Magnetic Sensing Detects Dangerous Defects in Metal:
New Technique Could Warn of Early Fatigue Damage

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 companies.

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 technique works.

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 magnetic fields.

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 metal.

"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 problems."

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 Mexico.