Investigating the need for a new standard/specification for DH and SuperG helmets.


 Judith Kelley
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1 Investigating the need for a new standard/specification for DH and SuperG helmets. By: Svein Kleiven and Peter Halldin Division of Neuronic Engineering, School of Technology and Health, Royal Institute of Technology, Stockholm, Sweden. Background The International Ski Federation (FIS) working group for technical equipment with Pernilla Wiberg as chairman asked for a recommendation if and how to make helmets safer for Downhill and SuperG. The reason is that several severe head injuries have occurred the last seasons. The helmets used today need to pass the EN 1077 test standard where the helmet is dropped vertically on a flat anvil at a speed of 5.4m/s. The pass fail criterion is a translational acceleration below 250G. The question is if this test results in helmets with the best possible head protection for the skier. FIS has therefor asked KTH (Royal Institute of Technology) together with NIH, Mavet and UIBK to evaluate the specification for the helmets used in high speed impacts in ski slopes prepared for Downhill or SuperG races. Goal Improve the helmet in order to reduce the number of injuries from Downhill and SuperG accidents. The project The project was initiated in August Here is first a short summary presented on the work conducted by KTH between August 2011 and March Then is the results presented conducted between April and May And finally is the plan for the future presented. Previous work August March 2012 TV footage from 11 accidents during has been collected by NIH, Oslo. Video analysis of the accidents has been conducted by KTH using the software Skillspector (Figure 1). The preliminary results show that impact speed in the gathered accidents are relative high compared to the velocities (5.4m/s) used in the current test standard. It is also shown that the impact angle is very steep (Around 20 degrees compared to current helmet standard where the impact angle is 90 degrees).
2 Figure 1 Left: Video analysis of Case F4. Right: Summary of preliminary impact speeds and impact angles in the accidents from FIS competitions between (Observe that the Mean values in the figure above are preliminary and not final). The next step was to use the head kinematics (vertical and horizontal linear velocity, rotational velocity and impact sited on the helmet) from the video analysis and then perform accidents reconstructions by use of the detailed KTH head model, Figure 2. The reason for the accident reconstruction was to evaluate if the steep impact angle is important to take into account when designing future helmets. Figure 2. Injury reconstruction from the video analysis. The input in this reconstruction is, except for the head kinematics, the FE model of the human head (Kleiven et al 2007), a model of a helmet validated against experimental tests at KTH and a FE model of the snow (Keller et al. 2004, Mössner 2011, Mellor 1977, Landauer Narita,
3 1980, Kirchner et al. 2001). The coefficient of friction was set to 0.1 representing dynamic dry friction for snow (Bowden Shimbo Barnes et al Evans et al Kuroiwa Colbeck and Glaciol. 1988). The initial results from reconstruction of three accidents showed that the rotational acceleration was very high for one of the accidents and quite low for two of the other accidents. Another study using just a FE model of a Hybrid III dummy head equipped with a helmet was conducted to analyze how the impact direction affected the results, Figure 3. It was found that the rotational acceleration transmitted to the head was very much depending on the stature of the skier, Figure 4. In the analysis was the impact speeds taken from the video analysis from the case F4. It was shown that the rotational acceleration in the head was around 5000 radians/s 2 for the Rear impact while radians/s 2 for the frontal impact as defined in Figure 4. It was a bit surprising that the rotational acceleration was as high as the coefficient of friction is so low as 0.1 for dry snow. The FE studies raised questions if the rotational accelerations and the rotational energy seen in the head could be caused by the tangential force from the snow or caused by offset impacts (impacts where the CG (center of gravity) of the head in the impact is offset the impact point on the helmet causing the rotation by the CG moment arm). So, the initial work did not lead to any conclusion but to questions. The first question to answer in order to believe in the results is how relevant the snow data from the literature data is for a modern DH and SuperG ski slope. It was therefore decided to validate the boundaries used in the simulations further. The most important boundary is the snow properties and the contact definition to the snow including the coefficient of friction. Figure 3 Showing the Linear horizontal (Vh) and Vertical (Vv) velocities and the rotational velocity applied on the FE model of the Hybrid III head and helmet.
4 Figure 4 Shows different impact directions analyzed. Recent work April May 2012 The project is now divided into the following tasks. Phase 1  Experiment of helmet impact on hard snow, Åre Sweden (Done). Phase 2  Simulation of snow impact (Almost done). Verification of snow/ice stiffness Verification of coefficient of friction Phase 3  Video analyze real accidents Definition of impact speed and impact angle in a typical accident. (Partly done) Phase 4  Reconstruction of the accident by use of a detailed FE model of the human head and brain (Kleiven 2007). Correlation to medical pictures. (Partly done) Phase 5  Conclusion and recommendation on how a DH and Superg helmet test standard should be designed. Phase 1 and 2 were added in the project in order to perform experiments on site in a ski slope prepared for competition. Experimental tests have been performed in Åre, Sweden (March 2012). A Hybrid III dummy head including accelerometers was equipped with ski helmets (RED Force). The helmet was chosen as it has a smooth outer surface of the shell, which is the case with a normal competition helmet. Acceleration data was collected from the tests and all tests was filmed with a high speed camera (600 fps). The helmets were dropped from 1.7m resulting in an impact speed of 5.8 m/s, Figure 5.
5 Figure 5. Photos from the experimental set up in Åre, Sweden. The experimental test was then compared to a FE model (Figure 6) in order to: 1. Validate the snow model by measuring the intrusion from the helmet in the snow. 2. Verify the coefficient of friction between the helmet and the snow. The results from this study will be presented in a separate report. In short, the findings are: 1. That the snow model that has been used so far is too stiff and produce linear accelerations that are around 50% too high compared to tests on real snow. New constitutive models and material properties are currently being evaluated. 2. A coefficient of friction of around 0.1 shows the best correlation so far. The final verification will be made when a new constitutive model for snow is validated. Figure 6. Showing a comparison between the experimental test and FE simulation. Conclusions so far! The test speed should be increased in current test standards from 5.4 m/s.
6 The rotational accelerations seen in the current FE simulations (Phase 2 and 4) and the experimental tests (Phase 1) are high. The high rotational accelerations could either be induced by the tangential force between the snow and the helmet or it could be induced by the inertial propertied of the head. It is therefore too early to give a final recommendation to FIS on how an advanced ski helmet test standard should be designed. Next step Phase 35 will continue when Phase 1 and 2 is completed. Some of the video analysis in Phase 3 needs to be controlled by external party in order to secure the quality. We are still waiting for medical pictures from the accidents, which is important to validate the injury outcome computed by the FE model of the human head and brain. Further simulations will be conducted analysing different impact situations in order to analyse the importance of the tangential force compared to the inertial effects from the CG of the human head. The absence of the neck and the body should be investigated. Previous work shows that the absence of the neck and/or the body does not affect the rotational acceleration in a helmet impact situation. This conclusion might not be relevant as the impact surface is slippery. The final report from this project will then be review by at least two objective external parties. References Coming soon.
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