Helmet Liner Research Articles

Response of a helmet liner under biaxial loading

Helmets are the most effective protective item for motorcyclists. The liner of the helmet is the part of the helmet which dissipates most of the impact energy and mitigates the risk of head injuries. It has been proposed that the helmet test standards should include assessment of the helmets for oblique impacts that is not currently addressed in the standards. A conventional uniaxial compression test method is still used for characterization of the helmet liner material. However, compressive tests of EPS foams provide reliable results for normal loading on EPS, but do not provide a realistic result for oblique impacts. Therefore, we carried out experimental tests to measure the response of EPS foams, which are commonly used for helmet liners, under biaxial loading. The result of our experiments show that the shear response of EPS foams is a function of axial compression, and increasing the axial strain leads to increased shear stiffness, and thus higher levels of shear stress. We also showed that including shear-stiffening of EPS in the FE assessment of helmets may change the headform rotational acceleration by 25%. Therefore, such behavior of EPS foams should be included in FE analysis of helmets in the case of oblique impacts for a more realistic assessment of their performance.

Cellular Helmet Liner Design through Bio-inspired Structures and Topology Optimization of Compliant Mechanism Lattices

Cellular Helmet Liner Design through Bio-inspired Structures and Topology Optimization of Compliant Mechanism Lattices

Feasibility study on the use of a hierarchical lattice architecture for helmet liners

Helmets are the most important piece of protective equipment for motorcyclists. The liner of the helmet is the main part of the helmet which dissipates the impact energy and mitigates the load transmitted to the head. Therefore, optimizing the material that absorbs most of the impact energy would improve the helmet’s protection capacity. It is known that the energy absorption of the helmet liner can be optimized by means of using liners with varying properties through the thickness, however currently the majority of used liners exhibit constant properties through the thickness. Advances in the field of topology optimization and additive manufacturing provide the ability of building complex geometries and tailoring mechanical properties. Along those lines, in the present work the feasibility of using a hierarchical lattice liner for helmets was studied. Finite element method was employed to study whether a hierarchical lattice liner could reduce the risk of head injuries in comparison to currently used liner materials. The results show that using a hierarchical lattice liner has the potential of significantly reducing the risk of head injury compared to a helmet with traditional EPS liner and could potentially be considered as the new generation of energy absorbing liners for helmets.

A design framework for the mass customisation of custom-fit bicycle helmet models

Mass customisation (MC) can provide significant benefits to the customers. For example, custom-fit design approaches can improve the users’ perceived comfort of products where the fit is an important feature. MC can also bring major value to the producers, where for instance, premium prices can be implemented to the products. Research show that MC can bring competitive advantages especially when the system is new. It is therefore surprising that MC of helmets has not been studied more extensively, especially given the advances in 3D scanning, computational analyses, parametric design, and additive manufacturing techniques. The purpose of this study was to present a novel MC framework for the design of custom-fit bicycle helmet models.In the proposed design framework, we first categorized a subset of the Australian population into four groups of individuals based on their similar head shapes. New customers were then classified inside one of these groups. The customisation took place inside these groups to ensure that only small variations of the helmet liner were implemented. During the design process, the inside surfaces of a generic helmet model was modified to match the customer's head shape. We demonstrated that all the customized models created complied with the relevant drop impact test standard if their liner thickness was between the worst and best case helmets of each group. Fit accuracy was verified using an objective evaluation method. Future work should include detailed description of the manufacturing methods engaged in our MC framework.

Evaluation of novel temperature-stable viscoelastic polyurea foams as helmet liner materials

Viscoelastic polyurea foams with densities of 98, 170, and 230kg/m3 were manufactured and integrated into helmet shells and tested using the NOCSAE and FMVSS 218 standards for football and motorcycle helmets, respectively. For football helmet testing, a Riddell Revolution helmet shell was used. The helmets with a foam liner thickness of 28mm were dropped using a NOCSAE medium headform (4.9kg) from heights ranging from 0.305 to 1.524m. All impact tests were done on the crown of the helmet and dropped on a 12.7mm modular elastomer programmer pad. Use of PU98 foam resulted in a reduction of 22% in peak g's and 25% in Severity Index and Head Injury Criteria values when compared to helmets with the original VN600 foams under ambient conditions. These tests were repeated under varying temperatures along with a reduced liner thickness of 22.2mm. In these tests, PU98 foam reduced the peak g's by 18% at 23°C, 26% at −15°C, and 20% at 50°C when compared to VN600 helmets. The FMVSS motorcycle helmet tests demonstrated the success of using an additional layer of polyurea foam liner on top of the existing expanded polystyrene liner in reducing the peak g's by 17%.

Anisotropic polyethersulfone foam for bicycle helmet liners to reduce rotational acceleration during oblique impact.

Although current standard bicycle helmets protect cyclists against linear acceleration, they still lack sufficient protection against rotational acceleration during oblique impact events. Rotational acceleration is correlated with serious traumatic brain injuries such as acute subdural haematoma and thus should be minimized. This study proposes using highly anisotropic polyethersulfone foam for bicycle helmet liners in order to limit the rotational acceleration. Helmet prototypes, made of polyethersulfone foam with cell anisotropy direction perpendicular to the head, have been produced and compared to a standard commercial helmet. Standard helmets consist of expanded polystyrene foam. Oblique impact tests were performed to measure both linear and rotational accelerations and impact pulse duration. Results demonstrate that the peak rotational acceleration of the polyethersulfone prototype helmet showed a decrease of around 40% compared to the reference expanded polystyrene helmet. Moreover, the peak linear acceleration showed an average decrease of about 37%. Upon impact, the polyethersulfone helmet showed improved head injury protection when analysed based on global biomechanical head injury criteria such as HIC15 and HICrot as well as generalized acceleration model for brain injury threshold, brain injury criterion and head impact power, with a predicted sixfold decrease in likelihood of concussion.

Novel Composite Foam Concept for Head Protection in Oblique Impacts

Rotational acceleration experienced by the head during oblique impacts is known to cause traumatic brain injuries. It is hypothesized that shear properties of a foam layer, used for head protection (e.g., protective helmet liners, headliners in cars) can be related to the extent of rotational acceleration transmitted to the head. Furthermore, it is hypothesized that by introducing anisotropy in a foam layer, rotational acceleration can be mitigated. In this study, an anisotropic composite foam concept is proposed to mitigate head rotational acceleration, hence reducing the risk of traumatic brain injuries. The composite foam concept introduces anisotropy in a foam at the “macro level”, combining different densities of foam in layered and quasi‐fiber/matrix configurations. The performance of expanded polystyrene (EPS) composite foams in quasi‐static compression and combined shear‐compression loading and also linear and oblique impact experiments, has been compared with the performance of single layer EPS foam of similar thickness and density. The results of oblique head impact have been analyzed by global head injury criteria such as HIC, HICrot, and HIP. The composite foam concept demonstrates a great potential to be utilized in applications such as protective helmets due to the significant mitigation of brain injury risk.

Head protection with cyclist helmet in impact against vehicle A-pillar

ABSTRACTIn vehicle–cyclist collisions, the head of cyclists impacts frequently against the windshield and the A-pillar of the vehicle. Accident analyses show that the head injury risk is high in impacts with stiff A-pillars. In this research, the head protection with cyclist helmets in impacts against the A-pillar was investigated from experiments and finite element (FE) analyses. The headform impact tests were conducted using the pedestrian headform with and without a cyclist helmet. In the A-pillar impact at 35 km/h, the HIC reduced substantially from without helmet (4815) to with helmet (3000) though it was far above the injury acceptance level (1000). The head protection by helmets in impacts against A-pillars might be limited because of high impact velocities of the head and A-pillar stiffness. The FE simulations of the headform impacts indicated that the helmet liner deformed locally and bottomed-out in the A-pillar impacts, whereas the A-pillar deformation was small. In the FE analysis of a human head model with a capping helmet, the skull fracture did not occur but the brain strain was large. From a simple model of the head acceleration in the A-pillar impact, it was shown that the helmet liner has little effect on the HIC, whereas a deforming A-pillar with a force level of 5–7 kN could reduce the HIC of the head with helmets less than 1500.

Improving fit of bicycle helmet liners using 3D anthropometric data

3D anthropometry has provided much-needed information about the size and shape of the head, which can be used to improve the fit of protective helmets. In this study, a new 3D head scan sizing method was implemented in a reverse engineering approach for bicycle helmet liner dimensioning. The inside liner of a commercially available helmet was modified to improve the fit for a selected size group of 30 participants. The fit of the standard and new liner were assessed and compared, using the Helmet Fit Index (HFI). The HFI scores showed a significant improvement of overall fit (Difference: 11.32 ± 7.82 (μ ± SD), p < 0.0005) and for each of five defined regions of the liner inside surface. The presented methodology for dimensioning helmet liners based on 3D anthropometry proved effective, resulting in improved fit for the end users.

Dynamic Response and Residual Helmet Liner Crush Using Cadaver Heads and Standard Headforms.

Biomechanical headforms are used for helmet certification testing and reconstructing helmeted head impacts; however, their biofidelity and direct applicability to human head and helmet responses remain unclear. Dynamic responses of cadaver heads and three headforms and residual foam liner deformations were compared during motorcycle helmet impacts. Instrumented, helmeted heads/headforms were dropped onto the forehead region against an instrumented flat anvil at 75, 150, and 195J. Helmets were CT scanned to quantify maximum liner crush depth and crush volume. General linear models were used to quantify the effect of head type and impact energy on linear acceleration, head injury criterion (HIC), force, maximum liner crush depth, and liner crush volume and regression models were used to quantify the relationship between acceleration and both maximum crush depth and crush volume. The cadaver heads generated larger peak accelerations than all three headforms, larger HICs than the International Organization for Standardization (ISO), larger forces than the Hybrid III and ISO, larger maximum crush depth than the ISO, and larger crush volumes than the DOT. These significant differences between the cadaver heads and headforms need to be accounted for when attempting to estimate an impact exposure using a helmet's residual crush depth or volume.

Effect of helmet liner systems and impact directions on severity of head injuries sustained in ballistic impacts: a finite element (FE) study.

The current study aims to investigate the effectiveness of two different designs of helmet interior cushion, (Helmet 1: strap-netting; Helmet 2: Oregon Aero foam-padding), and the effect of the impact directions on the helmeted head during ballistic impact. Series of ballistic impact simulations (frontal, lateral, rear, and top) of a full-metal-jacketed bullet were performed on a validated finite element head model equipped with the two helmets, to assess the severity of head injuries sustained in ballistic impacts using both head kinematics and biomechanical metrics. Benchmarking with experimental ventricular and intracranial pressures showed that there is good agreement between the simulations and experiments. In terms of extracranial injuries, top impact had the highest skull stress, still without fracturing the skull. In regard to intracranial injuries, both the lateral and rear impacts generally gave the highest principal strains as well as highest shear strains, which exceed the injury thresholds. Off-cushion impacts were found to be at higher risk of intracranial injuries. The study also showed that the Oregon Aero foam pads helped to reduce impact forces. It also suggested that more padding inserts of smaller size may offer better protection. This provides some insights on future's helmet design against ballistic threats.

The Helmet Fit Index – An intelligent tool for fit assessment and design customisation

Helmet safety benefits are reduced if the headgear is poorly fitted on the wearer's head. At present, there are no industry standards available to assess objectively how a specific protective helmet fits a particular person. A proper fit is typically defined as a small and uniform distance between the helmet liner and the wearer's head shape, with a broad coverage of the head area. This paper presents a novel method to investigate and compare fitting accuracy of helmets based on 3D anthropometry, reverse engineering techniques and computational analysis. The Helmet Fit Index (HFI) that provides a fit score on a scale from 0 (excessively poor fit) to 100 (perfect fit) was compared with subjective fit assessments of surveyed cyclists. Results in this study showed that quantitative (HFI) and qualitative (participants' feelings) data were related when comparing three commercially available bicycle helmets. Findings also demonstrated that females and Asian people have lower fit scores than males and Caucasians, respectively. The HFI could provide detailed understanding of helmet efficiency regarding fit and could be used during helmet design and development phases.

Innovative Fire- and Heat-Protective Knit Fabrics and Individual Protective Aids

The properties of fire- and heat-protective knit fabrics and individual protective aids, such as underand winter-wears, helmet liners, helmet guards, gloves, socks, etc., are studied. Knit fabrics and individual protective aids are developed using blended yarn comprised of fire-resistant modacrylic fibre Protex (55%) and cotton fibre (45%) and fire-resistant meta-aramid yarn. The special properties of the articles are studied at Scientific and Production Limited Liability Company Armokom-Tsentr with due regard for their performance conditions with respect to such parameters as resistance to open flame, heat insulating capacity, heat resistance, etc. It is demonstrated that the under- and winter-wears are resistant to open flame (no inflammation, residual burning, or smoldering, but destruction of the fabric is noticed). In terms of heat insulating capacity, these clothes meet the technical specifications in conformity with heat insulation standard GOST (Russian State Standard) 29104.14. Helmet liners and helmet guards are resistant to open flame (no inflammation, residual burning, or smoldering, but destruction of the fabric is noticed). The heat insulating capacity of these articles meets technical specifications. The performance properties of fire- and heat-protective gloves conform to GOST 12.4,246 and of socks, to GOST 8541. The gloves are resistant to open flame and are not destroyed, which make them usable without additional protection. The socks developed are resistant to open flame, have high heat insulating capacity, and are comfortable to use.

Feasibility of optimising bicycle helmet design safety through the use of additive manufactured TPE cellular structures

Bicycle helmets are designed to attenuate forces and accelerations experienced by the head during cycling accidents. An essential element of bicycle helmet design is, therefore, the appropriate manufacturing of energy-dissipating components. The focus of this study was to evaluate the feasibility of using thermoplastic elastomer (TPE) cellular structures (Duraform® Flex), manufactured via a laser sintering (LS) process, as the energy-dissipating inner liner of the bicycle helmet. This study is presented in two sections; the optimisation of the LS process capabilities for the manufacture of cellular structures and an evaluation of the effects of cellular structure density on helmet impact kinematics. Through the fabrication and testing of tensile and compressive specimens, each process parameter (laser power, scanning exposure, build temperature and part orientation) was optimised to maximise compressive strength. The energy-dissipating characteristics of helmet cellular structures, made from this optimised material, were evaluated during simulated helmeted headform impact tests. Reduced accelerations and increased pulse durations were reported for decreased structural densities, demonstrating improved energy-dissipating characteristics for this novel technique. This study demonstrates that cellular structure-based inner liners, manufactured via additive manufacturing processes, have exciting potential towards improving bicycle helmet safety.

Assessment of a bicycle helmet liner with semispherical cones

Helmets reduce the frequency and severity of head and brain injuries resulting from bicycle crashes. Generally, a bike helmet consists of the outer shell, liner, vents and straps. The liner is the major helmet component reducing head injury in an accident. Using superior energy absorbing structures and materials as helmet liners to meet the higher safety standards should thus be considered. In this study, a shock absorption test was performed in accordance with the EN1078 standard to evaluate the energy absorption capability of a helmet on the market. Then, the finite element analyses of helmet impact tests modelled using LS-DYNA software. To verify the accuracy of the model, the simulation results were compared with experimental data. Finally, an innovative design of helmet model with semispherical cone liner was considered in this study. To assess the dynamic energy absorption of this helmet, a simulation was performed in accordance with the EN1087 standard for shock absorption tests. The resultant simulation revealed that the helmet with the semispherical cone liner has the potential to protect the human head. This innovative design for a helmet liner can serve as a valuable idea to assist future development of safety-helmet technologies.

Finite Element Bicycle Helmet Models Development

Abstract Impact attenuation performance of three different range of commercial bicycle helmet were investigated in lateral drop impact test in accordance to AS/NZS 2063:2008, Australian/New Zealand Standard for bicycle helmet using numerical simulation and and experimental impact test. The aim of this research is to develop a simulation model of drop impact test, which to be used in further investigations of user-centred design approach of bicycle helmet. Three commercial bicycle helmet models were used in this study. All helmets and J headform were scanned using Flexscan 3D scanning equipment. Post-scan processing jobs of scanned geometry models such as helmet liner, shell and headform were conducted in Geomagic Studio 12. The experimental impact test is carried out using 2-wire drop test facility in accordance to the AS/NZS 2063:2008, Australian Standard for bicycle helmet. A few samples were cut from the liner of each helmet to determine the density of Expanded Polystyrene (EPS). Headform peak linear acceleration, impact duration and impact speed of each helmet were measured and recorded from the drop test. The scanned geometry models were imported into Abaqus. A drop impact simulation was developed based on the density and impact speed data obtained from the physical test. Inner liner of bicycle helmet, made from Expanded Polystyrene (EPS), was modeled using crushable foam properties, while headform and anvil were modeled as rigid bodies. Peak linear accelerations and impact duration of the headform on each helmet at three different impact locations of helmet were recorded. A robust correlation study using peak linear acceleration score, impact duration score and Pearson correlation coefficient between the data from physical test and numerical model was conducted. Good correlation scores (>80%) were achieved between the numerical model and experimental impact test in terms of headform peak linear acceleration and impact duration score, suggesting that the simulation model is in good correlation with those from physical test.

Impact Attenuation of Customized User-centered Bicycle Helmet Design

Bicycle helmets are currently available to cater to general head sizes, ranging from S/M and L/XL, but there is also a universal model that can fit all sizes through adjustable helmet strap. Numerous surveys addressed that wearing helmet is not comfortable and the current sizing did not accommodate various range of users, because the human head shapes and dimensions are different according to ethnic groups, age and gender. This paper describes impact attenuation of user-centred design approach, i.e by modifying the shape of the liner to improve fit of bicycle helmet in accordance to AS/NZS 2063:2008. Head scans of 15 participants from a selected control group were captured using Artec3D portable scanner, while bicycle helmets and J headform were scanned using Flexscan 3D scanning equipment. These participants were selected based on a grouping method referring to selected 37 landmarks on human head shape. A new headform for the control group was developed by aligning and combining all the involved head scans in Geomagic Studio 12. Several new helmets with different liner thickness were designed based on the new headform. A validated drop impact simulation model, developed using Abaqus FEA software, was used to conduct drop impact simulation of each customized helmet design. Thickness of each user-centred helmet models at 37 landmarks was also recorded and the peak linear acceleration (PLA) of the helmet at impact locations such as top, side and front area were measured. Results revealed that the PLA increases as the thickness of helmet liner decreases. The rate of increase of PLA as the helmet liner thickness decreases is different at each impact location; it was greater at front and side location compared to top impact location.

User Centred Design Customisation of Bicycle Helmets Liner for Improved Dynamic Stability and Fit

In order for bicycle helmets to maintain sufficient coverage and protection of riders, the helmets must be adequately positioned and secured on the head. This research investigates helmet's dynamic stability parameters in order to determine whether a customised liner can improve the retention and therefore safety. A specialist 3D scanner was used to scan a total of 17 male participants who volunteered for this study. Through 3D scanning the shape and depth of each section of a participant's head and facial structure was captured. All the heads scanned were orientated with respect to the same co-ordinates based on the Australian and New Zealand Standards in order to customize the liner design to fit the user's heads. The dynamic stability of the commercially available helmets and the customized helmet were assessed using relevant video and goniometer techniques. The results indicate that for frontal roll-off, the customised helmet liner passed in all simulations of the 160mm drop height. It consistently performed better in the lateral roll-off at height of 40mm and 80mm compared with the commercially available helmets. While the dynamic stability measurements and the design strategies offer the potential to improve fit for different user's head, further studies should take into consideration a larger population size to draw more accurate conclusions.

Comparative Study of Motorcycle Helmets Impact Performance

Two sets of new and in-service helmets were impact tested using a drop test machine, in accordance to established helmet test protocols. The first test for full helmets was executed in compliance with standard speed requirements of 5.9 m/s in which three of five new helmets performed poorly. The second set utilized lower impact speed of 4 m/s for individual helmet components test. New helmet liners absorbed 5 times more impact energy than the in-service liners while the new shell was 19.3% better in dispersing impact energy than the in-service shell. The undesirable changes in liner thickness have explicit effect on the liner density which is translated into reduction in energy absorbing potential. In brief, exposure to weathering stresses and use intensities has affected helmet impact performance, regardless of service duration.

Helmet liner evaluation to mitigate head response from primary blast exposure

Head injury resulting from blast loading, including mild traumatic brain injury, has been identified as an important blast-related injury in modern conflict zones. A study was undertaken to investigate potential protective ballistic helmet liner materials to mitigate primary blast injury using a detailed sagittal plane head finite element model, developed and validated against previous studies of head kinematics resulting from blast exposure. Five measures reflecting the potential for brain injury that were investigated included intracranial pressure, brain tissue strain, head acceleration (linear and rotational) and the head injury criterion. In simulations, these measures provided consistent predictions for typical blast loading scenarios. Considering mitigation, various characteristics of foam material response were investigated and a factor analysis was performed which showed that the four most significant were the interaction effects between modulus and hysteretic response, stress–strain response, damping factor and density. Candidate materials were then identified using the predicted optimal material values. Polymeric foam was found to meet the density and modulus requirements; however, for all significant parameters, higher strength foams, such as aluminum foam, were found to provide the highest reduction in the potential for injury when compared against the unprotected head.