Mechanics of Composite Materials, Vol. 48, No. 4, September, 2012 (Russian Original Vol. 48, No. 4, July-August, 2012)
DEVELOPMENT OF KNIFE- AND BULLET-IMPACT-RESISTANT COMPOSITE STRUCTURES
A. A. Levinsky,* S. B. Sapozhnikov, and T. S. Grass
Keywords: composite structures, puncture, stab, local impact Two types of layered composite structures — based on an aramid fabric covered with alumina microparticles and on polycarbonate — have been considered. Experimental investigation of the mechanical properties and puncture resistance of the first material was carried out. The results of quasi-static, dynamic, and firearm tests of the composite plates are presented. The minimum distance from a first puncture where the stubbing protection is safe was found by using the digital image correlation method. A FEM analysis of bending and tension of composite plates with a defect of complex geometry was performed. The analytical results obtained are compared with test data.
Introduction The existing armor vests made of fabric packages according to the NIJ-0101.06 standard [1] successfully resist attacks of firearms of the first and second classes, but cannot provide protection from low-speed stabs of piercing and cutting weapons even at a first-level threat (NIJ 0115.00 standard [2]). Fabric packages, irrespective of their thickness or number of aramid fabric layers, can be cut by standard P1-type knives at rather small forces and punctured by sharp tools (Spike pricker), by way of moving fabric fibers apart. The current state of affairs calls for the creation of such structures that ensure a combined protection, are concealable, and have minimum cost and weight. The market of means of individual protection offers a number of engineering designs meant to achieve some of the above-mentioned purposes. They include the impregnation and covering of fabrics with special compositions [3, 4], the use of sectional metal discrete TurtleSkin elements [5], and the impregnation of aramid fabrics with thermoplastics [6]. Frequently, to provide a simultaneous bullet- and knifeproof protection, inserts are used in pockets of the South Ural State University, Chelyabinsk, Russia * Corresponding author; e-mail:
[email protected],
[email protected]
Translated from Mekhanika Kompozitnykh Materialov, Vol. 48, No. 4, pp. 591-604 , July-August, 2012. Original article submitted March 30,2012. 0191-5665/12/4804-0405 © 2012 Springer Science+Business Media, Inc.
405
a
b
c
Fig. 1. View of a fabric before (a) and after (b) the surface treatment by a suspension; corundum microparticles under a microscope (c).
covers of standard bulletproof jackets. These inserts are made of stainless steel, titanium, polycarbonate, nonwoven materials, such as pressed DYNEEMA panels (fibers based on a weak solution of ultrahigh-molecular-weight polyethylene, UHMWPE), and fiberglass (biconvex Xtreme Force panels) [7]. However, all these structures have significant drawbacks. The fabrics impregnated with special compositions and thermoplastics show a high puncture resistance, but they have a high weight and a low stab resistance. It is also known that the impregnation of layers of aramid fabric make them less bullet-resistant. Therefore, in the present study, we consider structures based on a fabric covered with suspensions only on the front surface. The structures with metal discrete elements cannot provide protection against firearms, because, upon their rotation, the sharp edges of the metal elements can cut the fabric and seriously damage the body. The materials used in the abovementioned rigid inserts are rather expensive (pressed polyethylene fibers) or heavy (metals). The most economical ones seem to be protective panels made of polycarbonate. This material possesses hardness and rigidity sufficient to retard the sharp edges of cold steel, and it is plastic enough to efficiently absorb the energy of a high-speed striker. However, the utilization of homogeneous polycarbonate, even for the first-level protection (standard [2]), makes it necessary to use rather thick and heavy sheets. Therefore, in the present study, a composite material obtained by modification of polycarbonate sheets is considered. The ideas presented in this study make up the essence of the model in [8]. 1. Surface-Modified Aramid Fabric As already mentioned, one of the ways for increasing the resistance of barriers made of fabrics to the stabs of a cold steel is their impregnation with various binders. However, the impregnation of aramid fabrics dramatically reduces their ballistic properties [3] and significantly increases the weight of armor [6]. In this connection, it is suggested to modify only the front surface of the fabric by covering it with a suspension based on a 3% water emulsion of poly(vinyl acetate) (TU 2385-002406
b
a 30
P, N
P, N
2
2
250 200
20 150 100
10
1
1 0
10
20
30
, %
50
40
0
u, mm 2
4
6
8
10
12
14
Fig. 2. Experimental tension diagrams of fabric specimens (a) and puncture diagrams of fabrics (b); u is the displacement of lance.
45145919-97) and corundum microparticles (electrofilter dust, EFD). The impregnated composition is dewatered by placing its samples in a vacuum heat chamber at an elevated temperature. The corundum microparticles fastened to aramid-fabric fibers (Fig. 1) increase the friction factor between the fibers and steel and between the fibers themselves, increase the force necessary to pull apart the fibers in the fabric upon its puncture by a sharp tool, and can also blunt the sharp edge of a knife. All these factors raise the resistance of protective fabric structures to the action of stabbing tools. 2. Composite Plates Based on Polycarbonate Polycarbonate (PC) has already been successfully used for a long time in designing means of individual protection. Due to the high plasticity, this material well absorbs and dissipates the kinetic energy of strikers. However, PC has also some drawbacks. The main one is the fact that this material becomes brittle at high strain rates, but its ultimate strength in tension increases considerably [9]. Brittle fracture, such as splitting of a plate, absorbs much less energy than plastic deformation. Therefore, it is necessary to raise the crack resistance of such plates. The present study is dedicated to PC-based composite plates manufactured by gluing one layer of a high-modulus Twaron 709 aramid fabric to the back side of a plate made of monolithic PC. In bending, the neutral line of cross section of such a layered plate, with the more rigid layer at the bottom, is shifted downward. In this case, the size of the jamming zone of knife edge increases, the pressure on knife flanks rises, and the friction force between the blade surface and plate material also grows. In addition, the fabric-reinforced plate possesses a higher crack resistance because the fabric hinders the displacement of crack faces. All this increases the stab and puncture resistance of the plates. 3. Experiments The characteristics of puncture resistance of aramid fabrics without a covering and covered with a suspension of corundum microparticles and the rigidity of the plates in bending were investigated. The cutting resistance of the plates under quasi-static and dynamic loadings was determined. By using the digital image correlation method, the stress concentration factors in the neighborhood of a sharp defect — the trace of knife stab — were found. In all experiments, the data for PC plates were compared with those for plates reinforced with an aramid fabric. 3.1. Strength of fabric structures. To estimate the shear strength of fabrics before and after the surface treatment, we performed tensile tests on specimens in the form of rectangular (100 × 10 mm) strips of fabric whose fibers were oriented at 45° to the direction of tension. 20 specimens of each type were tested. 407
100
P, N
2
1
80 60 40 20
0
, mm 0.3
0.6
0.9
1.2
1.5
1.8
2.1
Fig. 3. Bending diagram of PC (1) and composite (2) plates; Δ is deflection.
1200
P, N 2
1000
1
800 600 400 200 0
l, mm 5
10
15
20
25
30
35
Fig. 4. Force Р as a function of knife displacement l in cutting armor plates made of PC (1) and composite (2) plates.
Quasi-static experiments on piercing and cutting of a fabric by a typical Spike lance and a typical P1 knife according to [2] were carried out by using an Instron 5882 universal testing machine. Figure 2a shows averaged tension diagrams of fabric specimens. It was found that the surface modification (a 10% increase in weight) by a suspension of the front layer of fabric twofold raised the shear strength of the fabric and fivefold increased the force necessary for a typical lance to pierce the Twaron 709 ballistic fabric (Fig. 2b). Here, the surface density of the fabric grew by no more than 10%. We should note that no significant increase in the stab resistance was observed in this case. However, a great advantage of such a method is that the ballistic characteristics of the fabric practically do not worsen. 3.2. Experimental investigation of the rigidity characteristics of composite plates. The experiments were carried out in the three-point bending of a plate 90 mm wide, 100 mm long, and 4 mm thick. Ten specimens of each type were tested. From the averaged deformation diagrams (Fig. 3), it is seen that the PC plate has a load–deflection curve with a smaller slope angle than the composite plate. It was found that the flexural rigidity of the composite plate grew by 25-30%, while its weight increased only by 5% compared with that of the PC plate. Indentation tests were carried out with specimens placed on a substrate prepared according to the standard [2] and located on a rigid basis. The experiments were performed on an Instron 5882 universal testing machine. As an indenter, a P1 knife was used. The specimens were plates 100 × 90 mm in size. Averaged stab force–knife displacement diagrams were constructed (Fig. 4). It is seen that the stab force of composite plates is higher than that of PC plates at the final stage of stabbing, which confirms the validity of the ideas suggested in Sec. 2. The depth of penetration beyond limits of the plate was the same (8.5 mm) for specimens of different types. As seen from the test results presented in Table 1, the stab resistance of composite plates 408
TABLE 1. Characteristics of PC and Composite Plates at a Stab Depth of 8.5 mm 4-mm plates composite (PC + 1 layer of polycarbonate Twaron 709)
Characteristic Maximum cutting force, N Total dissipated energy, J Weight of plate, g
Distinction, %
770 ± 50
1000 ± 75
28-30
15.3 ± 1.07 42 ± 3
19.6 ± 1.37 44 ± 3
22-25 4.5-5.0
TABLE 2. Results of Tests with a Freely Falling Weight 4-mm plates
5-mm plates
PC
composite
Distinction, %
Penetration depth beyond plate limits, mm
15.5
10
35.5
12
9.5
20.8
Penetration depth beyond AV limits, mm
9.5
4.0
57
6.0
3.5
41.7
Surface density, kg/m2
4.8
5.2
7.6
5.6
5.9
5.1
Obeys the standard [2]
No
Yes
Yes
Yes
Characteristic
700 600
Vk, m/s
700
a
600
500
500
400
400
300 200 100 0
PC
composite
Distinction, %
Vk, m/s
b
300
1 2
200
V0, m/s 150 300 450 600 750
100 0
1 2
V0, m/s 150 300 450 600 750
Fig. 5. Residual speed Vk of a striker as a function of its initial speed V0 for plates of thickness 4 (a) and 5 (b) mm.
exceeded that of PC plates by 5% on the average, while their weight increased by no more than 5%. When the knife penetrated beyond limits of the plate (8.5 mm), the value of dissipated energy approached 20 J. If we consider that the plates are usually located on an armor packet 5-6 mm thick, the penetration beyond the limits of an armor vest (AV) makes 2.5-3.5 mm, which is allowable according to the standard [2]. Tests with a freely falling weight were carried out on an Instron Ceast 9350 impact testing machine. We should note that the loading device used in this experiment did not fully correspond to the American standard [2] — a single-mass system was used according to the European standard [10]. As specimens, we used PC plates of thickness 4 and 5 mm and composite plates of thickness 4.03-4.07 and 5.03-5.07 mm; 15 specimens of each type were tested. Table 2 presents the maximum values of stabs for series of plates of different types. The maximum stabbing force for the 4-mm-thick PC plates was about 500 N and for the composite plates of the same thickness — about 900 N, which agrees with the results of quasi-static tests. In this case, the work spent for stabbing approached 23 J. The results obtained point to a considerable increase in the stab resistance of the modified plates in dynamic tests. 409
TABLE 3. Results of Ballistic Tests Initial speed of striker, m/s Residual speed of striker, m/s PC plates 4 mm thick 630 557 424 306 339 198 316 158 600 527 Composite plates 4 mm thick 441 344 569 525 PC plates 5 mm thick 275 0 426 283 406 300 628 572 370 201 290 97
Ballistic tests were performed by firing with spherical steel strikers 8 mm in diameter at different initial speeds, according to the procedure described in [11]. As specimens, PC and composite plates of thickness 4 and 5 mm were used. In the experiments, the initial and residual speeds of the striker were registered. It was observed that the higher the initial speed of the striker, the lower the resistance rendered by the barrier. This dependence can be estimated by the curves shown in Fig. 5, which can be described by the function [12]
0 at V0 < Vcr , Vk = V0 (1 − exp[−k (V0 − Vcr )]) at V0 > Vcr ,
where V0 is the initial speed of striker, Vk is its residual speed, Vcr is the critical speed (the ballistic limit, at which a breakthrough still not occurs), and k is an empirical factor. By using this function, the ballistic limit can be determined with an engineering accuracy. For example, for 4-mm-thick plates, this parameter was 192 m/s, but for 5-mm-thick ones — 257 m/s (Table 3). The 4-mm-thick plates made of PC could lower the striker speed twofold at an initial speed to 300 m/s and 1.5 times at 400 m/s. The 5-mm-thick plates reduced the striker speed 3.5 times at an initial speed of 300 m/s and 1.5 times at a speed of up to 420 m/s. In this test, the composite plates had no advantage over the PC ones. 3.3. Serviceability of a plate after repeated impacts. A first impact of a knife forms a sharp defect in the protective plate, and it is vital to investigate the ability of the damaged plate to efficiently restrain repeated impacts. To solve this problem, a contactless optical method for measuring strain fields — the method of digital image correlation (DIC, [13]) — was chosen. According to this method, the surface examined is covered with the so-called speckle pattern, which is a contrast image consisting of spots, dots, strips, or objects of arbitrary form chaotically distributed over the surface. Each element of the speckle is comparable in sizes with the other elements, and the distance between neighboring elements, on the average, is equal to their size. Thus, the speckle pattern is defined by only one parameter — the size of a spot. Upon deformation of the object examined, speckle spots become displaced. This process is registered by digital cameras with a given picture frequency. The set of the digital images obtained is analyzed by using a special software, which subsequently displays the time-varying field of deformations. This in essence corresponds to the application of a great number of small-size tensometers. A sharp 7-mm-long defect was made at the center of a specimen 50 mm wide and 100 mm long. During tension at a rate of 3 mm/min, a two-CCD-camera stereosystem registered images with a frequency of 1 picture per 1 s. The average size 410
a
b
c
Fig. 6. Strain field around a defect determined by the DIC (a) and FEM (b) methods; estimation of sizes of the zone outside which the defect affects the strength of the plate (c) insignificantly.
of a speckle spot was 1 mm. After processing the resulting images by the DIC method by using the Correlated Solutions Vic3D software package, deformations fields over the entire surface of a specimen were obtained. Figure 6a shows the strain field along the specimen axis and the longitudinal deformations on the line passing through the defect (perpendicularly to the longitudinal axis of specimen). At a nominal strain of 0.5%, the maximum value of the strain concentration factor near the defect was 2.54. Based on the results obtained, it is possible to distinguish a region near the defect where its weakening influence on the strength and rigidity of the plate (Fig. 6b) is considerable. This region has a diameter of about 40 mm. Thus, we may assert that the plate is able to effectively resist repeated knife impacts at a distance greater than 40 mm from an already existing defect. 4. Calculations By using the finite-element method (FEM), the processes considered in Sec. 3 (bending of plates on an elastic foundation and estimation of the strain concentration factor in tension of a plate with a defect of complex geometry) were simulated and calculated. Calculation results were compared with experimental data. In the calculations, we used material characteristics obtained from the tensile diagrams of polycarbonate specimens ([14], Fig. 7). Thus, an elastoplastic model with the following parameters was chosen: elastic modulus E = 1.8 GPa, yield point sy = 46 MPa, and hardening modulus Еhard = 45 MPa (Fig. 7). 4.1. The calculation of stresses in bending of plates on an elastic foundation was carried out in order to compare them with experimental data and to elucidate the reason for the increased stab resistance of the composite plates. The calculation scheme of the problem was a 4 × 90 × 100-mm plate lying on an elastic basis and loaded with a concentrated force applied to
411
, МPa
70
1
60
2
50 40 30 20 10
, %
0
5
10
15
20
25
30
Fig. 7. Tensile diagrams of specimens (1) and of material model (2).
а
1 p
Plan
e of
sym
met
ry
Plane
metry
of sym
2
3
b
x(h)
c
d
Fig. 8. Calculation of stresses in bending of a plate: a — calculation scheme, b — calculation result, c and d — distribution of normal stresses over the sections of PC and composite plates, respectively. 1 — PC plate of thickness 4 mm; 2 — PE foam layer of thickness 45 mm; 3 — layer of aramid fabric.
its center. Since we used the FEM, the pressure was assumed to be applied to a circular area 2 mm in diameter. The scheme had two planes of symmetry, therefore we considered only quarter of the plate (Fig. 8).
412
Figure 8b illustrates calculation results and the distribution of normal stresses in cross section of the plate. As seen, the experimental data are confirmed by the results of FEM calculations. The latter show that the compressive stresses increase in the zone where the jamming of knife occurs and that the compression zone also increases (see Fig. 8). 4.2. Calculation of strain fields in the vicinity of a complex-shape concentrator. In the SolidWorks package, the model of a plate with a defect was constructed. Figure 6c shows the field of strains εy along the specimen axis. According to these results, the maximum factor of concentration was equal to 1.6, which is somewhat less than the corresponding value obtained from calculations by the DIC method (k = 2.54).
Conclusions In the present study, fabrics covered with a suspension of corundum microparticles were examined and their puncture resistance was investigated experimentally. It was found that the method for modification of fabrics suggested allows one to improve their resistance to impacts from a sharp tool about fivefold. The application of such a technology to manufacturing armor vests will make it possible to improve their protective characteristics without a significant increase in weight and cost. In addition, for composite plates obtained by reinforcing PC plates with an aramid fabric, the characteristics of resistance to stabbing and to the breakthrough impacts of high-speed spherical strikers were investigated experimentally. It is proved that the stab resistance of composite plates is by 30-50% higher than that of PC plates of the same thickness and that they ensure the first-level protection (according to the standard [2]) from the action of cold steel at an increase in weight not exceeding 5%. PC plates of thickness 4-5 mm can lower the speed of a striker two- to threefold at its initial speed of 300-400 m/s. The DIC method was employed to investigate the deformed state of the plate region adjacent to a complex-shape defect. It was shown that the plates are able to restrain repeated local impacts at a distance of 20 mm from an existing defect. The use of the composite plates as rigid inserts makes it possible to raise the level of protection of AV by one class according to the standard [15] at an insignificant increase in their weight and cost.
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11. M. V. Forental’, Dynamics of Deformation and Fracture of Plates at a High-Speed Loading by Strikers of a Complex Structure, PhD, Izdat. YuUrGU, Chelyabinsk (2010). 12. S. B. Sapozhnikov, N. Yu. Dolganina, and S. A. Sakharov, “Simulation of the dynamics of interaction between a striker and a multilayer fabric package,” Vopr. Oboron. Tekhn., Ser. “Composite and Nonmetallic Materials in Mechanical Engineering,” Iss. 3 (140)-4 (141), 38-41 (2005). 13. M. A. Sutton, J. J. Orteu, and H. W. Schreier, Image Correlation for Shape, Motion and Deformation Measurements. Basic Concepts, Teory and Applications, Springer, USA (2009). 14. H. Q. Shah, “Impact resistance of a rectangular polycarbonate armor plate subjected to single and multiple impacts,” Int. J. Impact Eng., No. 36, 1128-1135 (2009). 15. GOST R 50744-95. Armor Wear. Classification and General Technical Requirements [in Russian], Moscow (2002).
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