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Simulation of Bullet Impact on Bullet Resistant Steel Plate

Abstract Godrej Security Solution Division has developed bullet resistant solutions like Bullet resistant doors, Mantraps, Bullet resistant screening for banks, and bullet resistant cabins etc which are designed as per the requirements mentioned in various standards like National institute of Justice (NIJ), Underwriters Laboratory (UL), etc. These agencies have established the performance criteria and classified it into various protection levels. To gain the confidence in these bullet resistant solutions it is decided to test performance of steel plate for its resistance against bullet. This paper will present the NIJ level III steel plate behavior against the dynamic loading of 7.62 x 39mm steel core AK47 ammunition using Radioss explicit non-linear analysis. For simulation, test setup was done according to the requirement of DIN EN 1522 and 1523 [1]. To verify the results of software simulation, the same steel plate was physically tested at Bundeswehr Technical Center for Protective and Special Technologies, Germany to prove its resistance against the mentioned ammunition. It was found that the steel plate behavior in Radioss explicit non-linear analysis is very much similar with actual test results. This study has shown that the predictive capability for penetration of steel core bullet is quite good with Radioss non-linear explicit analysis. With set simulation methodology and increased confidence level, same type of simulation can be tried out for other protection levels of different standards. It will save time and cost involved in actual testing.

Introduction The threats to the public security and premises security are on rise because of increasing terrorism and violence. Safety of individual is matter of concern, hence there is need to develop bullet resistant solutions for convenience stores, detention facilities, hospitals, government offices, schools and banks, public installations, military applications and sensitive premises. Generally, constructions in these premises can be made strong by using solution with first line of defense. However, the entrances need special attention to get sealed off and provide necessary protection against fatal attack. Thus, a variety of bullet resistant products with specified protection level need to be designed for range of applications. Bullet resistant door mainly consist of bullet resistant steel plate that resist bullet penetration. The term "Bullet Resistant" (henceforth called as BR) signifies that protection is provided against the complete penetration, passage of fragments of projectile, or spalling (fragmentation) of the protective elements to the degree that injury would not be caused to person standing directly behind the bullet resisting barrier. There are various protection levels in NIJ and UL standards as mentioned below:

Mr. Satish Ramavat Manager

Security Solution Division Godrej & Boyce Mfg. Co.

Ltd.

Mr. Devidas Thorat Deputy General Manager Security Solution Division Godrej & Boyce Mfg. Co.

Ltd.

Mr. Prashant C. Assistant VP

Security Solution Division Godrej & Boyce Mfg. Co.

Ltd.

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Sr. No. Protection Level

Test Round

Test bullet Equivalent UL 752 Ratings

1 IIA

1 9mm FMJ RN Level 1

2 0.40 S&W FMJ 2

II 1 9mm FMJ RN

Level 2 2 0.357 Magnum JSP

3

IIIA

1 0.357 SIG FMJ FN

Level 3 2 0.44 Magnum

SJHP 4 III 1 7.62mm NATO

FMJ Level 8

5 IV 1 0.30 Caliber M2 AP

Level 4

NIJ level III plate provides the resistance against the 7.62 soft core AK47 ammunition.

During the high velocity penetration that takes place where a projectile strikes a target, many significant phenomena appear to be experimentally intractable due to the complexity of materials deforming at extremely high strain rates. To better understand the physics of the penetration process during high velocity impact, finite element analysis methods have proven to be an invaluable diagnostic and design tool. In general, very good agreement between FEA experiments and theory can be obtained but care must be taken in the definition of the problem from the numerical and material modeling standpoints. In addition to defining an adequate finite element mesh, an important aspect of conducting successful penetration simulations is the use of adequate material failure models. Generally, for the dynamic analysis a simplified single-degree-of-freedom system is considered to simulate the dynamic responses of the steel plate under the impact load of bullet, by which the maximum distortion and time-history of acceleration can be investigated. The dynamic loading characteristics such as bullet speed and its weight need to be ensured prior to analysis.

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A. Finite Element Modelling:

For simulation FEM model is prepared exactly similar to the actual physical test setup mentioned in DIN EN 1523.

Fig1: Test setup as per standard.

In FEM model, size of steel plate and aluminum foil is scale down to reduce the file size. Also, the test distance is reduced to minimize the computation time.

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Fig2: Test setup in FEM model

Details about each part are as mentioned below: 1. Test Specimen - Steel Plate: The steel plate used in this simulation is basically certified as NIJ level-III. The material of plate has high yield stress and high stiffness value. The thickness of plate is 6.3mm. For simulation, steel plate is modeled, using 3D hexahedral elements, with elasto-plastic material (MAT/PLAS_JOHNS). In order to conduct a realistic simulation of the impact problem, the finite element mesh needs to be relatively dense in regions that will experience high stress gradients and large deformations. Hence the center part of plate is modeled with fine mesh. Properties of steel plate: Young's Modulus (E): 210000 MPa Density(ρ): 7.8e-9 Mg/mm3 Poisson ratio (ν): 0.3 Yield Stress (σ ): 1700 MPa Elongation (l) : 7%

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Fig3: HM model of Steel Plate

2. Bullet: The used bullet is 7.62 x 39mm steel core AK47 ammunition. Bullet type is Full Jacket (FJ)/ pointed bullet (PB)/Soft core (SC). Velocity of bullet is considered as 710 m/s. For simulation, bullet is modeled, using 3D hexahedral elements, with elasto-plastic material (MAT/PLAS_JOHNS). The meshing of bullet is one of the major challenges for this simulation. Bullet geometry was created using 3D modeling software. Initially, 3D hexahedral elements of bullet were generated using spin command which creates single node at tip, surrounded by wedge elements. Single node at the tip unable to identify slave nodes and thus not provided proper impact behavior. It shows the bullet penetration at the initial stages of impact.

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Fig 4.1: First HM model of Bullet

Thereafter, mesh was further refined. Some more elements and nodes are created at the tip using 2D mesh and same mesh was gradually projected along the length of the bullet. To get the better results, fine hexahedral mesh was done on tip of the bullet. Properties of steel plate: Young's Modulus (E): 16000 MPa Density(ρ): 11.3e-9 Mg/mm3 Poisson ratio (ν): 0.4

Fig 4.2: Revised HM model of Bullet

3. Aluminum Foil: Aluminum foil is of thickness 0.5mm, and it is modeled as 2D shell elements. It's mainly used to detect the splinters coming out of the steel plates. Any mark on this foil indicates that the splinters are coming out from the plate. Properties of steel plate: Young's Modulus (E): 70000 MPa Density(ρ): 2.7e-9 Mg/mm3 Poisson ratio (ν): 0.28

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Fig 5: HM model of Aluminum Foil

Highlights Of Meshing & Pre-Processing:

To model the impact, penetration and deformation processes occurring when the bullet impacts the target, and the subsequent deformation of the plate, it is necessary to divide the plate and the projectile into a finite number of regions called elements. The network of elements obtained is called a mesh. The computations are then performed by solving the constitutive equations for the deformation of the individual elements in the mesh. In order to conduct a realistic simulation of the impact problem, the finite element mesh needs to be relatively dense in regions that will experience high stress gradients and large deformations. Figures 3, 4 and 5 shows relatively coarse mesh constructed at the region, far from the impact zone. This was done in order to minimize the computational time that could otherwise be very large. For this study, a simple mesh sensitivity analysis was conducted. The mesh size was adjusted until the penetration results did not vary significantly as the mesh density was changed. The result was an optimum mesh size that was subsequently used for all the simulations performed in this study. To define the contacts between bullet and plate, different type of interfaces were tried out, which was evaluated after observing the impact behavior. Type 7 only supports surface to node contact and Type 20 supports surface to surface and surface to edge contact. In this case, surface and edge contact both play important role. Number of iteration were done and finally concluded on Type-20 interface between bullet and plate. Initial velocity was given to bullet using load collector as 710 m/s along X-direction. In output block, bullet node velocity is extracted to examine the bullet behavior after impact.

Acceptance Criteria: Bullet should not pass through the steel plates and also there should not be any splinter mark on aluminum foil. Dent marks are allowed on steel plates. Results of FEA: The study examines the effect of bullet impact on steel plate. Simulation was carried out with different size of meshing and finally concluded on the best size of mesh based on bullet penetration pattern and plate deformation. After simulation, it was found that bullet is unable to penetrate through the steel plate. In short, bullet shots were caught by plate. The bullet had not caused the failure of material on attacked side of plate

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and nearly left no dent on rear side of plate. It only caused surface damage on attacked sides. Splinters are not observed on aluminum foil.

Fig 6: Plate after impact

Fig 7: Bullet after impact

Thus, plate proves its resistance against the 7.62mm steel core AK-47 ammunition. Similarly, plate can be tested against 7.62mm lead core AK-47 ammunition which is considered as softer than steel core. Velocity and Kinetic energy observation for bullet has been plotted below for reference.

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Fig 8: Velocity graph for bullet during impact

Fig 9: Kinetic Energy graph for bullet during impact

B. Ballistic testing of Bullet Impact on steel plate: To carry out the actual testing of steel plate against the bullet impact, two plates of size 600 mm x 300 mm x 6.3mm were sent to Bundeshwehr Technical Center for Protective and special technologies, at Germany for testing. Test was carried out as per standard DIN EN 1522 & 1523 [1]. The mentioned standards define a test procedure to permit classification of the bullet resistance of windows, doors, and shutters.

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Experimental Setup:

For test setup, ref. figure 2 for details. The tests specifications are based on standard data sketches, and are drawn as per DIN EN 1523 standard to make it easier to perceive the test setup. Chosen Caliber according to DIN EN 1522:

Requirements and Definitions: Perforation: Piercing of a test specimen by a bullet or by bullet fragments, and/or creation of an opening from the attack face to the rear face. The following are considered to constitute perforation:

a. Passing through the test specimen by the bullet or any part of it; b. Splitting of the rear surface of the test specimen by the bullet or part of it, even if the bullet is

visibly retained in the rear of the test specimen. c. Creation of an opening right through the specimen, even if that opening closes again

afterwards. Perforation has not occurred if none of the above criteria are fulfilled. Splinter indicator: Aluminum foil : thickness = 0.5 mm : Mass per area = 54 g/m2 : Big enough to catch possible splinters Splinter indicator perforated: notation 'S' on test specimen (splinters) No. of shots: 3 hits on target area Minimal distance between hit points: 120mm Projectile velocity out of given tolerance (EN1523, table page 7)

a. Velocity lower + no perforation = additional shot needed. b. Velocity higher + perforation = shot repetition.

Result Survey: After each shot, examination of: a. Specimen (perforation, splinter emission) b. Splinter indicator (if penetrated- changing of indicator) c. Splinter collecting container (if splinters-removal of splinters + cleaning)

Analysis of test results: The shots were fired according to fig1. The projectile is pointed one and contains a steel core which is covered by metal jacket. The target points were shot as the numbers shown on fig 10. All additional shots were followed by the same pattern (starting with top triangle, rotating counterclockwise, switching to the triangle below).

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Fig 10: Bullet Shots position on plate.

The shots were easily caught by plates. The projectile nearly left no dent as one could feel on the rear. The plate proves its resistance against the mentioned ammunition. Results were shown below:

Fig 11 : Plate after bullet shots during ballistic testing

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Results & Discussion: Comparative chart for actual test data and simulation test data mentioned below:

Sr. no. Parameters Simulation Actual Test 1 Bullet type 7.62 x 39 soft core 7.62 x 39 soft core 2 Bullet velocity 710 m/s 710 m/s 3 Bullet mass 8 g (8e-6Mg) 8 g 4 Distance between Al foil &

BR steel plate 500 mm (0.5m) 500 mm (0.5m)

5 BR steel plate thickness 6.3mm 6.3mm 6 BR steel plate size 300 x 150 600 x 300 7 Results No through

penetration Splinters not observed on Aluminum foil

No through penetration Splinters not observed on test foil

After comparing the results of FEA simulation using Radioss and actual ballistic testing of BR steel, it's found that in both the cases, surface damage caused by the bullet on attack side is very much similar, with no dent marks and splinters on rear side of plate.

Fig 12: Comparison between actual testing and simulation result

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Further Scope of Study: The FEM model was tested against different plate thicknesses and different bullet velocity. Good agreement was obtained. The minimal plate protection thickness for no perforation of the projectile for different NIJ level like IV, IIIA, II, and IIA, has also been determined numerically and it can be validated with experiments in future. For further development work, Radioss can be used as efficient solving tool for complex models with good level of accuracy and reliability. The experiments can further be extended for bullet resistant glass & security glazing. It can also be extended for simulation of complete bullet resistant door to evaluate how the splinters can get absorbed by the infill material and thus reduce the damage. Conclusion:

The matching results of FEM simulation using Radioss and physical testing of steel plates are very much encouraging to use Radioss as a tool to check the performance of bullet resistance. This study has shown that the predictive capability for penetration of steel core bullet is quite good with Radioss non-linear explicit analysis. It also helps to get the steel plate behavior under dynamic impact loading. It has been shown that the simulations could catch the main features of the experiments (especially the velocity, and kinetic energy of the projectile).

ACKOWLEDEGMENTS The authors would like to acknowledge our company M/s. Godrej and Boyce Mfg. Co. Ltd. Security Solutions Division and Altair’s Technical support.

REFERENCES [1] DIN 1523 & 1522: Window, doors, shutters, and Blinds Bullet resistance -Test Methods. [2] Warren C Young, “Roark’s formula for Stress and Strain” [3] Altair HyperWorks Radioss v11 help Manual. [4] UL 752: Bullet resisting Equipments. [5] NIJ standard 0108.01 Ballistic Resistance Protective Materials. [6] NIJ standard 0101.06 Ballistic Resistance of Body Armor.