Collapsible fluid-filled fabric shock absorber with constant force

Abstract

Energy absorption is an important challenge shared by multiple different industries including manufacturing, transportation, and protective equipment. This paper presents a novel shock absorption system based on woven fabrics designed to fully collapse and absorb energy at a near-minimum force level. The proposed system exerts an approximately constant force across different impact velocities. This system utilizes a fixed-orifice hydraulic shock absorber with variable contact area over its displacement to provide a nearly-constant force which scales with impact energy. Using analytical fluid dynamics, the contact area needed to produce a constant minimum force is derived. A finite element model of this shock absorber is created to validate the concept. Different impact conditions are simulated. The results confirm that the proposed fabric shock absorber is capable of producing a nearly-constant force across different impact energies. In simulation, the fabric shock absorber follows the ideal constant force profile with an averaged efficiency of 77.8% ± 3.4% which is far above standard foams which have efficiency of only approximately 20%–40%. The proposed system is compared with a cylindrical damper to show performance improvements gained through variable area geometry. Potential applications of this technology include soft devices for space-constrained applications such as contact sport helmets.

Finite element evaluation of an American football helmet featuring liquid shock absorbers for protecting against concussive and subconcussive head impacts

“…The liquid shock absorber utilized throughout this study was modeled as a cylinder that can decelerate an impact mass in its axial direction (Figure 1), which was inspired by the cylindrical liquid shock absorber originally modeled by Vahid Alizadeh et al (2022). In an impact, the top surface of the cylinder is pressed downwards, and the liquid inside is forced to flow through the orifice.…”

“…Furthermore, multiple material angles were included to represent the different weaving directions of the fibers. *MAT_FABRIC was used to model the fabric material, which has a density of 777 kg/m 3 , longitudinal modulus of 76 GPa, transverse modulus of 0.6 GPa, and shearing modulus of 0.8 Gpa (Vahid Alizadeh et al, 2022). The side wall was meshed by 1,292 elements with an average length of 1.5 mm.…”

“…An internal incompressible fluid with water properties was used as the liquid inside the shock absorber (bulk modulus is 2.2 Gpa, density is 1,000 kg/m 3 ), and a discharge coefficient of 0.7 was used (Fanton et al, 2020;Vahid Alizadeh et al, 2022). The water was modeled as a control volume and no mesh was associated with the fluid part, but the inertia of the liquid was included.…”

“…In practice, many of these existing technologies exert a force based on the amount they are compressed, leaving them prone to high spikes in reaction force when they reach maximum compression or "bottom out" (Fanton et al, 2020). The impact force profile of an ideal shock absorber, on the other hand, displaces through its full stroke at a constant force, which scales based on the speed and energy of an impact speed (Baumeister et al, 1997;Spinelli et al, 2018;Vahid Alizadeh et al, 2022). Research has suggested that, in the context of a protective helmet, a constant force profile is optimal for preventing brain injury (Vahid Alizadeh et al, 2021).…”

“…Research has suggested that, in the context of a protective helmet, a constant force profile is optimal for preventing brain injury (Vahid Alizadeh et al, 2021). While gases and solids have yet to achieve such a response that is capable of scaling with impact velocity, prototype and computational models of liquid shock absorbers have demonstrated promising results on achieving a force profile near to that of an ideal shock absorber (Fanton et al, 2020;Vahid Alizadeh et al, 2021;Vahid Alizadeh et al, 2022).…”