evaluate impact the ballistic angle, projectile response shape of body and armor? materials, The etc.)investigation [10]. is done at macroHow doscale we and evaluate the tests the ballisticproduce response clear result of bodys: if the armor? target The resists, investigation the backface is done signa- at macroture could scale andreveal the the tests degree produce of protection clear results: and, if theif the target target resists, does the not backface resist, the signature residual couldvelocity reveal would the degree indicate of protectionhow far away and, the if the prot targetective does solution not resist, is from the residualthe requested velocity pa- wouldrameters, indicate but in how a qualitative far away way. the protective However, solution this scale is does from not the offer requested answers parameters, to improve butthe in ballistic a qualitative resistance, way. and However, the study thisof how scale projectiles does not and offer targets answers are destroyed to improve at lower the ballisticscale is resistance,of great interest. and the Thus, study researchers of how projectiles will investigate and targets the target are destroyedand projectile at lower failure scalemechanisms is of great using interest. optical Thus, and researchers scanning willelec investigatetron microscopes, the target high and speed projectile cameras failure and mechanismsspectrometers, using etc. optical and scanning electron microscopes, high speed cameras and spectrometers, etc. A synthesis of the damage mechanisms under impact is given in Figure 4. A synthesis of the damage mechanisms under impact is given in Figure4.

Figure 4. Damage mechanisms for packages based on fabrics.

Based on a general overview [10,24] on how the kinetic energy of the projectile is absorbed, this study aims to identify damage mechanisms for a particular package of 12 layers of LFT SB1plus when impacted with a 9 mm FMJ: Polymers 2021, 13, 2912 6 of 17

• Compression of the package directly under the bullet; • Compression in the target volume surrounding the impacted zone; • Conical deformation on the package backface; • Loading the main yarns that face the higher strain; these yarns tend to fail when the induced tensile strain of these yarns exceeds the ultimate strain; • Deformation and/or displacement of secondary yarns; • Delamination, especially on the layers situated towards the back of the target; • Shear plugging; • Spalling of the already broken layers (yarns); • Friction between the projectile and the target and friction between yarns and layers. The secondary yarns are close to the main ones, absorbing energy by their strain distribution within the yarns, and the highest values are found near the top face of the deformed cone. As the friction coefficient is difficult to measure for high impact velocities, simulation offers a fair evaluation by covering a large range of values for friction coefficient. The friction coefficient could have lower values, being more intense when the bullet is forced to be deformed inside the target. When the bullet hits the target, because of the sudden drop in contact force, only the upper few layers fail by shear as the shear wave propagates along the thickness direction. Then, the undamaged layer absorbs the residual kinetic energy of the projectile through creating a cone-shaped deformation. For woven fabrics, this deformation could be rhomboidal, the rhombus diagonals being almost oriented along the warp and weft yarns. Multiaxial unidirectional fabrics have an intermediate behavior, that is, the shape of deformation could be almost conical. Shear plugging is one of the major damage mechanisms during impact for energy absorptions by the targets. This process occurs due to the initial impact contact force between the projectile and the target, resulting shear stress through its thickness, around the periphery of the projectile. If shear plugging stress exceeds the ultimate shear plugging strength, the target will fail. The failure by shear plugging is not symmetrical as if the first yarns will be broken by shear, these yarns are de-stressed and able to be displaced when the projectile advances. During an experimental campaign, the investigation of failure mechanisms points out the ballistic performance of a specific target configuration when hit by different projectiles, with different impact velocities. The test results from nine fires on three packages made of 12 layers of SB1plus are plotted in Figure5, on a web chart, with the red line being the accepted limit of 44 m [ 23] and the blue points being the values measured as the depth of the imprint in the ballistic clay. Figure6 presents the shape, as taken with a mold resin, produced by a fire, in the ballistic clay behind the package in order to highlight the importance of this characteristic for body armor. From this study, the authors want to suggest a more detailed evaluation of the entire shape of the backface signature, not only its depth in the ballistic clay, with BFS being of 18 mm here. An investigation of the destruction of flexible packages was carried out with the help of a professional camera to highlight characteristics dependent on fabrics’ architecture and the number of layers. A detailed, photographic study on each layer of the tested ballistic packages was performed to point out the failure processes in stages of the layers. Polymers 2021, 13, 2912 7 of 18

1 45 40 Polymers 2021, 13, 2912 7 of 17 Polymers 2021, 13, 2912 9 35 2 7 of 18 30 25 20 15 1 10 8 45 3 5 40 0 9 35 2 30 25 20 7 15 4 8 10 3 5 0 6 5 Figure 5. Data for BFS measured for 9 fires on three packages made of 12 layers of SB1plus fabric. 7 4 Figure 6 presents the shape, as taken with a mold resin, produced by a fire, in the ballistic clay behind the package in order to highlight the importance of this characteristic for body armor. From this study, the authors want to suggest a more detailed evaluation 6 5 of the entire shape of the backface signature, not only its depth in the ballistic clay, with FigureBFS being 5.5. Data of for 18 BFSmm measured here. for 99 ﬁresfires onon threethree packagespackages mademade ofof 1212 layerslayers ofof SB1plusSB1plus fabric.fabric.

Figure 6 presents the shape, as taken with a mold resin, produced by a fire, in the ballistic clay behind the package in order to highlight the importance of this characteristic for body armor. From this study, the authors want to suggest a more detailed evaluation of the entire shape of the backface signature, not only its depth in the ballistic clay, with BFS being of 18 mm here.

Figure 6. Cavity shape in the ballistic clay, obtained with a molding resin. Figure 6. Cavity shape in the ballistic clay, obtained with a molding resin.

FiguresAn investigation7–11 show of photographs the destruction of LFT of flexible SB1plus packages layers fromwas carried a 12-layer out package.with the help It shouldof a professional be noted that camera the tests to werehighlight performed charac underteristics conditions dependent of a on small fabrics’ variation architecture for the and the number of layers.± A detailed, photographic study on each layer of the tested bal- bullet velocity (420 m/s 10 m/s). The angle of impact is normal on the target surface, thelistic deviations packages being was performed 5% at the mouth to point of theout barrel.the failure processes in stages of the layers. Figure 6. Cavity shape in the ballistic clay, obtained with a molding resin. InFigures the details 7–11 of show the photos, photograph for eachs of fire LFT the SB1plus main yarns layers (broken from a under 12-layer the package. projectile) It andshould the secondarybe noted that yarns the (those tests were affected performed by the penetration under conditions of the projectile,of a small byvariation twisting, for An investigation of the destruction of flexible packages was carried out with the help firingthe bullet andscattering) velocity (420 may m/s be ± counted. 10 m/s). The angle of impact is normal on the target surface, of a professional camera to highlight characteristics dependent on fabrics’ architecture the Onedeviations may see being the total5% at penetration the mouth of the barrel. first layer in Figure7: (a) general view from and the number of layers. A detailed, photographic study on each layer of the tested bal- the impactIn the direction details of and the (b), photos, (c) and for (d) each details fire the of each main of yarns the three (broken fires under on the the same projectile) target. listic packages was performed to point out the failure processes in stages of the layers. Theand second the secondary layer is presented yarns (those in Figure affected8: (a) by general the penetration view of the of designed the projectile, ballistic by package, twisting, Figures 7–11 show photographs of LFT SB1plus layers from a 12-layer package. It andfiring the and localized scattering) ruptures may of be the counted. yarns is highlighted, for each fire, in (b), (c) and (d). This typeshould ofOne failurebe maynoted occurs see that the whenthe total tests allpenetration thewere fibers performed of the threadfirst under layer break conditions in Figure in almost of7: (a)a the small general same variation place. view Thefrom for twothethe bullet causesimpact velocity of direction the breaking (420 and m/s (b), of ± the10(c) m/s).and yarns (d) The in details layerangle 1of of are each impact their of the stretchingis normal three fires alongon theon thethe target lengthsame surface, target. next tothe the deviations contact with being the 5% projectile at the mouth and the of shearingthe barrel. on their thickness, observed in Figure7 (detailIn b). the Theoretically, details of the the photos, fibers for in theeach yarn fire willthe main fail when yarns the (broken induced under strain the exceeds projectile) the strainand the at secondary break, but yarns the process (those is affected dynamic by (depending the penetration on strain of the rate) projectile, and statistic, by twisting, given thefiring large and number scattering) of fibers may inbe a counted. yarn. One may see the total penetration of the first layer in Figure 7: (a) general view from the impact direction and (b), (c) and (d) details of each of the three fires on the same target.

Polymers 2021, 13, 2912 8 of 18

The second layer is presented in Figure 8: (a) general view of the designed ballistic package, and the localized ruptures of the yarns is highlighted, for each fire, in (b), (c) and (d). This type of failure occurs when all the fibers of the thread break in almost the same place. The two causes of the breaking of the yarns in layer 1 are their stretching along the length Polymers 2021, 13, 2912 next to the contact with the projectile and the shearing on their thickness, observed in 8 of 17 Figure 7 (detail b). Theoretically, the fibers in the yarn will fail when the induced strain exceeds the strain at break, but the process is dynamic (depending on strain rate) and statistic, given the large number of fibers in a yarn.

(a) (b)

FigureFigure 7. 7. FrontalFrontal view view (a) of ( a )12-layer of a 12-layer LFT SB1plus LFT package: SB1plus (a) package: layer 1 (face) (a )and layer details 1 (face) of the fires and details of the fires on the first layer of the package: (b) fire 1, (c) fire 2 and (d) fire 3. BFS was noted as A on the photo. on the first layer of the package: (b) fire 1, (c) fire 2 and (d) fire 3. BFS was noted as A on the photo.

(a) (b)

FigureFigure 8. 8. FrontalFrontal view view of the of sample the sample with 12 layers with of 12 LFT layers SB1plus: of LFT (a) layer SB1plus: 2 (face),( witha) layer details 2 (face),of with details of the bullet holes of (b) fire 1, (c) fire 2 and (d) fire 3. the bullet holes of (b) fire 1, (c) fire 2 and (d) fire 3. Starting from layer 3 (Figure 9) and in layer 4 (Figure 10), the enlargement of the holes is observed (due to the lateral plastic deformation of the projectile when it starts to be stopped), and also the process of pulling the threads. The tear-off of the yarn (even partially) from its fabric architecture occurs when the yarns are not broken, sometimes only on one side of the projectile [6]. Layer 4 is the last layer in the SB1plus LFT package through which the bullets passed or stopped. In the current case of failure, the first four layers have the role of slowing down and even arresting and deforming the projectile. Starting from layer 3 (Figure 9) and in layer 4 (Figure 10), the widening of the holes is observed (due to the lateral deformation of the projectile when it starts to be stopped, and the process of pulling the yarns). The first fired bullet (Figure 9a) was found in the slot produced in the third layer, the second was found between the third layer and the fourth one (see the mark on Figure 10c) and the third bullet fired on the package had a similar position and shape, but advanced one layer (it was arrested between the fourth layer and fifth one). For body armor, it is very important that successive fires do not create failures with very different and progressive damages on the package. The authors consider this response promising in facing multiple and localized fires. The quality of the material SB1plus is noticed by the high degree of resemblance of the penetration zones and even by the size of zones with broken and torn-off yarns.

Polymers 2021, 13, 2912. https://doi.org/10.3390/polym13172912 www.mdpi.com/journal/polymers Polymers 2021, 13, 2912 9 of 17 Polymers 2021, 13, 2912 2 of 11

(a) (b)

Figure 9. Frontal view view of of the the sample sample with with 12 12 layers layers of of LFT LFT SB1plus: SB1plus: (a) ( alayer) layer 3 (face), 3 (face), with with details details of the of the bullet bullet holes holes of (b of) fires 1, (c) fires 2 and (d) fires 3. (b) fires 1, (c) fires 2 and (d) fires 3.

Starting from layer 3 (Figure9) and in layer 4 (Figure 10), the enlargement of the holes is observed (due to the lateral plastic deformation of the projectile when it starts to be stopped), and also the process of pulling the threads. The tear-off of the yarn (even partially) from its fabric architecture occurs when the yarns are not broken, sometimes only on one side of the projectile [6]. Layer 4 is the last layer in the SB1plus LFT package through which the bullets passed or stopped. In the current case of failure, the first four layers have the role of slowing down and even arresting and deforming the projectile. Starting from layer 3 (Figure9) and in layer 4 (Figure 10), the widening of the holes is observed (due to the lateral deformation of the projectile when it starts to be stopped, and the process of pulling the yarns). The first fired bullet (Figure9a) was found in the slot produced in the third layer, the second was found between the third layer and the fourth one (see the mark on Figure 10c) and the third bullet fired on the package had a similar position and shape, but advanced one layer (it was arrested between the fourth layer and fifth one). For body armor, it is very important that successive fires do not create failures with very different and progressive damages on the package. The authors consider this response promising in facing multiple and localized fires.

PolymersPolymers2021 13 2021, 13, 2912 3 of 11 , , 2912 10 of 17

(a) (b)

(c) (d) Figure 10. Front view of sample made of LFT SB1plus: (a) layer 4 (face), with details of the penetra- Figure 10. Front view of sample made of LFT SB1plus: (a) layer 4 (face), with details of the penetration: Polymers 2021, 13, 2912 tion: (b) fire 1, (c) fire 2 and (d) fire 3. 4 of 11 (b) fire 1, (c) fire 2 and (d) fire 3. Layer 5 (Figure 11) shows traces of crushing (compression), circular in shape, which would be justified by the architecture of the LFT SB1plus semi-finished product (with the orientation of the yarns 0°/90°/45°/−45°), inducing a tendency to uniformity of the properties of the fabrics in four sub-layers and, therefore, of its answer. Layers 6 and 7, theoretically, have the role of retaining (arresting) the projectile and dumping its tendency to advance together with the fabrics. These layers and the following ones are also responsible for reducing the BFS.

(a) (b)

(c) (d) Figure 11. Frontal view of a package made of layers of LFT SB1plus: (a) layer 5 (face), with details Figure 11. Frontal view of a package made of layers of LFT SB1plus: (a) layer 5 (face), with details of of (b) fire 1, (c) fire 2 and (d) fire 3. (b) fire 1, (c) fire 2 and (d) fire 3. Figure 12 presents the last layer of the package: (a) the general view being taken from the opposite direction of the impact (named back face in this text), (b) the print on the last layer of fire 1, where aramid fibers are not broken, only the auxiliary (white) yarns that keep the resistant aramid yarns in their aligned position, (c) for fire 2 the yarns are not broken but (d) for the last fire, there are several bundles of fibers broken due to the large deformation on this layer.

Polymers 2021, 13, 2912 11 of 17

The quality of the material SB1plus is noticed by the high degree of resemblance of the penetration zones and even by the size of zones with broken and torn-off yarns. Layer 5 (Figure 11) shows traces of crushing (compression), circular in shape, which would be justified by the architecture of the LFT SB1plus semi-finished product (with the orientation of the yarns 0◦/90◦/45◦/−45◦), inducing a tendency to uniformity of the properties of the fabrics in four sub-layers and, therefore, of its answer. Layers 6 and 7, theoretically, have the role of retaining (arresting) the projectile and dumping its tendency to advance together with the fabrics. These layers and the following ones are also responsible for reducing the BFS. Figure 12 presents the last layer of the package: (a) the general view being taken from the opposite direction of the impact (named back face in this text), (b) the print on the last layer of fire 1, where aramid fibers are not broken, only the auxiliary (white) yarns that keep the resistant aramid yarns in their aligned position, (c) for fire 2 the yarns are not Polymers 2021, 13, 2912 broken but (d) for the last fire, there are several bundles of fibers broken due to the5 largeof 11

deformation on this layer.

(a) (b)

Figure 12. Back view ofof packagepackage mademade of of 12 12 layers layers of of LFT LFT SB1plus: SB1plus: (a )(a layer) layer 12 12 (back), (back), with with details details of (ofb) ( fireb) fire 1, (c 1,) fire(c) 2fire and 2 and(d) fire (d) 3.fire 3.

After testing, it can be seen that the arching in the orthogonal yarns is more dominant in the back layers of the multilayer package, where the projectile tries to enter through an edge-like approach after being considerably slowed down by the initial layers [8]. The existence of the phenomenon of passing through the fabrics usually produces a hole smaller than the diameter of the projectile, with a smaller number of yarns being broken as compared to the number of yarns that intersect the projectile. In Figure 13a, two broken main yarns can be observed on the first layer of unidirectional yarns. The ends of the broken yarns relax after being tensioned, hence the appearance of scattered fibers. Figure 13b shows the deviation of the secondary yarns from layer 2 (from the four substrates of a layer) and the more disordered aspect of the fibers, as compared to layer 1.

Polymers 2021, 13, 2912 12 of 17 Polymers 2021, 13, 2912 6 of 11

After testing, it can be seen that the arching in the orthogonal yarns is more dominant in the back layers of the multilayer package, where the projectile tries to enter through an edge-like approach after being considerably slowed down by the initial layers [8]. The existence of the phenomenon of passing through the fabrics usually produces a hole smaller than the diameter of the projectile, with a smaller number of yarns being broken as compared to the number of yarns that intersect the projectile. In Figure 13a, two broken main yarns can be observed on the first layer of unidirectional yarns. The ends of the broken yarns relax after being tensioned, hence the appearance of scattered fibers. Figure 13b shows the deviation of the secondary yarns from layer 2 (from Polymers 2021, 13, 2912 the four substrates of a layer) and the more disordered aspect of the fibers, as compared6 of to11

layer 1.

(a) (b) Figure 13. Aspect of broken yarns from the ballistic protection package made of LFT SB1plus: (a) face of layer 1 (fire 1); (b) face of layer 2 (fire 1).

Some particular aspects of the failure of aramid fibers could be highlighted with the help of SEM images for the tested ballistic package of 12 layers. According to SEM images, breaking a fiber can be performed by fibrillation (Figure 14), shearing (Figure 15), necking (obviously, by tensile stress, as one may see in Figure 15 details (c) and (d) and twisting). In Figure 14, by the help of scanning electron microscopy, one may see the fibrillation, thinning and rupture of the aramid fibrils, resulting from the process of destroying a thread on the 2nd layer. The fibrillation (ata )the broken ends of the yarns (Figures 14a,b(b) and 15c) or along the yarn (Figure 15d) is a type of failure favored by the abrasive action of the projectile on the Figure 13. Aspect of broken yarns from the ballistic protection package made of LFT SB1plus: (a) lengthFigure 13.of theAspect fiber, of brokenbut it is yarns also from dependent the ballistic on protectionthe local traction package madecharacteristics of LFT SB1plus: of the (a )yarn face face of layer 1 (fire 1); (b) face of layer 2 (fire 1). fibers.of layer Of 1(fire course, 1); (b the) face location of layer of 2 (firethe fibrillation 1). is a defect of the molecular chains or their arrangement in the fiber. In layered systems, friction between layers results in abrasion SomeSome particularparticular aspectsaspects ofof thethe failurefailure ofof aramidaramid fibersfibers couldcould bebe highlightedhighlighted withwith thethe friction among fibers, inducing local scratches that diminish the fiber resistance. All pro- helphelp ofof SEM images for the tested ballistic packagepackage ofof 12 layers. According to SEM images, jectiles that have the ability to penetrate through the fabric also cause the scratching and/or breakingbreaking aa fiberfiber cancan bebe performed byby fibrillationfibrillation (Figure(Figure 1414),), shearingshearing (Figure(Figure 1515),), neckingnecking splitting of the fibers, which favors the appearance of fibrillation, a specific failure of ara- (obviously,(obviously, byby tensiletensile stress,stress, as as one one may may see see in in Figure Figure 15 15 details details (c) (c) and and (d) (d) and and twisting). twisting). mid fibers.In Figure 14, by the help of scanning electron microscopy, one may see the fibrillation, thinning and rupture of the aramid fibrils, resulting from the process of destroying a thread on the 2nd layer. The fibrillation at the broken ends of the yarns (Figures 14a,b and 15c) or along the yarn (Figure 15d) is a type of failure favored by the abrasive action of the projectile on the length of the fiber, but it is also dependent on the local traction characteristics of the yarn fibers. Of course, the location of the fibrillation is a defect of the molecular chains or their arrangement in the fiber. In layered systems, friction between layers results in abrasion friction among fibers, inducing local scratches that diminish the fiber resistance. All projectiles that have the ability to penetrate through the fabric also cause the scratching and/or splitting of the fibers, which favors the appearance of fibrillation, a specific failure of aramid fibers.

(a) (b)

FigureFigure 14. 14. DetailsDetails of of fibers ﬁbers from from the the ballistic ballistic package package LFT LFT SB1plus: SB1plus: (a) (fibrila) ﬁbril from from front front layer layer 2; (b 2;) detail(b) detail from from (a). (a ).

(a) (b) Figure 14. Details of fibers from the ballistic package LFT SB1plus: (a) fibril from front layer 2; (b) detail from (a).

Polymers 2021, 13, 2912 7 of 11

Figure 15 shows the details of some fibers damaged by fibrillation; there are noticed Polymers 2021, 13, 2912 discontinuities in the fiber and the thinning of the fibrils that supported the load and13 rup- of 17 tures of fibrils resulting from tensile and shearing loading (deformed breaks with less deformations, as a cut) during the destruction process.

(a) (b)

Figure 15. Details of the broken fibrilsfibrils from the 2nd2nd layerlayer ofof thethe LFTLFT SB1plusSB1plus ballisticballistic package.package. (a) layer 1, front viewview ofof the broken fibers, from the impact direction; (b) details of layer 1, front view of the broken fibers; (c) details of a higher the broken fibers, from the impact direction; (b) details of layer 1, front view of the broken fibers; (c) details of a higher magnification of a fiber; (d) details of (b) with non-uniform fibrillation and necking. magnification of a fiber; (d) details of (b) with non-uniform fibrillation and necking.

TheIn Figure values 14 ,for by BFS the help(backface of scanning signature) electron for microscopy,the LFT SB1plus one may package see the are fibrillation, given in Tablethinning 1 and and are rupture measured of the according aramid fibrils, to Ballisti resultingc Resistance from the of process Body Armor, of destroying NIJ Standard- a thread 0101.06on the 2nd [2] (Figure layer. 16). The fibrillation at the broken ends of the yarns (Figures 14a,b and 15c) or along the Tableyarn (Figure1. BFS of 15 a ballisticd) is a typepackage of failuremade of favored LFT SB1plus by the layers. abrasive action of the projectile on the length of the fiber, butLFT it is alsoSB1plus dependent Package on of the 12 local Layers traction characteristics of the yarn fibers. Of course,Fires the location of the fibrillation is a defectBSF of (mm) the molecular chains or their arrangement in the fiber. In layered systems, friction between layers results in 1 14 abrasion friction among fibers, inducing local scratches that diminish the fiber resistance. 2 15 All projectiles that have the ability to penetrate through the fabric also cause the scratching and/or splitting of the3 fibers, which favors the appearance of fibrillation,17 a specific failure of aramid fibers. Figure 15 shows the details of some fibers damaged by fibrillation; there are noticed discontinuities in the fiber and the thinning of the fibrils that supported the load and ruptures of fibrils resulting from tensile and shearing loading (deformed breaks with less deformations, as a cut) during the destruction process. The values for BFS (backface signature) for the LFT SB1plus package are given in Table1 and are measured according to Ballistic Resistance of Body Armor, NIJ Standard- 0101.06 [2] (Figure 16). Polymers 2021, 13, 2912 14 of 17

Table 1. BFS of a ballistic package made of LFT SB1plus layers.

LFT SB1plus Package of 12 Layers Fires BSF (mm) Polymers 2021, 13, 2912 1 14 8 of 11

2 15 3 17

FigureFigure 16. 16. MeasuringMeasuring BFS BFS in in agreement agreement with with Ballistic Ballistic Resistance Resistance of of Body Body Armor, Armor, NIJ NIJ Standard- Standard- 0101.060101.06 [23]. [23 ].

ImprintsImprints in in the the clay clay support support material, material, characterized characterized by the by BFS, the for BFS, a new for aarmor new pack- armor age,package, were wereanalyzed analyzed to determine to determine whether whether the thearmor armor will will provide provide adequate adequate protection protection againstagainst trauma trauma (without (without perforation/penetration perforation/penetration of of the the package). package). The The requirements requirements of of the the NIJNIJ [23] [23 ]state state that that all all depths depths of of the the measured measured traces traces in in the the support support material material obtained obtained from from firesfires falling falling within within the the firing firing requirements requirements mu mustst not not exceed exceed 44 44 mm, mm, or or if if the the BFS BFS exceeds exceeds 4444 mm mm then then there there must must be be a acoefficient coefficient of of 95 95%% confidence confidence that that 80% 80% of of the the depths depths of of the the tracestraces in in the the support support materialmaterial willwill be be 44 44 mm mm or or less. less. Under Under no no circumstances circumstances should should a trace a tracein the in supportthe support material material exceed exceed 50 mm. 50 mm. MeasuredMeasured BFSs BFSs for for a anew new armor armor package package we werere analyzed analyzed to to determine determine whether whether the the armorarmor will will provide provide adequate adequate protection protection against against trauma trauma (without (without perforation/penetration perforation/penetration ofof the the package). package). The The requirements requirements of of the the NIJ NIJ [23] [23 ]state state that that all all depths depths of of BFS BFS in in the the support support clay,clay, obtained obtained from from fires fires obeying obeying the the firing firing requirements, requirements, must must not not exceed exceed 44 44mm, mm, or if or theif theBFS BFSexceeds exceeds 44 mm 44 mmthen thenthere theremust mustbe a co beefficient a coefficient of 95% of confidence 95% confidence that 80% that of the 80% depthsof the of depths the imprints of the imprints in the support in the supportmaterial materialwill be 44 will mm be or 44 less. mm Under or less. no Under circum- no stancescircumstances should a should BFS in athe BFS support in the supportmaterial material exceed 50 exceed mm. 50 mm. FollowingFollowing the the extraction extraction of of the the bullets bullets from from a apackage package of of 12 12 LFT LFT SB1plus SB1plus layers, layers, the the followingfollowing characteristics characteristics of of the the recovered recovered projectiles projectiles could could be be highlighted highlighted based based on on the the analysesanalyses of of the the photographs photographs in in Figure Figure 17: 17 : • • BulletsBullets were were flatted flatted like like a amushroom mushroom cap; cap; • • BulletsBullets had had initiated initiated fragmentation fragmentation on on their their edges, edges, but but no no fragments fragments seemed seemed to to be be separated,separated, with with the the jacket jacket and and core core torn torn off off the the second second bullet; bullet; • • TheirTheir faces faces in in contact contact with with the the target target had had trapped trapped aramid aramid fibers, fibers, especially especially from from the the first and second hit yarns. first and second hit yarns. The number of layers penetrated in the package influences the way the projectile is damaged, and the bullets retained within the package (Figure 17) had a very flattened shape, with peripheral ruptures of the jacket and core.

Polymers 2021, 13, 2912 15 of 17 Polymers 2021, 13, 2912 9 of 11

(a) (b)

(e) (f)

FigureFigure 17.17. Bullets (9(9 mmmm FMJ)FMJ) asas recoveredrecovered afterafter testingtesting 12-layer12-layer ballisticballistic package:package: frontfront viewview ofof thethe bulletsbullets ((aa––cc);); backback viewview ofof thethe samesame bulletsbullets ((dd––ff).).

TheFigure number 18 presents of layers details penetrated of the impacted in the package face of influencesthe projectile the after way testing. the projectile One may is damaged,notice the andembedding the bullets of fragmented retained within aramid the ya packagerns on the (Figure projectile 17) had front. a very The flattenedfirst yarn shape,contacting with the peripheral bullet was ruptures broken of near the jacket its cont andacting core. length with the bullet due to high tensileFigure stress; 18 this presents central details fragment of the was impacted so compressed face of that the its projectile fibers were after forced testing. into One the mayprojectile notice material the embedding and remained of fragmented fixed on the aramid bullet, yarns even on if the this projectile one was front.intensively The first de- yarnformed contacting and cracked. the bullet The bullet was brokenfrom the near 3rd itsfire contacting kept a similar length aspect with of the the bullet bullet duefailure to high(local tensile fragmentation stress; this and central plastic fragment deformation), was so compressedbut only several that fibers its fibers were were “stuck” forced to into the thebullet. projectile Several material fibers were and trapped remained in fixedthe cracks on the of bullet,the projectile, even if thisboth one in the was lead intensively core and deformedcopper alloy and jacket. cracked. The bullet from the 3rd fire kept a similar aspect of the bullet failure (local fragmentation and plastic deformation), but only several fibers were “stuck” to the bullet. Several fibers were trapped in the cracks of the projectile, both in the lead core and copper alloy jacket.

Polymers 2021, 13, 2912 16 of 17 Polymers 2021, 13, 2912 10 of 11

(a) (b)

FigureFigure 18.18. SEMSEM detailsdetails of of bullet bullet of of 9 mm9 mm FMJ, FMJ, as as retrieved retrieved after after firing. firing. (a) Bullet(a) Bullet of 2nd of fire;2nd (b fire) bullet; (b) ofbullet 3rd fire.of 3rd fire.

5.5. Conclusions TheThe main objective objective of of this this study study focuses focuses on on designing designing and and testing testing protection protection systems sys- temsusing using advanced advanced materials materials based basedon aramid on aramid fibers for fibers high-impact for high-impact speeds speedsof up to of420 up ± to10 420m/s.± The10 m/sinvestigation. The investigation of the failure of the mechanis failurems mechanisms identifies current identifies issues current of protective issues of protectivematerials, major materials, challenges major challengesand technological and technological problems for problems developing for these developing types of these sys- typestems, ofincluding systems, the including fulfilling the of fulfilling ballistic ofimpact ballistic requirements impact requirements according accordingto NIJ Standard- to NIJ Standard-0101.060101.06 [23]. [23]. The purpose of the paper was to conduct an investigation on the failure processes The purpose of the paper was to conduct an investigation on the failure processes and destructive stages of a ballistic package made of successive layers of LFT SB1plus, and destructive stages of a ballistic package made of successive layers of LFT SB1plus, taking into account the particular test conditions from NIJ Standard-0101.06 [23]. The taking into account the particular test conditions from NIJ Standard-0101.06 [23]. The main parameter of interest was the backface signature (BFS), but also details of projectile main parameter of interest was the backface signature (BFS), but also details of projectile arrest and SEM investigation could offer an argument for using this material for individual arrest and SEM investigation could offer an argument for using this material for individ- protection. For the reported tests, the maximum and minimum values for BFS were 12 mm ual protection. For the reported tests, the maximum and minimum values for BFS were 12 and 24 mm, the mean value being 18.66 mm and the standard deviation being 3.8 mm. mm and 24 mm, the mean value being 18.66 mm and the standard deviation being 3.8 These values recommend the package for use as a protective panel for body armor for mm. These values recommend the package for use as a protective panel for body armor threat level IIA. for threat level IIA. Author Contributions: Conceptualization, C.P. and L.D.; methodology, C.P. and L.D.; software, C.P.; validation,Author Contributions: L.D.; formal Conceptualization, analysis, C.P.; investigation, C.P. and C.P.;L.D.; resources,methodology, C.P.; C.P. data and curation, L.D.; L.D.software, and C.P.; validation, L.D.; formal analysis, C.P.; investigation, C.P.; resources, C.P.; data curation, L.D. C.P.; writing—original draft preparation, C.P.; writing—review and editing, L.D.; visualization, C.P.; and C.P.; writing—original draft preparation, C.P.; writing—review and editing, L.D.; visualization, supervision, L.D.; project administration, L.D. and C.P.; funding acquisition, C.P. All authors have C.P.; supervision, L.D.; project administration, L.D. and C.P.; funding acquisition, C.P. All authors read and agreed to the published version of the manuscript. have read and agreed to the published version of the manuscript. Funding: This research was funded by the National Institute of Aerospace Research “Elie Carafoli” Funding: This research was funded by the National Institute of Aerospace Research “Elie Carafoli” INCAS, Bucharest. INCAS, Bucharest. Institutional Review Board Statement: Not applicable. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data presented in this study are available on request from the Data Availability Statement: The data presented in this study are available on request from the corresponding author. corresponding author. Acknowledgments: We acknowledge the support in the test campaign given by the CBRN Defense Acknowledgments: We acknowledge the support in the test campaign given by the CBRN Defense and Ecology Scientific Research Centre, Bucharest, Romania. and Ecology Scientific Research Centre, Bucharest, Romania. Conflicts of Interest: The authors declare no conflict of interest. Conflicts of Interest: The authors declare no conflict of interest.

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