The impact response of traditional and BMX-style bicycle helmets at different impact severities

Bicycle helmets reduce the frequency and severity of severe to fatal head and brain injuries in bicycle crashes. Our goal here was to measure the impact attenuation performance of common bicycle helmets over a range of impact speeds. We performed 127 drop tests using 13 different bicycle helmet models (6 traditional style helmets and 7 BMX-style helmets) at impact speeds ranging from 1 to 10 m/s onto a flat anvil. Helmets were struck on their left front and/or right front areas, a common impact location that was at or just below the test line of most bicycle helmet standards. All but one of the 10 certified helmet models remained below the 300 g level at an impact speed of 6 m/s, whereas none of the 3 uncertified helmets met this criterion. We found that the helmets with expanded polystyrene liners performed similarly and universally well. The single certified helmet with a polyurethane liner performed below the level expected by the Consumer Product Safety Commission (CPSC) standard at our impact location and the helmet structure failed during one of two supplemental tests of this helmet above the test line. Overall, we found that increased liner thickness generally reduced peak headform acceleration, particularly at higher impact speeds.     

Finite element analysis of impact damage response of composite motorcycle safety helmets

The energy absorption during impact provided by a motorcycle safety helmet is always of critical importance in order to protect the rider against head injury during an accident. In the present study, a parametric analysis has been performed in order to investigate the effect of the composite shell stiffness and the damage development during impact, on the dynamic response of a composite motorcycle safety helmet. This kind of parametric analysis may be used as a tool during helmet design for minimising testing needs.The LS-DYNA3D explicit hydrodynamic finite element code was used to analyse a detailed model of the helmet-headform system (composite shell/foam liner/metallic headform) and to simulate its dynamic response during impact. A significant part of the work was focused on the modelling of the mechanical behaviour of the composite materials, including damage and delamination development. The dynamic response of the different helmet-headform systems was judged in terms of the maximum acceleration monitored at the centre of gravity of the headform and the maximum value of head injury criterion.It was shown that composite shell systems exhibiting lower shear performance provide additional energy absorbing mechanisms and result to better crashworthiness helmet behaviour.     

The effectiveness of foams in bicycle and motorcycle helmets

The optimisation of the foam used for shock absorption in bicycle and motorcycle helmets is considered. It is shown that for the former, the ideal foam yield stress for impact with a rigid protruding object is higher than that for impact with a deformable flat object. Therefore, the design must be a compromise. For motorcycle helmets the shell stiffness is higher and the force to cause shell buckling plays a significant part in the response. Lower density foams can be used only if the impact test standards are rewritten with less emphasis on impacts with convex and pointed objects.     

A mixed logit analysis of two-vehicle crash severities involving a motorcycle

Using motorcycle crash data for Iowa from 2001 to 2008, this paper estimates a mixed logit model to investigate the factors that affect crash severity outcomes in a collision between a motorcycle and another vehicle. These include crash-specific factors (such as manner of collision, motorcycle rider and non-motorcycle driver and vehicle actions), roadway and environmental conditions, location and time, motorcycle rider and non-motorcycle driver and vehicle attributes. The methodological approach allows the parameters to vary across observations as opposed to a single parameter representing all observations. Our results showed non-uniform effects of rear-end collisions on minor injury crashes, as well as of the roadway speed limit greater or equal to 55 mph, the type of area (urban), the riding season (summer) and motorcyclist's gender on low severity crashes. We also found significant effects of the roadway surface condition, clear vision (not obscured by moving vehicles, trees, buildings, or other), light conditions, speed limit, and helmet use on severe injury outcomes.     

Fast fracture of rubber-toughened thermoplastics used for the shells of motorcycle helmets

The effects of outdoor ageing and of accelerated UV exposure on the grade of acrylonitrile butadiene styrene copolymer (ABS) used by UK manufacturers of motorcycle helmets were compared. Although the surface layer becomes embrittled the helmets still survive an impact test after conditioning at -20° C because the cracks are arrested before they penetrate the full thickness of the shell. A 4 mm thick rubber-toughened polycarbonate, used in helmets from continental suppliers, shows a transition from tough to brittle crack propagation behaviour at -15° C. For 4 mm thick ABS this transition had not occurred at the lowest test temperature of -20° C, but if 6 mm sheet is tested the transition is at -15° C. Temperature increases in the 40 sec delay between conditioning a helmet at -20° C and impact testing are shown to be significant.     

Oblique impact testing of bicycle helmets

The performance of bicycle helmets was investigated in oblique impacts with a simulated road surface. The linear and rotational accelerations of a headform, fitted with a compliant scalp and a wig, were measured. The peak rotational accelerations, the order of 5 krad s⁻² when the tangential velocity component was 4 m s⁻¹, were only slightly greater than in comparable direct impact tests. Oblique impact tests were possible on the front lower edge of the helmet, a site commonly struck in crashes, without the headform striking the ‘road’. Data characterizing the frictional response at the road/shell and helmet/head interfaces, were generated for interpretation via FEA modelling.     

Impact dynamics of metal foam shells for motorcycle helmets: Experiments & numerical modeling

To reduce the weight of motorcycle helmet, metal foam for outer shell in place of conventional thermoplastics was tested. The dynamic behaviour of this new helmet was studied through experiments and numerical modeling. Open-face motorcycle helmets were designed with metal foam shell and impact experiments were performed with these helmets fitted on a headform. A finite element model was developed and the predicted acceleration of headform from this model was validated against the experiments. The mechanical behaviour of full-face helmets with metal foam shell was investigated next. The FE analysis was performed separately with rigid and deformable heads. Head injury criterion (with rigid head) and stresses in brain (with deformable head) were evaluated separately for metal foam shell and ABS shell helmets. The helmet impact performance is examined with two separate densities of metal foam. The shell with low-density metal foam (150 kg/m³) gives a better performance compared to ABS shell. The metal foam shell showed significant visible plastic deformation in the impact region.     

Evaluation and replication of impact damage to bicycle helmets

A group of 72 impacted bicycle helmets were collected, primarily from manufacturers with a crash replacement policy that encourages the return of damaged helmets. Each damaged helmet was thoroughly inspected and measured to determine the construction details and collision damage. Laboratory replication tests were then performed on selected samples using exemplar helmets to determine impact velocity and peak headform aceleration. The predominant impact location was the front left quarter and the replication studies indicate that the majority of impacts took place on flat surfáces from drop heights of 1 meter or less. Overall, it is evident that a large number of bicycle helmet users who have benefited from the use of a bicycle helmet, and future bicycle helmet standards must incorporate the protective requirements of this unique group.     

FEA of oblique impact tests on a motorcycle helmet

In accidents, motorcycle riders full-face helmets often make oblique impacts with road surfaces. Finite element analysis was used to predict the rotational and linear acceleration of a Hybrid II headform, representing a motorcyclist's head, in such impacts, considering the effects of friction at the head/helmet and helmet/road interfaces. Simulations of the oblique impact test in British Standard BS 6658 were validated by comparison with published data. This showed that COST 327 experimental data was largely determined by the friction coefficient (0.55) between the helmet shell and abrasive paper, and hardly affected by that between the head and helmet. Slip was predicted at the shell/paper interface throughout the impact, due to the high angular inertia of the helmet, and the normal force remaining below 3.5 kN. Simulations of more severe motorcycle helmet impacts explored the effects of impact site and direction, impact velocity components, helmet fit and the scalp. In these impacts, the higher velocity component normal to the road caused high frictional forces on the helmet shell, eventually causing it to roll on the road. The peak headform rotational accelerations, at some impact sites, were potentially injurious. The most effective method of reducing head rotational acceleration could be a reduction in the linear acceleration limit of the helmet standards.     

The effect of temperature on the capability of industrial safety helmets to absorb impact energy

The fundamental document specifying the requirements and testing methods applicable to industrial safety helmets in European Union member states is the standard EN 397:2012. According to that standard, one of the most important parameters of a helmet is shock absorption, determined for an impact of a striker with a kinetic energy of 49 J. The shock-absorbing performance of a safety helmet involves absorbing the energy of a striking object associated with a deformation of the shell and cradle, as well as an increase in the force transferred to the user’s head. The paper presents a study conducted with the aim to estimate the actual amount of energy absorbable by various helmet types without exceeding the threshold value of the force acting on the user’s head. A method of testing helmet deformation and the force acting on the helmet during an impact exerted by a falling object is presented. The effect of the temperature used for conditioning various helmet types on their capability to absorb impact energy was determined. The causes of deterioration of that capability due to temperature changes are analyzed for various designs of helmets made of different materials, and possible solutions to that problem are offered.     

Issues in motorcycle sensory and cognitive conspicuity: The impact of motorcycle low-beam headlights and riding experience on drivers’ decisions to turn across the path of a motorcycle

Background

Crashes involving a passenger car and a motorcycle, where the car is turning across the path of the motorcycle, are a major crash type of motorcycle riders. It has been proposed that the incidence of such crashes could be reduced through improvements in motorcycle conspicuity. Operation of low-beam headlights on motorcycles has been discussed as one approach for improving the “sensory conspicuity” of motorcycles during daylight hours, whilst previous experience as a rider may serve to heighten “cognitive conspicuity” through raised awareness of motorcyclists on our roads.

Method

Twenty-three experienced car drivers with no riding experience (“drivers”) and 20 experienced car drivers who were also motorcycle riders (“driver–riders”) completed a series of trials in a driving simulator where their task in each trial was to turn ahead of an oncoming vehicle if they felt that they had sufficient room to do so safely. A key manipulation across trials was whether the oncoming vehicle was a motorcycle with headlights on, or a motorcycle with headlights off. Time gap (short, medium, long) was also manipulated.

Results

Results indicate that, at time gaps defined in the current study as short, low-beam headlights may confer some benefit in gap acceptance by encouraging drivers to accept fewer gaps ahead of a motorcycle with headlights on than ahead of a motorcycle with headlights off. No statistically significant differences in gap acceptance between the headlight conditions were found at either the medium or long time gaps. Irrespective of time gap, driver–riders were found to adopt a more efficient turn strategy than drivers with no direct riding experience.

Conclusions

Overall, the present research provides support for the use of low-beam headlights and riding experience as tools through which to augment the sensory and cognitive conspicuity of motorcycles, respectively. It is proposed that further research aim to explore directly the precise mechanisms underlying the observed effects.     

Rainfall effect on single-vehicle crash severities using polychotomous response models

As part of the Wisconsin road weather safety initiative, the objective of this study is to assess the effects of rainfall on the severity of single-vehicle crashes on Wisconsin interstate highways utilizing polychotomous response models.Weather-related factors considered in this study include estimated rainfall intensity for 15 min prior to a crash occurrence, water film depth, temperature, wind speed/direction, stopping sight distance and deficiency of car-following distance at the crash moment. For locations with unknown weather information, data were interpolated using the inverse squared distance method. Non-weather factors such as road geometrics, traffic conditions, collision types, vehicle types, and driver and temporal attributes were also considered. Two types of polychotomous response models were compared: ordinal logistic and sequential logistic regressions. The sequential logistic regression was tested with forward and backward formats. Comparative models were also developed for single vehicle crash severity during clear weather.In conclusion, the backward sequential logistic regression model produced the best results for predicting crash severities in rainy weather where rainfall intensity, wind speed, roadway terrain, driver's gender, and safety belt were found to be statistically significant. Our study also found that the seasonal factor was significant in clear weather. The seasonal factor is a predictor suggesting that inclement weather may affect crash severity. These findings can be used to determine the probabilities of single vehicle crash severity in rainy weather and provide quantitative support on improving road weather safety via weather warning systems, highway facility improvements, and speed limit management.     

The mechanical response of Rohacell foams at different length scales

The quasi-static mechanical response of polymethacrylimide (PMI) foams of density ranging from 50 to 200 kg m⁻³ is investigated in order to provide experimental data to inspire and validate numerical constitutive models for the response of polymer foams. The macroscopic mechanical response is characterised by conducting quasi-static compression, tension, shear and indentation experiments, whereas microscopic deformation mechanisms are identified by conducting in situ SEM observations during static compression and tension tests; it is shown that foams of low density collapse by cell wall buckling while foams of high density undergo plastic cell-wall bending. As a result, both the elastic and plastic macroscopic response of the foam display a tension/compression asymmetry.     

A different perspective on conspicuity related motorcycle crashes

The most common type of conflict in which a motorcyclist is injured or killed is a collision between a motorcycle and a car, often in priority situations. Many studies on motorcycle safety focus on the question why car drivers fail to give priority and on the poor conspicuity of motorcycles. The concept of ‘looked-but-failed-to-see’ crashes is a recurring item. On the other hand, it is not entirely unexpected that motorcycles have many conflicts with cars; there simply are so many cars on the road. This paper tries to unravel whether – acknowledging the differences in exposure – car drivers indeed fail to yield for motorcycles more often than for other cars. For this purpose we compared the causes of crashes on intersections (e.g. failing to give priority, speeding, etc.) between different crash types (car–motorcycle or car–car). In addition, we compared the crash causes of dual drivers (i.e. car drivers who also have their motorcycle licence) with regular car drivers. Our crash analysis suggests that car drivers do not fail to give priority to motorcycles relatively more often than to another car when this car/motorcycle approaches from a perpendicular angle. There is only one priority situation where motorcycles seem to be at a disadvantage compared to cars. This is when a car makes a left turn, and fails to give priority to an oncoming motorcycle. This specific crash scenario occurs more often when the oncoming vehicle is a motorcycle than when it is a car. We did not find a significant difference between dual drivers and regular car drivers in how often they give priority to motorcycles compared to cars.     

The Cochrane Collaboration and bicycle helmets

Effective interventions for care of health need to be based on scientific evidence. To this end, the Cochrane Collaboration insists that its reviews should be based on reliable data, normally obtained by randomised controlled trial. To constitute evidence, data should also support a hypothesis in accord with scientific laws and knowledge. From these considerations, an appraisal is made of the conclusion of the Cochrane review Helmets for preventing head and facial injuries in bicyclists, that it establishes scientific evidence that all types of standard helmet protect against injuries to the brain. It is concluded that the review takes no account of scientific knowledge of types and mechanisms of brain injury. It provides, at best, evidence that hard-shell helmets, now rarely used, protect the brain from injury consequent upon damage to the skull. The review therefore is not a reliable guide to the efficacy of helmets and to interventions concerning their use.     

Adult headform impact tests of three Japanese child bicycle helmets into a vehicle

The head is the body region that most frequently incurs fatal and serious injuries of cyclists in collisions against vehicles. Many research studies investigated helmet effectiveness in preventing head injuries using accident data. In this study, the impact attenuation characteristics of three Japanese child bicycle helmets were examined experimentally in impact tests into a concrete surface and a vehicle. A pedestrian adult headform with and without a Japanese child bicycle helmet was dropped onto a concrete surface and then propelled into a vehicle at 35 km/h in various locations such as the bonnet, roof header, windshield and A-pillar. Accelerations were measured and head injury criterion (HIC) calculated. In the drop tests using the adult headform onto a concrete surface from the height of 1.5 m, the HIC for a headform without a child helmet was 6325, and was reduced by around 80% when a child helmet was fitted to the headform. In the impact tests, where the headform was fired into the vehicle at 35 km/h at various locations on a car, the computed acceleration based HIC varied depending on the vehicle impact locations. The HIC was reduced by 10–38% for impacts headforms with a child helmet when the impact was onto a bonnet-top and roof header although the HIC was already less than 1000 in impacts with the headform without a child helmet. Similarly, for impacts into the windshield (where a cyclist’s head is most frequently impacted), the HIC using the adult headform without a child helmet was 122; whereas when the adult headform was used with a child helmet, a higher HIC value of more than 850 was recorded. But again, the HIC values are below 1000. In impacts into the A-pillar, the HIC was 4816 for a headform without a child helmet and was reduced by 18–38% for a headform with a child helmet depending on the type of Japanese child helmet used. The tests demonstrated that Japanese child helmets are effective in reducing accelerations and HIC in a drop test using an adult headform onto a relatively rigid hard surface, i.e., simulating a road surface or concrete path. However, when the impact tests are into softer surfaces, the child helmet’s capacity to decrease accelerations is accordingly reduced. Impacts into the windshield, while below the critical HIC value of 1000, indicated higher HIC values for a headform with a child helmet compared to an adult headform without a child helmet. The unpredictable nature of the results indicates further research work is required to assess how representative the stiffness of an adult headform is when compared to an actual head.     

The efficacy of bicycle helmets against brain injury

An examination is made of a meta-analysis by Attewell, Glase and McFadden which concludes that bicycle helmets prevent serious injury, to the brain in particular, and that there is mounting scientific evidence of this. The Australian Transport Safety Bureau (ATSB) initiated and directed the meta-analysis of 16 observational studies dated 1987-1998. This examination concentrates on injury to the brain and shows that the meta-analysis and its included studies take no account of scientific knowledge of its mechanisms. Consequently, the choice of studies for the meta-analysis and the collection, treatment and interpretation of their data lack the guidance needed to distinguish injuries caused through fracture of the skull and by angular acceleration. It is shown that the design of helmets reflects a discredited theory of brain injury. The conclusions are that the meta-analysis does not provide scientific evidence that such helmets reduce serious injury to the brain, and the Australian policy of compulsory wearing lacks a basis of verified efficacy against brain injury.     

The protective performance of bicyclists' helmets in accidents

A study of the injuries sustained by 1,892 bicycle riders during accidents indicated that 432 of the bicyclists had been wearing a helmet and 64 of the latter group had sustained an impact to the helmet. The 64 helmets were evaluated in this project to relate the nature and severity of the impact they had sustained to the head injury experienced by the wearer. The protective performance of the helmet shells, impact absorbing liners, and retention systems were evaluated, and the severity of the impacts sustained by the helmets was simulated in the test laboratory. The simulation was performed by dropping sample helmets from progressively greater heights in a test apparatus until the damage observed on a sample helmet matched that observed on an accident damaged helmet. The severity observed in the simulated impacts was compared with the severity of test impacts prescribed in established helmet performance standards (ANSI 1984; Snell 1984; AS 1986). It was found that all of the impacts occurred against flat objects; a high proportion of helmets sustained more than one impact; most impacts occurred on areas of a helmet which were not tested during certification to a standard; and many impacts were more severe than those stipulated in performance standards. The predominant form of head injury recorded was low severity concussion--AIS-1, AIS-2, AIS-3. All serious head injuries occurred when the helmet came off the rider's head and collapsed due to a material defect or was struck predominantly below the rim. A high proportion of helmets worn by young riders had been misused, and many helmets displayed defects in the impact-absorbing liners. Recommendations have been made for improving helmet construction and altering current standards to reflect the conditions encountered in the field.     

Erosion mechanisms of aluminium nitride ceramics at different impact angles

In this paper, erosion wear behaviour of aluminium nitride (AlN) ceramics is studied. The influence of particle hardness and shape on erosion of the AlN surface is examined. The effect of varying the impingement angle on the weight loss and the roughness parameters of AlN ceramics testing sample is also determined. Therefore, erosive wear behaviour of AlN ceramics was investigated using SiC and SiO₂ particles as erodents, at following impact angles: 30°, 45°, 60°, 75° and 90°. Scanning electron microscopy (SEM) was used to analyze the eroded surfaces in order to determine erosion mechanisms. The roughness parameters (R_a, R_z and R_max), before and after erosion with SiO₂ and SiC particles at 30° and 90° angles of impingement, respectively, were determined using a profilometer. It was found that the impact angle is influencing the erosion wear of the AlN ceramics and maximum erosion takes place at impact angle of 90°. The results indicate that hard, angular SiC particles cause more damage than softer, more rounded SiO₂ particles.