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Vol. 7. Num. 4.October - December 2018
Pages 403-616
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Vol. 7. Num. 4.October - December 2018
Pages 403-616
Original Article
DOI: 10.1016/j.jmrt.2018.06.019
Open Access
Effect of the impact geometry in the ballistic trauma absorption of a ceramic multilayered armor system
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Fábio de Oliveira Bragaa,b,
Corresponding author
fabio_obraga@yahoo.com.br

Corresponding author.
, Fernanda Santos da Luza, Sergio Neves Monteiroa, Édio Pereira Lima Jr.a
a Military Institute of Engineering – IME, Department of Materials Science, Praça General Tibúrcio 80, URCA, 22290-270 Rio de Janeiro, RJ, Brazil
b Faculty of the National Service of Industrial Apprenticeship (SENAI Rio), Rua Mariz e Barros, 678, 20270-003 Rio de Janeiro, RJ, Brazil
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Tables (4)
Table 1. Characteristics of the ceramic.
Table 2. Characteristics of the 5052 H34 aluminum alloy.
Table 3. Properties of the aramid fibers and fabrics [23].
Table 4. Backface signature (BFS) measured in the ballistic tests for both ceramic geometries.
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Abstract

Ceramic armors are frequently used for protection against high energy projectiles, such as the 7.62mm and 5.56mm. Recently, it has been demonstrated that a modification in the geometry of the impact face, from flat to convex, enlarges the stress distribution zone created by the projectile-target interaction. This effect raises the projectile's energy absorption and might improve the user's safety. In the present work, the objective is to characterize ceramic armor plates with convex impact face, by means of the NIJ-0101.06 (2008) standard methodology, aiming to provide an eventual application in armor vests. The characterization is based on the measurement of the backface signature, a deformation behind armor imprinted in a reference material that simulates the consistency of the human body. The results showed significant improvement in the ballistic performance after the impact geometry modification.

Keywords:
Ballistic test
Ceramic armor
Multilayered armor system
Impact geometry
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1Introduction

Ceramic materials are widely employed for ballistic protection of individuals (ballistic vests) and structures, as alternative to reduce the weight of the armor, which has conventionally been made of steel. They integrate the multilayered armor systems (MAS), acting in conjunction with other materials for protection against high kinetic energy projectiles, such as the 7.62 and 5.56mm ammunitions [1–3].

The ceramics are of great importance to a MAS. They are commonly used as the front layer, due to their high strength and hardness, having the ability of eroding the tip of the projectile. In this case, the stresses generated through the impact are distributed to a larger area, and thus a significant portion of the material responds and resists to the dynamic load. Also, the ceramic spalling characteristics enables the absorption of a great amount of the projectile's energy, in the production of fracture surfaces, which can reach 50% of the total kinetic energy of a 7.62mm moving bullet [2–6]. In fact, ceramics are usually brittle materials with a tendency to shatter under impact loading. In many engineering situations, such as the case of concrete subjected to high strain rates [7–10] the impact impairs its integrity. By contrast, ceramic fragmentation under ballistic impact favors the dissipation of energy [11–16].

In a previous work, Monteiro et al. [17] investigated a ballistic ceramic based on alumina (Al2O3) doped with niobia (Nb2O5), either with flat or convex-shaped (80mm radius) strike face. The authors measured the absorption of the projectile's kinetic energy by the ceramic target, by measuring the penetration of the projectile in an 6061-T6 aluminum block positioned in the back face of the target (known as “depth of penetration test” or DOP). They found an improvement of 16% in energy absorption when the convex-faced ceramic replaces the flat one.

Finite element simulations were also performed by Monteiro et al. [17]. The result is illustrated in Fig. 1. This shows the decaying of the projectile's energy after impacting the front ceramics, together with images of the damaged zones 25μs, also relative to the impact. According to the authors [17], the impact interaction ceased at 30μs, which represents perforation of the target. At this time, the difference in absorbed energies is 0.23kJ, which represents 16% difference between flat and convex-faced ceramics. Fig. 1 also shows that the convex face contributes to enlarge the damaged area of the target, resulting in higher energy absorption. Those features contribute to the understanding that the difference in observed energy absorption (DOP test) is related to the impact geometry modification.

Fig. 1.
(0.19MB).

Decaying dissipated impact energy of the 7.62mm bullet [17].

The oblique impact has been studied by several authors [18–21]. The motivation is mainly to develop lighter and safer protection systems. Therefore, the objective of the present work is to characterize the behavior of a MAS with convex ceramic front, when subjected to ballistic impact with 7.62mm ammunition. It has been applied the methodology of the NIJ-0101.06 standard [22], which specifies the measurement of the indentation behind armor (also called “backface signature” or “trauma”) in a reference clay witness that simulates the consistency of the human body.

2Materials and methods

The ceramic material investigated in the present work consists of alumina (Al2O3) containing 4wt.% of niobia (Nb2O5). The alumina has been provided by the company Treibacher Schleifmittel and the niobia by Companhia Brasileira de Metalurgia e Mineração (CBMM). The ceramic processing included the mixing and milling of the powder in water suspension using polyethylene glycol as binder. The suspension was then dried at 60°C for 48h and sifted until 0.355 (42mesh). The resulting powder was cold pressed (30MPa), heat treated at 158°C for binder evolution and sintered at 1400°C. Table 1 shows some properties of the ceramics produced.

Table 1.

Characteristics of the ceramic.

Characteristic  Average value  Standard deviation 
Density (g/cm33.51  0.06 
Vickers microhardness (HV)  386  40 
Grain size (μm) 

The ceramic powder pressing was performed in a hexagonal mold, shown in Fig. 2a. An adaptation has been made to the mold using epoxy putty (Fig. 2b), aiming to produce convex faces in the ceramic pieces. The result can be seen in Fig. 2, which shows the produced flat-faced (Fig. 3a) and convex-faced (Fig. 3b) ceramic tiles.

Fig. 2.
(0.24MB).

Ceramic powder pressing: (a) hexagonal steel mold; (b) epoxy putty adaptation for the production of convex-faced pieces.

Fig. 3.
(0.29MB).

Ceramic tiles used in the front layer of the MAS: (a) flat-faced and (b) convex-faced ceramics.

For the ballistic tests, the 10mm thick ceramic pieces were integrated to a MAS as shown schematically in Fig. 4. Besides the front ceramic, the MAS is composed of an intermediate layer of aramid laminate and a back layer of aluminum alloy 5052 H34. The thickness of the back plates, aramid and aluminum, were 10 and 5mm, respectively.

Fig. 4.
(0.26MB).

Schematic diagram showing the multilayered armor system of the present work mounted for the backface signature test.

The aluminum alloy sheets were provided by the Brazilian company Metalak Metais. Some of their characteristics are shown in Table 2.

Table 2.

Characteristics of the 5052 H34 aluminum alloy.

Mechanical property  Average value  Standard deviation 
Tensile strength (MPa)  244 
Total deformation (%)  19 
Rockwell B hardnessa  20.0  0.7 
Chemical composition  Al  Mg  Ag  Cr 
Element content (%)b  96.7  2.3  0.7  0.2 
a

Using 5mm steel sphere and 750g as load.

b

Estimated by energy-dispersive spectroscopy (EDS).

The aramid fabric was provided by the Brazilian company LFJ Blindagens. It consists in a plain weave fabric composed by Kevlar 29® fibers, with areal density of 450g/m2. The laminates contained 18 fabric layers joined together with polychloroprene rubber. Some mechanical properties of the aramid fibers and fabrics are shown in Table 3.

Table 3.

Properties of the aramid fibers and fabrics [23].

Aramid fiber type  Fiber diameter (μm)  Fiber density (g/cm3Tensile strength (GPa)  Tensile modulus (GPa)  Elongation at break (%) 
Kevlar 29®, DuPont  12  1.43  2.9  70 
Kevlar 129®, DuPont  12  1.43  3.4  99  3.3 

The MAS were subjected to ballistic impact with 7.62×51mm M1 ammunition, with 9.7g of weight, commercially provided to the Brazilian Army. The shooting device, available in the Brazilian Army Assessment Center (CAEx), consists in a gun barrel with laser sight (Fig. 5a), located 15m from the target (armor specimens). The shooting was performed horizontally, with the targets positioned in front of a Roma Plastilina type clay witness (Fig. 5b), with 1.7g/cm3 density, simulating the consistency of the human body. The consistency of the clay witness is validated by the drop weight tests, which was performed following the procedure specified by the NIJ Standard [22]. The deformation behind armor, known as trauma or backface signature (BFS), was measured and used as parameter to compare the MAS specimens. This methodology is also specified in the NIJ-0101.06 standard [22]. Fig. 5 also shows the MAS with strike face with flat (Fig. 5c) or convex ceramic (Fig. 5d) fixed in the clay witness for the test.

Fig. 5.
(0.39MB).

Ballistic test: (a) gun barrel; (b) Roma Plastilina type clay witness; (c) MAS with flat-faced ceramic front; (d) MAS with convex-faced ceramic front.

The impact velocity (vi) of each projectile was measured using a HPI B472 optical barrier. This is specified by the NIJ standard [22] to avoid large variations in the impact kinetic energy, influencing the results.

After the ballistic tests, fragments of the MAS in the impact zone were microscopically evaluated by scanning electron microscopy (SEM), using a Quanta FEG 250 FEI equipment, using secondary electrons contrast.

3Results

Table 4 shows the values of impact velocity (vi) and backface signature (BFS) for the MAS with flat [12] and convex ceramic strike face. None of the MAS were perforated, and all the BFS values were below the 44mm (1.73in.) specified by the NIJ-0101.06 [22] for the level III of protection. Therefore, in terms of BFS, the performance can be considered satisfactory for both MAS.

Table 4.

Backface signature (BFS) measured in the ballistic tests for both ceramic geometries.

Strike face geometry  vi (m/s)BFS (mm)
Flat840.4517.56
842.7821.79
849.9023.16
842.0725.04
856.1620.10
Average  846±7  Average  21±3 
Convex853.7916.5
846.1016.3
844.2018.3
Average  848±5  Average  17±1 

By comparing the BFS values, the MAS with convex strike face had a better performance, with 19% lower average BFS than the flat one (decreasing the BFS from 21±3 to 17±1mm). It is important to emphasize that, the lower the BFS, the better the ballistic performance, since less energy would be transmitted to the user, improving safety.

The general aspect of the different MAS were very similar, with total fragmentation of the ceramic tiles and partial penetration of the projectile in the aramid layer, as shown in Fig. 6. The projectile, as well as the ceramic, was totally fragmented. As can be seen in Fig. 6c and d, aramid yarns were broken in the impact zone, and ceramic fragments can be visualized all over fiber surfaces.

Fig. 6.
(0.66MB).

Aspects of the MAS after the ballistic test: general aspect of the MAS with (a) flat-faced and (b) convex-faced ceramic front; details of impact zone of the (c) flat-faced and (d) convex-faced ceramic MAS.

Larger ceramic fragments were collected and observed in the SEM, and the result is shown in Fig. 7. For both specimens (Fig. 7a and b), the fracture is essentially intergranular. This is attributed to grain boundary precipitates which are formed by the reaction of the ceramic components (alumina and niobia).

Fig. 7.
(0.62MB).

Microscopic fracture aspect of the (a) flat-faced and (b) convex-faced ceramic.

Fig. 8a and b shows the microscopic aspect in the aramid impact zone, for the flat-faced and convex-faced ceramic MAS, with high magnifications (5000× and 4000×, respectively). In these figures, the main features are the broken aramid fibers and the ceramic deposition over the fibers.

Fig. 8.
(0.68MB).

Ceramic deposition on the aramid fibers in the (a) flat-faced and (b) convex-faced MAS.

4Discussion

The highest energy absorption of the convex-faced ceramic against the flat-faced, shown in Table 4, had already been demonstrated by Monteiro et al. [17], as previously discussed. The improvement in the performance is attributed to the fact that most of the convex impact surface is oblique relative to the projectile, and then the probability of orthogonal impact is almost negligible [17]. Besides that, the obliquity of the surface makes the impact energy to be distributed to a larger volume of the ceramic. This phenomenon can be visualized in the simulation (Fig. 1) by the longer and more numerous radial cracks in the (25μs) stress calculations. According to Tasdemirci et al. [24], a larger energy absorption can be expected when the ceramic is being fragmented in smaller pieces. Since more cracks are being nucleated and propagated, more of the projectile's energy is being consumed in the convex-faced material. That would be the same reason that the intergranular cracking could be beneficial to the ballistic performance of the ceramic. In this case, the cracks go through the grain boundaries, following a long and tortuous path, as can be seen in Fig. 7. Therefore, the niobia addition to improve brittleness and promote intergranular cracking is also important for a high energy absorption [17].

The fracture of the aramid threads, observed in Fig. 6c and d, can be attributed to the ceramic and projectile fragments moving cloud that impacts the fabric layer at a very high speeds. The detail of the fiber surfaces filled with ceramic particles can be better observed in Fig. 8a and b. Differentiation between ceramic and projectile fragments is relatively easy, however, fragments of the projectile are very scarce and could not be found among the ceramic particles. According to Braga et al. [25], in a similar ceramic MAS, the approximately 20mm in length projectile is fragmented to particles smaller than 50μm. In this previous work [25], the authors observed a small projectile fragment bonded together with ceramic particles in a similar MAS, after 7.62mm ballistic impact.

5Summary and conclusions

In the present work, the ballistic behavior of a ceramic multilayered armor system (MAS) with either flat, or convex strike face, was investigated. The MAS was subjected to level III backface signature (NIJ-0101.06 tests), using 7.62×51mm commercial ammunition.

  • The ballistic performance of both MAS could be considered satisfactory in terms of backface signature tests, performed following the NIJ Standard 0101.06 [22], for the level III of protection.

  • The convex-faced ceramic showed superior performance relative to the flat-faced, decreasing the backface signature of the armor in 19%. This system can thus be considered a safer solution in terms of trauma absorption.

  • The observation of the fracture mechanisms together with a finite element simulation [17] could elucidate the reason for the higher energy absorption for the convex-faced ceramic. In this case, the impact is mainly oblique, and thus the stresses are distributed to a larger volume of material. This makes a larger number of intergranular cracks propagate and reach a greater distance from the point of impact, consuming more energy.

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgements

The authors of the present work wish to thank the Brazilian supporting agencies CAPES, CNPq and FAPERJ for the funding, and the CAEx, for performing the ballistic tests.

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Paper was part of technical contributions presented in the events part of the ABM Week 2017, October 2nd to 6th, 2017, São Paulo, SP, Brazil.

Copyright © 2018. Brazilian Metallurgical, Materials and Mining Association
Journal of Materials Research and Technology

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