Variable area, constant force shock absorption motivated by traumatic brain injury prevention

Abstract

Compact and efficient energy absorption is desirable for numerous applications including manufacturing, transportation, and protective equipment. An ideal shock absorber is a smart material or structure that can adapt its force-displacement properties to minimize the peak impact force regardless of the impact energy. While traditional shock absorbers can produce precisely-tuned ideal force profiles, they are rigid devices that only compress half of their total length, limiting utility in space-constrained applications. Energy absorbers that are soft and collapsible, such as foams, do not have ideal force profiles and generally have insufficient viscoelasticity to adapt to different impact energies. Here, we present a smart structure concept-variable area shock absorption (VASA)-that leverages a changing contact area in a hydraulically damped collapsible system to passively adapt the force to the minimum necessary to absorb the energy of an impact. Using an analytical fluid dynamics model, we derived the contact area as a function of compression to produce a constant force over the entire stroke of a fixed-orifice damper, and validate this concept experimentally using a preliminary 3D-printed prototype. The VASA prototype follows the constant force profile with a NRMSE between 0.02-0.25 at impact speeds between 2.3 and 4.3 m s −1 . This new approach for absorbing energy is compatible with full collapse of the absorber, enabling soft devices for space-constrained applications in future work. Potential applications include helmets that must absorb energy at a near-minimum force level across multiple impact energy levels.

Collapsible fluid-filled fabric shock absorber with constant force

“…For the majority of impact velocities, the orifice Reynolds number is sufficiently large to represent the turbulent flow. In the turbulent flow the orifice discharge coefficient is constant and equal to 0.7 (Fanton et al, 2020). Moreover, the viscosity of the fluid in an incompressible turbulent flow regime has negligible effect on the discharge coefficient of the orifice.…”

“…An ideal shock absorber exerts a near-constant force over its full stroke length to absorb the energy of an impact (Baumeister et al, 1997;Fanton et al, 2020;Spinelli et al, 2018;Vahid Alizadeh et al, 2021). The necessary constant force level is inversely proportional to the usable stroke length; a longer stroke length allows the shock absorber to spread the force out over a longer displacement, allowing for a lower force level.…”

“…The necessary constant force level is inversely proportional to the usable stroke length; a longer stroke length allows the shock absorber to spread the force out over a longer displacement, allowing for a lower force level. The magnitude of the constant force must scale with the impact energy, such that the ideal minimum force is provided regardless of the initial impact speed (Fanton et al, 2020). Hydraulic shock absorbers can be tuned to absorb a given energy at the minimum force level through both passive and active systems (Dixon, 2008;Fang et al, 2013;Hundal, 1977).…”

“…Given the inverse relationship between force level and usable stroke length, losing half of the usable stroke length necessitates a doubled force level, thus making existing hydraulic dampers inefficient for use in many space constrained applications (Lamb and Hoshizaki, 2009;Zhang et al, 2017). This paper builds off of the concept of variable area shock absorption (Fanton et al, 2020). Previous studies have investigated various other approaches to achieving an optimal constant shock absorption force.…”

“…In prior work, we presented the concept of telescoping variable-area shock absorption (VASA)-a fixedorifice hydraulic shock absorber with a contact area profile that increases in a piecewise constant manner throughout its stroke length to provide a nearlyconstant force (Fanton et al, 2020). In this prior work, we validated the physics of the VASA concept through a bench-top rigid 3D printed telescoping prototype which represents a piecewise approximation of an ideal variable-area shock absorber.…”