The Influence of Headform/Helmet Friction on Head Impact Biomechanics in Oblique Impacts at Different Tangential Velocities

Abstract

Oblique impacts of the helmet against the ground are the most frequent scenarios in real-world motorcycle crashes. The combination of two factors that largely affect the results of oblique impact tests are discussed in this work. This study aims to quantify the effect of the friction at the interface between the headform and the interior of a motorcycle helmet at different magnitudes of tangential velocity. The helmeted headform, with low friction and high friction surface of the headform, was dropped against three oblique anvils at different impact velocities resulting in three different magnitudes of the tangential velocity (3.27 m/s, 5.66 m/s, 8.08 m/s) with the same normal component of the impact velocity (5.66 m/s). Three impact directions (front, left-side and right-side) and three repetitions per impact condition were tested resulting in 54 impacts. Tangential velocity variation showed little effect on the linear acceleration results. On the contrary, the rotational results showed that the effect of the headform’s surface depends on the magnitude of the tangential velocity and on the impact direction. These results indicate that a combination of low friction with low tangential velocities may result into underprediction of the rotational headform variables that would not be representative of real-world conditions.

Oblique impact responses of Hybrid III and a new headform with more biofidelic coefficient of friction and moments of inertia

“…The results showed that the headform–liner CoF plays an important role in headform’s rotational acceleration. These conclusions were further confirmed by a recent study, investigating the headform’s CoF under the oblique impact with varying tangential velocities ( Juste-Lorente et al, 2021 ). Recently, Trotta et al (2018b) attached a porcine scalp on both EN960 and HIII headforms and compared their kinematics with the bare headforms.…”

“…The relative differences are similar or smaller than those between the 3D-printed headform and human head (I xx : 10.2%, I yy : 2.5%, and I zz : 16.1%) ( Connor et al, 2018 ). Therefore, the Cellbond headform can better represent the MoIs of an average (western) male rider than the HIII, EN960, and 3D-printed headforms in previous studies ( Connor et al, 2018 ; Juste-Lorente et al, 2021 ).…”

“…In contrast to this headform, the MoIs and CoF of the Cellbond headform are in better agreement with those obtained from measurements on live and postmortem human subjects ( Trotta et al, 2018a ; Connor et al, 2020 ). Previous research has studied helmet response with headforms that have more biofidelic MoIs ( Kendall et al, 2012 ; Connor et al, 2020 ) or CoF ( Trotta et al, 2018b ; Juste-Lorente et al, 2021 ; Zouzias et al, 2021 ) but not a headform that has both. For the first time, this study used a headform that has both biofidelic CoF and MoIs.…”

“…These differences may be explained by the different MoIs about the rotation axes dominant at different impact locations. Most previous studies included impact 3 (lateral) and impact 4 (frontal) but not impacts 1, 2, or 5, in their oblique impact tests ( Trotta et al, 2018b ; Connor et al, 2018 ; Abayazid et al, 2021 ; Juste-Lorente et al, 2021 ). Interestingly, our results showed that impact 5 produced the highest PTA, and impact 1 produced the highest PRA for both headforms.…”

“…They found that the 3D-printed headform produced much higher rotational acceleration and velocity than the EN960-M headform under the oblique impact, but it is not clear whether this difference is due to their different MoIs or CoFs, or both. Similarly, other studies have used headforms with realistic CoF but not MoIs ( Trotta et al, 2018b ; Juste-Lorente et al, 2021 ; Zouzias et al, 2021 ). The rotational kinematics of a headform that has both biofidelic MoIs and CoF remains unknown.…”