Characterization of Fluid-Filled Interconnected Dual-Cells for Reduction of Force and Linear Acceleration due to Blunt Impact

Impact reduction mechanisms are important for reducing sport-related injuries. Major known factors that contribute to increased likelihood of injury in impact scenarios are high forces, high linear and rotational accelerations, and shorter impact duration. Foam materials, air cells, and a combination of both are widely used in protective gears (helmets and pads) for prevention of injuries in sports. Many of the top ranked football helmets today apply similar concepts, for instance Xenith X2E uses foam cells and shock absorbing air cells, while Schutt Air and Riddell SPEED utilize interconnected foam cells and multilayer foam cells, respectively. This research is in continuation of a concept presented in an earlier study, where an interconnected fluid-filled cells approach has been proven to reduce linear acceleration and reaction force. The generic architecture consists of a primary cell with connecting channel(s) to a secondary cell. Only the primary cell experiences impact, in which it will deform and transfer fluid from its own cell to a secondary cell. This action aids in the absorption of impact energy by the secondary cells as they expand, thus reducing the transfer of impact forces to the head/body.  In the current study, simulation research is being performed on a variety of new cell arrays and geometries for the interconnected fluid-filled cells concept. The study will demonstrate how changing the geometry and materials of the air cells will affect the reaction force, duration of impact, and linear acceleration due to impact. The new design variations explored are intentional modifications to accommodate for its potential use in a football helmet; however, this design would be able to adapt for other protective gears. Three major variables were examined to observe the differences in the efficiency of the interconnected dual-cells design: primary and secondary cell architecture, channel geometry, and type of materials. Finite element analysis was performed for design evaluation and optimization. Some variations of the primary and secondary cell architectures include mixed combinations of cell locations (same plane or multilayer), cell geometries (height, thickness, diameter), and added corrugations to the side walls and/or top membrane of the cells. Side wall corrugations were intended to be useful in shock absorption during compression (primary cell) and expansion (secondary cell), while top membrane corrugation was added to allow for increased expansion of the secondary cell. Results from this simulation study demonstrated the significance of these parameters in changing the impact reduction characteristics. For instance, different combinations of corrugated and non-corrugated pairs of the primary and secondary cells were explored and resulted in the lowest force reaction with a non-corrugated primary and corrugated secondary cell combination. Channel geometry was also explored, demonstrating that the diameter of the channel had a noticeable effect on impact reduction capabilities of the cells. Hyperelastic materials Silicone (XIAMETER® RTV-4234-T4) and polyurethane (Smooth-On PMC 724) were used as a cell material to study the effects of different elastic moduli. In comparison to the interconnected dual-cell design, the Xenith X2E helmet liner uses impact absorbing cells of similar notion, with major differences being that Xenith cells are foam-filled, made of TPU, and not interconnected. For a comparison study, the dual-cells will be evaluated against Xenith. In the proceeding paper, Xenith X2E helmet mandible compression shock part will be tested on a custom-build pendulum impact testing apparatus at 5m/s impact speed. The reaction force, acceleration, and duration of impact results will be contrasted with simulation results by using the commercial software ANSYS. The future efforts on this work will investigate experimental studies of the interconnected cell variations using the custom impact testing apparatus.