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The cell phone user as chemical detective
In the not-too-distant future, says Miltos Hatalis, the ubiquitous cell phone will acquire a new, silent function.
Fitted with arrays of gas sensors, it will monitor the air for leaks of toxic chemicals.
In fact, says Hatalis, professor of electrical and computer engineering, groups of cell phones will serve as dynamic, wireless networks of chemical sensors – first for police and other first responders and later for average phone users.
“What we see,” says Hatalis, “is the day when every cell phone will be a mobile chemical lab detecting and analyzing harmful chemicals. The phones will send data to a central location, which will correlate data to achieve a realistic picture of a regional environment.”
None of this will come to fruition until sensors can be integrated cheaply into cell phones.
Hatalis’ group is working with NASA to make multi-channel sensors that will be integrated with carbon-nanotube (CNT) sensing materials. The CNTs will be functionalized to absorb certain chemicals selectively. This will cause changes in their electrical resistance that signify gases have been detected.
One goal is to increase the number of sensors that can be fitted into a phone and to ensure their accurate performance.
“As the number of sensors increases,” says Hatalis, “it becomes impractical to wire each sensor directly to the circuit that measures the change in electrical resistance.
“We’ve developed an array of 64 sensors that requires only 16 wires. We’re exploring a device with 256 channels and 24 wires. Our goal is to have arrays of thousands of sensors that can be read with just a few input-output wires.
“By reducing the number of wires and utilizing glass substrates, we’ll end up with a device that is smaller, cheaper and much easier to integrate into a cell phone.”
Taking the sting out of explosions
Terrorist attacks have motivated engineers to design structures that withstand explosions without collapsing while also minimizing danger from flying debris.
Clay Naito, associate professor of structural engineering, has worked six years with the Air Force Research Laboratory and Portland Cement Association to assess the blast resistance of precast concrete panels. His group conducts full-scale blast tests to evaluate the resistance to explosions of standard wall panel construction techniques. The goal is to design the panels more accurately and efficiently.
Wall panels are typically designed to resist loads from handling, construction, wind pressure, thermal expansion and shrinkage, and from floors and roofs. Flexural (bending) resistance is also a concern.
During an explosion, a wall sustains a pressure increase of up to 28,000 pounds per square foot, which falls to a negative pressure, creating suction on the panel, before returning to ambient conditions. All of this happens in less than a 20th of a second.
Even a small explosion can cause a pressure increase 20 times greater than the maximum static load a panel is designed to support. Inertial and flexural resistance, says Naito, help a structure withstand a blast.
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| Magnetorheological (MR) dampers significantly reduce the vibration and drift of a structure during an earthquake, say ATLSS researchers. |
In an NSF project, Naito is attempting to improve the flexural performance of precast concrete sandwich panels by using reinforcement strategies that combine bonded and unbonded wire strands with an internal layer of insulating foam.
“An analytical study is under way to determine the most effective arrangement of bonded to unbonded strands,” says Naito.
Under close-proximity detonations, the pressure of the initial impact generates a compression wave that penetrates the thickness of the panel and reflects off an interior face as a tension wave. If this wave exceeds the tensile capacity of the concrete, fragments break off, or spall. If the spall depth exceeds half the thickness of the panel, a breach typically occurs.
“The propagation of the compression wave is reduced by the presence of low-density foam insulation,” says Naito. “We plan to study this effect and assess the potential for supplemental reinforcing materials such as carbon fibers, nylon and other synthetic fibers to improve the tensile strength of the concrete layers.”
Naito also collaborates with Auburn University. Blast testing is carried out at the Tyndall Air Force Base in Florida.
Treating Bridges as a system
The ambient vibrations caused by wind, river flow and car traffic are not the most dramatic loads a bridge sustains, says Shamim Pakzad, assistant professor of structural engineering. Forced vibrations from large trucks and earthquakes cause the most significant stresses.
But an assessment of ambient vibrations can paint a revealing portrait of a bridge’s structural health and enable engineers to evaluate more precisely the effect of an extreme event.
Pakzad and his students use networks of wireless sensors to study three truss bridges near Lehigh. On the Northampton Street Bridge connecting Easton, Pa., to Phillipsburg, N.J., the students installed 28 sensor units. In one day, they collected 3 million data points, or more than 100,000 information bits per sensor.
The sensors record ground vibrations to the bridge’s foundation as well as response vibrations by beams, columns and bridge deck. The baseline data will help engineers detect damage caused over the long term by truck traffic or over the shorter term by an extreme event.
The three bridges, says Pakzad, function as a system whose performance affects the social and economic life of eastern Pennsylvania.
“If one bridge is taken out of service, its traffic has to be taken up by other bridges in the region. This increases the risk to the other bridges and requires us to look at the behavior of the overall system.
“Instead of looking at one bridge and evaluating its prognosis, you look at all the bridges as a system. A decision about one affects the others.”
Pakzad recently helped lead a team that installed 64 wireless sensor units on San Francisco’s Golden Gate Bridge. The units worked as effectively as conventional wired sensors at a fraction of the cost.
Each wired unit on the Golden Gate cost thousands of dollars, says Pakzad. Each prototype installed by his group cost $200. Mass production and new design could cut that to $10.
