Crack detection

Citation

(2002), "Crack detection", Aircraft Engineering and Aerospace Technology, Vol. 74 No. 1. https://doi.org/10.1108/aeat.2002.12774aab.005

Crack detection

Keywords: Aircraft, Lasers

A new method of detecting tiny cracks in ageing aircraft has been developed by researchers at the University of Cincinnati, under a programme sponsored by the US Air Force. Conventional inspection techniques, such as ultrasonic inspection, are sensitive to cracks, but can confuse them with other discontinuities, such as rivet holes and corrosion pits.

However Peter Nagy, an associate professor of aerospace engineering, and graduate student Zhongyu Yan combined ultrasonic inspection with laser heating to identify fatigue crack much earlier than before and distinguish them from other types of discontinuities.

It's often extremely difficult to detect early signs of cracking. "The material is degrading on a microscopic level".

Cracks occur in critical areas, such as around a rivet hole or at sharp corners and often where it is not easy to detect. "Cracks associated with long-term service would always be in the worst possible location. They will be in the corners or just hiding on the rivet hole. So you have to distinguish them from something that produces a bigger signal."

Shining an infrared laser through a special lens on a suspected crack heats up the surface. The skin wants to expand but cannot, because the body of the material remains cold. This causes the crack to close for a fraction of a second. This "blinking" can be used to better identify fatigue cracks which are very sharp, narrow discontinuities.

The researchers tested their method using a series of aluminium and titanium specimens with cracks ranging from 0.5 to 1mm in length and specimens with no cracks in them. Their results indicated that the method not only found the cracks, but could measure the difference in the severity of the cracking.