Rensselaer Researchers Use T-Rays to Uncover Defects in Space Shuttle Foam Sample

Could Lead to More Effective Method of Pre-flight Quality Control of Insulating Foam

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.

About Rensselaer

Rensselaer Polytechnic Institute, founded in 1824, is the nation’s oldest
technological university. The school offers degrees in engineering, the
sciences, information technology, architecture, management, and the humanities
and social sciences. Institute programs serve undergraduates, graduate students,
and working professionals around the world. Rensselaer faculty members are known
for pre-eminence in research conducted in a wide range of research centers that
are characterized by strong industry partnerships. The Institute is especially
well known for its success in the transfer of technology from the laboratory to
the marketplace so that new discoveries and inventions benefit human life,
protect the environment, and strengthen economic development.

IMAGE CAPTION:
[http://www.rpi.edu/web/News/Pressimgs/THz12-copy.jpg]
Rensselaer Polytechnic Institute graduate students Xie Xu (rear) and Hua Zhong
examine the insulating space shuttle foam sample. (Photo courtesy of Rensselaer
Polytechnic Institute.)