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 micrometeoroid 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.

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

Related releases:

* Monitoring system to be integral part of future spacecraft fuel tanks
http://news.uns.purdue.edu/UNS/html4ever/2004/040714.Adams.spacetanks.html
* Method aims to improve aviation safety by monitoring rivets
http://news.uns.purdue.edu/UNS/html4ever/2004/040714.Adams.rivets.html

PHOTO CAPTION:
Douglas E. Adams, an assistant professor of mechanical engineering at Purdue
University, strikes a spacecraft heat-shielding panel with a special hammer
equipped with a sensor, while graduate student Harold Kess, foreground, watches
data showing how the panel responds to the vibration caused by the impact. In
research funded by the U.S. Air Force, Adams is developing a system that uses
vibration to constantly monitor the special panels for damage during simulated
spaceflight. He has discovered that such a system works best while the panels
are heated as they would be during reentry. At that time, the system records
subtle damage that would not be detected when the spacecraft is on the ground
under normal temperatures. Consequently, future spacecraft equipped with the
panels could be constantly monitored from takeoff to landing to ensure damage
does not go undetected. Unlike heat tiles on the space shuttle, which are
ceramic and must be glued to the spacecraft’s underbelly, the experimental
metallic panels could be replaced within minutes simply by bolting on a
replacement panel. (Purdue News Service photo/David Umberger)

A publication-quality photo [2MB] is available at
http://ftp.purdue.edu/pub/uns/+2004/adams.vibration.jpg

ABSTRACT

Loads, Damage Identification and NDE/SHM Data Fusion in Standoff Thermal
Protection Systems Using Passive Vibration-Based Methods

Mr. R. Jason Hundhausen, Prof. Douglas E. Adams, Mr. Mark Derriso, Mr. Paul
Kukuchek, Mr. Richard Alloway

Standoff thermal protection system (TPS) panels are critical structural
components in future aero-vehicles. Thermal shock, acoustic pressure and
transient foreign object impact loading during launch and re-entry can cause
degradation in the health of mechanically attached metallic TPS panels in the
form of, for example, face sheet buckling, deformation/cracking of standoff
bolts and standoffs or wrinkling to thermal seals. To reduce turnaround times of
such vehicles, the TPS must be quickly inspected and repaired using data from
both pre-/post-flight nondestructive evaluation (NDE) and in-flight structural
health monitoring (SHM) technologies. In this work, simulated in-service
transient loads are identified experimentally using physics-based models of the
TPS and damage is identified experimentally using passive acceleration
transmissibility measurements. It is demonstrated in simulations that certain
types of damage are more apparent using SHM techniques during operation than
off-line NDE techniques. Transmissibility functions are shown in experiments to
be effective at detecting and locating damaged standoff bolts in panels
subjected to acoustic loading (~130 dB). A framework for NDE/SHM data fusion in
which SHM features are calibrated using NDE and subsequently used to interpolate
between inspections is also developed.