"Blast-proofing" structures studied

By ELLEN GOLDBAUM
Contributing Editor

Could structures be built with shock-absorption systems so powerful that jet planes would literally bounce off them?

A system modeled in a paper authored by UB theoretical physicists and published in the current issue of Physica A demonstrates that it may one day be possible to protect bridges, ships, skyscrapers, highway structures and even automobile bumpers from extremely powerful impacts.

In the paper, "Thermalizing an Impulse," Surajit Sen, associate professor of physics, and Felicia S. Manciu and Marian Manciu, former doctoral candidates in the Department of Physics, describe a system they envision that is capable of reducing the amplitude of a physical impact it receives by at least 95-98 percent.

The work shows further, Sen said, that it may be possible to turn the dissipated energy from a shock wave into usable thermal energy. It also suggests that in a similar way, energy may be able to be harnessed from natural phenomena, such as ocean waves and geothermal sources.

The theoretical research is supported by the National Science Foundation. Sen is collaborating with researchers at NASA to create an experimental system based on this work.

In the wake of the Sept. 11 terrorist attacks, Sen said the engineering applications of the research are most relevant. He added that the work also provides an important step in the physics of how shock waves travel through granular systems.

"Granular materials, such as sand or soil, have long been used in shock-absorbing systems, but they have had only mixed results," said Sen, who also conducts research on how weak shock waves penetrate through soil, information that he is applying to land-mine detection systems.

"Impulses simply will pass through systems where sizes of individual grains are about the same. In systems like ours, where grain sizes are altered in specific ways, a granular assembly can efficiently absorb the impulse," he said.

The shock-absorption system modeled by the UB physicists consists of a long, cone-shaped chain of spheres. The sphere positioned closest to the expected source of a shock is the largest, while each subsequent sphere is slightly smaller; the sphere closest to the structure—the building, bridge or ship—that the system is designed to protect would be the tiniest of all.

"The design is such that if a large shock hits the wide end of this tapered cone-shaped chain, then the shock would be broken down into an extremely large number of tiny shocks that would be received at the tapered end of the chain," he said.

"This very simple system demonstrates that theoretically, any size shock can be absorbed with assemblies of appropriately tapered chains," explained Sen.

He and his colleagues performed the research by first developing a model of the tapered-chain system and then by precisely solving Newton's equations of motion on the computer to describe the dynamics of the tapered chain system.

"What the solutions did was to describe the system's particle dynamics, that is, to describe precisely what type of motion is being experienced by each sphere in time," said Sen.

The calculations demonstrate that by tailoring the way in which the spheres are tapered, the size of the spheres and the length of the chain, the system could reduce the amplitude of literally any size shock for any type of material.

"The crux of the argument is that as mass gets reduced, the energy of the impulse gets distributed so that no single sphere is carrying too much energy," he said. "That is what reduces or absorbs the shock."

Sen envisions a shock-absorption material or a structural material, such as brick, in which these chains of spheres are embedded. At the same time, he said, the theoretical calculations provide a strong basis for the reuse of the mechanical energy of an absorbed shock wave. A similar reuse of impulses from nature, such as geological activities and ocean waves, also might be possible, Sen said.

"It could create a situation where not only could structures be made shock-absorbent, but it might be possible for the building, the ship or the bridge that received the impact to take advantage of that energy for its own internal systems," he said.

Sen said that it is likely that a system based on the present research could be used in conjunction with existing technologies for shock absorption.

He acknowledged that issues such as whether or not such a system would be cost-effective from an engineering-design viewpoint remain to be sorted out, and noted that much remains to be learned about shock absorption.

"How well do we know the science of shock absorption?" he asked. "I would contend that we do not know it well enough because the study of nonlinear effects, which is what a shock wave produces, is still so much in its infancy."

Sen is developing workshops and symposia for physics and materials-science conferences on a whole new field: new shock-absorbing systems that are able to harness the energy of any impact.