Working Group 3: Impact Engineering

Introduction

One of the primary ways to improve bicycle helmet technology is through the review and revision of the standards under which those helmets are tested. Current helmet testing for impact properties is not based on real accident conditions. Testing methods include a linear shock absorption test, during which a helmet is dropped vertically onto a horizontal flat surface, as well as a kerbstone-shaped surface. However, actual accident reports indicate that angled impact is far more common, and results in more serious brain injury. Compared to linear shock absorption, the rotational movement of the neck and head that results in the angled impact can cause both concussion and more severe brain injury, such as subdural hematoma and diffuse axonal injury.

In order to create truly effective bicycle helmets that protect against the most common and severe types of head injury, new testing conditions and pass/fail criteria are essential. The resulting design improvements can, in turn, protect cyclists in both accidents due to falls, and collisions with other vehicles.


Working Group Focus

In relation to COST Action TU1101, the primary focus of Working Group 3 was to:

  • Analyse the motion of the head (kinematics) at the moment of impact in real-world accident conditions;
  • Explore helmet impact conditions in order to improve helmet testing;
  • Review existing head injury criteria based on global kinetic parameters;
  • Achieve progress in the field of Finite Element Model-based head injury criteria;
  • Propose new pass/fail criteria for helmet testing;
  • Present a new helmet test method and suggest areas for further research;

Overall Result

After thorough review of existing data, accident reports and testing data, Working Group 3 conducted real-world accident simulations, in which accident data was used to accurately reproduce actual reported accidents. In this way, the shortcomings of the current testing methods were exposed, thereby leading to the Working Group’s recommendations for testing improvement. The recommended improvements have been realised within this COST Action, and the test apparatus exists. Initial tests clearly show that testing could be vastly improved by the inclusion of angled impact analysis and measurement of the rotational kinematics transferred through the helmet to the head. There is also a need to improve brain injury criteria in order to assess the head injury risk in a more realistic way.

The Working Group’s own testing and biomechanics research also led to recommendations for a new bicycle helmet test method, including improved pass/fail testing criteria. These improvements could contribute to more advanced helmet design.


Implications for Industry

Working Group 3’s primary output is the proposal of a new bicycle helmet test method, which considers tangential helmet impact and advanced head injury criteria. This progress will allow enhanced protection of bicyclist’s heads.

The novel test method can be, and already is, considered to be current by the industry. New helmet design can now be evaluated under tangential head impact conditions. In this domain, no less than three helmet evaluation programmes have already be launched in Germany, France and Sweden.

With both improved testing and refined pass/fail criteria, helmets can be designed in a more effective way. This, in turn, can have a positive impact on the general perception of helmet efficacy, and thereby help improve helmet usage amongst cyclists. Most importantly, the improved helmet design can significantly reduce serious head injury in accidents.


Implications for Legislators

Working Group 3’s output can aid in the improvement of both testing standards and helmet certification. In the near future, the proposal of new testing standards and pass/fail criteria should be discussed with the relevant standard bodies. In order to protect cyclists from serious head injury even better, the current industry standard is simply not enough. Helmet design, testing and certification must first be improved in order to protect against the most common – and often most devastating – types of injury. Standards like EN 1078 and EN 1080 are not enough to ensure that helmets are truly effective in preventing serious head injury in traffic accidents. Modifications to these standards can have a tremendous impact on cyclist safety and on injury prevention.


Bicycle Accident Reconstruction

Working Group 3’s investigations began with a review of the typical impact situations for a bicyclist. This proved to be a complicated endeavour, as accidents can happen in a nearly infinite number of ways, under a multitude of different conditions, including – but not limited to – rate of travel, weather conditions, angle of impact, the involvement (or not) of other parties, and more. There is a limited amount of valid data available, but enough exists to make preliminary evaluations of accident conditions, based on the results of six recognised studies.

WG3 table 1

Impact speed and angle from detailed accident reconstruction studies.

The six studies evaluate accident reconstructions based on the GIDAS database, the French Accident Database, and the simulation programme MADYMO. Parameters such as accident type (single fall or collision with car), rate of speed, angle of impact and road surface type were included in the studies. In total, data from 5,534 accident reports were evaluated, and 46 accidents were reconstructed.  

Although the ‘most common’ impact angle is still difficult to evaluate exactly, it is clear from the reconstructions that impact angles are usually far removed from the direct, 90-degree impact conditions under which helmets are currently tested. There remains a lack of additional, detailed accident reconstructions, which would provide a statistically based decision on the most common types of bicycle accidents. Based on existing data and literature review, Working Group 3 proposes to keep the shock absorption tests defined in EN 1078 and EN 1080, but to complement these with three angled (oblique) tests, using an impact angle of 45 degrees at a speed of 6.5 m/s.


New Helmet Impact Conditions

Current testing is shown to be deficient in three key areas. First, it does not include testing for impacts at an angle, although the majority of accident data indicates that angled impacts are common and cause significant injury. Second, testing does not currently account for realistic brain injury criteria, despite the fact that this is shown to have an effect on impact conditions. Third, data is available to analyse the impact location on the helmet after an actual accident. This data indicates that the testing line defined in EN 1078 is too high. Working Group 3, in conjunction with Working Group 1, therefore proposes to lower the test line, so that the helmet covers more of the head, whilst still creating a design that is attractive and accepted by the end consumer.

Working Group 3 proposes new test methods for improving testing in bicycle helmets from these three critical points of view. First, impact testing to accommodate the 45-degree-angle impact condition. Second, testing to account for the effect of a more realistic pass/fail criteria. The third testing improvement would be to lower the testing line for the impact location on the helmet.


Pass/Fail Criteria

Next, Working Group 3 has evaluated and analysed current pass/fail criteria for helmet testing, and discovered a number of antiquated or inadequate parameters for these tests. In an extensive evaluation, the Working Group has examined head injury criteria at a number of different levels. These include an evaluation of existing head injury criteria, as well as a review of specific head injury criteria based on:

  • Translational acceleration;
  • Rotational acceleration and velocity;
  • Combined rotational and translational acceleration;
  • Finite Element (FE) Head Model-based head injury criteria;

The group also introduced advanced head models, which enable the computation of axon strain at the time of impact. The simulation of 115 real-world head trauma incidents were used to define initial limits for model-specific based brain injury criteria, as well as post-processing tools for end-users. Finally, a coupled experimental versus numerical test method was defined.

WG3 figure 25

Illustration of the coupled experimental versus numerical head impact test method based on head FE modelling

After the evaluations were complete, the Working Group reached the conclusion that it is possible to use both global pass/fail criteria, such as BRIC, RIC, HIP or PrHIC, or to use the more advanced, model-based pass/fail criteria that were developed within the group. Both methods, however, do need further work in order to be tuned to a final pass/fail criteria used in a helmet test standard.


A New Helmet Test Method

As a result of this extensive investigation, Working Group 3 concludes by proposing an advanced, scientific helmet test method, based on real-world data and new biomechanical research results. The primary aspects of this new bicycle helmet test method are the introduction of tangential helmet impact and improved pass/fail criteria that can predict brain injuries, in addition to predicting skull fractures as it does today. It can easily lead to changes to the way helmets are certified and improve helmet efficacy in accidents. By adding real accident data and new biomechanical knowledge to the current regulation tests EN 1078 and EN 1080, the protective aspects of bicycle helmets can be vastly improved. Specific new testing parameters and methods, based on actual accident data and new biomechanical criteria, would bring the certification standards to a much higher, more effective level.


Further Research

Working Group 3 has been closely interacting with Working Group 11 of the CEN TC158, which is investigating the specifications for a new testing method. Certain details of that study provide ample opportunity for further study, such as the definition of calibration tests for HIII head forms and the spread and variation between helmets and different test labs. Research in the domain of brain injury criteria also need further efforts: first, to continue efforts to define injury criteria based on six-dimensional global head kinematic parameters. Second, for benchmark studies between different FE head models in order to achieve harmonisation of model-based injury criteria.

Further research in the field will also be organised within a variety of EU projects, including HEADS, MOTORIST, SmartHelmets, Safe2Wheelers, as well as national projects in Sweden, Germany and France.

 
 
 
 

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