Keywords: bumpers, granular medium, impact, energy, dissipation, collisions.

Automotive bumpers are essential components, installed to minimize harm to both the car and passengers during collisions. The bumper is one of the most frequently replaced and repaired automobile part [1]. A good bumper system is functionally expected to withstand low speed impacts without damage to safety systems, viz; lights, brakes and steering systems. Most bumpers comprise of three basic components: an outer skin (mainly for aesthetic purposes), a rigid inner beam of steel, aluminium, or reinforced plastic, and an energy absorber of some kind [2].

Energy absorber designs may store or dissipate energy [3]. Storing implies a subsequent return of the impact energy in a rebound reaction (for example, springs). Dissipation on the other hand can be either destructive - metal flow - or non-destructive - shearing of a working fluid. Automobile bumper systems are of maximum value from a performance and aesthetic standpoint only when at the design position, implying "self-restoration" after impact.

To date, more research effort has gone into energy absorbing systems as compared to energy dissipating systems. Due to weight considerations, foam materials, such as polyurethane, ethylene vinyl acetate (EVA) copolymer, expanded polypropylene, and aluminium foam have been used in energy absorption. Geometry aspects of varied designs have also been explored, such as the use of egg crate design and honey-comb structures.

The effectiveness of current bumpers lays in harnessing properties of material from which their constituent components are made. An attempt has been made to shift from this traditional design platform by concentrating on the energy dissipation dynamics of the system's components. A conceptual bumper system simulating a human ergonomic phenomenon has been modelled. By closely matching energy implications in the human context to the model's impact energy dissipation mechanisms, desirable performance expectations have been identified. Functional aspects have been evaluated in terms of impact energy dissipation, packaging space, conservation of geometry, and minimization of repair costs.

Results from experimental testing of the conceptual system assembly showed that encapsulated granular media can be used to effectively manage impact energy by manipulating boundary conditions using lattice dimensionally-structured plates. Of the Bravais lattice structures, the BCC pattern proved to work best with the conceptual energy management system, giving even better results than random packing, which characterizes naturally existing granular media such as sand. The system's lubrication with an appropriate lubricant improved functional performance and recovery of the geometry.

1

Consumer Bumper Quality Disclosure Bill., Online, 2004, www.smartmotorist.com/bum/bum.htm, Accessed 6 January 2007.

2

Drew Winter, 1994, The Bumper Materials War - Plastics Increasingly Used in Automobile and Light Truck Bumpers, Online, www.findarticles.com/p/articles/mi_m3165/is_n1_v30/ai_14979251, Accessed 6 January 2007.

3

Carpenter K.H., Kerr L.L., "The 1973 General Motors Hydraulic-Pneumatic Energy Absorber Bumper System", SAE Preprints, (n730031): 8p, 1973.

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