Helmet-Head Interactions under Ballistic Impacts - A Study on the Effects of Padding
M. Salimi Jazi, A. Rezaei, G. Karami*, F. Azarmi and M. Ziejewski
Mechanical Engineering Department, North Dakota State University, Fargo, ND 58108-6050.
* Corresponding Author: Mechanical Engineering Department, NDSU, email: email@example.com
Ballistic impacts are a major cause of traumatic brain injuries (TBIs). Although, ballistic helmets are highly effective in preventing severe injuries, the skull and brain can still be injured under ballistic attacks, as well by other types of insults such as blasts, or kinematical impacts. The efficiency of a helmet mostly depends on the types of materials used for its shell and padding. This paper presents the results of a computational study on the effect of the padding materials in helmet-head interaction and the severity level of the brain injury. The helmet, head and the padding are modeled in the study by finite elements (FEs). Appropriate contact conditions between the head and the padding, as well as the padding and the helmet, are defined. Kevlar is considered to be the material for the helmet, even though various types of padding materials have been used to evaluate the response on the human head during ballistic impact insults. In order to model a realistic situation, the helmet is placed on a fidelity head-model that includes the complete skull and brain components. The scenario of a bullet impacting the helmeted head is then simulated. Results of brain intracranial pressure (ICP) and kinematical parameters of the brain motion are determined for different padding materials. The results indicate the degree of injury to the brain and, thus, the efficiency of the padding during ballistic impacts.
Keywords: brain injury, helmet, padding, ballistic impact, finite elements
Traumatic brain injury (TBI) often happens when the brain experiences a rapid kinematical motion, or strain. The intensity of TBI can be categorized as mild, moderate, or severe, depending on the severity of the injury. The most common causes of brain injury for civilians are motor vehicle crashes (MVCs), violence related injuries, collisions in sports, and falls. Most military personnel are, however, at risk of a TBI when they are exposed to shock waves due to blasts in combat regions, or ballistic impacts. Both have different masses and velocities than MVCs.
To increase safety and reduce the possibility of injury, ballistic helmets are used because they are often highly effective in preventing severe injuries. It is obvious that in many situations (particularly blast scenarios), wearing a helmet reduces the severity of injury to the brain. TBIs can, however, still occur due to the loading size that goes beyond the limit of protection and tolerance. Using energy absorbing materials, pads in the helmets can reduce the amount of transferred load considerably. Foam, for example, is a good energy absorbing material that is widely used in helmet pads, as well as in industry.
Since 1700, helmets made from materials such as leather, brass, bronze and iron have been used to protect soldiers’ heads from cutting blows with swords and flying arrows in the battlefield. The introduction of firearms in the battlefield, however, ended the usage of these helmets by soldiers. In 1915, the French Adrian was the first known modern helmet made from steel. It was used extensively during World War I. The first United States (U.S.) army helmet, called the M1, was also made of steel, and served the U.S. soldiers not only during World War II, but was also used for the next four decades . With concerns, in the 1980s, regarding material, ergonomic design, and protective aspects of the helmet, the M1 was finally substituted by the 29-layer Kevlar Personal Armor System of Ground Troops (PASGT). In 2003, the PASGT was replaced by a lighter Kevlar, called the Advanced Combat Helmet (ACH). The introduction of composites, in modern industry, has definitely improved the overall character of the helmets. An important component of any helmet is its padding which is widely dependent on its material, shape and geometry for purposes of efficiency.
Over time, there has been considerable research and interest in studying TBIs from both a medical and biomechanical perspective. Modeling the brain injury through biomedical engineering, due to mechanical impact loads and blasts, has been the focus of many research works [2, 3, and 4]. In particular, helmet protection capacity, and its mechanical behavior, has been an area of research for the safety of the human head. The response of different helmets to ballistic impacts, both experimentally and numerically, has been studied in recent years. Van Hoof et al.  simulated the impact on a Kevlar helmet which was placed on a simple head model. They used advanced dynamic finite element (FE) software (LS-Dyna) and selected Mat 22 (Chang-Chang composite failure model) in LS-Dyna in order to consider the failure of the helmet during the impact. Baumgartner et al.  considered a head model, including its principle anatomic components, and studied the contact between the helmet interior and the skull surface. They modeled an aluminum helmet and subjected it to an impact by a steel bullet. Aare et al.  studied the response of the human head to a ballistic impact by using a complex and complete model of the human head and ballistic helmet. They also used Mat 22 in LS-Dyna for the helmet. They suggested that in order to minimize the stress and strain levels in the skull, the helmet material could not be made from extremely stiff, or extremely soft, material. Gerald  developed a numerical model to study the interaction between the helmet, the interior pad system, and the human head during a ballistic impact for different bullet velocities. His results indicated that the difference in the density of the foam pads influences the level of stress and pressure in the head.
In the work presented here, two different types of foam material were used for the interior pad system of the ballistic helmet and their effects on the impact load on the skull and brain was studied.
FE Modeling of the Head and Helmet
To simulate the impact of the bullet with the helmeted head model, the head, padding and helmet, the LS-Dyna software was used .
The Human Head
The geometrical data from Horgan’s et al.  head model was used to develop a detailed FE head model with all the necessary components (Figure 1). Two and four layers of 8-node brick elements were chosen for simulating the scalp and skull, respectively. All of the membranes (i.e. the dura mater, pia mater, flax, and tentorium) were modeled with 4-node shell elements. Cerebrospinal fluid (CSF) was modeled by using 8-node brick elements with fluid like properties. The brain was also modeled with 8-node brick elements. A facial bone was added to the model to find a realistic head shape. Appropriate contacts were applied to each component of the head . The material properties of the head components were taken from a study conducted by Li et al. . They assumed that the material properties of different parts of the head, except for the brain, are homogenous, isotropic and linearly elastic. The brain was considered to have viscoelastic behavior.
Figure 1: Model of the human head with all of its components
In the study presented here, the geometry of ACH derived accurately from a coordinated measuring machine, was used. It was meshed using four layers of 8-node brick elements and placed on the head model (Figure 2). Although the helmet material is considered to be a laminated type composite material [2, 4] this study assumed that it was a homogeneous type material with elastic properties close to its composite component, with the highest elastic properties. (Young’s modulus is 18.5 GPa, and Poisson’s ratio is 0.28.) The density of the material was assumed to be that of a composite helmet material density (1230).
Figure 2: Helmet and head model with the bullet approaching the helmet before the strike.
The Helmet Pads
Helmet pads are usually made of foam materials with low density to absorb shocks and kinematical motion to the head. Pads, therefore, reduce the amount of transformed load to the head and also increase the level of safety. The material behavior of the foams under the compress loading is recognized in three types of regimes: 1) an initial linear elastic behavior, followed by 2) a stress plateau, and finally 3) a rapid increase in the stress versus the strain. In the design of pads out of foams, the behavior at the initial stages, and at its plateau strength, is important. The area under the stress-strain curves indicates the amount of energy that can be absorbed by the foam. The amount of absorbed energy is also dependent on other parameters such as the foam density and strain rate, as well as the environment temperature .
In the work presented here, two different expanded polypropylene (EPP) closed cell foams were selected in order to study the effect the foam properties had on the response of the head to the ballistic impact. EPP foam is a closed cell foam with a reasonable energy absorption capacity that is traditionally used in industries to absorb kinematic energy. The foams were designated foam A and foam B with the respective densities of 44 and 27 kg/m3 and with moduli of 9.2 and 7.2 MPa. The stress-strain curves of these two foams are presented in Figure 3, showing a rather big difference between the amounts of absorbed energy.
Figure 3: The stress-strain behavior of the two types of foams.
In Figure 2, a human head is covered with a helmet that is being struck with a cylindrical bullet weighing approximately 8 grams. Two types of foams (A and B) separate the head from the helmet, as shown in Figure 4. The velocity of the bullet at the time of impact was assumed to be 360 m/s for both types of foam. The effect the impact had on the helmet is shown in Figure 5. The pressure time history in the region, where that bullet struck the helmet (as designated by the element number in Figure 5) is plotted from the time of the strike to several milliseconds after the strike occurred. Sudden pick pressures of about 650 and 40 MPa were created in the helmet for foams A and B, respectively. The pressure suddenly dropped to low values, with time, and was finally reduced to zero, as expected.
Figure 4: The layout of the padding system in the helmet.
Figure 5: The pressure-time history in the region of the bullet strike in the helmet and the distribution at an instant (t=0.04 ms) in the region of the strike.
As was mentioned earlier, the foams absorbed a noticeable part of the energy produced by the bullet strike. The pressures created, due to the strike, were transformed to the head, skull and brain. In Figure 6, the time-history of the pressure transformed to the skull is plotted together with its contour distribution at an instant (t=0.16 ms). The maximum pressure measured from this strike was recorded to be around 18 and 15 MPa for foams A and B, respectively. The size of the pressure decreased considerably as it moved from the helmet to the skull.
Figure 6: The pressure time history transformed to the skull due to the impact of the bullet and the pressure distribution in the skull at an instant (t=0.16 ms).
The created motion and pressure were transferred to the brain consecutively. This motion and external pressure produced various types of stress within the different regions of the brain. In Figure 7, the brain pressure time history, after the strike, is plotted together with its contour distribution that occurred at an instant. The brain pressure time-history plotted was for the area around the element designated 4796 as shown in Figure 7. Due to the constitutive material properties of the brain, which were assumed to be viscoelastic  compared with those of the helmet  and the skull, the level of pressure was, comparatively, much lower. The level of the pressure should, however, be compared to the injury thresholds of the brain material . Other types of stresses could also be monitored within the brain region. In Figure 8, the created time-history of the shear stress in the same region of brain (element 4796 as shown in Figures 7 and 8) is plotted again for both types of foams. The size of the stresses and pressures in the brain, due to this strike, for both types of the foams, can be taken as an indication of the efficacy of the particular foam material under the circumstances of the assumed loading.
Figure 7: The brain pressure time history created due to the motion and the loading transformed from the skull due to the bullet impact and the brain pressure distribution at t=0.16 ms.
Figure 8: The brain shear stress time history created due to the motion and the loading transformed from the skull due to the bullet impact and the brain pressure distribution at t=0.16 ms.
In this study the modeling of a helmeted human head, under the strike of a ballistic bullet, was presented. The strike created pressure and motion in the helmet, skull and brain. The focus was to study the influence of the padding material of the helmet on the transformation of the loadings and motion to the brain. Two types of foams with different stiffness and density were assumed. The pressure time-histories, as well as their maximum values, for the helmet, skull and brain were compared. It was concluded that the foam with lower stiffness and density reduces the pressure and stresses to the brain the greatest amount. From the analysis, it can also be concluded that the level of pressure drops considerably as it moves from the helmet, to the skull, to the brain. This is due to the energy absorption of the padding, as well as to the constitutive material properties. These conclusions hold true for a considerable range of loadings inflicted upon the head. The two foams were different in their stiffness levels, as well as in their levels of density. In a quasi static analysis, the influence of the density cannot be observed. In the dynamic analysis used in the study presented here, however, both the density stiffness of the padding played their roles in absorbing the energy and reducing the size of the transferred loads. In the case of quasi-static loading, it is a fact that increasing the density increases the absorbed energy capacity explicitly. In particular, it can be concluded that foam A performed better because under high strain rates, the amount of energy absorption capacity of foams increases noticeably, and the effect of the strain rate on the lighter foams is more than that on the heavier ones.
The authors would like to acknowledge the Army Research Office (ARO) for the financial support of this work.
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