Abstract
This study aims to explore the effects of helmet structure designs and wearing modes on the protective performance of safety helmets under the impact of falling objects. Four helmet types (no helmet, V-shaped, dome-shaped, and motorcycle helmets) and five wearing modes (left and right tilt by 5 deg, backward tilt by 15 deg, 0 deg without chin strap, 0 deg with chin strap) were included in this study. The axial impact of a concrete block under various impact velocities was simulated. The results indicate that the energy absorption and shock mitigation effects of the foam cushion are superior to those of the suspension system in traditional industrial safety helmets. The structure of the top of V-shaped helmets is designed to withstand greater impact. Regarding the wearing mode, the helmet strap's deflection angle increases stress in the brain tissue and skull, heightens intracranial pressure, and causes pressure diffusion toward the forehead.
1 Introduction
Falling objects frequently cause injuries, posing significant risks to both lives and property. A large number of falling object accidents occur in densely populated high-rise residential areas and construction sites [1]. According to data released by the Research Office of the Supreme People's Court of China, Chinese courts concluded over 1200 civil cases related to falling objects from high altitudes from 2016 to 2018, with nearly 30% resulting in personal injury [2]. At the same time, according to a data survey of 751 cases of injuries caused by falling objects in the previous decade (January 2011–November 2020), Construction debris, including cement bricks and wall attachments, account for approximately 52% of all high-altitude falling objects. Moreover, the majority of injuries caused by falling objects concern the head and neck, comprising about 59% of all injured body parts [3]. High-velocity impacts to the head can generate significant forces, leading to concussions, cervical fractures, dislocations, and other traumatic injuries [4–8].
Industrial safety helmets (ISHs) are the most common protective equipment for the head, which can ease and disperse the instantaneous impact within a short period of time [9,10]. Consequently, numerous scholars have conducted extensive research on the protective capabilities of ISHs. The impact resistance of ISHs is significantly affected by their material and structure. To evaluate the impact resistance of hardhat helmets, climb-style safety helmets, and helmets with novel rotation-damping technologies, Bottlang et al. conducted helmet impact tests with falling objects. Their research demonstrated that compared to traditional helmets, modern helmets are not necessarily better at preventing impact and fall [11]. In another study, Krzysztof selected helmets with different designs, structures, and materials to perform impact tests. They proposed a method for testing helmet deformation and the force acting on it upon impact by a falling object, aiming to estimate the actual energy absorbed by the different helmets without exceeding the threshold of the force acting on the user head [12]. Li et al. performed a drop hammer impact experiment to study different environments and materials. It was found that the protective effect of a helmet made from the same material is limited to an impact energy of 55 J [13], beyond this threshold, the helmet loses its effectiveness. Furthermore, long-term use leads to a significant decrease in the impact strength of aged helmets. The above studies have generally adopted traditional protective helmet performance testing methods (e.g., drop hammer tests), which only consider the effect of a single factor, and cannot assess the internal response of the brain, e.g., stresses and strains.
The finite element (FE) method can effectively solve the problem that the information of brain injury cannot be obtained experimentally. Therefore, several scholars have studied the protective performance of the helmet by developing FE models of the helmet and the head. For example, Salimi Jazi et al. designed four distinct bullet-proof helmets utilizing various padding materials and developed a helmet-human brain FE model. Their model was utilized to investigate how different helmet padding materials affect the biomechanical response of the human brain under ballistic impact [14]. To further investigate head injuries in contact sports such as football, William and his team built a FE model of the Schutt Air XP Profootball helmet [15]. The helmet model was mounted on a Hybrid III dummy head, which was validated through a series of representative impact experiments. On this basis, Chang et al. built a FE model based on the actual geometric characteristics of a motorcycle helmet to simulate the dynamic response under different collision speeds. The protective performance of the helmet was evaluated based on the peak acceleration of the head model and the standard value of head injury [16]. Currently, the research on the difference in the protective performance of helmets with different structural designs is not sufficient. For instance, in traditional ISHs, a suspension structure is generally used for energy and shock absorption, while in motorcycle helmets, a foam liner structure is used to resist impact. The FE method can study head and neck injury from the perspective of biomechanical response inside the human body, helping us better understand the changes of stress, strain, and acceleration. The changes of these biomechanical parameters also affect the degree of brain tissue damage, therefore, the FE method can help us to consider the impact of different factors on the helmet comprehensively.
At the same time, studies have revealed that the way a helmet is worn may have a certain impact on its protective effect [17]. Common incorrect ways of wearing a helmet include not wearing it in the correct position and not fastening the chin strap, which may cause the helmet to slip or move under impact. However, it has been reported that, due to problems such as wearing discomfort and weak protection awareness of workers, helmets are not worn according to the regulations in dangerous environments such as construction sites and factories [18]. There is no clear investigation on the risks and hazards that may be caused by wearing the helmet in the wrong way, and there is mainly speculation about its hazards based on accident investigation reports.
To this end, this study investigates the effects of different helmet structure designs and wearing methods on the protective performance. FE models of different helmet types and helmet wearing modes are developed utilizing the THUMS human model [19] and the protective effect is studied by analyzing the biomechanical response of the head. The outcomes of this work can increase people's understanding of head impact injury and provide new design ideas and a theoretical basis for protective equipment engineers. At the same time, it is expected that this study will provide better protection and awareness of proper way of helmet wearing for workers in the construction industry, mining industry, and related industries.
2 Material and Methods
2.1 THUMS Human Finite Element Model.
Toyota Oshita et al. developed the first-generation THUMS human FE model in 2002, which has now evolved to the sixth generation, THUMS 6.1 [20]. This model can be controlled using ls-dyna, a FE analysis software. THUMS has been extensively validated through crash tests and compares well with real humans, providing high biological fidelity. In this study, the base model is an adult male with a height of 175 cm and a mass of 77 kg. The sixth-generation THUMS includes shoulders, chest, lumbar spine, internal organs, and muscles [21,22].
The THUMS craniocerebral model consists of the skull, brain, and skin. The skull model is made of spongy bone and cortical bone, meshed with solid and shell elements, respectively, while an isotropic elastoplastic material (LS-DYNA MAT_24) has been used to describe the material properties. The brain model consists of hexagonal solid elements representing the brain, cerebellum, brainstem with distinct white and gray matter, and cerebrospinal fluid. The brain is modeled as a viscoelastic material using a Maxwell-type equation (LS-DYNA MAT61), while the cerebrospinal fluid is modeled as an elastic material with fluid-like behavior (LS-DYNA MAT_1). The skull and brain are connected through shared nodes. The material properties and element type of the different parts of the head and neck are listed in Tables 1 and 2 [23,24].
Material type | ρ (kg/m3) | K (MPa) | GO | GI | E (MPa) | μ | σy (MPa) | |
---|---|---|---|---|---|---|---|---|
Skull | Isotropic elastoplasticity | 2120 | — | — | — | 1490 | 0.22 | 95.87 |
Cerebrospinal | Elastic material | 1000 | 2000 | 0.0005 | 0.0001 | — | — | — |
Brain | Viscoelastic material | 1000 | 2160 | 0.006 | 0.0012 | — | — | — |
Skin | Elastic material | 1050 | — | — | — | 22 | — | — |
Cortical bone | Elastoviscoplasticity | 2000 | — | — | — | 1302 | 0.3 | 80 |
Cancellous bone | Elastoviscoplasticity | 1210 | — | — | — | 40 | 0.45 | 1.8 |
Nucleus pulposus | Isotropic plastic model | 1000 | 21.7 | 0.013 | 0.013 | — | — | 0.013 |
Annulus fibrosus | Foam material | 1000 | — | — | — | 2.09 | — | — |
Material type | ρ (kg/m3) | K (MPa) | GO | GI | E (MPa) | μ | σy (MPa) | |
---|---|---|---|---|---|---|---|---|
Skull | Isotropic elastoplasticity | 2120 | — | — | — | 1490 | 0.22 | 95.87 |
Cerebrospinal | Elastic material | 1000 | 2000 | 0.0005 | 0.0001 | — | — | — |
Brain | Viscoelastic material | 1000 | 2160 | 0.006 | 0.0012 | — | — | — |
Skin | Elastic material | 1050 | — | — | — | 22 | — | — |
Cortical bone | Elastoviscoplasticity | 2000 | — | — | — | 1302 | 0.3 | 80 |
Cancellous bone | Elastoviscoplasticity | 1210 | — | — | — | 40 | 0.45 | 1.8 |
Nucleus pulposus | Isotropic plastic model | 1000 | 21.7 | 0.013 | 0.013 | — | — | 0.013 |
Annulus fibrosus | Foam material | 1000 | — | — | — | 2.09 | — | — |
Note: ρ is the density; σy is the yield stress; E is the Young's modulus; μ is the Poisson's ratio; GO is the short-term shear modulus; GI is the long-term shear modulus; and K is the elastic bulk modulus.
Element type | Material keyword | |
---|---|---|
Skull | Solid and shell element | MAT_PIECEWISE_LINEAR_PLASTICIY |
Cerebrospinal | Solid element | MAT_FLUID_ELASTIC_FLUID |
Brain | Solid element | MAT_KELVIN_MAXWELL_VISCOELASTIC |
Skin | Shell element | MAT_ELASTIC |
Cortical bone | Solid element | MAT_GENERAL_VISCOELASTIC |
Cancellous bone | Solid and shell element | MAT_GENERAL_VISCOELASTIC |
Nucleus pulposus | Shell element | MAT_GENERAL_VISCOELASTIC_MOISTURE |
Annulus fibrosus | Beam element | MAT_CABLE_DISCRETE_Beam |
Ligaments | Shell element | MAT_SIMPLIFIED_RUBBER |
Element type | Material keyword | |
---|---|---|
Skull | Solid and shell element | MAT_PIECEWISE_LINEAR_PLASTICIY |
Cerebrospinal | Solid element | MAT_FLUID_ELASTIC_FLUID |
Brain | Solid element | MAT_KELVIN_MAXWELL_VISCOELASTIC |
Skin | Shell element | MAT_ELASTIC |
Cortical bone | Solid element | MAT_GENERAL_VISCOELASTIC |
Cancellous bone | Solid and shell element | MAT_GENERAL_VISCOELASTIC |
Nucleus pulposus | Shell element | MAT_GENERAL_VISCOELASTIC_MOISTURE |
Annulus fibrosus | Beam element | MAT_CABLE_DISCRETE_Beam |
Ligaments | Shell element | MAT_SIMPLIFIED_RUBBER |
The THUMS neck model includes the first to seventh cervical vertebrae. Each cervical vertebra model is constructed with solid elements for the spongy and cortical bone as well as shell elements. Each cervical spine bone is connected to a disk including the annulus fibrosus and nucleus pulposus, and its properties are described by a nonlinear viscoelastic material (LS-DYNA MAT76). Among them, the fibers of the fiber ring are modeled using bundle elements modeled with an elastic material (LS-DYNA MAT71) with only nonlinear tension. In addition, ligaments such as the anterior longitudinal ligament, posterior longitudinal ligament, Flava ligament, and Nuchae ligament are modeled with a rubberlike material (LS-DYNA MAT181) [23,24].
2.2 Head-Helmet Finite Element Model Development
2.2.1 Different Helmet Types.
This study utilized models of two helmet types: traditional safety helmets and motorcycle helmets. Traditional helmets include V-shaped helmets with a raised top and dome-shaped helmets, both of which are commonly available in the market, as shown in Fig. 1. They consist of a cap shell, cushion, cap liner, and lower chin strap. Their geometry was designed using solidworks, where basic shapes were sketched and features were added [25,26].
The solidworks models were imported into HyperMesh for meshing and quality checks, as shown in Table 3 [27]. Solid elements were used for the cap shell and cushion, while shell elements with a thickness of 1 mm were used for the cap liner and chin strap. Unlike traditional helmets, motorcycle helmets have a foam buffer layer to absorb energy. Acrylonitrile butadiene styrene plastic and expanded polystyrene (EPS) foam were employed to simulate the cap shell and foam buffer layer, respectively, and their stress–strain curves were defined in ls-dyna, as shown in Fig. 2 [28,29]. According to “Head Protective Helmet” of China (GB 2811-2019), the geometric and material parameters of the different helmets are listed in Tables 4 and 5, respectively. The helmet elements type parameters are shown in Table 6.
Length-to-width ratio | <5 | Inner angle of a triangular element | 30–120 deg |
---|---|---|---|
Buckling angle | 15 deg | Inner angles of a quadrilateral element | 45–135 deg |
Length-to-width ratio | <5 | Inner angle of a triangular element | 30–120 deg |
---|---|---|---|
Buckling angle | 15 deg | Inner angles of a quadrilateral element | 45–135 deg |
Material type | Density (kg/m3) | Modulus of elasticity (MPa) | Poisson's ratio | σy (MPa) | |
---|---|---|---|---|---|
Helmet shell | Piecewise linear plastic | 1210 | 2000 | 0.37 | 68 |
Helmet liner | Plastic kinematic | 1270 | 3580 | 0.3 | 110 |
Lower chin strap | Plastic kinematic | 1200 | 3000 | 0.3 | 110 |
EPS foam | Foam material | 60 | 7.5 | 0.1 | 0.3 |
Material type | Density (kg/m3) | Modulus of elasticity (MPa) | Poisson's ratio | σy (MPa) | |
---|---|---|---|---|---|
Helmet shell | Piecewise linear plastic | 1210 | 2000 | 0.37 | 68 |
Helmet liner | Plastic kinematic | 1270 | 3580 | 0.3 | 110 |
Lower chin strap | Plastic kinematic | 1200 | 3000 | 0.3 | 110 |
EPS foam | Foam material | 60 | 7.5 | 0.1 | 0.3 |
Element type | Material keyword | |
---|---|---|
Helmet shell | Solid element | MAT_PIECEWISE_LINEAR_PLASTICIY |
Helmet liner | Shell element | MAT_PLASTIC_KINEMATIC |
Lower chin strap | Shell element | MAT_PLASTIC_KINEMATIC |
EPS foam | Solid element | MAT_CRUSHABLE_FOAM |
Element type | Material keyword | |
---|---|---|
Helmet shell | Solid element | MAT_PIECEWISE_LINEAR_PLASTICIY |
Helmet liner | Shell element | MAT_PLASTIC_KINEMATIC |
Lower chin strap | Shell element | MAT_PLASTIC_KINEMATIC |
EPS foam | Solid element | MAT_CRUSHABLE_FOAM |
The FE simulation software ls-dyna was used to assemble the THUMS model with the different helmet types. According to “Safety helmet test Method” of China (GB/T 2812-2006), the vertical distance between the helmet shell and the helmet liner should be between 25 mm and 50 mm. As depicted in Fig. 3, the helmet liner is restricted and fitted with the THUMS cranial model to represent the wearing state of the helmet under standard working conditions. According to the experience of construction workers, the spacing between head and helmet is at least 320 mm [30]. Friction settings refer to the study of Ievgen Levadnyi et al. “AUTOMATIC_SURFACE_TO_SURFACE” is adopted between the head and the helmet lining, and the friction coefficient is set to 0.2. The contact between the falling object and the contact object is also surface-to-surface contact, and the friction coefficient is set to 0.4. The helmet shell and the helmet lining are connected with a common node. Other parameters are set by default. Typical function stiffness = 0 contact stiffness is set to 0 by default, slave surface thinkness = 0 and master surface thinkness = 0 optional master and slave plane thickness are set to 0 by default. The contact magnification diagram is shown in Fig. 4.
2.2.2 Helmet Wearing Variations.
Dome-shaped safety helmet, as a common safety protection equipment, has exhibited a high degree of practicality and cost-effectiveness in design and manufacture. Therefore, to study the effect of different helmet wearing ways on head injury, the dome-shaped helmet was selected as the research object. This work mainly focused on studying the effects of different helmet wearing angles and the presence or absence of a chin strap on the protective performance. To this end, three different helmet wearing angles were first designed, aiming to simulate the way workers might wear the helmet in a real working environment. As depicted in Fig. 5, the helmet was tilted 15 deg backward and 5 deg left and right. At the same time, the effect of the presence or absence of a chin strap on the protective performance of the helmet was also taken into consideration. The chin strap is an important part of the helmet, its role is to ensure that the helmet is tightly fitted to the head, preventing it from falling off or shifting under impact.
Two helmet models were designed, one with and one without a lower chin strap. The chin strap exerts a preload on the helmet when it is fastened, ensuring that the helmet is tightly fitted to the head. As regards the preload of the chin strap, the-higher-the-better principle is not applicable, since a too high a preload may lead to limited movement of the wearer or even cause injury. In Fig. 6, the helmet-chin strap restraint system is illustrated. According to the national standard GB 2811-2019 “Head Protection Helmet,” the width of the chin strap of ISHs shall not be less than 15 mm. In this study, the width of the chin strap was 25 mm, and the chin strap was meshed with two element types: one part was meshed with shell elements with a thickness of 1 mm to simulate the connection function (Table 5); the other part was modeled with beam elements, which can simulate the preload of the lower chin strap by defining the spring material model *MAT_CABLE_DISCRETE_BEAM in ls-dyna. The preload of the lower chin strap of the helmet is usually 30–40 N. In this study, the magnitude of the preload force was set as 30 N. The contact between the lower chin strap and the head was set to “AUTOMATIC_SURFACE_TO_SURFACE” and the dynamic friction coefficient was set to 0.2. The tightening process of the lower chin strap is exhibited in Fig. 7.
2.3 Boundary and Loading Conditions.
When objects are falling from high altitude, only the height of the fall affects the impact speed. According to China's Housing Design Code (GB50096-1999), residential floors can be divided into: low-rise (1–3 floors), multistorey (4–6 floors), midrise (7–9 floors), and high-rise (10 floors and above). In this study, concrete blocks falling from 2 to 10 floors with impact velocities of 8–25 m/s were simulated. The falling objects was concrete blocks weighing 2 kg, which are common in construction sites. James et al. used 2 kg steel blocks and 5 kg boards. On the other hand, for authenticity, the mass of a brick is about 1.5–3 kg; thus, choosing a 2 kg concrete block as the falling object is more in line with reality. Material No. 84 *MAT_WINFRITH_CONCR ETE in ls-dyna was used to define its material properties; its main parameters are listed in Table 7 [31]. As depicted in Fig. 8. During the entire simulation process, the six degree-of-freedoms of atlanto-occipital joint were all constrained. And a concrete block with a mass of 2 kg impacted the dummy head from the top. The duration of simulation was set to 50 ms and the time-step was set to 0.1 ms. For the simulation of the different helmet wearing methods, the 2 kg concrete blocks were also used as the falling objects, and their impact speed was set at 8–18 m/s. The remaining boundary conditions were kept the same.
Material number | Density (kg/m3) | Modulus of elasticity (MPa) | Poisson's ratio | Compressive strength | Tensile strength | Crack control parameter |
---|---|---|---|---|---|---|
84 | 2320 | 3000 | 0.19 | 40 | 4 | 0.07043 |
Material number | Density (kg/m3) | Modulus of elasticity (MPa) | Poisson's ratio | Compressive strength | Tensile strength | Crack control parameter |
---|---|---|---|---|---|---|
84 | 2320 | 3000 | 0.19 | 40 | 4 | 0.07043 |
2.4 Head Injury Criterion Injury Criteria.
Formula (2.1) is the linear acceleration of the head centroid (m/s2), which represents the time interval when HIC reaches the maximum value. The time interval can be 15 ms and 36 ms, representing HIC15 and HIC36, respectively. In this study, HIC15 was selected as one of the judgment bases for measuring head injury under the impact load of falling objects. Literature has shown that most countries in the world regard HIC15 injury value equal to 700 as the threshold of head injury for adults and children over 6 years old [34].
3 Results
3.1 Head Injury Criterion Value.
Figure 9 presents the HIC values for various helmet types. It can be observed that the HIC value increases gradually with increasing speed, and the HIC value of the three protective helmets is significantly lower than that without a helmet. Without wearing a helmet, the HIC value is 703 when the impact speed of the concrete block reaches 18 m/s. When the impact speed is less than 25 m/s, the HIC value of the three protective helmets is controlled within 800, and the dome-shaped helmet has the highest HIC value of 760. When the speed is below 18 m/s, the HIC values for V-shaped and dome-shaped helmets are nearly identical, but with the increase of speed, the HIC value of the dome-shaped helmet increases abruptly, while the HIC damage trend of the V-shaped helmet is much slower. When the speed is not higher than 20 m/s, the HIC value of the motorcycle helmet is slightly lower than that of the other two safety helmets.
Figure 10 demonstrates the maximum HIC value under different helmet wearing modes. In general, the HIC value increases with increasing speed. At speeds over 12 m/s, the HIC growth is the lowest when the chin strap is securely fastened. The HIC value for the other four incorrect helmet-wearing modes is significantly higher compared to the securely fastened one. The maximum HIC value was reached when the helmet was tilted backward by 15 deg. At 18 m/s, the HIC value increases by over 20% compared to the securely fastened chin strap. When the helmet is tilted to the right, the HIC value increases more slowly. When the helmet is tilted by 5 deg to the right, there is little difference in the HIC value, which is about 300 when the speed is 18 m/s. During impact, the protective performance of the helmet is poorest at a 15 deg backward tilt, but similar for the 5 deg left/right tilted cases and when not using the chin strap. The helmet offers optimal protection when worn at the correct angle with a securely fastened chin strap. Although improper helmet wearing reduces it protective effect, the damage value remains below 700, highlighting its importance.
3.2 Von Mises Stress in Brain Tissue.
Figure 11 presents the maximum stress values of the brain under different conditions. The von Mises stress in brain tissue is positively correlated with the impact speed, remaining below 6 kPa (maximum 5.93 kPa) for speeds up to 16 m/s, regardless of helmet use. If the speed exceeds 18 m/s, the stress without helmet increases significantly, reaching 13.5 kPa at 25 m/s. When the impact speed is lower than 25 m/s, the brain tissue stress with the dome-shaped helmet is notably higher than that with the other helmets, peaking at 9.47 kPa and remaining below 10.2 kPa. When comparing the effect of helmets on reducing the brain tissue stress, the motorcycle helmet significantly outperforms the others, with its von Mises stress value being nearly half that of the traditional industrial helmets. At speeds below 18 m/s, the stress value of the three helmet types increases smoothly; however, once the speed exceeds 18 m/s, the stress value increases abruptly.
Figure 12 shows the maximum von Mises stress values in the brain under different helmet wearing modes, exhibiting an upward trend with increasing speed. The von Mises stress in the brain with chin strap is significantly lower, while the other helmet wearing modes exhibited an interweaving trend in the stress curves. Under a normal helmet wearing angle, the brain stress increases smoothly. However, increasing the helmet tilt leads to a steep increase in the maximum von Mises stress. This is evident in the stress curve under a 15 deg backward tilt and 18 m/s speed, where brain von Mises stress surged. The von Mises stress in the brain reached 7.56 kPa, exceeding the 50% threshold for moderate head injury. When the helmet was tilted by 5 deg to the left/right and without the chin strap, the brain stress ranged from 6 to 6.5 kPa, also exceeding the 50% threshold, but being lower than that under a 15 deg backward tilt.
3.3 Skull Strain Energy.
By examining the maximum strain energy of the skull (Fig. 13), it is possible to assess the risk for skull fracture. Deck and Willinger [35] stated that a 50% chance of skull fracture occurs when the skull strain energy reaches 865 mJ. At 18 m/s, the cranial strain energy without helmet is 809 mJ. The strain energy with the dome-shaped helmet is 384 mJ, half of the unprotected level and increasing with speed. All helmets significantly reduced the skull strain energy, but their values were similar. The motorcycle helmet with foam cushion achieved the lowest strain energy (499 mJ), followed by the V-shaped (631 mJ) and dome-shaped (685 mJ) helmets.
Figure 14 presents the maximum cranial strain energy values under different helmet wearing modes. At speeds below 12 m/s, the skull strain energy for the different helmets was nearly identical, exhibiting similar upward trends, which is consistent with the predicted HIC and brain stress values. With increasing speed, the cranial strain energy exhibited varying trends. Wearing the lower chin strap, the strain energy increased steadily and its value was the lowest. At 18 m/s, the cranial strain energy peaked at 300 mJ. The skull strain energy of the four other helmet wearing modes increased with increasing impact speed. The steepest increase was observed when the helmet was tilted backward by 15 deg, reaching a maximum value of 463 mJ, over 50% higher than with normal wearing. When the helmet was tilted by 5 deg to the left/right, the skull strain energy was nearly identical, peaking at about 400 mJ, which was slightly higher than without the lower chin strap (maximum 383 mJ).
3.4 Maximum Intracranial Pressure.
Figure 15 shows the distribution of the maximum intracranial pressure over a cross section of the brain tissue under different helmet wearing modes. The intracranial pressure decreased gradually from the frontal lobe to the brain stem, with the impact location experiencing the highest pressure. Compared to the cerebellar and brain-stem pressure, the opposite side of the impact point also exhibited an increasing trend of the stress. The most severe brain contusion occurred in the frontal lobe, with the cerebellar part opposite to the impact point being also at high risk. Comparing the maximum intracranial pressure for different helmet wearing modes, the order at the impact location was: 15 deg backward > 5 deg left > 5 deg right > without chin strap > with chin strap. When the helmet was tilted backward by 15 deg, the intracranial pressure at the impact location peaked at 2.7 MPa, exhibiting a wider pressure distribution. When the chin strap was tightly tied, the minimum intracranial pressure at the impact point was 1.8 MPa, and the maximum pressure distribution was mainly located at the top of the brain tissue, similar to the distribution without the chin strap. With the lower chin strap, the maximum intracranial pressure was reduced by nearly 0.5 MPa compared to that without it, and by nearly 1 MPa compared to that under the other helmet wearing modes. Comparing the intracranial pressure under different helmet wearing angles, normal helmet wear resulted in the pressure being mainly concentrated over a small range near the impact point. However, under the other three angles, the maximum intracranial pressure was more diffused, e.g., on the forehead, and increased by varying degrees.
3.5 Helmet Maximum Offset.
Figure 16 shows the maximum offset of the helmet under different wearing modes. Compared with the condition without the jaw strap, the maximum deflection of the helmet was significantly reduced to 4.126 mm with the jaw strap tightened, a decrease of about 68.4%, indicating that the jaw strap fixed the helmet position and enhanced the helmet stability. When the helmet is tilted back by 15 deg, the maximum offset increases significantly to 18.663 mm, and the tilt of the helmet, especially the backward tilt, will significantly increase the offset, which may reduce the protective effect of the helmet.
4 Discussion
Safety helmets, essential protective gear for workers, have garnered significant attention and research efforts. Suderman et al. found that hard helmets reduce head acceleration and injury risk under vertical impact from large objects, echoing this study [36]. Long et al. performed numerical simulations to study helmet damage thresholds. Notably, when the impact speed exceeds 20 m/s, the HIC value for dome-shaped helmets increases rapidly [30], similar to the findings by James, possibly due to failure of the shell structure.
Shreiber et al. concluded that there is a risk of brain contusion when the von Mises stress in the brain tissue is between 6 and 11 kPa, and the risk of brain contusion is relatively low when the impact speed is lower than 16 m/s, regardless of helmet protection or not [37]. Yao et al. concluded through simulations and calculations that, when the von Mises stress in the brain tissue reaches 14.8±4.5 kPa, there is a 50% probability of moderate and severe brain injury [38]. According to results of this study, the maximum von Mises stress in the brain tissue when the helmet is worn is 9.4 kPa, which is still below 10.3 kPa (the minimum for a 50% probability of moderate brain injury). When the speed is 25 m/s, the dome-shaped helmet has a maximum HIC value of 760, indicating a high risk for moderate head injury based on the Prasad–Mertz curve [39]. Helmets are crucial in reducing fatalities and disabilities. The skull strain energy analysis predicts the risk for skull fracture. Deck and Willinger found a 50% skull fracture risk at a strain energy of 865 mJ. In this study, the use of helmet reduced the skull strain energy below 865 mJ, which is consistent with other injury predictions [35]. At the same time, the HIC values of the three helmets were analyzed and compared to those of two other traditional ISHs (V-shaped and dome-shaped helmets). In most cases, the damage values of the three motorcycle helmets were lower; especially, as regards the von Mises stress in the brain tissue, the reduction was nearly double. Therefore, the energy and shock absorption effects of using a foam cushion were due to the suspension system. The damage values of the two traditional ISHs were very similar, with the V-shaped helmet damage values being slightly lower, especially for speeds higher than 18 m/s, which confirms the effectiveness of the convex structural design of the helmet top.
Moreover, this study revealed that the helmet wearing mode significantly affects the brain response under impact. Proper helmet wear (tightening the chin strap) reduces head injury, brain stress, and skull strain energy, enhancing helmet stability and protection. While improper helmet wear, such as tilting or leaving it unsecured, increases these factors and reduces the helmet's effectiveness. In addition, the intracranial pressure distribution from the prefrontal lobe to the brain stem, as well as at the opposite side of the impact point, exhibited a gradually increasing trend. The wearing mode of the helmet had significant effect on the intracranial pressure response. The maximum intracranial pressure at the impact location reached 2.7 MPa when the helmet was tilted 15 deg backward, and its distribution range was wide. The minimum intracranial pressure was 1.8 MPa when the chin strap was fastened, and the distribution range was small. Ward and Thompson found that when the intracranial pressure reached 173 kPa, it may lead to minor brain tissue damage; when the intracranial pressure exceeds 235 kPa, it may pose a fatal risk to humans [40]. Further research by Newman and colleagues showed that when intracranial pressure exceeds 300 kPa, it leads to traumatic brain injury [41]. Under normal helmet wear, the intracranial pressure is concentrated at the point of impact, while when wearing it incorrectly, the pressure spreads to the forehead and increases in magnitude. In summary, tightening the chin strap plays an important role in enhancing the stability of the helmet and reducing the intracranial pressure. The correct helmet wearing angle also plays a key role in inhibiting the distribution and diffusion of intracranial pressure and reducing the pressure magnitude. Consequently, when wearing a safety helmet, one should try to maintain the correct wearing angle and fasten the chin strap to enhance its stability and protective effect.
It should be mentioned that this study comes with certain limitations. First, the helmets used in this study was the simplest safety helmets on the market, their structure was also simplified, maintaining only the most basic geometric characteristics and the final damage result value. Therefore, there is still plenty room for improvement in the selection of safety helmets. At the same time, there are several other possibilities regarding the choice of falling objects, such as other objects commonly falling at construction sites, e.g., steel products or other large debris. Therefore, future work will further improve the structure of the helmets and include different falling objects, in order to investigate their effect on the human head. The head and neck are a complex biomechanical system, the helmet design should consider the head protection and the overall stability of the head and neck in motion. However, existing models only focus on a single protective indicator of the head and ignore the neck-head interaction. Future studies should consider the overall protection needs of the head and neck. To sum up, this study developed different types of craniocerebral helmet FE models to simulate real injuries caused by falling objects, studied the effect of different helmet types on head injury severity, and compared the effect of different wearing modes. This study advocates for enhancing the safety helmet's design in several key aspects. First, the incorporation of advanced energy-absorbing materials as the helmet's lining, akin to the foam padding found in motorcycle helmets, is proposed to amplify shock absorption capabilities and mitigate direct head injuries during impacts. Second, the study suggests optimizing the helmet's top design, drawing inspiration from the low-damage performance of V-shaped helmets under high-speed collisions. Specifically, it recommends incorporating or refining a convex structure atop the helmet, as this feature has demonstrated remarkable protection against high-speed impacts. Additionally, to further bolster the helmet's stability and protective efficacy, relevant regulations should mandate that wearers maintain the helmet at a horizontal angle and ensure the chin strap is securely tightened. This work can provide improvement ideas for relevant individual protection personnel, help people to gain a deeper understanding of the head injury threshold and injury mechanisms in accidents related to falling objects, and offer new ways and ideas for injury treatment and assessment.
5 Conclusion
Safety helmets effectively reduce head acceleration and injury risk from falling objects, which is crucial for lowering fatality and disability rates. Comparing the suspension type of traditional ISHs and the foam cushion of motorcycle helmets, it has been found that the energy and shock absorption effect of the foam cushion are significantly better than that of the suspension system. In addition, the design of the top of V-shaped helmets can make them withstand greater impact loads. Different wearing angles have significant effects on the impact response of brain tissue. Fastening the chin strap plays an important role in enhancing the stability of the helmet and reducing the intracranial pressure magnitude. Proper wearing angles play also a key role in inhibiting the distribution and diffusion of the intracranial pressure and reducing its magnitude.
Acknowledgment
This work has been supported by the National Natural Science Foundation of Tianjin and Tianjin Graduate Research and Innovation Project (2022SKYZ115).
Funding Data
Natural Science Foundation of Tianjin City (No. 21JCYBJC01210; Funder ID: 10.13039/501100006606).
Data Availability Statement
The authors attest that all data for this study are included in the paper.
Impact Statement
This study significantly impacts the safety helmet industry by revealing the superior protective performance of foam cushioning over traditional suspension systems. The discovery of V-shaped helmets' enhanced impact resistance encourages manufacturers to adopt innovative designs that offer improved protection. Moreover, the insights into the effects of wearing modes underscore the importance of proper fitting and wearing practices, potentially leading to industry-wide changes in training and standards. These findings have the potential to reduce workplace head injuries, enhancing the safety of workers and fostering a culture of safety in the industry.