WEST LAFAYETTE, Ind. — Researchers have demonstrated how a "structural health
monitoring" system will likely be used to pinpoint damage in a new class of
large metal fuel tanks for future spacecraft.

The monitoring system would be needed for proposed "space operations vehicles,"
which would fly many more missions than the current space shuttles, said Kumar
Jata, an engineer at the U.S. Air Force Research Laboratory/Materials and
Manufacturing Directorate.

The research is related to the field of "non-destructive evaluation," commonly
referred to as NDE, which involves inspection methods that enable technicians to
analyze structures without taking them apart.

"If you are going to be performing many missions, that means the tank is going
to be used over and over again, so it’s critical to have a health monitoring
system that constantly checks for damage," said Jata, who specializes in metals
development and processing and is a technical adviser in the Metals, Ceramics
and NDE Division at Wright-Patterson Air Force Base in Ohio.

The system uses a high-frequency "actuator," which is a miniature loudspeaker,
to produce sound waves that travel through a material. The sound creates
vibration waves that are picked up by an array of sensors. The sound waves
behave differently when passing through damage caused by cracks and other flaws,
producing differing vibration patterns, said Douglas E. Adams, an assistant
professor of mechanical engineering at Purdue.

An onboard monitoring system that looks for signs of damage in vibration wave
patterns would be critical because the fuel tanks would undergo extreme changes
in pressure and acceleration during launch and while re-entering the Earth’s
atmosphere, said Adams, who is working with the Air Force to develop the
monitoring system.

Engineers have recently demonstrated that the system effectively detects and
locates subtle damage in a new lightweight alloy that will likely be used to
create fuel tanks for future spacecraft and satellites.

Findings from the Air Force-funded research were detailed in a paper presented
July 7 during the Second European Workshop on Structural Health Monitoring in
Munich, Germany. The paper was written by Adams, Purdue doctoral students
Muhammad Haroon and Shankar Sundararaman, and Jata.

The experimental fuel tanks are manufactured using a new type of welding in
which a rotating pin "stirs" the metal from opposing plates until they form into
a single piece. The method, called friction-stir welding, creates welds many
times stronger than conventional welds, which weaken materials by melting them,
Adams said.

"The rotating pin causes the metal to plastically deform, and it stirs it,
literally," Adams said. "It looks like you’re making a milkshake. As you make
this milkshake along the weld, the material comes together and joins."

Unlike conventional welding, the two plates being welded are not heated to the
point of melting.

"When you melt a material and it recrystallizes, you are weakening the
material," Adams said. "You can get voids, and when something breaks, very often
it breaks at a weld. The new friction-stir welding method gives you much better
strength and toughness than competing welding methods that have been in
existence for many years.

"The tanks, made from a very lightweight aluminum-lithium alloy, will hold
cryogenically cooled fuel and/or liquid oxygen for rocket motors. These tanks
are enormous and they are quite thin-walled, which means they are quite
flexible. They flex, squeeze and undergo acoustic loads from the extremely loud
noise of rocket launches."

The tank walls contain a machined grid that looks like a continuous pattern of
adjacent, rib-like triangles that provide extra strength without adding much
weight to the structure.

Jata helped develop the alloy, working with the Alcoa Technical Center near
Pittsburgh.

"The Air Force has been the driving force behind that material because it
provides you with a lot of weight savings," Jata said.

He said the onboard monitoring system could save time and money by telling
technicians when a part was damaged or worn out, cutting down on unnecessary
scheduled maintenance.

Technicians would still have to perform routine non-destructive evaluation on
the spacecraft after each flight. Such tests include using a dye that changes
color if damage is present in a material, hand-held devices that use
high-frequency sound waves to detect damage and "eddy current sensors" that use
electromagnetic fields to analyze material.

"We have very reliable NDE techniques, but they take time and increase the
operations costs," Jata said. "If you have a good, robust health monitoring
system in place, then perhaps you could reduce the inspection time after each
flight."

Adams has shown in related research that future spacecraft and "hypersonic"
aircraft that will travel several times the speed of sound must be equipped with
a structural health monitoring system that constantly records vibration patterns
to detect subtle damage as it occurs in real time. Otherwise, this "incipient
damage" will not be detected, he said.

Findings in the paper show the system was able to not only detect such incipient
damage, but also to pinpoint its location on a flat piece of the alloy, in
research at Purdue’s Ray W. Herrick Laboratories. Adams developed an algorithm,
or software that uses mathematics to analyze vibration patterns with so-called
"wavelet transformations," that breaks data into pieces to help detect and
pinpoint tiny changes in the signals.

"The incipient damage is smaller than a crack but, if left undetected, could
eventually become larger and pose a safety threat," Adams said. "We simulate
this sort of damage by heating a very small spot of material with a localized
heat source. The heating is not high enough to melt the metal, but it
temporarily creates changes in the microscopic structure of the metal — the
same kind of changes seen in incipient damage.

"The heating does not create permanent damage, so we are able to conduct
numerous tests in different locations simply by applying the heat source to
those locations."

Jata said such metallic cryogenic tanks with onboard health monitoring systems
could be used within the next 10 to 15 years in the military spacecraft.

The next step in the research will be to test the monitoring system on curved
pieces of the alloy, instead of the flat pieces used in the current work. The
curved segments will be similar to the curved walls of actual tanks, said Jata,
who worked with Purdue researchers while on a recent sabbatical at the university.

Note to Journalists:
An electronic or paper copy of the research paper is available from Emil Venere,
(765) 494-4709, venere@purduel.edu .

ABSTRACT

Incipient Damage Identification Using Elastic Wave Propagation through a
Friction Stir Welding Al-Li Interface for Cryogenic Tank Applications

Shankar Sundararaman, Muhammad Haroon, Douglas Adams, Kumar Jata

Organic matrix composite and metal alloy tanks are being considered by the
United States Air Force for next generation space vehicles. In particular, Al-Li
alloys can offer substantial weight reduction with reduced susceptibility to
leakage. Unlike in expendable tanks, reusability and quick turnaround time
between missions are key considerations for future space operations. Therefore,
identification, quantification and location of damage on the reusable cryotanks
have become key drivers in achieving safety and life-cycle cost objectives. This
work investigates acoustic elastic wave propagation sensing and data
interrogation methodologies for extracting features that can potentially be used
to identify incipient damage in the form of gradients in dislocation densities
in a friction stir Al-Li butt weld. Localized temperature gradients are used to
simulate spot changes in weld material density in static tests to mimic
dislocation gradients along the weld and elastic waves with various narrow and
broadband spectra are propagated through the weld to detect these incipient
forms of damage. It is demonstrated that discrete Fourier transforms and
harmonic wavelet transforms can be used in conjunction with elastic waves to
identify low-level incipient damage in metal cryotank material systems.