Brain injury caused by rifle bullet impacting 1 bulletproof plate: Experiment and Simulation

10 Background: In wars, when bullets impact the bullet-proof helmet, kinetic energy 11 will be transferred from the skull to brain tissue, resulting in the rapid deformation, 12 stretching, shearing and final destruction of the soft tissue. In recent years, with the 13 continuous upgrading of protective equipment, the penetration ability of bullets into 14 protective equipment has gradually decreased, but the problem of head injuries caused 15 by deformation of the back of the helmet has become increasingly prominent. It is of 16 great significance and value to study the brain trauma caused by the bullet impact of 17 the bullet-proof helmet. 18 Methods: First proceeded the rifle bullet impact physical brain model experiment and 19 the results were used to verify the simulation process of high-speed bullet impact, 20 simulated the bullet hitting brain model from different directions (front, side, rear) and 21 different incident angles (0°, 15°, 30°), then evaluated the craniocerebral injury by 22 analyzing skull stress, intracranial pressure, principal strain, and shear strain. 23 Results: When impact from the rear, the peak intracranial pressure and skull stress 24 increase by 20%-25% compared to the front impact, and the principal strain and shear 25 strain are 1.5-2.2 times than that of the front impact. In the same impact direction, the 26 severity of brain injury will increase with the increase of incident angle. When the 27 incident angle increases from 0° to 15°, the intracranial pressure and skull stress both 28 increase, the principal strain and shear stress increase sharply with 6-7 times. 29 Conclusions: Under different shock conditions, the dynamic response of the brain is 30 sensitive, and the impact position and angle of the bullet have important influences on 31 the brain. It is more likely to be caused injury during rear impact, and as the incident 32 angle increases, the severity of the injury will become more serious. 33


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Traumatic brain injury is caused by external mechanical force or head movement 37 caused by rapid acceleration, deceleration, and rotation [1]. In modern warfare, when 38 the kinetic energy or shock wave of bullets and fragments impacts the bulletproof 39 helmet, the kinetic energy will be transmitted from the skull to the brain tissue, shapes. The researchers also analyzed the helmet shell material and helmet lining or 50 cushioning system [9][10][11]. In early experimental studies, the main target was live 51 anesthetized animals (such as pigs, dogs, and sheep). Oukara et al. [12] developed 52 corpse and animal impact test models, dummy tests, and finite element brain models 53 for numerical simulations of brain risk assessment, among which numerical models 54 can predict different types of brain injuries. Rafaels et al. [13] conducted statistical 55 analysis on the brain damage caused by bullet impact bulletproof helmets through 56 cadavers brain injury experiment. Due to moral restrictions and legal prohibitions in 57 most countries, human tissue simulants (such as gelatin, soap, etc.) have been 58 introduced into experiments in recent years. Gelatin is used in wound ballistic 59 research because its mechanical properties are considered to be similar to those of 60 human tissues [14][15][16]. Freitas  helmet liner materials on brain injury. Different liner materials are related to brain 70 acceleration, intracranial pressure, and shear stress.

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In recent years, with the continuous upgrading of protective equipment, the 72 penetration ability of bullets into protective equipment has gradually decreased, but 73 the problem of head injuries caused by deformation of the back of the helmet has 74 become increasingly prominent. It is of great significance and value to study the brain    density is 45kg/m 3 , elastic modulus is 4.16Mpa, and load its stress-strain curve.

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The 5.56mm bullet finite model is established and meshed in Hypermesh. As 135 shown in Fig.1f, the grid size is 0.5mm*0.5mm*0.5mm, and the number of grids is  Combine bullet, bulletproof plate, cushion foam, and brain model into an impact  Fig.2d. 168 Refer to the installation position of acceleration sensor in craniocerebral physical 169 model (Fig.1a), select the same position in finite element model (Fig.2e) to output the 170 skull acceleration value and compare it with the sensor data (Fig.2f). . Therefore, the skull in this simulation will 204 not be fractured, and the grid will not be deleted. Galbraith et al. found that when the 205 principal strain of brain tissue is greater than 0.25, structural damage will occur. When 206 the principal strain is greater than 0.2, the brain will experience functional damage 207 and cause irreversible damage. When the principal strain of the brain is greater than 208 0.1, the brain will suffer damage but can be restored to normal state [27]. Zhang et al.

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[28] found that when the shear strain of brain is greater than 0.14, there is a 25% and shear strain value of brain during front impact are the smallest, both less than 0.1.

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During side impact, the principal strain and shear strain of brain are larger than those 215 of front impact, but they are also less than 0.1, so there will be no damage. In the rear 216 impact, the principal strain of the brain reaches 0.017, and the shear strain reaches 217 0.0138. Therefore, the rear impact is more likely to cause head injury during the 218 impact.

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It can be seen that the peak skull stress value is 62.83MPa at 15° impact, the peak 245 skull stress value at 30° impact is 66.84MPa, and the peak skull stress value at 0° with an incident angle of 15°, the principal strain and shear strain of the brain are less 251 than 0.1, but they are greater than impact at 0°. When a bullet impacts at an incident 252 angle of 30°, the principal strain and shear strain of the brain are also less than 0.1, 253 but they are greater than the impact at 15°, exceeding 0.05. The simulation results

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show that as the incident angle of the bullet increases, the principal strain and shear 255 strain of brain tissue will increase, which is more likely to cause brain injury.

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-15 - angles. It can be seen from the curve that the peak intracranial pressure at 0° impact is 258 the smallest with 52.8KPa, the peak intracranial pressure at 15° impact is 60.3Kpa, 259 the peak intracranial pressure at 30° impact is the largest with 63.6KPa.   and it has good sensitivity to the brain dynamics response of different load conditions.

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The stress of skull is used to assess whether the skull fracture is caused, and the 291 brain injury is assessed by the intracranial pressure, principal strain and shear strain of 292 the brain. There are significant differences in the damage caused by bullets impacting used as the protective liner, it did not cause brain damage. It can be seen from the 301 peak intracranial pressure and peak stress of skull that when impacting the brain in 302 different directions, the intracranial pressure did not reach the threshold of moderate 303 injury and skull fracture. When impact from the rear, the peak intracranial pressure 304 and skull stress increase by 20%-25% compared to the front impact, and the principal 305 strain and shear strain are 1.5-2.2 times than that of the front impact. In the same 306 impact direction, the severity of brain injury will increase with the increase of incident 307 angle. When the incident angle increases from 0° to 15°, the intracranial pressure and 308 skull stress both increase, the principal strain and shear stress increase sharply with 309 6-7 times. At the same time, it is found that the transmission of shock waves in the 310 model will not only gradually attenuate along the impact direction, but will also be 311 transmitted in the same tissue, which may cause superimposed damage.

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The finite element model used in this reasearch can reflect the biomechanical 314 response of human brain and is sensitive to the dynamic response of brain under 315 different impact conditions, the impact position and angle of the bullet have important 316 influence on the response of the brain. It is more likely to be caused injury during rear 317 impact, and as the incident angle increases, the severity of the injury will become 318 more serious.