Laser heating reveals aircraft cracks

17 Jun 2002

Aerospace engineers from the University of Cincinnati, US, have combined an infrared laser with an ultrasonic inspection system to find and measure tiny cracks in aircraft components ten times as accurately as previous techniques. This allows earlier detection of aircraft fatigue.

The technique, developed by Zhongyu Yan and Peter Nagy, is based on a conventional ultrasonic flaw detector with a pulsed Nd:YAG laser that 'highlights' the parts of the ultrasonic echo coming from fatigue cracks.

The infrared beam is pulsed at 50 hertz, half the rate of the ultrasound signal, and heats the near-surface region of the sample so that the metal undergoes thermal expansion. This causes stresses that momentarily close the relatively tight fatigue cracks but do not have much effect on features like corrosion pits, coarse grains and rivet holes, which also scatter the ultrasound signal. Nagy explained, "As a result, the crack signals are modulated at the irradiation frequency as they 'blink' after each pulse, which can be exploited for positive identification of them." The laser beam allows rapid heating of localized areas without producing electromagnetic noise that would interfere with the weak ultrasonic echoes.

The technique is more complicated and expensive than standard systems, but Nagy says that it is only intended for the most crucial inspection applications. He added that it should become simpler with further technical development.

The researchers presented this work earlier this month at the annual meeting of the Acoustical Society of America. They are currently carrying out a feasibility study to verify how the system works. Nagy says that development of a commercial version could take one or two years and that Physical Acoustics of Princeton, New Jersey, is interested in the device. The researchers are also trying to get some additional support from the US Air Force.

SH