Experimental and Numerical Investigation on the Layering ... - MDPI

metals Article

Experimental and Numerical Investigation on the Layering Configuration Effect to the Laminated Aluminium/Steel Panel Subjected to High Speed Impact Test Najihah Abdul Rahman 1, * , Shahrum Abdullah 1, *, Mohamad Faizal Abdullah 2 , Wan Fathul Hakim Zamri 3 , Mohd Zaidi Omar 3 and Zainuddin Sajuri 3 1 2 3

*

Centre for Integrated Design for Advanced Mechanical Systems, Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia, Bangi 43600, Selangor, Malaysia Department of Mechanical Engineering, Faculty of Engineering, Universiti Pertahanan Nasional Malaysia, Kem Sg Besi, Kuala Lumpur 57000, Malaysia; [email protected] Centre for Materials Engineering and Smart Manufacturing, Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia, Bangi 43600, Selangor, Malaysia; [email protected] (W.F.H.Z.); [email protected] (M.Z.O.); [email protected] (Z.S.) Correspondence: [email protected] (N.A.R.); [email protected] (S.A.)

Received: 16 August 2018; Accepted: 14 September 2018; Published: 17 September 2018







Abstract: This paper presents the effect of laminated aluminium-steel panel with different configurations in a high-speed impact test. Layering aluminium plate with high strength steel has become an interest in reducing the overall density of armour vehicle body while improving the ballistic resistance. Different layering configurations differ in laminated panel performance. Two layering configurations of double-layered panel achieving 25% of existing panel weight reduction were tested using experiment and computational method to investigate their behaviours when impacted with 7.62-mm full metal jacket at velocity range of 800–850 m/s. The ballistic performance of each configuration plate in terms of ballistic limit velocity, penetration process and permanent deformation was quantified and considered. Laminated panel with aluminium as the front layer reduced the ballistic performance of existing panel to 50% and the other panel maintained its performance. Thus, the laminated panel with aluminium as the back layer can be used in designing a protective structure for armoured vehicle while maintaining the performance of the existing vehicle in achieving weight reduction. Keywords: ballistic impact; ballistic test; double-layered plate; failure; numerical simulation

1. Introduction Material failure characterisation can be determined using several methods and engineering methods, such as using the finite element analysis. The finite element method is an analysis to predict the state of a product or material when encumbered to test the strength of the material before it fails. The penetration depth of the material, as one example of the failure of the material when subjected to a high-speed impact, can be analysed using this method. High-speed impact, also known as ballistic impact, is one of the measures used to determine the material resistance to prevent the projectile from penetrating the panel [1]. Failure of a material by ballistic impact is influenced by several factors such as the projectile nose shape, projectile materials, impact velocity, panel layering configuration and mechanical properties of materials constituting the laminated panel [2]. The ballistic test is often performed to test the strength of the materials used in military applications such as vehicle

Metals 2018, 8, 732; doi:10.3390/met8090732

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panel shield [3]. The use of a single material made of armour steel has been widely used as the main material in designing armoured vehicle panel due to the combined features of high strength, toughness, good formability, weldability and excellent ballistic performance [4]. However, the disadvantage of this material is from the point of its heaviness. Weight is an important parameter in the manufacture of armoured vehicles because higher weight leads to higher energy consumption to drive the vehicle and more difficulty in manoeuvring the vehicle on challenging topography [5]. Therefore, many studies have been done to reduce the weight of the materials used in these applications. One way to achieve lighter weight is to combine two different types of materials by laminating the light weight material with the existing steel. According to previous studies, most researchers found that the single material of armour steel provided better ballistic performance compared to the layered material. Børvik et al. [6] conducted a study on the ballistic performance of the layered material and compared it with a panel of single material using Weldox 700E. The result showed that monolithic panel of same thickness and material has better performance compared to the layered panel as a result of reduction in the structure leading to substantiality of bending behaviour. Restriction of vehicle manoeuvrability due to heaviness characteristics of an armoured vehicle has directed researchers to study the performance of lightweight materials under ballistic impact. Wei et al. [7] also concluded that achievement of ballistic impact of monolithic lightweight materials has been found to be always better than the layered panel of similar material and thickness. However, monolithic lightweight materials such as aluminium alloy panel could not give a comparable ballistic performance with monolithic existing armour steel panel. It has been suggested that the ballistic performance of an aluminium alloy can be improved by layering the alloy with the existing armour steel as a laminated panel. Gupta et al. [8] suggested that the performance of aluminium panel with various thicknesses and layering configurations gave different performance. Forrestal et al. [9] reported the performance of laminated armour steel and aluminium alloys of Al7075-T6 and Al5083-H116 can be integrated with armour steel to serve as vehicle protective structures and demonstrated relatively good performance as existing material. Flores-Johnson et al. [10] performed finite element analysis of the impact of a 7.62 mm APM2 projectile on multi-layered armour plates to investigate the effect of different layer configurations, thicknesses and material properties on ballistic performance. Übeyli et al. [11] investigated the effect of laminate configuration on the behaviour of aluminium laminated composite against 7.62 AP projectiles and found that using hard material on the first layer and aluminium alloy on the back layer can improve the ballistic performance of armour steel. Layering configuration is one of the important parameters affecting the ballistic performance of a laminated dissimilar metal panel. Moreover, researchers recently focused on implementing finite element method to study the behaviour of multi-layered panels consisting of aluminium alloys and steels under high velocity impact. The literature shows that the penetration effects of multi-layered panels are categorised as a complex problem. For the design of a ballistic resistant panel, the factors such as layer configurations and thickness should be well considered to obtain optimum protective structures. Since different layering configuration would give different ballistic performance, the study for different layering configuration has become necessary to find the best configuration as an option for an armoured vehicle application. Therefore, the aim of this study is to analyse the effect of different layering configuration to the failure of the laminated panel in terms of permanent deformation and penetration process using both experiment and simulation approaches. The study performed on a double-layered panel consisting two different materials, aluminium alloy Al7075-T6 and steel Ar500 which has similar total weights and initial impact velocities but different layering configuration. Double-layered panels were subjected to the ballistic tests using powdered gun and the tests then were simulated using an explicit non-linear finite element program against a 7.62-mm FMJ projectile at a velocity of 800–850 m/s. The ballistic performance of laminated layered plates made of combination of Ar500 steel and Al7075-T6 was evaluated based on the perforation mechanism, the depth of penetration and the crater diameter.

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2. Materials Method 2. Materials and and Method methodology framework in this study is given in Figure 1. Combination of Ar500 The The methodology framework usedused in this study is given in Figure 1. Combination of Ar500 steelsteel and Al7075-T6 aluminium is an interesting option leading to weight-saving and ballistic performance and Al7075-T6 aluminium is an interesting option leading to weight-saving and ballistic performance improvement based on their material properties in terms of tensile strength, bending strength improvement based on their material properties in terms of tensile strength, bending strength and and hardness. At first, the mechanical tests by means of the tensile, bending and hardness tests hardness. At first, the mechanical tests by means of the tensile, bending and hardness tests werewere conducted to find the respective material properties. types of layering configuration panels conducted to find the respective material properties. TwoTwo types of layering configuration panels werewere prepared. Ballistic carried out using experimental simulation works. thenthen prepared. Ballistic test test was was carried out using bothboth experimental andand simulation works.

Figure 1. Flow diagram of a methodology usedused for this Figure 1. Flow diagram of a methodology for work. this work.

2.1. Material Characterisation 2.1. Material Characterisation In this study, a seriesofofmechanical mechanical tests such hardness testtest andand three-point bending In this study, a series suchas astensile tensiletest, test, hardness three-point test was to determine the mechanical properties of material constituting the laminated bending test performed was performed to determine the mechanical properties of material constituting the panel, aluminium alloy Al7075-T6 and steeland Ar500, the original steel, Rolled Homogeneous laminated panel, aluminium alloy Al7075-T6 steeland Ar500, and thearmour original armour steel, Rolled Armour (RHA). The specimen used in used the tensile test was according to the size of Homogeneous Armour (RHA). The specimen in the tensile testprepared was prepared according to the × 140 mm × 6× mm in in compliance with thethe ASTM E8.E8. The strain rate and thethe crosshead speed size 6ofmm 6 mm × 140 mm 6 mm compliance with ASTM The strain rate and crosshead were setset at at 0.001 mm/min,respectively respectively[12]. [12].This This carried to determine speed were 0.001s−s1−1 and and 1.8 mm/min, testtest waswas carried out out to determine the material properties as Young’s modulus, yield strength and ultimate strength. the material properties such such as Young’s modulus, yield strength and ultimate tensile tensile strength. In In measuring the hardness the tested materials, Rockwell hardness was used B scale measuring the hardness of the of tested materials, Rockwell hardness testertester was used for Bfor scale hardness measurement. Rockwell is a hardness based on indentation hardness hardness measurement. The The Rockwell scalescale B is aB hardness scalescale based on indentation hardness of a of a material. specimen prepared according to ASTM with a size mm × mm 10 mm 10 mm material. The The specimen waswas prepared according to ASTM E18E18 with a size of of 1010 mm × 10 × 10×mm to test materials resistance against penetration three-point bending to test the the materials resistance against penetration [13].[13]. The The three-point bending test test was was alsoalso performed to determine the mechanical properties of materials when subjected to the on impact performed to determine the mechanical properties of materials when subjected to the impact the on panel and to identify energy absorption capability each material [8]. test, In this frontthe of front panelofand to identify energy absorption capability of each of material [8]. In this the test, the specimen cut to 5 mm 50 mm in × accordance 10 mm in accordance with ASTM E290tested and was specimen was cut was to a size of a5 size mmof × 50 mm× × 10 with ASTM E290 and was by the universal testing machine at a speed of 5 mm/min which is commonly used to test metal bending behaviour [14]. The machines used for all above tests are shown in Figure 2.

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tested by the universal testing machine at a speed of 5 mm/min which is commonly used to test metal 4 of of 16 16 4 used for all above tests are shown in Figure 2.

Metals 2018, 2018, 8, 8, x x FOR FOR PEER PEER REVIEW REVIEW Metals bending behaviour [14]. The machines

(a) (a)

(b) (b)

(c) (c)

Figure mechanical test performed for: (a) test; test; three2. Machines Machinesfor mechanical performed (a) tensile test;hardness (b) hardness test;(c) (c) Figure 2. 2. Machines forfor mechanical testtest performed for:for: (a) tensile tensile test; (b) (b) hardness test; and and (c)and threepoint bending test. three-point bending point bending test. test.

2.2. Experimental Ballistic Ballistic Test 2.2. 2.2. Experimental Experimental Ballistic Test Test The dimensions of ballistictest test plates of double-layered for both configurations The dimensions dimensions of of ballistic ballistic plates of double-layered double-layered for both both configurations were were in 100 100in × The test plates of for configurations were in × 2 following the NATO Stanag 4569 standard. This dimension is adequate to ensure 2 100 × 100 mm 100 mm following the NATO Stanag 4569 standard. This dimension is adequate to ensure there is 2 100 mm following the NATO Stanag 4569 standard. This dimension is adequate to ensure there is there is no force induced on the bulletthe from the reflected wave by the edge of plate during the impact. no no force force induced induced on on the the bullet bullet from from the reflected reflected wave wave by by the the edge edge of of plate plate during during the the impact. impact. All All All layered plates were in direct contact with each other and plate spacing effect or adhesion effect on on layered plates were in direct contact with each other and plate spacing effect or adhesion effect layered plates were in direct contact with each other and plate spacing effect or adhesion effect on ballistic resistive performance were not considered in this study. The target plates were mounted onona ballistic ballistic resistive resistive performance performance were were not not considered considered in in this this study. study. The The target target plates plates were were mounted mounted on stiff frame in in thethe ballistic testtest set-up as in Figure 3 and subjected to three shoots each.each. The ballistic tests a stiff frame ballistic set-up as in Figure 3 and subjected to three shoots a stiff frame in the ballistic test set-up as in Figure 3 and subjected to three shoots each. The The ballistic ballistic were performed using NATO Stanag 4569 standard level threat two threat. The ammunition for the tests tests were were performed performed using using NATO NATO Stanag Stanag 4569 4569 standard standard level level threat threat two two threat. threat. The The ammunition ammunition for for level threat two is named as 7.627.62 mm full full metal jacket (FMJ) which is composed of a copper jacket and the the level level threat threat two two is is named named as as 7.62 mm mm full metal metal jacket jacket (FMJ) (FMJ) which which is is composed composed of of aa copper copper jacket jacket a lead core. core. The velocity of theof bullet was kept 830 m/s830 as m/s stated Stanag The velocity and a lead lead The velocity velocity the bullet bullet wasaround kept around around asin stated in 4569. Stanag 4569. The and a core. The of the was kept 830 m/s as stated in Stanag 4569. The measurement system and test were placed at 3.5 m away and 5.0 m away from the target. velocity measurement system and test were placed at 3.5 m away and 5.0 m away from the target. velocity measurement system and test were placed at 3.5 m away and 5.0 m away from the target.

Figure 3. The schematic of the ballistic test system used. Figure Figure 3. 3. The The schematic schematic of of the the ballistic ballistic test test system system used. used.

The ballistic tests were carried out to investigate the behaviour of two different configurations The ballistic were carried to the behaviour of two different configurations The ballistic tests tests were carried out to investigate investigate theand behaviour of of two different configurations of double-layered panel which haveout same areal density thickness each constituting material of double-layered panel which have same areal density and thickness of each constituting material of double-layered panel which have areal density and of each material subjected to high velocity impact. Thesame geometric model of thethickness layered plates is constituting shown in Figure 4a,b. subjected to velocity impact. The model is Figure 4a,b. subjected to high highlayered velocitypanel impact. The geometric geometric model of of the theAlayered layered plates is shown shown in15-mm Figure thick 4a,b. The first double designated as Configuration (Figureplates 4a) consists of ain The first layered panel designated Configuration (Figure 4a) consists aa 15-mm thick The first double layered panel designated asAl7075-T6 Configuration Aback (Figure 4a)Meanwhile, consists of of second 15-mm thick Ar500 as double the front layer and a 10-mm thickas as theA layer. double Ar500 as the front layer and a 10-mm thick Al7075-T6 as the back layer. Meanwhile, second double Ar500 the front layer a 10-mmas thick Al7075-T6 Basconsisting the back layer. Meanwhile, second double layeredaspanel (Figure 4b)and is presented Configuration of a 10-mm thick Al7075-T6 as the layered layered panel panel (Figure (Figure 4b) 4b) is is presented presented as as Configuration Configuration B B consisting consisting of of aa 10-mm 10-mm thick thick Al7075-T6 Al7075-T6 as as the front front layer layer and and aa 15-mm 15-mm thick thick Ar500 Ar500 as as the the back back layer. layer. These These panels panels were were designed designed to to be be for for the armoured armoured vehicle vehicle application application to to achieve achieve the the NATO NATO Stanag Stanag 4569 4569 ballistic ballistic application application of of level level threat threat 22 [14]. [14].

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front layer and a 15-mm thick Ar500 as the back layer. These panels were designed to be for armoured vehicle application to achieve Metals 2018, 8, x FOR PEER REVIEWthe NATO Stanag 4569 ballistic application of level threat 2 [14]. 5 of 16

(a)

(b)

Figure 4. The cross-section geometrical model of the 7.62-mm FMJ projectile double-layered panel for: Figure 4. The cross-section geometrical model of the 7.62-mm FMJ projectile double-layered panel for: (a) Configuration A; and (b) Configuration B. (a) Configuration A; and (b) Configuration B.

2.3. Computational Ballistic Test 2.3. Computational Ballistic Test Computational ballistic test was performed using the explicit dynamic finite element method Computational test was performed dynamic finite element (FEM). In solving theballistic problem modelled in FEM,using there the areexplicit four established methods that method can be (FEM). In solvingmethod, the problem modelled in FEM, there are four established methods that can be utilised: Eulerian Lagrangian method, arbitrary-Lagrangian–Eulerian (ALE) method and utilised: Eulerian method, Lagrangian method, arbitrary-Lagrangian–Eulerian method and smooth-particle hydrodynamics (SPH) method. Eulerian method involves volume (ALE) constraint to solve smooth-particle (SPH) method. Eulerian method involves constraint to solve problem equationshydrodynamics through the conservation of mass, momentum and energy.volume ALE method is extended problem equations through conservation of mass, momentum and energy. method from the Lagrangian method,the in which some computation steps have been addedALE to solve gridis extended from the Lagrangian method, in which some computation steps have been added to solve movement problem using simulation. It can give stability during computation distortion but lengthy grid movement problem usingissimulation. It can give is stability duringof computation but computational time and space needed. SPH method an extension Lagrangiandistortion method and lengthy computational time and space is needed. SPH method is an extension of Lagrangian method suitable for complex behaviour during simulation. However, it is difficult to determine the boundary and suitable formethod complexasbehaviour simulation. it on is difficult determine the condition of this there is noduring geometry defined. However, Lagrangian the othertohand can solve boundary condition of minimum this method as there is for no geometry defined. theproblem other hand complex problem with requirement computational timeLagrangian and space.onThe of can solve complex problem minimum requirement for computational time and space. The element distortion arisen from with this method can be solved by introducing the geometry erosion model problem of element during finite element distortion modelling.arisen from this method can be solved by introducing the geometry erosion model commercial during finitesimulation element modelling. A specific software package was used to develop a two-dimensional specific commercial software was package usedvelocities to develop a two-dimensional modelAfor ballistic tests. A 7.62simulation mm FMJ projectile used was at initial ranging from 810 m/s for ballistic tests. A 7.62 FMJaccording projectile was used at initial4569 velocities ranging from 810 tomodel 850 m/s. Initial velocity wasmm chosen to NATO Stanag ballistic protection levelm/s 2 to 850 m/s. Initial velocity was chosen according to NATO Stanag 4569 ballistic protection level which is 830 ± 20 m/s, for both layout configurations. The target plate was modelled as 50 mm2 which is circular 830 ± 20plate m/s,and for fully both clamped layout configurations. The targetasplate was modelled as 50 mm diameter at the edge boundaries in Figure 5a,b. The projectile diameter circular plateindependent and fully clamped the edge as in Figure The projectile was modelled in two parts: at metal jacketboundaries and lead core, which 5a,b. has outer diameterwas of modelled in two independent parts: metal jacket leadThe core,target whichplate has outer diameter as of 7.7 mm, 7.7 mm, inner diameter of 6.2 mm and length of and 35 mm. was modelled 50 mm inner diameter 6.2 mm of 35 mm. Theboundaries. target plateThe wastarget modelled mm diameter diameter circularofplate and and fullylength clamped at the edge plateas for50computational circularwas plate and fully at thewhich edge boundaries. plate for computational method method designed asclamped circular plate differs from The thattarget of experimental method, because the was designedmethod as circular plate which differs from of experimental simulation method, because the computational requires asymmetrical model forthat the two-dimensional modelling. computational requires asymmetrical for the two-dimensional This is importantmethod to save computational time andmodel computer memory required for simulation simulation.modelling. However, This on is important save computational memory for simulation. based Forrestal et to al. [9] and Flores-Johnsontime et al.and [10],computer ballistic results for a required 50-mm diameter circular However, on×Forrestal al. [9]plate and Flores-Johnson et al. [10], ballistic results for aby50-mm plate and a based 100-mm 100-mmetsquare did not vary because the deformation caused high diameter circular platelocally. and a 100-mm × 100-mm square plate did not vary because the deformation speed impact happens caused by high speed impact happens locally. In finite element analysis, the problem was considered as axisymmetric model as the bullet rotation is not considered because, in this study, the main interest is the behaviour of impacted panel. A constant mesh size of 0.5 mm was selected to finely resolve each problem and make the through thickness elements for the penetration process to be 50 elements. This size was found to adequately

of geometric erosion. This technique was able to remove the elements experiencing large distortion and, consequently, simulation failure can be avoided [10,15]. Developing glue or bonding model in finite element modelling is another issue occurred frequently in obtaining accurate result [16]. At this stage, no bonding material was included to avoid large error during analysis. Therefore, to imitate closely finite element model behaviour, the panels were simply clamped without any joining material Metals 2018, 8, 732 6 of 16 in between during the experiment.

(a)

(b)

Figure 5.5.FEA FEA model of 7.62-mm FMJ projectile and double-layered for: (a) Figure model of 7.62-mm FMJ projectile and double-layered target platetarget for: (a)plate Configuration Configuration A; and (b) A; and (b) Configuration B. Configuration B.

In element analysis, problem considered as axisymmetric model as the Allfinite panels have same totalthe thickness of was 25 mm. The important factor in the sample sizebullet is its rotation is not considered because, in this study, the main interest is the behaviour of impacted thickness because the selected thickness of 25 mm is the standard thickness of existing armour panel panel. A constant mesh size 0.5accordingly mm was selected to finely resolve eachofproblem make the [15]. Each layer thickness wasofset to the 25% weight reduction originaland armour panel through for the be 50 elements. size foundand to to ensurethickness the bodyelements maintaining onpenetration the groundprocess duringtoextreme conditionsThis such aswas blasting adequately produce good results basedpanels on previous work by Rahman et al.constitutive (2016) [15]. The platesmodel were explosion. Both projectile and target used the Johnson–cook (JC) material modelled node-to-node usingimpact pure langrage algorithm implementation of which hasusing been commonly usedconnectivity for high velocity simulation [9,17,18].with The JC model is applied geometric erosion strain to avoid The penetration mechanics at very high velocitymodels does involve to determine the strain rate and failure. temperature dependence of viscous-plastic material and is in temperature effects. Therefore, expressed as [18] in Equation (1) the Johnson–Cook material constitutive models were suitable to represent the target plate and projectile in the finite element𝐶analysis. 𝑛 ∗ (1) (1 − 𝑇 ∗𝑚 ) 𝜎𝑒𝑞 used = (𝐴 between + 𝐵𝜀𝑒𝑞 )(1plates + 𝜀̇𝑒𝑞 )and Besides, the contact condition projectile was defined as trajectory contact. connectivity was applied using pure langrage algorithm with implementation where 𝜎Node-to-node 𝑒𝑞 is the equivalent stress; 𝜀𝑒𝑞 is the equivalent plastic strain; 𝐴, 𝐵, 𝑛, 𝐶 and 𝑚 are the ∗ technique of geometric erosion. This able to remove the elements experiencing large material constants; and 𝜀̇𝑒𝑞 = 𝜀̇𝑒𝑞 ⁄𝜀̇0 was is the dimensionless strain rate where it is a ratio ofdistortion the strain ∗𝑚 and, simulation can be [10,15]. Developing glue orisbonding model rate consequently, and a user-defined strainfailure rate. 𝑇 is avoided the homologous temperature and given by 𝑇 ∗𝑚 in = finite modelling is another issue occurred frequently in obtaining accurate result [16]. At this (𝑇 − 𝑇element )(𝑇 ), 𝑟 𝑚 − 𝑇𝑟 where 𝑇𝑟 and 𝑇𝑚 represent the room temperature and the melting temperature, stage, no bonding material was includedmodel to avoid during implemented analysis. Therefore, to imitate respectively. This modified JC material has large been error successfully to model impact closely model behaviour, panels were simply clamped any joining on steelfinite [16] element and aluminium targets [9].the The JC parameters used in this without study are shown in material Table 1. in the experiment. JCbetween materialduring constitutive models were utilised to avoid the stresses transmitted to the steel core were All panels have same thickness of 25 mm.material The important factor in the sample size is limited by the flow stress of total the lead and brass jacket during high velocity impact [19]. its thickness because the selected thickness of 25 mm is the standard thickness of existing armour panel [15]. Each layer thickness was set accordingly to the 25% weight reduction of original armour panel to ensure the body maintaining on the ground during extreme conditions such as blasting and explosion. Both projectile and target panels used the Johnson–cook (JC) constitutive material model which has been commonly used for high velocity impact simulation [9,17,18]. The JC model is applied to determine the strain rate and temperature dependence of viscous-plastic material models and is expressed as [18] in Equation (1)    .∗ C σeq = A + Bεneq 1 + εeq (1 − T ∗m )

(1)

where σeq is the equivalent stress; ε eq is the equivalent plastic strain; A, B, n, C and m are the .∗ . . material constants; and εeq = εeq /ε0 is the dimensionless strain rate where it is a ratio of the strain rate and a user-defined strain rate. T ∗m is the homologous temperature and is given by T ∗m = ( T − Tr )( Tm − Tr ), where Tr and Tm represent the room temperature and the melting temperature, respectively. This modified JC material model has been successfully implemented

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to model impact on steel [16] and aluminium targets [9]. The JC parameters used in this study are shown in Table 1. JC material constitutive models were utilised to avoid the stresses transmitted to the steel core were limited by the flow stress of the lead and brass jacket material during high velocity impact [19]. Table 1. Material properties and modified Johnson–Cook model parameters, adopted from [9,16] with permission from Springer Nature, 2010 and Elservier, 2009. Material Properties

RHA

Ar500

Al7075-T6

Copper Jacket

Lead Core

(kg/m3 )

7800 7.690 0.33 780 780 0.106 0.004 1 1800

7860 7.69 0.33 1250 362 1 0.0108 1 1800

2804 2.69 0.3 480 520 0.52 0.001 1 893

8520 3.7 0.31 206 206 0.42 0.01 1 1189

10600 0.56 0.42 24 24 1 0.1 1 760

Density, ρ Young’s Modulus E (GPa) Poisson’s ratio, ν Yield Strength, A (MPa) Strain Hardening, B (MPa) Strain Hardening exponent, n Strain rate constant, c Thermal softening constant, m Melting temperature, Tm (K)

Failure is modelled using a criterion proposed by Johnson and Cook. It depends on the effect of stress triaxiality, temperature and strain rate. The model was defined as in Equation (2) assuming that the damage accumulates in the material element during plastic straining and it breaks immediately when damage reaches a critical value. ( D=

0, when  ε p ≤ εp,d Dc / ε f − ε p,d , when ε > ε p,d

(2)

where Dc is the critical damage, ε p,d is the damage threshold and ε f is the fracture strain given by the Johnson–Cook, as presented in Equation (3), whereby D1 –D5 are the material constants given in Table 2. These parameters were adopted from previous research works that possess similar material properties.  . ε f = [ D1 + D2 exp( D3 σ )] 1 + D4 ln ε [1 + D5 T ]

(3)

Table 2. Johnson–Cook fracture model constants for target materials. Damage Constant

Ar500 [18]

Al7075-T6 [9]

D1 D2 D3 D4 D5

0.05 0.8 −0.44 −0.046 −2.9

−0.068 0.451 −0.952 0.036 0.697

3. Results and Discussion 3.1. Determination of Material Properties Tensile testing was performed three times for each material and the average value of the results obtained were taken as the reference for identifying the mechanical properties of the material. It can be observed from the condition of specimens resulted in Figure 6 that these three materials possess the elastic properties due to the necking process taken place before the material fracture. These materials have elastic to plastic deformation prior to failure and fracture at the same place that is 1/3 of the length gauge as a result of consistent pull off action to cause similar spot of stress concentration [20]. The results from experimental tensile tests are provided in Figure 7 where the ultimate strength of Ar500 and Al7075-T6 are 1687 MPa and 545 MPa, respectively. A slight difference in elongation of

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these 2018, materials where Metals 8, x FOR PEER Ar500 REVIEWexhibits 12.07% and Al7075-T6 is at 11.71%. The summary of mechanical of 16 strain curve in Figure 6, steel Ar500 shows similar mechanical properties to RHA and is stronger8 than properties of the high strength steel and aluminium alloy are tabulated in Table 3. Based on the aluminium alloy Al7075-T6 due to the higher carbon content in the material. stress–strain curve in Figure steel Ar500 mechanical properties to RHA is stronger strain curve in Figure 6, steel6,Ar500 showsshows similarsimilar mechanical properties to RHA and isand stronger than than aluminium alloy Al7075-T6 due to the higher carbon content in the material. aluminium alloy Al7075-T6 due to the higher carbon content in the material.

(a) (a) (b) (b) (c) (c)

Figure 6. After tensile test fracture condition for: (a) RHA; (b) Ar500; and (c) Al7075-T6. Figure 6. After tensile test fracture condition for: (a) RHA; (b) Ar500; and (c) Al7075-T6. Figure 6. After tensile test fracture condition for: (a) RHA; (b) Ar500; and (c) Al7075-T6.

2000 2000 1600

(MPa) Stress (MPa) Stress

1600 1200 1200

Al 7075-T6

800 800

Ar500 Al 7075-T6 RHA Ar500

400

RHA

400 0 0

0

2

4

0

2

4

6 8 Elongation 6 8(%)

10

12

14

10

12

14

Elongation (%) Figure Figure7.7.Stress–strain Stress–straincurves curvesfor forRHA, RHA,Ar500 Ar500and andAl7075-T6 Al7075-T6atat1.5 1.5mm/min. mm/min. Table 3. The mechanical properties of RHA, Ar500 and Al7075-T6. Figure 7. Stress–strain curves for RHA, Ar500 and Al7075-T6 at 1.5 mm/min. Table 3. The mechanical properties of RHA, Ar500 and Al7075-T6. Modulus Yield Ultimate Tensile Stress at Elongation at Yield Ultimate Stressσ fat Elongation at Table 3. The mechanical properties of RHA, Tensile Ar500 and Al7075-T6. Material Elasticity Strength, σ Strength, σ Break, Break y uts Modulus (GPa) (MPa)σy (MPa) σuts (MPa) σf (%) Material Strength, Strength, Break, Break

Elasticity (GPa)

Modulus RHA 213 Material 150 RHAAr500 Elasticity 213(GPa) Al7075-T6 Ar500 150 70 RHA Weldox 400E [9] 213 145 Al7075-T6 70 Al7075-T651 [16] 150 70 Ar500

Yield (MPa) 1230 Strength, σy 1410 1230 (MPa) 472 1410 1230 1250 472 477 1410 472 1250

Ultimate Tensile (MPa) 1737 Strength, σuts 1687 1737 (MPa) 545 1687 1737 1680 545 485 1687 545 1680

Stress at (MPa) 1257 Break, σf 1293 1257 (MPa) 516 1293 1257 1200 516 500 1293 516 1200

Elongation at (%) 12.70 Break 12.07 12.70 (%) 11.71 12.07 12.70 12.00 11.71 11.00 12.07 11.71 12.00

Weldox Al7075-T6 70 145 400E [9] Weldox Rockwell hardness from the hardness test for Al7075-T6 and RHA Al7075145values obtained 1250 1680 1200alloy, Ar500 12.00 70 respectively are 477summarised in Table 485 4. The RHA500 11.00 400E [9] and 114 HRB, areT651 87, 105, hardness value is higher [16] Al7075than other materials because of its material Molybdenum 70 477composition, RHA 485 containing Molybdenum. 500 11.00 T651 [16] Rockwell hardness values obtained from the hardness test for Al7075-T6 alloy, Ar500 and RHA are 87, 105, and 114 HRB, respectively are summarised in Table 4. The RHA hardness value is higher Rockwell hardness values obtained from the hardness test for Al7075-T6 alloy, Ar500 and RHA are 87, 105, and 114 HRB, respectively are summarised in Table 4. The RHA hardness value is higher

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9 of 16 because ofof itsits material material composition, composition, RHA RHA containing containing Molybdenum. Molybdenum. than other materials because Molybdenumisisa amaterial materialoften oftenused usedininalloying alloyingmaterials materialsfor forhigh highstrength strengthcharacteristics. characteristics.Figure Figure8 8 Molybdenum shows the condition of specimen after five punching points for hardness test and Figure 9 represent shows the condition of specimen after five punching points for hardness test andFigure Figure89shows represent is a material often used in alloying materials for high strength characteristics. the theRockwell Rockwellhardness hardnessvalue valueagainst againsteach eachpunching punchingpoint. point.The Thestrength strengthororhardness hardnessofofthe thematerial material the condition of specimen after five punching points for hardness test and Figure 9 represent the Rockwell anypoint pointdepends dependson onthe the materialcapability capabilitytotoresist resistthe thepenetration penetrationofofthe theprojectile. projectile.Each Eachpoint point atatany hardness value against each material punching point. The strength or hardness of the material at any point represents a different value for the material microstructure changes occurred during cutting process. represents different value for the material occurred Each during cutting process. depends ona the material capability to resist microstructure the penetrationchanges of the projectile. point represents a Through this test, it can be observed that RHA has the best penetration resistance compared Ar500 Through value this test, can be observed that RHAchanges has the best penetration compared totoAr500 different for itthe material microstructure occurred during resistance cutting process. Through this andAl7075-T6 Al7075-T6and andthe thehardness hardnessfeature featureisisimportant importanttotoprevent prevent theprojectile projectile frompenetrating penetratingthe the and test, it can be observed that RHA has the best penetration resistancethe compared to from Ar500 and Al7075-T6 plate. plate. and the hardness feature is important to prevent the projectile from penetrating the plate. Characterisationtest testwas wasalso alsoperformed performedby bycomparing comparingthe thematerial materialproperties propertiesofofmaterials materials Characterisation studied with similar materials of Forrestal et al. (2010) and Flores-Johnson et al. (2011) to adopt JC studied with similar materials Forrestal al. (2010) and for Flores-Johnson Table 4.of Summary of et average hardness each material. et al. (2011) to adopt JC properties from these research works to be used in this study. The yield strength and ultimate properties from these research works to be used in this study. The yield strength and ultimate Material Rockwell Hardness Valuewith (HRB) strengthofofAr500 Ar500steel steeland andAl7075-T6 Al7075-T6obtained obtained were compared thoseofofWeldox Weldox400E 400Esteel steeland and strength were compared with those Al7075-T651, respectively. The similarities between these materials are quite high, between 94.6% 114 materials are quite high, between 94.6% Al7075-T651, respectively. TheRHA similarities between these Ar500 of Weldox 400E and 105 Al7075-T651 can be used to represent the and95.8%. 95.8%.Thus, Thus,the theJCJCproperties properties and of Weldox 400E and Al7075-T651 can be used to represent the Al7075-T6 87 materials used in this study. materials used in this study.

Figure 8.After Afterhardness hardnesstest testcondition condition with different punching points. Figure 8. with different punching points.

Al7075-T6 Al7075-T6

Rockwell Hardness (HRB) Rockwell Hardness (HRB)

140 140 120 120 100 100 80 80 60 60 40 40 20 20 00

00

11

22

Ar500 Ar500

33

RHA RHA

44

55

ReferencePoint Point Reference Figure 9. Rockwell hardness value at different punching point for each material. Figure9.9.Rockwell Rockwellhardness hardnessvalue valueatatdifferent differentpunching punchingpoint pointfor foreach eachmaterial. material. Figure

Characterisation test was also performed by comparing the material properties of materials studied with similar materials of Forrestal etaverage al. (2010) and Flores-Johnson Table4.4. Summary hardness for eachmaterial. material. et al. (2011) to adopt Table Summary ofofaverage hardness for each JC properties from these research works to be used in this study. The yield strength and ultimate Material RockwellHardness HardnessValue Value(HRB) (HRB) Material Rockwell strength of Ar500 steel and Al7075-T6 obtained were compared with those of Weldox 400E steel and RHA 114 114 are quite high, between 94.6% and Al7075-T651, respectively.RHA The similarities between these materials Ar500 105 Ar500of Weldox 400E and Al7075-T651 105 95.8%. Thus, the JC properties can be used to represent the materials Al7075-T6 Al7075-T6 8787 used in this study. Bending and Ar500, totostudy the the Bendingtest testwas wasperformed performedon oneach eachmaterial, material,Al7075-T6 Al7075-T6and andAr500, Ar500,to studythe theeffect effectof the Bending test was performed on each material, Al7075-T6 study effect ofofthe maximum force exerted on on the the material before it stopped and the deformation of theseofmaterials when maximum force exerted material before it stopped and the deformation these materials maximum force exerted on the material before it stopped and the deformation of these materials subjected to bending tests. Each material was deformed as in Figure 10 Figure producing maximummaximum bending whensubjected subjected bending tests. Eachmaterial material wasdeformed deformed producing when totobending tests. Each was asasininFigure 1010producing maximum stress, bending strain and bending modulus as tabulated in Table 5. Results show that Ar500 steel

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bending stress, bending strain and bending modulus as tabulated in Table 5. Results show that Ar500 steel shows bending performance than Al7075-T6 can befrom observed from its lower shows better better bending performance than Al7075-T6 which canwhich be observed its lower deformation deformation Al7075-T6 alloy. The test was to conducted study the mechanical compared to compared Al7075-T6to alloy. The bending testbending was conducted study thetomechanical properties properties and the ability of eachconstituting material constituting panel when subjected to the impact and the ability of each material laminatedlaminated panel when subjected to the impact from the from the front. Tensile testing is inadequate to the represent the resistivity characteristics of to a material to front. Tensile testing is inadequate to represent resistivity characteristics of a material the impact the impact effect thewas tensile test was on a uniaxial at different effect because the because tensile test conducted onconducted a uniaxial direction and atdirection differentand direction of the directionimpact of thedirection. normal impact direction. In addition, bending test results can provide of normal In addition, bending test results can provide indication of theindication appropriate the appropriate material to front be placed inthe thelayered front part of the Material layered material. with high material to be placed in the part of material. with highMaterial elastic modulus is elastic modulus is not suitable to be placed on the front panel because it will cause damage to the not suitable to be placed on the front panel because it will cause damage to the back material structure backconsequently material structure consequently it can be easily penetrated by projectiles [21]. and it can and be easily penetrated by projectiles [21].

(a)

(b) Figure 10. 10. After After three-point three-point bending bending test test condition condition for: for: (a) (a) Ar500 Ar500 steel; steel; and and (b) (b) Al7075-T6 Al7075-T6 alloy. alloy. Figure Table 5. Summary of three-point bending test results for each material. Table 5. Summary of three-point bending test results for each material. Maximum Bending Stress, σ B Bending Strain, Bending Modulus, EB Material Maximum Bending Stress, σB Bending Strain, Bending Modulus, EB (MPa) ε (MPa) B Material Ar500

Ar500 Al7075-T6 Al7075-T6

(MPa) 1818 1818 624 624

εB

0.07 0.07 0.1

0.1

(MPa) 54 54 34 34

3.2. Effect of Ballistic Impact on Different Layering Configuration 3.2. Effect of Ballistic Impact on Different Layering Configuration The final conditions of the double layered panels with Configuration A and Configuration B The final theconducted double layered panels Configuration A and Configuration B resulting from conditions the ballisticoftest are shown in with Figure 11a,b and Figure 12a,b, respectively. resulting from the ballistic test conducted are shown in Figures 11a,b and 12a,b, respectively. Ballistic Ballistic test results show that the projectile partially penetrated both panels with depth of penetrations test results thatmm the for projectile partiallyA,penetrated both10panels with of penetrations between 1.3show and 1.7 Configuration and between and 11 mmdepth for Configuration B. between 1.3 and 1.7 mm for Configuration A, and between 10 and 11 mm for Configuration B. and The The average after three shoots was calculated as 1.5 mm and 10.3 mm for Configuration A average after three shoots was calculated as panels 1.5 mm andConfiguration 10.3 mm forAConfiguration A and Configuration B, respectively. Double-layered with and Configuration B Configuration B, respectively. Double-layered panels with Configuration A and Configuration B possess similar areal density and weight. Only layering configuration differs and significantly affected possess similar areal density and weight. Only layering configuration differs and significantly the panel behaviour during the high velocity impact. At high velocity, maximum stress occurred at the affected the panel behaviour during the high velocity impact. At high velocity, maximum stress early stage of penetration process because the panel had to withstand the high pressure and kinetic occurred at the early stage of penetration process because the panel had to withstand the high energy from this projectile and then it slowly degraded as the plate distributed the stress all over the pressure and kinetic energy from this projectile and then it slowly degraded as the plate distributed panel [22]. Both configuration panels have caused the projectile to completely shatter. Deformation the stress all over the panel [22]. Both configuration panels have caused the projectile to completely occurred on the projectile nose during penetration led to an immense heat generation and the material shatter. Deformation occurred on the projectile nose during penetration led to an immense heat of the panel locally melts and loses all mechanical strength [23]. The Configuration A panel which has generation and the material of the panel locally melts and loses all mechanical strength [23]. The front panel with 20% higher hardness than the front panel of Configuration B successfully stopped Configuration A panel which has front panel with 20% higher hardness than the front panel of the projectile from penetrating the panel at 1.3 mm depth. This phenomenon is related with energy Configuration B successfully stopped the projectile from penetrating the panel at 1.3 mm depth. This absorption capability and the strength of the material. The Configuration B panel allowed projectile phenomenon is related with energy absorption capability and the strength of the material. The penetrating the front layer panel and then successfully stopped the penetration at the back layer. Configuration B panel allowed projectile penetrating the front layer panel and then successfully stopped the penetration at the back layer.

crater fracture diameter. The cause this layer big mismatch is due to lack of wave symmetry ofisthe crater diameter. The caused in theoffront is due to reflected tensile which called spallation and usually fracture caused inshape the front due to reflected tensile wavetest which is called spallation and usually is circular [24].layer The is projectile during the ballistic is not in normal direction to the target is circular The subjected projectile during the ballistic test isThus, not in normal direction to the target panelshape and is[24]. usually to a degree of obliquity. it has affected the penetration process panel by andwhich is usually subjected to a degree of obliquity. Thus, itbut has affectedthe thecrater penetration process the obliquity reduces the depth of penetration increases diameter. Metals 2018, 8, 732 reduces the depth of penetration but increases the crater diameter. 11 of 16 by which the obliquity

(b)

(a) (b)

(a)

Figure 11. Final condition of plates constituting Configuration A panel: (a) front layer of Ar500 steel; Figureand 11. Final condition plates Configuration A panel: front(a)layer Ar500 Figure Final condition of constituting plates constituting Configuration A (a) panel: frontoflayer of steel; Ar500 steel; (b) 11. back layer ofofAl7075-T6 alloy. and (b)and back layer of Al7075-T6 alloy. (b) back layer of Al7075-T6 alloy.

(b)

(a) (a)

(b)

Figure 12. Final condition of plates constituting Configuration B panel: (a) front layer of Al7075-T6 Figure 12. Final condition of plates constituting Configuration B panel: (a) front layer of Al7075-T6 alloy; and (b) back layer of Ar500 steel. Figurealloy; 12. Final of plates constituting Configuration B panel: (a) front layer of Al7075-T6 and condition (b) back layer of Ar500 steel. alloy; and (b) back layer of Ar500 steel.

The results from simulation were compared with ballistic test results in terms of depth of penetration and crater diameter tabulated in Table 6. The percentage differences of the depth of penetration and crater diameter range between 10.7% and 20%, and between 9% and 20%, respectively. Although the differences seem to be high in term of percentage for Configuration A, the differences in term of real value were quite small: 0.3 mm for depth of penetration and 2 mm for crater diameter. The cause of this big mismatch is due to lack of symmetry of the crater diameter. The fracture caused in the front layer is due to reflected tensile wave which is called spallation and usually is circular shape [24]. The projectile during the ballistic test is not in normal direction to the target panel and is usually subjected to a degree of obliquity. Thus, it has affected the penetration process by which the obliquity reduces the depth of penetration but increases the crater diameter.

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Table 6. Comparison between ballistic test and simulation results for depth of penetration and crater Table 6. diameter. Comparison between ballistic test and simulation results for depth of penetration and crater diameter.

Ballistic Test

Simulation

Percentage Difference

Ballistic Test PercentageDepth Difference Depth of Crater Simulation Depth of Crater of Crater Plate Plate Crater Depth of Crater Depth of Crater Depth of Penetration Diameter Penetration Diameter Penetration Diameter Diameter Penetration Diameter Penetration Diameter Penetration (mm)(mm) (mm) (mm) (mm)(%) (%)(%) (%) (mm) (mm) (mm)

Configuration A

1.5 10 10.3 11

Configuration A 1.5 Configuration B10.3 Configuration B

10 1.8 11 11.4

1.8 12 11.4 12

12 20 12 10.7

20 20 10.7 9

20 9

Simulation results in Figure 13a,b show that the 7.62-mm FMJ projectile was deformed and Simulation results in Figure 13a,b show that the 7.62-mm FMJ projectile was deformed and shattered completely when striking each configuration panel at a velocity of 800 m/s. However, shattered completely when striking each configuration panel at a velocity of 800 m/s. However, penetration process taking place differed by which it completely penetrated front layer of Al7075-T6 penetration process taking place differed by which it completely penetrated front layer of Al7075-T6 in Configuration B panel and failed to penetrate through the front layer of Ar500 in Configuration A in Configuration B panel and failed to penetrate through the front layer of Ar500 in Configuration A panel. Nevertheless, the projectile has also stopped at the back layer of Ar500 in Configuration B panel. Nevertheless, the projectile has also stopped at the back layer of Ar500 in Configuration B which which is caused by the high tensile strength and high hardness properties of Ar500 material. Time is caused by the high tensile strength and high hardness properties of Ar500 material. Time taken by taken by the Configuration A panel to stop the projectile was 16.7% shorter than time taken by the the Configuration A panel to stop the projectile was 16.7% shorter than time taken by the Configuration Configuration B panel (Figure 14). Both panels caused the projectile to shatter after the impact and B panel (Figure 14). Both panels caused the projectile to shatter after the impact and the projectiles the projectiles debris moved from the panel at opposite of initial direction, as shown in Figure 14, debris moved from the panel at opposite of initial direction, as shown in Figure 14, after 0.05 ms and after 0.05 ms and 0.06 ms for Configurations A and B, respectively. The duration time of penetration 0.06 ms for Configurations A and B, respectively. The duration time of penetration process is associated process is associated with the amount of energy being absorbed during the impact occurred. Figure with the amount of energy being absorbed during the impact occurred. Figure 15 shows the energy 15 shows the energy being absorbed by the front and back layer for each configuration panel at being absorbed by the front and back layer for each configuration panel at projectile initial velocity of projectile initial velocity of 800 m/s. A large amount of energy, about 1400 J, was absorbed by the 800 m/s. A large amount of energy, about 1400 J, was absorbed by the Configuration B panel due to Configuration B panel due to the perforation process that occurred on the front layer. Meanwhile, the perforation process that occurred on the front layer. Meanwhile, Configuration A panel absorbed Configuration A panel absorbed 58.6% less energy, about 580 J, to stop the projectile. 58.6% less energy, about 580 J, to stop the projectile.

(a)

(b)

Figure 13. Penetration of FMJ projectile at 0.07 ms and initial velocity of 800 m/s for: (a) Configuration Figure 13. Penetration of FMJ projectile at 0.07 ms and initial velocity of 800 m/s for: (a) Configuration A; and (b) Configuration B. A; and (b) Configuration B.

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Figure Figure 14. 14. Projectile Projectile velocity velocity as as aa function function of of time time for for each each plate plate at at initial initial velocity velocity of of 800 800 m/s. m/s. Figure 14. Projectile velocity as a function of time for each plate at initial velocity of 800 m/s.

Figure 15. Energy absorbed by front and back layers of each plate at initial velocity of 800 m/s. Figure 15. Energy absorbed by front and back layers of each plate at initial velocity of 800 m/s. Figure 15. Energy absorbed by front and back layers of each plate at initial velocity of 800 m/s.

Numerical studies of the ballistic impact with penetration show that the absorption capabilities Numerical studies of the ballistic impact with penetration show that the absorption capabilities areNumerical increased studies at higher velocities, absorption has its at higher velocities of impact the ballistic impactand withenergy penetration show that thelimit absorption capabilities are increased at higher impact velocities, and energy absorption has its limit at higher velocities regardless of which laminated panel is used as the target. These phenomena are related to an elastic are increased at higher impact velocities, and energy absorption has its limit at higher velocities regardless of which laminated panel is used as the target. These phenomena are related to an elastic response of ofwhich the panel because of essential role the phenomena deformationare of related the laminated panel. regardless laminated panel is used as theplayed target. by These to an elastic response of the panel because of essential role played by the deformation of the laminated panel. Figure 16 shows that double-layered panel with Configuration A exhibits lesser energy absorption than response of the panel because of essential role played by the deformation of the laminated panel. Figure 16 shows that double-layered panel with Configuration A exhibits lesser energy absorption Configuration panel, and RHA panel absorbed the least energy to double-layered panels Figure 16 showsBthat double-layered panel with Configuration A compared exhibits lesser energy absorption than Configuration B panel, and RHA panel absorbed the least energy compared to double-layered with Configuration A or B.and This happens to thethe effectiveness ofcompared ballistic resistance properties than Configuration B panel, RHA panel due absorbed least energy to double-layered panels with Configuration A or B. This happens due to the effectiveness of ballistic resistance of monolithic panel whichAisor higher thanhappens that of multi-layered panels [25].ofThese double-layered panels with Configuration B. This due to the effectiveness ballistic resistance properties of monolithic panel which is higher than that of multi-layered panels [25]. These doublepanels which have similar areal density with only layering configuration however properties of monolithic panel which is higher thandifferent that of multi-layered panels [25]. Theseperformed doublelayered panels which have similar areal density with only different layering configuration however differently. strength steel when the front as in Configuration A prevented any layered panelsHigh which have similar arealplaced densityatwith only layer different layering configuration however performed differently. High strength steel when placed at the front layer as in Configuration A penetration happening to the layered panel because of the ability of the front layer to absorb most performed differently. High strength steel when placed at the front layer as in Configuration A prevented any penetration happening to the layered panel because of the ability of the front layer to of the kinetic energy fromhappening the projectile. hardness alsoability playsof anthe important role prevented any penetration to theBesides, layeredmaterial panel because of the front layer toin absorb most of the kinetic energy from the projectile. Besides, material hardness also plays an increasing the ballistic resistance. Configuration A panel has shattered the projectile completely with absorb most of the kinetic energy from the projectile. Besides, material hardness also plays an important role in increasing the ballistic resistance. Configuration A panel has shattered the projectile 6% less and more energy be absorbed by the panelAcompared to Configuration B panel important role24% in increasing the needed ballisticto resistance. Configuration panel has shattered the projectile completely with 6% less and 24% more energy needed to be absorbed by the panel compared to and existing RHA respectively. completely with 6%panel, less and 24% more energy needed to be absorbed by the panel compared to Configuration B panel and existing RHA panel, respectively. Configuration B panel and existing RHA panel, respectively.

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(J)(J) Energy Absorbed Energy Absorbed

RHA RHA 2000 2000 1500 1500 1000 1000 500 500 0 0 700 700

14 of 16 14 of 16 14 of 16

Configuration A Configuration A

Configuration B Configuration B

800 800 Projectile Initial Velocity (m/s) Projectile Initial Velocity (m/s)

900 900

Figure 16. 16. Total Total energy energy absorbed absorbed by by the the laminated laminated panel panel at at different different initial initial projectile projectile velocity. Figure velocity. Figure 16. Total energy absorbed by the laminated panel at different initial projectile velocity.

Figures 17 17 and and 18 18illustrate illustratethe thestress stressdistribution distributionpattern patternfor for each panel time from to Figures each panel at at time from 10 10 µ s µs to 70 Figures 17 and 18 illustrate the stress distribution pattern for each panel at time from 10 µ s to 70 70 µs for an initial velocity of 800 m/s. The concentration of stress for each time observed was around µ s for an initial velocity of 800 m/s. The concentration of stress for each time observed was around µthe s for velocity 800 m/s.retained The concentration of stress for each time observed wastip around the tip of projectile. Theofprojectile projectile retained its shape shape when when penetrating the plate plate and the the tip of the tipan ofinitial projectile. The its penetrating the and of the the tip of projectile. The projectile retained its shape when penetrating the plate and the tip of the projectile was progressively deformed at the same time as the material in panel was displaced and projectile was progressively deformed at the same time as the material in the panel was displaced projectile was progressively deformed at the same time the material the panel was displaced a hole formed. Large amount of energy inasfront panel of in Configuration A associated and a was hole was formed. Large amount of absorbed energy absorbed in front panel of Configuration A and a hole was formed. Large amount of energy absorbed in front panel of Configuration A with large stress was distributed on the front panel. This front panel is can withstand the projectile associated with large stress was distributed on the front panel. This front panel is can withstand the associated with large stress was distributed on the front panel. This panel is can withstand the penetration. A different occurred on the front of front Configuration B. The frontB.panel projectile penetration. Aphenomenon different phenomenon occurred onpanel the front panel of Configuration The projectile penetration. A different phenomenon occurred on the front panel of Configuration B. The made of aluminium failed to absorb and distribute the stress throughout its volume, as shown in front panel made of aluminium failed to absorb and distribute the stress throughout its volume, as front panel made of aluminium failed to absorb and distribute the stress throughout its volume, as Figure 18. The back played the played role of withstanding the projectilethe penetration, and stoppedand the shown in Figure 18.layer The back layer the role of withstanding projectile penetration, shown in Figure 18. The back layer played the role of withstanding the projectile penetration, and projectile at its surface. stopped the projectile at its surface. stopped the projectile at its surface.

t = 10 μs t = 10 μs

t = 20 μs t = 20 μs

t = 30 μs t = 30 μs

t = 40 μs t = 40 μs

t = 50 μs t = 50 μs

t = 60 μs t = 60 μs

t = 70 μs t = 70 μs

Figure 17. Stress distribution distribution patterns patterns of of Configuration Configuration A A panel panel at at time time between between 10 10 μs µs and and 70 μs µs at Figure 17. Stress distribution patterns initial projectile m/s. of Configuration A panel at time between 10 μs and 70 μs at velocity of 800 m/s. initial projectile velocity of 800 m/s.

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t = 10 μs

t = 20 μs

t = 30 μs

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t = 40 μs

t = 50 μs

t = 60 μs

t = 70 μs

Figure18. 18.Stress Stress distribution distribution patterns patterns of Figure of Configuration Configuration BB panel panel at at time timebetween between10 10μs µsand and70 70μs µsatat initialprojectile projectilevelocity velocityof of800 800m/s. m/s. initial

4.4.Conclusions Conclusions Numerical results show thatthat lightweight material Al7075-T6 alloy when Numericaland andexperimental experimental results show lightweight material Al7075-T6 alloy placed when atplaced back at layer double-layered panel panel has a has better ballistic performance than when it is placed backoflayer of double-layered a better ballistic performance than when it is placedat at the front layer a similar areal densitypanel. panel.The Thedifference differencein in terms terms of the depth the front layer forfor a similar areal density depth of ofpenetration penetration is quite significant (600%), while in terms of energy absorption, the difference is marginal is quite significant (600%), while energy absorption, the difference is marginal(6%). (6%). However,the theperformance performance of of the the double-layered double-layered panel However, panel is is 20% 20% less lessthan thanthat thatof ofexisting existingRHA RHApanel. panel. Considering the the 25% thethe armour shield made of these two materials with Considering 25% weight weightreduction reductionachieved, achieved, armour shield made of these two materials Configuration A, whereby Ar500 steel and Al7075-T6 alloy are placed at front and back layer, with Configuration A, whereby Ar500 steel and Al7075-T6 alloy are placed at front and back layer, respectively,could couldpotentially potentiallyperform perform better better than the equivalent respectively, equivalent areal arealdensity densitymonolithic monolithicsteel steelplate. plate. However,the the results results in in this this study study are are limited limited and and further However, further research research has has to to be be carried carriedout outtotofully fully understand this type of target configuration. It is concluded that layering configuration does matter understand this type of target configuration. It is concluded that layering configuration does matter improving the laminated panel consisting different typestypes of materials. This ininimproving the ballistic ballisticperformance performanceofof laminated panel consisting different of materials. study could be further performed to investigate the behaviour of stress distribution on the plates This study could be further performed to investigate the behaviour of stress distribution on the plates duringthe theprojectile projectileimpact impactand andpenetration. penetration. during Author Contributions: N.A.R. carried out the experimental and computational works, analysis and writing Author Contributions: N.A.R. carried out the experimental and computational works, analysis and writing under under the supervision of S.A., M.Z.O., Z.S. and W.F.H.Z., while M.F.A. provides guidance on the experimental the supervision of S.A., M.Z.O., Z.S. and W.F.H.Z., while M.F.A. provides guidance on the experimental works works the write-up. and the and write-up. Funding: This by by Ministry of Higher Education Malaysia [LRGS/2013/UPNMFunding: This research researchwas wasfunded funded Ministry of Higher Education Malaysia [LRGS/2013/ UKM/DS/04]. UPNM-UKM/DS/04].

to the the Ministry Ministryof ofHigher HigherEducation EducationMalaysia, Malaysia, Acknowledgments: Acknowledgments:The Theauthors authors wish wish to to express express their gratitude gratitude to Universiti Malaysia and andUniversiti UniversitiPertahanan Pertahanan Nasional Malaysia for financial the financial assistance Universiti Kebangsaan Kebangsaan Malaysia Nasional Malaysia for the assistance and and support. support. Conflicts of Interest: The authors declare no conflict of interest. Conflicts of Interest: The authors declare no conflict of interest.

References References 1. 1. 2. 2. 3.3. 4.

Jena, P.K.; Ramanjeneyulu, K.; Siva Kumar, K.; Balakrishna, B.T. Ballistic studies on layered structures. Jena, P.K.; Ramanjeneyulu, K.; Siva Kumar, K.; Balakrishna, B.T. Ballistic studies on layered structures. Mater. Des. 2009, 30, 1922–1929. [CrossRef] Mater. Des. 2009, 30, 1922–1929. Teng, X.; Dey, S.; Børvik, T.; Wierzbicki, T. Protection performance of double-layered shields against projectile Teng, X.; Dey, S.; Børvik, T.; Wierzbicki, T. Protection performance of double-layered shields against impact. J. Mech. Mater. Struct. 2007, 2, 1309–1329. [CrossRef] projectile impact. J. Mech. Mater. Struct. 2007, 2, 1309–1329. Volgas, J.P.; Alonso, Alonso,J.E. J.E.Ballistics: Ballistics: A primer surgeon. 36, 373–379. Volgas,D.A.; D.A.; Stannard, Stannard, J.P.; A primer for for the the surgeon. InjuryInjury 2005, 2005, 36, 373–379. [CrossRef] Jena, P.K.; [PubMed] Mishra, B.; Siva, K.K.; Bhat, T.B. An experimental study on the ballistic impact behavior of some metallic armour materials against 7.62 mm deformable projectile. Mater. Des. 2010, 31, 3308–3316.

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4. 5. 6.

7.

8. 9. 10. 11.

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16. 17. 18.

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