M3 - The effect of anvil surface friction on head kinematics when evaluating hard hat impactperformance during occupational falls from height

Introduction: : Concussion awareness has gained significant attention in sports, with numerous studies emphasizing the role of helmets and headgear in reducing concussion risk. This increased focus has led to substantial advancements in helmet design and head protection for athletes. However, comparatively less attention has been given to occupational head injuries and improvements in safety helmet design within the workplace. Work-related traumatic brain injuries (WRTBIs) are estimated to account for 4–24% of all traumatic brain injuries (TBIs) reported annually.
Despite falls being the leading cause of both fatal and non-fatal WRTBIs, current safety helmet standards often fail to address impact conditions that realistically mimic fall scenarios. To better understand these risks, oblique impact tests were conducted using a helmeted NOCSAE headform against two surfaces of varying friction: a low-friction commercial floor tile and a high-friction 80-grit sandpaper. These tests aimed to investigate the influence of surface friction on peak head kinematics. The results of this work will help inform how to best represent work-related head impacts during standardized laboratory testing and further optimize occupational head protection.

Materials and

Methods: : Friction testing on concrete was conducted using a standard sled test setup with three common helmet shell materials. A sled, with samples of each helmet shell, was placed on the concrete slab. One end of a cable was attached to the sled, while the other end was connected to a bucket over a pulley. The bucket was loaded until the sled began to move. The bucket weight at the onset of sled motion was recorded. The static coefficient of friction (COF) for each material against concrete was then calculated.
To determine the COF of helmet materials against other potential impact surfaces, the same procedure was repeated with the concrete slab replaced by either sandpaper or commercial floor tile. Sandpaper was selected for its common use in oblique helmet testing, while commercial floor tile was chosen to approximate the COF of concrete while offering greater repeatability and ease of use when switching surfaces.
Surface friction tests were done during oblique drop tests to determine its effect on head kinematics. Five representative safety helmet models were selected: four Type II and one Type I model. A helmeted was dropped onto a 25° steel anvil covered with either 80-grit sandpaper or commercial floor tile (Figure 1).
A linear mixed effects model with helmet model as a random effect was conducted in R to quantify the effects of impact speed, impact location, and impact surface on PLA and PRA. A significance level was selected, and post hoc comparisons were completed using least squares means.

Results, Conclusions, and Discussions:: Static friction testing showed that the average coefficient of friction (COF) between concrete and all helmet shell materials was 0.4. This value was noticeably lower than the average COF on 80-grit sandpaper (0.89) and higher than the average COF on commercial floor tile (0.34).
Impact testing revealed substantial variability in peak rotational velocities (PRVs) and peak rotational accelerations (PRAs) both within and across helmet models during impacts onto the tile surface (Figure 2). Impact location and impact speed were found to have significant effects on peak linear acceleration (PLA) (p < 0.001) and PRA (p < 0.001). The tile impacts were associated with an average of 14.9 g lower PLA compared to sandpaper (CI: –41.3 to 11.4 g, p = 0.271). In contrast, tile impacts produced significantly higher PRAs, with an average increase of 2099 rad/s² (CI: 878 to 3320 rad/s², p = 0.002) compared to the sandpaper condition.
High-speed video analysis revealed differences in headform rotation when comparing tile and sandpaper surfaces at the same impact location and helmet model (Figure 2). During tile impacts, some helmet models and locations caused the headform to slip on the anvil and rotate toward the impact surface, unlike the typical rotation away from the anvil observed with sandpaper (Figure 2). However, this slipping behavior was not consistent across all impacts, contributing to the wide range of PRVs and PRAs observed for the tile condition (Figure 3). These differences highlight important considerations when establishing a laboratory test method that represents work-related head impacts.

Acknowledgements and/or References (Optional)::