Format and Guidelines for Manuscript Preparation and Submission to Journal of Advanced Manufacturing Technology (JAMT) A REVIEW ON FLEXIBLE THERMOPLASTIC COMPOSITE LAMINATE FOR ANTI-STAB APPLICATIONS M.Y., Yuhazri1, N.H.CM.H, Nadia2, H., Sihombing3, S.H., Yahaya4, and A., Abu5 1, 2, 3, 4, 5 Faculty of Manufacturing Engineering, Universiti Teknikal Malaysia Melaka, Hang Tuah Jaya, 76100 Durian Tunggal, Melaka, Malaysia. Email: *[email protected]; [email protected]; [email protected]; [email protected]; [email protected] ABSTRACT: Research on anti-stab utilizing various types of fiber reinforcement and matrix materials in the production of laminated composite have been increased very rapidly over the past decade to investigate the performances and behavior of anti-stab. This review summarized recent research on production of laminated composite, thickness, design of laminates, various types of materials used, international requirement standards and the failure mechanism on the composite after the anti-stab testing. This review focused on thermoplastic as matrix materials due to good flexibility, stiffness and strength. The review showed that a proper hybridization through laminate system may make the hybrid panel very promising for anti-stab enhancement; however, lot of studies is still needed in the fields of preparation and stability, characterization and applications to overcome the challenges. Thus, the study propose a new flexible composite laminate for anti-stab applications by combining two types of reinforcement materials and embedding them into a proper matrix material. KEYWORDS: Flexible, Thermoplastic, Laminate, Anti Stabbing. 1.0 IN TRO D UCTIO N Composite materials are commonly used in engineering and military applications based on their characteristics in providing benefits in mechanical properties, especially towards the impact of performance characteristics to absorb the impact of energy [1]. As the first impact and damage failure in composite laminate product becomes an ultimate concern of engineers and manufacturers, Aktacs et al. [2] state that the real concern is on the behavior of the impact loading which can occur during the manufacture process, normal operation, and maintenance in industry. Based on Mayo et al. [3], one of the challenging applications is for the product to have stab and puncture resistance, especially on a product like body armor which must be flexible and comfortable to protect law enforcement officers and security personnel against the stab attack. In European countries, stab

Journal of Advanced Manufacturing Technology assaults are higher compared to the firearm attack. In addition, the most common test for anti-stab behavior and performance is quasi static stab testing using universal testing machine and stab drop tower test as reported by Li et al. [4] and Stojanović et al. [5]. As a flexible characteristic of composite laminate is its capability of bending easily on certain load without breakout, a higher stiffness characteristic is required to resist elongation when an external load is applied. Therefore, a composite required in this case should have a combination of strength, toughness, and flexibility. Strength is an amount of stress that materials can withstand before it breaks while stiffness is an ability to resist elongation when the load is applied, Kim and Nam [6] argue that the flexibility and toughness of the reinforcing materials required for stab resistant performance and behavior should be viewed from the perspective of manufacture process and materials.. Adel [7] finds that thermoplastic has different properties compared to thermoset related to high ductility and toughness, facilities of processing and recycling potential. According to Kaw [8], thermoplastic can also be formed at high temperature and pressure because of the weak bonding known as Van Der Waals bonding. Meanwhile, thermoplastic elastomers have become a significant part of the elastomers industry since they were first produced about 55 years ago [9]. Physical properties of thermoplastic elastomer are similar to rubber; soft, flexible and resilient. Thus, among these three materials, thermoplastic is more focused because it can enhance mechanical properties compared to thermoset and elastomers. Besides, thermoplastic has more advantages than others. Tsai and Melo [10] assert that the composite materials must emphasize those materials which contain matrix constituents that bind together and provides stronger and stiffness reinforcement constituents. The composite materials used as suggested by previous researchers are Kevlar/polyester [4], Aramid/ LDPE [6], Aramid/Glass Fiber/Epoxy [12] and Kevlar/PE [13]. Hence, aramid fiber is commonly used as reinforcement materials on industry application and human body protection against the ballistic and stabbing threats. According to Tien et al. [14], an important parameter for a flexible and toughness composite is the types of resin and fiber used which is related to the stab resistant performance. In addition, the shape of the impactor used is also important to determine a depth penetration against thickness and layer. However, there are only few researchers who focus on the flexibility and air permeability of the composite, especially thermoplastic materials. Therefore, this review is conducted to investigate the main factor needed when fabricating a flexible composite laminate based on the orientation and numbers of plies to handle force applied towards bending and damage on the surface of the composite laminate through the destructive testing. Hence,

this review also needs to find the right selection and decision on thematerials, orientation, number of ply, and other important criteria for a newflexible thermoplastic composite laminate. Particularly, the flexiblecomposite laminate which can withstand stab impact using low costmaterials.2.0 S T A B B I N G A N D ANTI-STABCampbell [15] discovers that in England, about 30 % increase has been notedin stabbing assaults.. Compared with Asia, stabbing attacks are moreprevalent threats because of the ban on gun possession. In a different study,Li et al. [11] examine the increasing of uses in body armor with stabprotection in recent years. Previous cases involve gun or bullets but recentlycases such as using swords, knives or other sharp objects become moreprevalent. According to Tien et al. [14] personnel body protection can bedivided into two groups according to the protection object. Firstly, againstthe kinetic impact and secondly against the intrusion of a sharp edge.Besides, the stab threats can be classified into two categories which arepuncture and cut. Puncture refers to the penetration of an object withpointed tips through a target, for example, ice picks or awls. Cut refers to thedestruction of the target with a continuous sharp edge such as knife edge.3.0 M A T E R I A L S F O R A NTI-STABMany types of materials can be used in making composite laminates forstabbing testing. Tien et al. [14] indicate that the form of Aramid and Basaltfiber are in filament and metal fiber is in staples. The specifications of thefilament and staples are shown in Table 1. Furthermore, Stojanović et al. [5]also use p-aramid fiber type of Twaron (Teijin Aramid) and the laminates onthe side of the p-aramid use thermoplastic polyurethane film.Table 1: Specification of the filament and staplwes based on Tien et al. [14] Materials Fiber Specificationp-aramid filament 22.2 tex (13fil) Basalt filament 50 tex (200fil)Metal staple yarn (stainless steel 319- L) 65 tex, -Mean fiber length: 150mm Mean fiber diameter: 12µmCotton Roving 2 x 400 tex-Mean fiber length: 31.5mm Micronaire value: 4.38 Staple yarn 17.72 tex (2-plied)In another study, Li et al. [11] examine Kevlar fiber which has a high countand high density of fabrics which are commonly used as soft body armorafter multiple layer lamination. Besides that the interlayer of nonwoven withglass fabric or Kevlar fabric are called glass compound fabric and Kevlarcompound fabric. Table 2 shows mechanical and physical properties of glassand Kevlar fabric.

Journal of Advanced Manufacturing TechnologyTable 2: Mechanical and physical properties of glass fiber and Kevlar fabrics [11]Fabric Structure Density Tensile Force (N) Elongation (%) Modulus (GPa) (in.) Warp Weft Warp Weft Warp Weft direction direction direction direction direction directionGlass Plain 34x26 606.10 514.45 5.26 3.18 1.46 2.05Kevlar plain 28x28 2627.52 7.00 4.76Yong [16] use soft rubber compound in order to overcome the high rigidityproblem by enhancing the flexibility of the laminates. At the same time, highfiber loading can also impact on strength of the laminates. Thus, the mixingof the fibre with either thermosetting or thermoplastic matrix are a wellknown combination to produce fibre based composite. Table 3 shows acomparison among materials commonly used for anti-stabbing testing. Table 3: Materials commonly used in anti-stab research Reinforcement / Fiber References Aramid Fiber/p-aramid [5] & [6] Aramid, Basalt and metal fiber [4] Recycle Kevlar Fiber [11] Rubber Wood [16] Aramid, Glass fiber [12] PET [17]4.0 T H I C K N E S S F O R A NTI-STABAccording to Tien et al. [14] an armor thickness of a specimen is essential forproviding comfort in wear ability. Thus, the thickness of the specimen candetermine its penetration depth behavior. Besides, several studies showwhen the penetration depth increases, the thickness decreases. The causemay be due to an influence of fabric density in each specimen. In addition, Liet al. [4] discover that when number of layers increases the thickness alsoincreases automatically. This scenario leads to a penetration throughcompound fabrics and offers more chance to contact with compound fabrics.Table 4 shows the thickness based on the materials used and the layers ofthe composites in anti-stab research.Table 4: The thickness and number of layer used in anti-stab research Materials Thickness Layer References (mm) Aramid/Kevlar 8 [13] P-aramid 0.571 4 [6] Aramid Fiber p-aramid 4 [5] Aramid, Basalt and metal fiber 17.6, 17.3, 14.2 - [14] Recycle Kevlar Fiber 0.32 5 [11] Rubber Wood 11.0 - [16] UHMWPE 5µm single [4] PET 2.2 single [17]

5.0 A NTI-STAB TESTINGStojanović et al. [5] shows that most stabbing effect testing use quasi-staticknives penetration tests based on modified versions of NIJ standard 0115.00,with engineered knife S1 for the multiaxial composite. Figure 1(a) shows thetesting of knives penetration toward the composite. Kim and Nam [6] alsofind that quasi-static stab resistance testing of the p-aramid and resinreinforced fabrics is performed using a Universal Testing Machine (Instron,Series IX automated material testing system). The knife and spike impactorsare mounted in the upper grip, and a single sheet of the target fabric istightly placed below the impactor using a compression jig. The compressionjig for quasi static stab resistance testing is shown in Figure 1(b & c). (a (b) (cFigure 1: Quasi static knives penetration (a) after, and compression jig (b) [5], (c) [6]A study by Tien et al. [14] show another method in conducting stabbingtesting; using a drop mass with standard blades equipped with an impactoras depicted in Figure 2. The blades consist of two types; knives and spikes.Basically, the knife was dropped onto the unclamped fabric which was placedon the top of damped backing materials by certain height. The mass, speed,and damping characteristics of the backing material for the experiment havebeen designed to mimic the biomechanical process of stabbing assaults. Table5 illustrates the types of standard and machine researchers used for the anti-stab testing application. (a) (b) Figure 2: Anti-stab testing (a) drop tower, (b) the schematic of drop tower [14]

Journal of Advanced Manufacturing TechnologyTable 5: Standard and equipment used for anti-stab researchStandard Type Machine/ Equipment ReferencesNIJ 0115 Stab Resistance Test Machine [16]NIJ 0115 Stab Impact Testing Instron 9250HV (Boston USA) [4]ASTM F1342.05 Static Puncture Test Instron 5566 Universal Tester (Instron, USA) [11]NIJ 0115 Stab Testing Frame, Conventional Drop Tower [14]NIJ 0115 Instron Universal Testing Machine [5]NIJ 0115 Quasi-static stab resistance testing using UTM (Instron, USA) [6]Kim and Nam [6] analyze two types of knives which are double edge bladeknown as S1 and spike. These knives are used as impactors for the droptower testing following the NIJ standard. Figure 3 shows the two types of theimpactors. According to NIJ standard, to set the point, it must strike theenergy. The total drop mass including impactor is fixed to 1.8 kg and thedrop height is adjusted to 1.36 meter. The depth of penetration that strikes totarget is detected by the displacement sensor. Before the stab test, theimpactor is placed to the drop mass in a rail-guided drop tower, and then itwill drop off from a fixed height to impact the target. Tien et al. [14] also findthat two types of impactors that NIJ recommends are knife, and spike. Figure3 are example of the impactor with various dimensions. Figure 3: Type of impactor nose (a) S1, (b) P1, and (c) spike [14]Stojanović et al. [5] recommend using eight layers of the PU/aramid/PVBfabrics composite plus the 5 wt.% Silicon Oxide (SiO2) which can provide anexcellent resistance to penetration by a ballistic projectile and stab resistance

from the front impact. Thus, adding only 5 wt.% SiO2 can improve thethermo mechanical properties and provide more comfort and flexibility.Hou et al. [17] find that the woven structure has a numerical calculation onthe damage that happen to the surface after testing. Parameters that are usedin this calculation are warp yarn, weft yarn, the matrix and the relevantcomposite fiber used for an experiment. However, Yan et al. [18] find that a3D structure can support the drop weight impact and due to the differentfailure mechanism, the static and dynamic puncture test is observed. It alsoshows that a hybrid composite can perform exceptionally in static anddynamic impact resistance and areal density can influence impact behavior.A schematic diagram of the static and dynamic puncture is shown in Figure4. Tien et al. [14] also provide a formula to measure value in the stab testingfor penetration depth, but the performance of the specimen against thestabbing cannot be interpreted. When penetration depth increases, thetendency of areal density and thickness decreases. Thus, the penetrationdepth can protect the human body against stabbing treat assigned by the NIJstandard. The anti-stabbing index formula (ASI) is known as thickness timesdensity armor specimen for penetration depth respectively. (a) (b)Figure 4: the puncture (a) close-up impact, (b) dynamic [15]Kim and Nam [6] also find that materials composed with textile structure forstab resistance can provide high flexibility and process ability compared toprotective materials such as metal or ceramic plate. The higher performanceof fiber is due to aromatic polyamide, ultrahigh molecular weightpolyethylene, and polybenzoxazole. All these can prevent the penetration orcut by a sharp impactor on the stab resistance testing. Table 6 shows theresults for the resin reinforced p-aramid fabrics.Table 6: Penetration depth for knives and spikes at fixed number of layer [6] Thickness Penetration depth (mm) (mm)Sample Layer Knives SpikesUntreated 20 11.4 Over 36.0 Over 36.0Single side 20Both side 20 12.6 13.0 Over 36.0 15.7 16.7 24.0In contrast Yong [16] finds that in general, rubber wood fiber shows a goodreinforcement behavior for making laminate composite. It also has strong

Journal of Advanced Manufacturing Technology elastomeric features and this behavior can be found in other fibers such as jute and glass fiber. According to him, edge blade and spike only pass the testing on energy level of 1 and 2 and when reaching energy level 3 it shows 100 % failure as shown in Table 7. However, Li et al. [4] find that a knife has a sharp face, so it can only cut fiber along the stab direction during the stab impact, for UHMWPE UD composite stab behavior is directionally independent compared to other fiber reinforced composites which is anisotropy. The structure of the UHMWPE UD composite must be designed first before doing stab resistance test to get better results.Table 7: Stab resistance test at different energy level, angle and threat weapon [16]Energy Level Angle of incidence Threat weapon Penetration depth (mm) 0 P1 3.4 0 S1 4.11 45 P1 3.7 45 S1 4.5 0 Spike 6.0 45 Spike 7.2 0 P1 4.9 0 S1 5.42 45 P1 5.1 45 S1 5.8 0 Spike 7.3 45 Spike 8.1 0 P1 7.2 0 S1 7.53 45 P1 7.9 45 S1 8.1 0 Spike 9.0 45 Spike 9.86.0 F A I L U R E M E C H A N I S MDecker et al. [19] find that when impact energy increases, the penetrationdepth on the backing materials also increases. Hence, the drop tower stabperformance of the Kevlar and STF Kevlar targets against the spike impactor,resulting in STF-Kevlar provides more exceptional stab resistance comparedto Kevlar. Furthermore, at this highest energy level against the STF–Kevlartarget, the spike impactor is plastically bent. Figure 5 shows fabric damageon the STF-Kevlar and Kevlar based on the mass 2.34 gram and the heightwas 0.75 meter.

Figure 5: Fabric damage on the STF-Kevlar and Kevlar [19]Li et al. [4] also find the morphology of failure in UHMWPE UD after anti-stabbing testing using Scanning Electron Microscopy (SEM). The mainfailure is the tensile failure instead of cutting. Hence, the failure is caused bythe puncturing tip in the first step. Figure 6 shows the failure morphology.Tam et al. [20] state that chain of ultra-high molecular weight polyethylenehave a high strength. It is suitable to be a ballistic resistance compositefabricated from a high strength synthetic fiber exhibited a varying degree ofresistance to penetration by a high speed impact from projectiles such asbullet, shells, and shrapnel. (a) (b) (c) (d) Figure 6: SEM pictures (a) near the tip, (b) in the middle of the stab position, (c) near the blunt sides, (d) near the tip [4]In a different study, Li et al. [11] find that using glass compound fiber andKevlar compound fiber makes the damage area elongate. This can affect thelarger area after dynamic puncture and the fracture fiber becomes higher

Journal of Advanced Manufacturing Technology against the dynamic puncture energy. This statement is verified by the fact that the damage region found is regular and quadrilateral static damage area on the specimen. As highlighted by Raja and Hari [21] the definition of the density is average number of matrix crack in each hybrid layer per unit length in the longitudinal direction. In this experiment, the maximum stress of cracking hybrid sample is 403.42 MPa. The matrix cracks of hybrid specimens are nucleated in the central 45° plies of the angle-ply specimen and the delamination of hybrid specimens are observed to appear from the specimen surface. The damage initiation is delayed by the interleaving in the angle-ply hybrid specimens. Kevlar is more ductile than glass fibers as it creates more damage compared to glass fiber due to more energy required to breakout the Kevlar. 7.0 R E V I E W S U M M A R Y Based on extant literature, many findings show the flexibility of thermoplastic laminate composite for anti-stab application. Many researches have used various materials on various experiments related to stab resistance, stab properties, stab behavior, stab performance, and puncture behavior. Based on these studies, a lot of information is overlapped or similar. Besides, discussion is generated on how to make composite laminate more flexible and higher in strength. Most importantly, the thickness, the layer, types of materials used for fiber and resin, and the effect on the composite after testing are highlighted. Furthermore many researchers use Aramid fiber, glass, rubber wood, flexible woven for the reinforcement on their experiments and research. This Aramid fiber is combined with thermoplastic resin to give some flexibility and high in strength. Most common used material for thermoplastic resin is epoxy but other types of materials are also found in thermoplastic such as TPU, PET, PP or UHMWPE. Other than that, the making of the laminate requires two directions; bidirectional or unidirectional. This review shows that the polyethylene (PE), high density polyethylene (HDPE) and low density polyethylene (LDPE) are not used at all in previous studies for anti-stab research. Moreover, the most suitable method and machine to test the composite laminate on anti-stabbing is Universal Testing Machine (UTM) using NIJ standard 0.115. Many researchers use this machine for the testing of the specimen. Some researchers use static puncture based on ASTM standard. Important parameters for the testing include the height of the impactor used, the types of impactor and the results show the penetration depth between the thickness and the impact on the composite laminate. Some researchers who investigate the structure of the specimen after the testing use Scanning Electron Microscopy. It can show the structure damage on the specimen based on the drop mass and velocity of the impactor.

In short,, a few researchers use a thermoplastic as a matrix materials. Thus,thermoplastic composite laminate has a high potential which include usingthermoplastic as a matrices combined with glass and aramid fiber asreinforcement materials. From the combination of materials, the compositelaminate can be formed and stab drop tower and quasi static stab testing canbe conducted. In addition, flexural testing can be done to investigate thelevel of the laminate composite which can be bent to prove its flexibility andelasticity. The Figure 7 shows the overview of studies on anti-stab testing. 4 Number of Layer 8 20.0 17.5 Basalt [14] [14] [13]Thickness, mm 15.0 Cotton yarn [14] Latex 12.5 Metal Rubber 10.0 p-Aramid, [16] wood rAramid & [5] PP, PE 7.5 Aramid [14] [17] PET 5.0 Core [6] [11] [4] TPU 2.5 spun UHMWPE yarn Matrix Materials Thermoset Thermoplastic Figure 7: Overview of study on anti-stab8.0 CO N C LUS IONIn conclusion, a few important points can be highlighted as follows.(a) Proper thickness of laminated composite for anti-stab might achieve their highest strength but increasing the thickness will reduce the flexibility performance reduced.(b) The flexibility of laminated composite can be improved by adding the element of rubber properties in matrix materials beside using the reinforcement materials with good ductility properties. Good candidates to be explored extensively are LDPE and HDPE as matrix materials, whereas the reinforcement materials are Aramid Fiber and glass fiber.(c) Currently, there is no standard value to account for “flexibility value/index”. Thus this is a good departure point to setup proper and new scientific data base for flexibility in anti-stab research.(d) The dependency on ballistic materials like Kevlar as anti-stab material can be reduced by integrating other materials to create a so- called hybrid system.

Journal of Advanced Manufacturing Technology AC KN O W L E D G M E N T S The authors would like to thank the Faculty of Manufacturing Engineering for a permission given to use the facility in the Composite Engineering & Technology Laboratory. REFERENCES [1] C. Evci and M. G\"ulgecc, “An experimental investigation on the impact response of composite materials”. Int. J. of Impact Engineering, Vol.43, pp.40-51, May 2012. [2] M. Aktacs, C. Atas, B. Iccten and R. Karakuzu, “An experimental investigation of the impact response of composite laminates”. Composite Structures, Vol.87, No.4, pp.307-313, Feb. 2009. [3] J.B. Mayo, E.D. Wetzel, M.V. Hosur and S. Jeelani, “Stab and puncture characterization of thermoplastic-impregnated aramid fabrics”. Int. J. of Impact Engineering, Vol.36, Iss.9, pp.1095-1105, Sep. 2009. [4] C. Li, X. Huang, Y. Li, N. Yang, Z. Shen and X. Fan,”Stab resistance of UHMWPE fiber composites impregnated with thermoplastics”. Polymers Advanced Technologies, Vol.25, No.9, pp.1014-1019, Jun. 2014. [5] D.B. Stojanović, M. Zrilić, R. Jančić-Heinemann, I. Živković, A. Kojović, P.S. Uskoković and R. Aleksić, “Mechanical and anti- stabbing properties of modified thermoplastic polymers impregnated multiaxial p-aramid fabrics, Polymers Advanced Technologies, Vol.24, No.8, pp.772-776, Aug. 2013. [6] H. Kim and I. Nam, “Stab resisting behavior of polymeric resin reinforced p-aramid fabrics”. Journal of Applied Polymer Science, Vol.123, Iss.5, pp.2733-2742, Mar. 2012. [7] K.A. Allen, Processing of Thermoplastic Composites, Polymer and Composites Processing, pp.1-10, 2012. [8] A. Kaw, Mechanics of composite materials. Boca Raton: CRC Press, 1997. [9] M. Kutz, Applied plastics engineering handbook. Amsterdam: William Andrew, 2011. [10] S.W. Tsai and J.S.D. Melo, “An invariant-based theory of composites”. Composites Science and Technology, Vol.100, pp.237-243, Aug. 2014.

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