Magnetic Sensing Detects Dangerous Defects in Metal:
Date: Feb. 5, 2001
New Technique Could Warn of Early Fatigue Damage
By: Chris Curran
Phone: (513) 556-1806
Photos by: Lisa Ventre
Archive: Research News
Cincinnati -- University of Cincinnati aerospace researchers have
demonstrated that a simple magnetometer can identify tiny defects in
metal such as copper, aluminum, and titanium - the first steps toward
developing a highly sensitive technique for spotting metal fatigue in
airplane parts and power generators.
Peter Nagy, professor of aerospace engineering, has spent the last
several years developing new nondestructive evaluation techniques
under projects funded by the Air Force Office of Scientific Research, the
Air Force Research Laboratory, the Department of Energy and private
The newest one is based on a very simple premise. Many metals such as
titanium and aluminum aren't magnetic by nature; however, cracks and
other defects change the metal's properties. "If there's any magnetism at
all, something's wrong," explained Nagy.
A temperature gradient is established throughout the specimen by
heating and cooling. In the presence of material imperfections, the
resulting thermoelectric currents in the metal create a very weak
magnetic field. Nagy and his collaborators reported recently (Journal of
Applied Physics, Dec. 2000) that a basic magnetometer is
sufficient to detect that change. Nagy's research is at the early stages. He
and his graduate students use solid metal bars of copper or titanium
disks, then intentionally damage them. Since the researchers know the
exact state of the metal, their results clearly demonstrate that the
A magnetic scan of undamaged or annealed metals turns up nothing.
Damaged metals reveal colorful "hot spots" precisely where the damage
occurs. Eventually, Nagy plans to adapt his system to use SQUIDs
(superconducting quantum interference devices) which are
approximately a 100 times more sensitive at detecting changes in
A key advantage to Nagy's system is its ability to detect defects at a
distance. For example, General Electric in Schenectady is interested in a
better method for spotting defects in huge copper coils used in electric
power generators. Like an X-ray zooming in on bone, the magnetic
scanning method finds buried defects as well as those on the surface of a
"We don't have to be very close," said Nagy. "Other methods use heat or
electrical currents, but these coils have to be insulated. So, those
methods don't work. Magnetically, we can do it."
From a safety aspect, Nagy's work with titanium has even more
important implications. Showing off a sample turbine blade from a giant
jet engine, he pointed to the small piece of metal that connects with the
rotor. "This is how the blade is hanging on for dear life," said Nagy.
"The rubbing is terrible."
In aerospace terms, titanium "frets." The constant rubbing of blades and
rotors causes damage over time. It's extremely difficult to spot this kind
of damage, but essential. The heat produced by the fretting doesn't
dissipate easily. Titanium simply doesn't conduct heat well. "The parts
weld together from the heat, tear apart, weld again. If the blade breaks
off, it flies off and breaks everything else." And that's just the beginning
of a deadly chain reaction, according to Nagy. "The blade kills the
engine which kills the whole plane."
Nagy's work is just moving from copper into titanium. He believes it will
take one or two years of fundamental research before the research moves
into the applied stage. "At this point, we're just happy to detect the
damage." In fact, Nagy's system could detect damage after just a few
minutes of rubbing and fretting.
Ultimately, the technique might also prove to be a more economical
method for nondestructive testing of critical jet engine or generator
parts. For example, the most sensitive residual stress measurements
available today (based on X-ray diffraction) cost about $1,000 per
measurement. If Nagy succeeds in developing a magnetic method, the
cost could drop to less than $100.
The work has taken on increased importance as the nation's military and
commercial jets remain in service longer and longer. "There used to be a
quick turnover. Now, planes fly for more than 20 years. The important
problems today are corrosion, fatigue, rubbing very down-to-earth
Simple sounding problems but it will take new technology and
innovative thinking to solve them, the type of thinking that thrives in
Peter Nagy's lab.
The other researchers working on this problem in his lab include Curtis
Fox Research Associate and Hector Carreon, a graduate student
supported by a prestigious government scholarship from his native