Design, manufacture, and analysis of ceramic-composite armor

Design, manufacture, and analysis of ceramic-composite armor

Design, manufacture, and analysis of ceramic-composite armor 12 L. Bracamonte 1 , R. Loutfy 1 , I.K. Yilmazcoban 2 , S.D. Rajan 3 1 MER Corporation,...

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Design, manufacture, and analysis of ceramic-composite armor

12

L. Bracamonte 1 , R. Loutfy 1 , I.K. Yilmazcoban 2 , S.D. Rajan 3 1 MER Corporation, Tucson, AZ, United States; 2Sakarya University, Turkey; 3 Arizona State University, Tempe, AZ, United States

12.1

Introduction

During active conflicts and peace-keeping times, protection of the soldier is a major challenge for defense departments all over the world. A typical US soldier in the battlefield is protected by an armored tactical vest that weighs between 30 and 35 lb (Beidel, 2011). It provides full, 360-degree protection to the torso and also includes detachable protection for the upper arms and groin. Its other functionalities include quick don and doff capabilities and comfort over a wide temperature and climatic range. To make the soldier agile, lethal, and able to survive, advanced body armor materials are needed. Currently, body armor is fielded only in specific high-risk scenarios and is typically limited to chest and head protection. The common theme in meeting ballistic and blast threats is the ability of the material and the system to neutralize the threat by dissipating the energy of the projectile and the blast waves in the most appropriate manner (Rajan, 2011). A number of conflicting criteria must be considered when selecting materials for use in an armor system. Armor systems that meet the performance requirements are typically evaluated in terms of their weight, thickness, and cost. Since the type of threat and the performance requirements vary widely for personal and vehicular armor systems, there is no single best armor material for all applications. The designer must select the most appropriate material or combination of materials to defeat the perceived threat. Select material properties of some of these materials are summarized in Table 12.1 (Rajan, 2011). In general, as shown in Table 12.1, armor materials can be divided into four basic categories: metals (aluminum, steel), polymers (polyethylene, aramid), ceramics (aluminum oxide, silicon carbide, boron carbide, titanium diboride), and their composites. Armor systems made of ceramic and composite materials are widely used in ballistic applications to defeat armor-piercing projectiles. Alumina (Al2O3), boron carbide (B4C), boron silicon carbide (BSiC), and silicon carbide (SiC) are some of the ceramics that are commonly used (Holmquist et al., 2005). The range of composite materials used as backing and spall-minimizing materials include

Lightweight Ballistic Composites. http://dx.doi.org/10.1016/B978-0-08-100406-7.00012-X Copyright © 2016 Elsevier Ltd. All rights reserved.

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Table 12.1

Select material properties of various armor materials Designation

Young’s modulus (GPa)

Ultimate tensile strength (MPa)

Hardness

Density (g/cm3)

Aluminum

5083-H112a

70

300

50 (Rockwell)

2.66

Steel

Alleghany Ludlum K12 dual-hardness steela

200

1020

48e54 (Rockwell)

7.86

Polyethylene

Spectra 2000b

113e124

3250e3340

NA

0.97

179

3450

NA

1.47

370

379

83 (Rockwell)

3.9

410

380

2800 (Knoop)

3.13

380

280

1550 (Knoop)

2.56

540

265

2100 (Vickers)

4.50

Aramid Aluminum oxide Silicon carbide Boron carbide Titanium diboride a

http://www.matweb.com. http://www51.honeywell.com/. http://www.hexoloy.com/. d http://www.mmmt.com/. b c

a

Kevlar 149

a

Coorstek CAP-3 Hexoloy

c d

RBBC-751

a

Ceralloy 225

Lightweight Ballistic Composites

Material

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ultrahigh-molecular-weight polyethylene (UHMWPE) materials, aramid woven fabrics such as Kevlar® and Twaron®, fiberglass materials such as S2-glass and E-glass, and so on. In this chapter we will take a look at ceramics as an armor material and discuss the manufacture, usage, and computer modeling of the material.

12.2

Ceramics as an armor material

Ceramic armor is continually evolving to meet ever-increasing threat levels, particularly for body armor for which light weight is a necessity. The initial vehicle and aircraft armor solutions were based on metals (5083 Al, RHA (rolled homogeneous armor-grade steel) or HH (high-hard steel)). However, as the threats in the battlefield changed and, particularly, harder direct fire and fragment threats appeared, metals and polymer matrix composites (PMCs) were inadequate because of their low hardness (and or high weight). Hence it became necessary to use other materials such as ceramics (Chhdea et al., 2006; Elliott, 2007; Aghajianian et al., 2002, 2005; Flinders et al., 2003; Roberson and Hazell, 2003a,b; Woolmore et al., 2003; Lillo et al., 2003; Hazell et al., 2006; James, 2002; Normandia and Gooch, 2002). For ceramics, damage usually occurs with the scission of ionic or covalent bonds and the resulting formation of atomic-sized microcracks. These cracks grow in size until they lead to catastrophic failure. Clearly, ceramics that have superior mechanical properties including strength, fracture toughness, and hardness have improved ballistic performance. For lightweight armor applications, the highest performing ceramics are produced by pressure-assisted processes such as hot pressing or hot isostatic pressing as described in the next subsection. Such ceramic materials have been extensively researched and developed to the extent that properties have been nearly optimized for the monolithic materials which have the best ballistic performance. Ceramic composites have potential for improvements assuming the secondary reinforcing components have properties superior to those of the ceramic that promote good ballistic performance. This combination of superior mechanical properties and hardness is not easily met by nonceramic materials, and as such composites have not typically shown improved ballistic performance compared to the monolithic ceramic materials. However, carbon nanotubes (CNTs) have unique properties unrealized by any other material which could be beneficial for armor applications. For example, the tensile strength of CNTs is w100 times greater than that of steel of the same diameter, the Young modulus is about five times greater than that of steel, and because they are elastic, the bonds in their atomic lattice do not break when bent or compressed. In addition to improving the properties of ceramic armor components, researchers have been looking for extraneous means to improve the performance of the ceramic, through synergistic design of the entire armor system. One approach that has significant potential is confinement of the ceramic, which can serve two functions. If the

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confinement is all around the ceramic, it can serve to keep the broken ceramic pieces in place to provide for protection from subsequent hits, and if a significant compressive stress is imparted to the ceramic, the confinement can also serve to improve ballistic performance of the ceramic. In fact, the greater the degree of confinement of a ceramic material, the more it behaves like a ductile one (Ernst et al., 1994), thus resulting in improved ballistic performance. Ceramics are strongest in compression, and it has been shown that an increase in ceramic fracture strength from 0.3 to 0.9 GPa results in a 9% decrease in required areal density (Anderson, 2002). Thus, ceramics in the compressive state offer the greatest potential for lengthening the duration of the shattering process of the projectile, thus resulting in the greatest amount of fracture of the projectile and a lesser amount of damage to the rest of the armor package. Unfortunately, while the substantial benefits of ceramic confinement are well known, implementation into practical applications has not been realized owing to the perception that substantial weight and bulk will be added owing to the use of the confinement systems. However, there are examples in the literature (Sarva et al., 2007) which describe the effects of a lesser extent of confinement with fiberreinforced polymers on the front face only, where thin layers of E-glass/epoxy improved the ballistic efficiency by almost 25% while increasing the overall areal density by only 2.5%. Flash radiology also showed that the front-face confinement of ceramic tiles resulted in a much greater mushrooming and erosion of the projectile than was observed without the confinement (Nemat-Nasser et al., 2002). Another consideration that affects the performance of ceramic armor is the type of bonding between the ceramic and the backing material. The range of materials used as backing and spall-minimizing material includes metals, UHMWPE materials, aramid woven fabrics such as Kevlar® and Twaron®, fiberglass materials such as S2-glass and E-glass, and so on. In general, any interface between two dissimilar materials will give rise to a set of both shear and longitudinal reflected and transmitted waves, even if there is a very close match in acoustic impedance. The use of low-impedance glues and adhesives compounds this problem, resulting in strong tensile reflections into the ceramic, which fracture the ceramic and limit ballistic performance (James, 2002). One approach to improving the ballistic performance of ceramic armor with metal backings is to create a strong metallurgical bond, such as was demonstrated for laminated ceramicetitanium composites (Leighton et al., 1997) and functionally graded composites in which metal layers transition to the ceramic layers without abrupt interfaces (Gooch et al., 2001). For ceramic armor to have the greatest utility, it must be capable of multihit protection. Confinement of the ceramic could accomplish this, or a multipiece configuration might be the best option in which the stresses involved in the fracture of one ceramic tile are not propagated in the form of a crack through the surrounding tiles. Accordingly, the overall configuration and means of contact or not of ceramic components have a significant influence on the system’s ballistic performance. The next subsection describes some alternative lighter-weight and volumeconfinement systems that are being investigated for multipiece ceramic armor systems and the manner of fabricating these systems.

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12.3

353

Manufacture of ceramics

Ceramic materials for armor applications must have near 100% density, ie, no porosity, for best performance. A wide variety of fully densified ceramic monolithic materials including oxides, borides, and carbides have been developed to the stage at which microstructures are well controlled and properties optimized for the particular application. There is a plethora of literature describing the processing of ceramics (Reed, 1995; Richerson, 2006). Many monolithic ceramic materials are consolidated by first fabricating “green” bodies, which have varying amounts of porosity, with all requiring a second sintering step to fully densify. Processes used to make green bodies include dry pressing, cold isostatic pressing, slip casting, tape casting, gel casting, injection molding, and extrusion. Consolidation of these green bodies is then achieved by a heat-treatment process such as pressureless sintering, liquid-phase sintering, or reaction sintering. Full densification of covalently bonded ceramics such as 100% boron carbide typically requires a combination of high temperature and pressure using processes such as uniaxial hot pressing and hot isostatic pressing. Lightweight ceramic body armor for personnel is typically in the form of multicurvature small-arms-protective inserts or enhanced small-arms-protective inserts, which are usually one-piece ceramics composed of lighter ceramics such as sintered SiC or hot-pressed B4C inserted into ballistic vests such as shown in Fig. 12.1. While these one-piece ceramic inserts must have multihit capabilities, their thickness and thus weight are greater than might be possible for a multipiece configuration in which each element must withstand only one hit. The concept of multipiece ceramic armor has been explored for some time, and there are many theories as to which type of configuration provides the best compromise of cost and ballistic performance. For example, different sized and shaped tiles could be used, and they could be directly abutted or some distance between them

SAPI plates

Figure 12.1 Photograph of modular tactical vest disassembled into the individual components. SAPI, small-arms-protective insert. “Modular Tactical Vest components” by Bahamut0013down work. Licensed under CC BY-SA 3.0 via Wikimedia Commonsdhttps://commons.wikimedia.org/wiki/File:Modular_ Tactical_Vest_components.jpg#/ media/file: Modular_Tactical_Vest_components.jpg.

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(a)

Lightweight Ballistic Composites

(b)

Figure 12.2 Photograph after 40-mm-shaped charge impact with ceramic tiles with (a) direct contact between the tiles and (b) intermediate layer between the tiles (Lanz, 2002).

maintained with or without some type of damping material at the joint. Lanz (2002) investigated two types of alumina (Al2O3) tile configurations against both model-shaped charges and heavy metal rods. Fig. 12.2(a) shows the tiles with no damping elements at the boundaries, while Fig. 12.2(b) shows the tiles with a 1-mm-thick rubber layer in between the ceramic tiles. Fig. 12.2(b) shows a smaller damage radius after impact with a 40-mm projectile, indicating that damping components between ceramic tiles might reduce the amount of damage in a multipiece ceramic armor system. This type of research suggests that for multipiece ceramic armor, the joints between the ceramic pieces have a critical effect on ballistic performance. The options are (1) no joint, (2) filler but no metallurgical bond, and (3) no distinct interface owing to full reaction of joint material with ceramic. Any interface between two different materials will result in both shear and longitudinal reflected and transmitted waves, which can lead to large tensile reflections into the ceramic, which can cause the ceramic to fracture. It is desirable to have acoustic impedance in the joint which matches that of the ceramic and other typical armor components such as metals; however, there are no adhesives other than ceramic adhesives (which do not create a strong bond) with high enough impedance. Accordingly, there remains a strong demand for higher strength adhesives with matching acoustic impedance, or another approach needs to be developed to eliminate this problem. Possible bonding solutions include brazing and soldering materials, which typically have higher modulus and densities that decrease the elastic impedance mismatch between ceramics and metals in comparison to adhesives. Perhaps the best solution would be to form a metallurgical bond between the two components to be joined. This approach to join ceramic components has been investigated with many studies performed for joining B4C to itself with metal interlayers such as

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Figure 12.3 (a) 2  2-inch B4C tiles joined with 20-mm Al foil, (b) scanning electron micrograph of joint, and energy-dispersive X-ray spectroscopy elemental scans of (c) aluminum and (d) carbon. Courtesy of MER Corporation.

aluminum (Sekine et al., 2012a,b, 2014), copper (Aizenshtein et al., 2013), and nickel (Vosughi and Hadian, 2008). An example of the type of interface that can be achieved with boron carbide tiles bonded with 20 mm aluminum foil is shown in Fig. 12.3, which shows high reactivity and no residual aluminum remaining after sufficiently long time at temperature for full diffusion and reaction to occur. Instead, AleBeC ceramics are formed, which results in an all-ceramic body with high-strength bonding which, although multipiece, could simulate having a single piece of ceramic armor and eliminate ballistically inferior weak spots at the joints or at least improve joint ballistic performance. Producing multipiece all-ceramic armor also represents an opportunity to achieve a much wider range of configurations with more extensive complex-shaped, curved components for harder-to-protect body parts. A further advantage is that joining of smaller ceramic pieces reduces the size of ceramic pieces that must be fabricated, thus decreasing processing equipment footprint requirements and capital equipment costs for furnaces and hot presses. The ongoing and current drive for ceramic armor development is to make it thinner and/or lighter, no matter the application. One approach to accomplish this and which has been used for decades for metal armor is to incorporate perforated or slit armor. This concept for cast metal plates in which the slotted holes are cast at obliquity was patented (Gooch et al., 1991). This is one of many patents in which significant weight decreases were achieved, while the perforated armor design acts to effectively break an incoming projectile or at least divert it from its incident trajectory and thus substantially reduce its residual penetration capability through the basic armor (Ravid and Hirschberg, 2006). There are other patents for perforated ceramic armor (Bocini et al., 2013), which describes monolithic ceramic tile with cutout and/or through-thickness channels spaced from one another at a distance greater than a dimension of the channels, with a few examples shown in Fig. 12.4.

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Figure 12.4 Schematics of surface of suggested ceramic perforated armor configurations in Bocini et al. (2013).

The difficulty in any of these perforation approaches for the highest performance ceramics such as boron carbide (which needs to be hot pressed to achieve full densification) is that postmachining would most likely be required, which is too expensive and time-consuming. One approach currently being explored is to combine the metal interlayer-joining approach of Al, Cu, or Ni with the use of multipiece configurations to eliminate the need for any machining of the slots. Fig. 12.5(a) shows some actual B4C pieces that were hot pressed to net shape to produce the type of angled slots shown in Fig. 12.5(b). There is evidence in the literature that alteration of surfaces of ceramic armor in the form of straight or angled grooves similar to perforated armor could have a significant positive influence on ballistic performance through degradation of the projectile. Shukla and coworkers (Shukla et al., 2003) explored eight different geometries, and they found that three were effective against APM2 armor-piercing projectiles by inducing considerable damage to the core of the projectile. Interesting, only slight changes in the

Figure 12.5 Concept for producing multipiece perforated ceramic armor without postmachining, with (a) showing hot-pressed B4C pieces and (b) showing a schematic of potential armor configuration. Courtesy of MER Corporation.

Design, manufacture, and analysis of ceramic-composite armor

(a)

357

2

2

2

(b)

1

2 1.5

(c)

1 1

2

Figure 12.6 Schematics of different surface configurations investigated for the ability to damage APM2 projectiles. (a) and (c) were no better than a flat B4C surface, while (b) was more effective in defeating the threat (Shukla et al., 2003).

configurations of different depths and widths of the grooves produced different ballistic results. For example, features Fig. 12.6(b) resulted in superior ballistic performance, while the features in Fig. 12.6(a) and (c) were not found to be effective in damaging the projectile and fragmenting the steel core. Another type of material that shows promise for lightweight armor applications is a cermet, which is a composite mixture consisting of ceramic and metal phases. Depending on whether the ceramic is a continuous or noncontinuous phase, the cermet is termed a metal matrix composite or ceramic matrix composite. The metal is typically aluminum, titanium, magnesium, or silicon, and the ceramic components include silicon carbide, boron carbide, alumina, or titanium diboride. Some well-known examples of cermets include aluminumesilicon carbide from Lanxide Armor Products and siliconesilicon carbide and siliconesilicon carbideeboron carbide from M Cubed Technologies. Titanium (Ti)etitanium diboride (TiB2) is higher in density but offers a unique microstructure due to the dissolution of TiB2 into the Ti matrix, which then reprecipitates in the matrix in the form of TiB needles as shown in Fig. 12.7. Accordingly, the compositions and relative amounts of metal and ceramic in cermets can be tailored for specific applications and are a trade-off between cost and performance. One application for cermets that might offer improved ballistic performance is to utilize the cermet as an encapsulating layer around ceramic plates. The difficulty in encapsulating ceramics with all-metal layers arises from the extreme differences in the coefficient of thermal expansion (CTE). If the metal is melted in contact with the ceramic, as it cools and shrinks at a much greater rate than the ceramic, the stresses imposed on the ceramic cause it to catastrophically fracture. For example, the CTE of B4C is w5  106 0C1, while the CTE of Al is w24  106 0C1. However, if an AleB4C cermet has 50% B4C particulates, the CTE mismatch is much less, ie, the CTE of 50% B4Ce50% Al is w14  106 0C1. Care must still be taken in using controlled and slow heating and cooling rates to achieve crack-free samples.

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Figure 12.7 Scanning electron micrograph of TieTiBeTiB2 composite. Courtesy of MER Corporation.

Figure 12.8 Scanning electron micrograph of AleB4C-encapsulated B4C composite. Courtesy of MER Corporation.

The microstructure of such a sample showing good bonding between the B4C and the Al without cracking is shown in Fig. 12.8. Clearly there are a multitude of potential armor materials and designs. The modeling described in the next subsection provides a means to quickly identify preferred systems for defeating a particular threat.

12.4

Finite element analysis of a ceramics-based ballistic package

Computer-based numerical tools such as the finite element method provide the means of modeling a complex system and understanding the role of the various constituents in

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the system. For example, the impact of a projectile on the surface of the ceramic material generates compressive shock waves that propagate through the ceramic plate (Kauffmann et al., 2003). These stress waves are reflected back as tensile waves once they reach the free surface. The ceramic material fractures if the magnitude of the reflected tensile wave exceeds the dynamic tensile strength of the material. Radial cracks are formed at the bottom of the ceramic material owing to the initial impact and travel from the bottom to the top of the ceramic plate. Meanwhile a fracture cone (conoid) is formed at the impact zone on the top of the ceramic tile and grows toward the back face of the ceramic. In the case of a composite armor system in which the ceramic tile is backed by a ductile material, part of the compressive waves is transmitted into the ductile backing. The rest of the waves are reflected back into the ceramic plate. The amount of stress waves that are transmitted depends on the mechanical impedance of the ductile backing. The thickness of the adhesive layer, used to bond the ceramic tile to the ductile backing, also determines the percentage of reflected and transmitted stress waves. It should be noted that while finite element models may yield results, researchers modeling a complex system do so systematically with the results verified and validated at every stage of the analysis. Lee and Yoo (2001) discuss the numerical modeling and experimental study of ceramic metal armor systems with a metal backing. Using smoothed-particle hydrodynamics, the ceramic (alumina) is modeled using the MohreCoulomb strength model and linear equation of state (EOS) in AUTODYN. Lundberg (2004) used the JohnsoneHolmquist (JH1 model for SiC and JH2 model for alumina and boron carbide) model in AUTODYN. The determination of transition velocity (transition from interface defeat to penetration) for various combinations of projectile, target material, and target configuration was studied. Simha et al. (2002) developed and used a constitutive model for ceramic (alumina) and implemented it into EPIC Lagrangian finite element code. The model consists of a strength model based on the Hugoniot elastic limit for compression, viscoelastic flow rule, damage model for compression and tension, and MieeGruneisen EOS. The most popular models are the JH1 and JH2 (Johnson and Holmquist, 1994, 1999; Holmquist and Johnson, 2005) ceramic constitutive models. They describe the response of brittle materials to large deformations. The constitutive model comprises three parts: strength, pressure, and damage. The model includes a representation of the intact and fractured strength, a pressureevolume relationship that can include bulking, and a damage model that transitions from an intact state to a fractured state. In a 2003 study (Johnson et al., 2003), a new computational ceramic model was developed that allows for the effects of a phase change. Model constants were obtained for aluminum nitride, which include a distinct phase change. Some of the constants can be determined directly from published experimental data, but others must be inferred by matching computed results to experimental penetration data. As stated earlier, some armor packages include ceramics and a backing material. There are different ways of bonding the two materials, including the use of spray-on adhesive, adhesive tape, autoclaving/vacuum bagging, etc. Zaera et al., (2000) studied the effect of the adhesive layer thickness on the performance of the ceramic/metal armor. They showed that the adhesivesda soft adhesive (polyurethane) and a hard

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adhesive (rubber-modified epoxy)dshow strain-rate-dependent behavior. In the follow-up publication (Lopez-Puente et al., 2005), the adhesive was modeled in AUTODYN using the SteinbergeGuinan model and the MieeGruneisen EOS. By analyzing the depth of penetration and the projectile residual velocity, the authors concluded that the thickness of the epoxy resin adhesive significantly affects the performance of the system. Gooch (2011) provides an interesting history of the evolution of ceramic armor since 1970. In this section we discuss the details of the two-step procedure to develop and use a finite element model. The first step is to choose an appropriate material model for the three different components used: the bullet, the ceramic plate, and the backing material, UHMWPE. The second step is to calibrate the material and the finite element model by matching the finite element response to experimental results.

12.4.1

Material models

Two of the most popular constitutive models for ceramics are JH1 and JH2. The JohnsoneHolmquist (Johnson and Holmquist, 1994, 1999) ceramic constitutive model was proposed to describe the response of brittle materials to large deformations. The constitutive model comprises three parts: strength, pressure, and damage. The model includes a representation of the intact and fractured strength, a pressureevolume relationship that can include bulking, and a damage model that transitions from an intact state to a fractured state. The strength and damage relationships are given by the following set of equations (LSTC, 2009). The normalized intact strength is given by si

  $  ¼ AðP þ T Þ 1 þ C ln ε 

 N

[12.1]

The fractured material strength is defined as sf

  $  ¼ BðP Þ 1 þ C ln ε  M

[12.2]

The hydrostatic pressure, before the onset of damage, is P ¼ K1 m þ K2 m2 þ K3 m3

[12.3]

The plastic strain to fracture under a constant pressure P is εpf ¼ D1 ðP þ T  ÞD2

[12.4] . $ $ $ where P ¼ P=PHEL ; T  ¼ T=PHEL ; ¼ ε ε0, where ε is the actual strain rate and $ ε0 ¼ 1:0 is the reference strain rate; and m ¼ r=r0  1 for current density r and initial density r0. $ ε

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One of the approaches for constructing the material model for UHMWPE material was discussed in chapter “Numerical analysis of ballistic composite materials.” The material model for a typical bullet is relatively straightforward since bullets are made of well-established metals (as we will show next).

12.4.2 Model calibration Most material model parameters can be found through experimentation. However, some material parameters are difficult to obtain for a variety of reasons. In addition, some parameters used in describing a finite element model also need to be obtained in numerical experiments. In this section, we will show how this task can be tackled using an example. The depth-of-penetration (DOP) test methodology (Moynihan et al., 2000) was used to evaluate the performance of the various ceramic materials and has been successfully used to characterize and rank armor ceramics for vehicle protection (Woolsey et al., 1989). The first step is to calibrate the bullet model. Fig. 12.9 shows a coarse and a fine mesh for the bullet that is made of three materials: steel core, lead filler, and a copper jacket. Details of the DOP test to calibrate the bullet are shown in Fig. 12.10. A large aluminum block is shot by the bullet as shown in Fig. 12.10(a). The finite element model containing the bullet and the aluminum is constructed as shown in Fig. 12.10(b) and (c). Erosion strains for the components of the bullet and aluminum

(a)

(b)

Figure 12.9 The bullet finite element model. (a) Coarse finite element mesh. (b) Fine finite element mesh.

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Lightweight Ballistic Composites

(a)

(c)

(b)

(d)

Figure 12.10 (a) Aluminum block shot by a bullet. (b) Plan view of the bullet and the aluminum block. (c) Close-up view of the bullet and the impact point on the aluminum block. (d) Cutout view of the aluminum block showing the final state of the bullet.

are calibrated simultaneously using several metrics such as the depth of penetration, exit velocity (for uncontained bullets), damage to the bullet and aluminum, and final state/orientation of the bullet (Krishnan et al., 2010). The final state of the aluminum block and the bullet in the finite element model are shown in Fig. 12.10(d). The second step is to calibrate the JH2 material constants. Once again the DOP test setup is used, except the ceramic plate is mounted on top of the aluminum block and covered with a black spall layer as shown in Fig. 12.11(a). The ceramic is attached to the aluminum block using a high-strength adhesive. The finite element model of the DOP test is shown in Fig. 12.12 at various stages of the finite element simulation. A fine mesh is required to capture failure involving spall and crack propagation. Using the erosion strains for the aluminum block and the bullet materials from the first DOP test, and the very fine mesh, the JH2 strength and damage parameters A, B, C, M, N, D1, and D2 are calibrated such that the DOP and the extent of

(a)

(b)

(c)

Figure 12.11 (a) DOP test with ceramic plate, aluminum block, and spall layer. (b) Shot system. (c) Close-up of the point of impact.

(a)

(b)

(c)

Time = 0 ms

(d)

Time = 0.06 ms

(e)

Time = 0.045 ms

Time = 0.08 ms

Figure 12.12 (a) Finite element model of the DOP test. (b) Close-up view of the point of contact. (c) State of the ceramic, the bullet, and the aluminum block at 0.045 ms, (d) 0.06 ms, and (e) the end of the simulation (0.08 ms).

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damage incurred by the ceramicealuminum assembly match closely the experimental results.

12.4.3

An example

We finally show how the calibrated models can be used to help design an armor package containing a ceramic strike face, a UHMWPE backing, an adhesive bonding the ceramic to the backing layer, and a (black) spall layer around the package. One shot is fired at the center of the ceramic tile using 7.62  54Re155-grain, B32 API at 2880  30 ft/s. Fig. 12.13 shows the tested package and the finite element simulation of the package. While the multiple cracks in the ceramic are not captured in the finite element model, the model is able to accurately show that the bullet is captured and the resulting back-face deformation.

(a)

(b)

(c)

Figure 12.13 (a) Top side of the tested package. (b) Underside of the tested package showing contained bullet. (c) Finite element simulation of the tested package showing contained bullet.

Design, manufacture, and analysis of ceramic-composite armor

12.5

365

Concluding remarks

Ceramic armor has greatly evolved over many decades and at this point the characteristics and properties of the ceramic itself have been optimized. Further improvements in ballistic performance will require a combination of identifying preferred armor configurations while continuing to strive to achieve the lowest possible areal densities. Variations in armor could include the use of perforated or multipiece ceramic components, encapsulation or not in metals or cermets, efficient means of joining ceramic pieces at low temperatures resulting in high-performance ceramic bonds, functional gradation of microstructures, etc. The significant number of possibilities for armor design suggests that modeling of armor systems will continue to grow in importance as a necessary means to accelerate the ongoing development of lighter and improved armor materials. Current modeling approaches utilize more sophisticated codes resulting in a much better match to experimental results. The role of modeling is expected to continue to grow in the search for the lightest and most efficient armor configurations for a variety of threats.

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