WEST LAFAYETTE, Ind. – Heat-shielding panels on future spacecraft could be constantly monitored from liftoff to landing to ensure safety, according to engineers who are developing a technique using vibration and sound measurements to detect subtle damage in a variety of structures.
"Future space vehicles and hypersonic aircraft may be equipped with a structural health monitoring system that constantly records how vibration and sound waves travel through materials and structures to detect damage as it occurs in real time," said Douglas E. Adams, an assistant professor of mechanical engineering at Purdue University. "Otherwise, large numbers of ground inspectors would need to spend a great deal of time looking for damage between flights.
"There is also a possibility that subtle damage just beginning to form, which could later lead to accidents, will not be detected."
Moreover, his research shows that such a detection system would be most effective during periods of highest stress – while the vehicle is taking off and reentering the atmosphere – when it is subject to the greatest pressures and temperatures. During those times, because of the way vibrations travel through the heated metal panels, certain kinds of damage are easier to detect in flight than while the spacecraft is sitting on the ground.
The research was funded by the U.S. Air Force Research Laboratory Structures Division.
A technical paper about the new findings was presented July 7 during the Second European Workshop on Structural Health Monitoring in Munich, Germany. The paper was written by Adams; Purdue graduate student R. Jason Hundhausen; Mark Derriso, an engineer leading the work in structural health monitoring at the U.S. Air Force Research Laboratory in Dayton, Ohio; and Paul Kukuchek and Richard Alloway, engineers for Goodrich Corp's Aerostructures Group in Chula Vista, Calif.
The health-monitoring system has been tested on a new generation of metal heat-protection panels developed by Goodrich Aerostructures.
"The fundamental advance we have made is that we have shown that unless you monitor for damage and loads while the vehicle is in the most severe part of the mission, you will likely miss incipient damage," Adams said. "We are developing mathematical models and data-analysis methods that overcome challenges to identify damage in real time.
"It's very important to note that damage is much easier to detect while the metal panels are heated during flight. That's because extremely hot temperatures reduce the stiffness of the metal, changing how the panels vibrate and making the flaws easier to detect with our techniques."
The panels have to withstand temperatures ranging from roughly minus 250 degrees Fahrenheit in space to 1,800 degrees during reentry.
"And you are talking about that change occurring in a relatively short period of time," Adams said. "On top of those rapid temperature changes, you have extreme acoustic loads – noise loud enough to burst your ear drums. These sound levels are much higher than at a rock concert.
"The loud noise causes vibration and sound pressure that continuously pulsate and produce forces on the panel."
Unlike the current space shuttle's ceramic tiles, the metallic panels could be easily replaced within minutes. Tiles on the space shuttle must be glued onto the orbiter using "strain-isolation pads" in a process that takes days. Future spacecraft and hypersonic aircraft that travel several times the speed of sound will likely have heat panels that are bolted in place. Replacing the panels would be a snap – simply a matter a unbolting the old panel and attaching a new one, Derriso said.
The panels are made of a "metallic sandwich" material capable of withstanding high temperatures and pressures. Each panel consists of two outer face sheets bonded to an inner honeycomb core.
Adams is helping the U.S. Air Force and NASA develop a structural health monitoring system, which uses sensors to record how vibration and sound waves travel through materials and structures. These waves respond differently when passing through damage caused by cracks and other flaws, producing differing patterns.
Researchers at Purdue's Ray W. Herrick Laboratories used the monitoring system to detect impact forces such as those exerted by a heavy tool being dropped on a panel – simulating and identifying resulting damage to bolts and the panel itself.
"If a micro-meteoroid or other form of debris strikes a panel, we want to identify how hard it hit that panel because designers know how much force the panels can withstand," Adams said. "If a force goes above a certain level, then we know that we ought to replace that panel the next time around."
The Air Force is developing technologies for a proposed "space operations vehicle," which will need a new kind of thermal protection system for reentering the atmosphere.
"One of the main goals is for this new vehicle to have a fast turnaround time from one mission to the next," Derriso said. "Obviously, a critical advantage of these heat panels is that they are mechanically attached.
"Right now the shuttle uses an adhesive to bond the tiles onto the airframe. Even if you detect damage in a particular tile, it's going to take a long time to replace that tile because you have to clean and prep the surface and re-glue these tiles back on – all of which takes time."
The innovative metallic heat-protecting panels were tested as part of NASA's experimental X-33 spacecraft program, one early concept for a space operations vehicle. The proposed spacecraft never flew but was tested in specialized chambers that recreate the extreme conditions of launch, space flight and reentry. The chambers, located at Wright-Patterson Air Force Base in Ohio, are the only ones capable of simultaneously simulating all of the conditions, including extreme pressures, temperatures and noise, Derriso said.
Goodrich Aerostructures created about 1,300 of the special panels for the X-33. The panels performed well and are available for future space vehicle applications, Kukuchek said.
One advantage of the panels is that they could be replaced in space. However, because many of the panels have unique shapes, the crew would have to haul hundreds of replacement panels to ensure a match for a specific damaged panel. A more practical approach, Kukuchek said, might be for crew members to repair damaged panels in space and reattach them before reentry.
Meanwhile, in two other papers also presented during the conference in Munich, researchers showed how structural health monitoring systems could be used to check for damage in future spacecraft fuel tanks made from a new lightweight alloy and also to record the quality of rivets in commercial and military aircraft.
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.
Researchers also showed how to use structural health monitoring to identify inferior rivets in aircraft, some of which contain as many as a million rivets. Inferior rivets lead to corrosion, cracks and potentially serious structural failure. The Purdue-developed method uses a rivet gun equipped with sensors that record data on every rivet installed. The data could be used to create "maps" that indicate locations of inferior rivets so that ground crews in the future could concentrate on those areas during routine inspections.
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