LIKE humans, steel beams can only take so much stress before they
snap鈥攐ften with dangerous consequences. But for steel beams, at least,
scientists have found a new way of detecting when everyday strain causes too
much fatigue and might lead to a breakdown.
Metal fatigue is an industrial nightmare. 鈥淔atigue is the number one
mechanical failure problem,鈥� says David Jiles, a physicist at the US Department
of Energy鈥檚 Ames Laboratory in Iowa. 鈥淪omething like 90 per cent of all
industrial failures due to mechanical problems are due to fatigue.鈥�
But what makes the problem so insidious is that it is almost impossible to
tell whether a steel part is in great shape or whether it is about to snap. 鈥淭he
terrible news is that there鈥檚 nothing you can do,鈥� says Thomas Erber, a
physicist at the Illinois Institute of Technology. Though there are a number of
techniques to tell whether a part is defective, spotting when it has outlived
its useful life is more difficult. 鈥淭he only thing to do is to look for cracks,鈥�
says Erber. But by the time an inspector spots cracks鈥攂y ultrasound, by
X-ray diffraction, or by eye鈥攖he metal has already been a safety hazard
for some time.
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In the Journal of Physics D: Applied Physics (vol 30, p 2818), Erber
and his colleagues describe a way to measure a beam鈥檚 fatigue through magnetism.
A steel bar鈥攅ven one which has been 鈥渄emagnetised鈥濃�攈as little
packets of atoms whose magnetic spins are pointed in roughly the same direction.
As the bar is crushed or bent, these magnetic domains shift positions with
respect to each other and scientists can detect the associated changes in the
bar鈥檚 magnetic field. As defects begin to appear in the steel鈥檚 structure, the
flutter in the bar鈥檚 magnetic field becomes more pronounced. And as the bar
begins to crack, the researchers see spikes in its magnetic
field鈥攕ignalling the beam鈥檚 imminent failure.
This discovery might give engineers a way to measure the integrity of
critical steel parts such as aircraft engines, bridge struts, and supports in
nuclear power plants. 鈥淚n a stress test, your doctor temporarily overloads your
cardiovascular system and sees how you recover from the overload,鈥� says Erber.
鈥淲e hope to do the same for steel.鈥�
Though Jiles believes that magnetic inspection will be the best way to detect
fatigue in steel, he cautions that Erber鈥檚 method might be sensitive to slight
changes in the part鈥檚 initial magnetisation or other environmental factors.
鈥淚t鈥檚 a good technique; it鈥檚 highly sensitive to structure,鈥� says Jiles. 鈥淏ut
you can鈥檛 just slap a sensor on a steel part and say `Next week, it鈥檚 going to
蹿补颈濒鈥�.鈥�
