Introduction:: Helmet-testing headforms are used to replicate the human head’s impact response, enabling the assessment of helmet protection and evaluation of injury risk. Accurate representation of rotational kinematics is critical as they are key predictors for brain injury risk [1]. Studies have suggested that peak rotational acceleration (PRA) and peak rotational velocity (PRV) are sensitive to individual headforms’ friction during oblique impacts [2-5]. However, there has been no quantification or comparison of the overall effect across the three commonly used headforms in helmet testing: EN960, Hybrid III, and NOCSAE. Each headform also has different frictional characteristics, and only one study has compared headforms with different moments of inertia (MOI) and frictional coefficients, finding associations in PRV and PRA [6]. Nonetheless, there was no specification if the response changes were from friction or MOI differences in the headforms [6], and MOI plays a crucial role in oblique impact response [7-8]. Therefore, the actual effect of friction on oblique impact measures is unknown, and recommendations for which headform is most biofidelic have not been made. This study aimed to quantify the influence of headform coefficient of friction (COF) and inertial properties on oblique impact response, including peak linear acceleration (PLA), PRV, and PRA.
Materials and Methods:: A total of 100 oblique impact testing was completed using guided drop tests of a helmeted headform (helmet, KASK Protone Icon) onto a 45-degree anvil with 80-grit sandpaper. Each headform (EN960, Hybrid III, NOCSAE, Hybrid III with skull cap, NOCSAE with skull cap) was tested at two speeds (4.8 and 7.3 m/s) and two orientations (y-axis and x-axis rotation) with five repetitions (Figure 1). Using a specially designed tribometer, the static COF of each headform was measured against the helmet’s lining material. Each headform was instrumented with a six-degree-of-freedom sensor package at its center of gravity. Data were collected at a sampling rate of 20 kHz and filtered using a 4-pole Butterworth low-pass filter with cut-off frequencies of 1650 Hz (CFC 1000) for accelerometer signals and 289 Hz (CFC 175) for angular rate sensor signals. Resultant PLA, PRA, and PRV were calculated for each test. ANOVAs with type II sums of squares (SS) were used to quantify the influence of COF, inertia (published [7-9]), and location on PLA, PRA, and PRV at the two different impact speeds.
Results, Conclusions, and Discussions:: Against the helmet lining material, the static COF were 0.92 ± 0.04 NOCSAE, 0.83 ± 0.03 Hybrid III, 0.48 ± 0.04 EN960, 0.38 ± 0.02 NOCSAE with skull cap, and 0.37 ± 0.02 Hybrid III with skull cap (Figure 2). In a previous study, we determined that the EN960, NOCSAE with a skull cap, and Hybrid III with a skull cap have COFs closest to a human head. COF affected all measures at high-speed impacts, but friction had a smaller effect than inertial properties for PRA and PRV, but not PLA (Table 1). Our model suggests that an increase in COF by 0.1 increases PLA by 3.5 g, PRA by 204 rad/s2, and PRV by 0.43 rad/s. In comparison, a decrease of 25 kg*cm2 in MOI decreased PRA by 1379 rad/s2 and PRV by 4.1 rad/s. However, at the low-speed impacts, friction only had a meaningful effect on PRV, agreeing with previous literature that friction’s effect depends on tangential velocity. However, MOI still had large effects on PRA and PRV at the low-speed impacts (Table 1). At the low-speed impacts, an increase of 0.1 COF caused an overall minor alteration in PRV by 0.23 rad/s, while a 25 kg*cm2 decrease in axial MOI decreased PRA by 729 rad/s2 and PRV by 3.0 rad/s. Overall, we recommend that MOI and COF be considered for their respective influences when comparing helmet testing data between different headforms. This recommendation is based on the strong relationship between rotational impact response and brain injury risk models. These results also provide a framework for cross-comparative analysis between studies that use different headforms and headform alterations and for investigating the implications in injury prediction between headforms.
Acknowledgements (Optional): : KASK S.p.A. ad Unico socio provided funding support for the completion of this project.
