The shape of things to come
by Kathleen Ricker

One might say that Harry Hilton’s work is beyond cutting-edge—in the sense that the materials with which he works...well, simply don’t exist yet.

The USS Asheville, a Los-Angeles-class fast attack submarine, on its way to conduct high-speed surface drills off the coast of Southern California. Hilton is researching what would be required to create designer materials that, among other things, will prevent submarine hulls from generating oscillations that could be audible to enemy sonar. view larger

Hilton is Professor Emeritus in UIUC’s Aerospace Engineering Department and Senior Academic Lead for Computational Structural/Solid Mechanics at NCSA. As an engineer and physical scientist, one of his lifelong motivations has been better design—not merely of buildings, aircraft, and bridges, but also of the materials that constitute them. One of Hilton’s particular interests is in how to protect the integrity of structures subjected to aerodynamic “noise,” highly turbulent frequencies caused by high-velocity air flow that can dramatically weaken structural integrity.

Obviously, many current structures do work successfully and safely when subjected to the intense oscillations of high air flows. Panels don’t regularly fly off airplanes. Skyscraper windows don’t pop out of their frames. Bridges don’t commonly fly apart in the manner of the famous Tacoma Narrows incident, now a classic textbook engineering case study. However, many such structures only work because they’re over designed, says Hilton. “In other words, you know the problem is there, so you fix it by making the structure more rigid, making it thicker, any number of things. So it’s not going to fail, but it’s still a bad design, because you could be innovative and do much better.”

Over designing happens, according to Hilton, because the materials engineers have to work with are not necessarily the best suited for their purposes. He takes a different approach: working backwards, he investigates not what the best existing materials for a particular kind of loading condition are, but what kinds of qualities new materials—what he refers to as “designer materials”—would need to have in order to meet those conditions.

“It’s sort of like playing creation,” says Hilton. “What I do is totally speculative, in the sense that I’m looking to the future and seeing what kind of material performance we need. We need them today, but I say “future” because someone still needs to manufacture these new designer materials.”

With seed funding from TRECC and a UROP grant from NASA Hilton and his aerospace engineering student Hank Lee are currently focusing on simulating the effects of aerodynamic or fluid noise on the kind of plates used on fuselages of planes and hulls of submarines. One of their objectives is to ascertain what properties a material would need to have in order to minimize the impact of turbulent air—or water, in the case of a submarine—flowing over panels of a fuselage or ship’s hull that could cause deflection. Deflection, which causes metal plates to bend back and forth, is a particularly serious problem for aircraft in that it creates “flutter,” amplifying the effects of aerodynamic noise, creating an aerodynamic feedback loop that can cause catastrophic structural failure—of precisely the sort that destroyed the Tacoma Narrows Bridge in 1940 and caused windows to pop out of the John Hancock Tower in Boston in 1973.

In the case of submarines, the critical issue is stealth, in addition to safety. Submarines have double hulls. While the outer hull is by necessity rigid, the inner hull is more flexible, which causes them to oscillate at different rates as a result of the air and water trapped between them. These oscillations can then be picked up by sonar. Hilton suggests that materials used to construct then, should either minimize the oscillation to an undetectable point or mask (dampen) the sound so that listeners can’t pinpoint the submarine’s location.

Hilton’s problem involves first working out the analysis—“the messy operations of complicated equations,” as he describes it—and then making pilot computations on his own desk machines on a small scale, using codes he and his students have developed in combination with commercial packages. Some of the parameters he will be evaluating include how mass is distributed over a plate, what causes deformations to be smaller or larger, what primary properties result in low failure probability and long survival times, and how the properties might vary throughout a single plate. To increase the amount of data, he then expects to scale the analysis up and to parallelize it to run on TRECC’s cluster.

Hilton anticipates that the kind of material that might minimize deflection could very well require concentrating different properties in different areas and therefore have a non-homogenous composition, like concrete or polymer nano-composites. “It’s not just the material properties you have that matter,” says Hilton. “It also important how you arrange them within the structure.”

The current research carries further the work begun by Hilton in collaboration with Professor Sung Yi of Portland State University and Dr. Cristina Beldica, currently program manager at NCSA. It will be presented this July in Southampton, UK, at the Ninth International Conference on Recent Advances in Structural Dynamics.