Author(s): RIETH, V., BOURDET, N., MEYER, F., DECK, C., Institution: UNIVERSITY OF STRASBOURG; ICUBE LABORATORY, Country: FRANCE, Abstract-ID: 1263

INTRODUCTION:
Expanded polystyrene (EPS) is the most widely used foam in helmets for shock attenuation due to its high energy absorption capability [1]. Modeling the behavior of EPS remains a critical challenge because of its complex mechanical response. Nevertheless, it is the primary component of helmet responsible for energy absorption during an impact [2]. Accurate helmet modeling can improve the understanding of injury mechanisms and support design optimization based on biomechanical criteria.
METHODS:
More than 500 experiments were conducted on energy-absorbing samples over a wide range of strain rates, from quasi-static conditions (0.001 s⁻¹) to dynamic loading (200 s⁻¹), to characterize the behavior of expanded polystyrene (EPS) through stress–strain curves across various densities and strain rates. The robustness of the experimental database was evaluated using statistical analysis based on an integrated coefficient of variation (CV). The resulting experimental stress–strain curves were implemented in LS-DYNA using the *MAT_083 Fu–Chang Foam material model. Numerical simulations replicating the experimental tests were then performed to validate the material laws in terms of acceleration and displacement time histories.
Subsequently, the calibrated material properties for EPS and other helmet components were assigned to the corresponding parts of a developed ski helmet finite element model, which was coupled with a Hybrid III headform. Final validation of the ski helmet model was achieved by reproducing and comparing numerical and experimental results in accordance with the CERTIMOOV test protocol.
RESULTS:
The experimental campaign on energy-absorbing materials demonstrated high robustness, with stress–strain curves exhibiting a coefficient of variation below 10%. CORA analysis confirmed the validity of the numerical EPS material model, showing strong agreement with experimental acceleration and displacement time histories (CORA > 0.80). Furthermore, the close correspondence between experimental ski helmet tests and numerical simulations conducted in accordance with the CERTIMOOV protocol confirms the predictive capability of the model.
CONCLUSION:
This study proposes a new methodology for the experimental characterization and modeling of helmet energy-absorbing materials based on quasi-static and dynamic EPS experiments. Beyond the validation of material laws, a numerical ski helmet model was developed and experimentally validated, demonstrating strong agreement between predicted and measured responses.

[1] J. Zhang, et al., « Constitutive modeling of polymeric foam material subjected to dynamic crash loading », International Journal of Impact Engineering, vol. 21, no 5, p. 369‑386, mai 1998, doi: 10.1016/S0734-743X(97)00087-0.
[2] B. Leng,et al., « Recent bicycle helmet designs and directions for future research: A comprehensive review from material and structural mechanics aspects », International Journal of Impact Engineering, vol. 168, oct. 2022.