July 14, 2004
Monitoring system to be integral part of future spacecraft fuel tanks
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.
Sources: Douglas Adams, (765) 496-6033, firstname.lastname@example.org
Kumar Jata, (937) 255-1304, email@example.com
Purdue News Service: (765) 494-2096; firstname.lastname@example.org
Note to Journalists: An electronic or paper copy of the research paper is available from Emil Venere, (765) 494-4709, email@example.com.
Incipient Damage Identification Using Elastic Wave Propagation through a Friction Stir Welding Al-Li Interface for Cryogenic Tank Applications
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.