June 12, 2003 TROY, N.Y. -- Using a technique pioneered by researchers at Rensselaer Polytechnic Institute, terahertz (THz) radiation has been used to uncover small defects in a sample of space shuttle foam. This nondestructive method could help National Aeronautical and Space Administration (NASA) officials examine the insulating foam that is applied to each shuttle's fuel tank prior to launch.
Fuel tank manufacturer Lockheed Martin Space Systems (New Orleans) approached X.-C. Zhang, the J. Erik Jonsson Professor of Science at Rensselaer, and requested a study of a sample of the foam material. Zhang and his research team have employed terahertz radiation (T-rays) to spot defects, including air bubbles and separations, purposely embedded in a specially prepared sample. Such defects have proved difficult to locate using X-rays or ultrasound. Zhang's team (including doctoral students, Hua Zhong, Xie Xu, Tao Yuan, and Shaohong Wang) has been working closely with Lockheed Martin to study the sample. The sample is composed of material identical to that which would be applied to the shuttle fuel tank. In contrast to the continuous layer of foam normally applied to the tank, the sample is a block measuring two feet square and approximately four inches thick. An aluminum plate serves as the base for two different insulating materials: A one-inch layer of dense, cork-like Super-Lightweight Ablator (SLA) is applied on top of a three-inch layer of closed-cell Sprayed-On Foam Insulation (SOFI).
A total of eight man-made defects of various sizes were scattered throughout the sample. The embedded imperfections mimic defects that could potentially occur in a normally produced foam application on the fuel tank. Two types of defects were hidden in the sample: voids (or air bubbles), ranging from one-quarter inch to one inch in size, and debonds (separations between layers of foam or between a foam layer and the aluminum base).
NASA investigators believe that the Columbia space shuttle crash may have been caused by foam insulation breaking away and striking the left wing of the craft.
A New Way to See
The technology behind the emitters and detectors used to produce and sense the T-rays was developed at Rensselaer. The researchers use electro-optic crystals and a femtosecond laser to generate and detect the THz signal. They are able to locate and identify defects in the insulating foam sample by measuring the signal amplitude, temporal delay, and waveform distortion of the signal.
Optimal THz scan sensitivity also depends on the material being looked at. Thickness and density of an object can affect how far the T-rays will penetrate and how widely they will scatter. Both the SLA and SOFI materials making up the insulating foam sample happen to be excellent subjects for THz radiation, says Zhang. "The foam has a lower attenuation, allowing the terahertz waves to penetrate to a depth of many inches."
T-rays lie within the far-infrared region of the electromagnetic spectrum – the large range between microwaves and visible light. The unique properties of THz radiation make it a potentially excellent complement to existing imaging methods such as X-rays and ultrasound. The safety and sensitivity of T-rays may allow the technology to someday play a part in security searches for weapons and toxins, and could improve detection of breast and skin cancer.
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