Strain Rate Dependent Behavior of Vinyl Nitrile Helmet Foam in Compression and Combined Compression and Shear

Abstract

Vinyl nitrile foams are polymeric closed-cell foam commonly used for energy absorption in helmets. However, their impact behavior has never been described in isolation. This study aims to characterize the strain rate dependent behavior of three VN foams in compression and combined compression and shear. Vinyl nitrile samples of density 97.5, 125, and 183 kg/m3 were submitted to quasi-static compression (0.01 s−1) and impacts in compression and combined compression and shear (loading direction of 45°). For impacts, a drop test rig was used, and a method was developed to account for strain rate variation during impactor deceleration. Young’s modulus and stress at plateau were correlated with foam density in both compression and combined loading. Vinyl nitrile foams were strain rate dependent: The absorbed energy at the onset of densification was two to four times higher at 100 s−1 than at 0.01 s−1. In combined loading, the compressive stress at yield was reduced by 43% at a high strain rate. Compared to expanded polypropylene, vinyl nitrile foams transmitted less stress at the onset of densification for equivalent absorbed energy and presented a larger ratio between the compression and shear stresses in combined loading (0.37 at yield). This larger ratio between the compression and shear stresses might explain why vinyl nitrile helmet liners are thought to be better at reducing head rotational acceleration than expanded polypropylene helmet liners.

Effect of Compressive Strain Rate on Auxetic Foam

“…Impact testing auxetic closed cell foam, with similar stiffness (1 MPa, Figure 6d) to foam in sporting PPE [5,6,23,29], could show whether the established benefits of NPR to quasi-static indentation resistance (Equation (1) [32,[41][42][43][44][45]) and peak force under impact [34][35][36][37][38], inferred but not tested here, could improve sporting PPE. Further high-speed impact [81] and indentation testing could assess theoretical improvements by NPR. As the magnitude of NPR can decrease between quasi-static and high strain rate tests, high-speed indentation studies should focus on testing auxetic foam that has high magnitude quasi-static NPR (e.g., [42,54,[59][60][61]63,71]).…”

Finite element-based optimisation of an elastomeric honeycomb for impact mitigation in helmet liners

“…This, however, is contrary to the fact that the most commonly occurring helmet impact occur at an angle [54]. The forces that arise, therefore, have components of compression and shear [55], [56], [57] which ultimately leads to a rotational velocity and acceleration. It is widely accepted that the human head is susceptible to rotational kinematics [58] and that these loading regimes are more closely linked to traumatic brain injury [59], [60].…”

Optimisation of an elastomeric pre-buckled honeycomb helmet liner for advanced impact mitigation

“…At least 10 elements, with an average size of 1.85 mm, were meshed through the thickness to prevent shear locking. VN's strain-rate dependant mechanical behaviour for two densities (125 kgm −3 and 183 kgm −3 ) was obtained from the literature, to calibrate a numerical material model [24]. The foam liner elements were modelled as a hyperelastic material, using the Hyperfoam model available in the Abaqus material library [62].…”

“…Conventional helmet liners typically mitigate impact by being constructed from expanded polystyrene (EPS) or vinyl nitrile (VN) foam [21], with their energy absorbing properties defined by the density of the base material [22]. VN holds a unique advantage over EPS due to its ability to sustain several impacts without compromising its energy absorption capability [23,24]. Accordingly, VN is highly desirable for protective devices that must respond effectively to consecutive impact events, like in snow sports, ice hockey and American football [25].…”