This paper, which was accepted for publication in October 2008, has finally appeared in print. About bloody time.
Title: Optimal Design of Composite Granular Protectors
Authors: Fernando Fraternali, Mason A. Porter, and Chiara Daraio
Abstract: We employ an evolutionary algorithm to investigate the optimal design of composite protectors using one-dimensional granular chains composed of beads of various sizes, masses, and stiffnesses. We define a fitness function using the maximum force transmitted from the protector to a "wall" that represents the body to be protected and accordingly optimize the topology (arrangement), size, and material of the chain. We obtain optimally randomized granular protectors characterized by high-energy equipartition and the transformation of incident waves into interacting solitary pulses. We consistently observe that the pulses traveling to the wall combine to form an extended (long-wavelength), small-amplitude pulse.
Note that this paper is in many senses an engineering paper. Some of the results are definitely things that would be nice to understand in a more "fundamental" fashion.