Scanned pulsed eddy current instrument for nondestructive inspection of aging aircraft

The aging of commercial and military aircraft fleets in the US has led to increased emphasis on detecting accumulated damage such as corrosion and wide-spread fatigue damage in multi-layer airframe structures. Eddy currents are the method of choice for this task, since they can penetrate multiple layers whether or not the layers are mechanically bonded. We have developed a new pulsed eddy-current instrument that is fast, rugged, portable, and relatively inexpensive to build. It has the ability to acquire wide- bandwidth information that can quantitatively characterize the thickness of multi-layered airframe structures. The instrument can also detect cracks growing from fastener holes in the second or third layers of a lap-joint structure. The probe is mounted in a rugged scanner that easily attaches to an aircraft. Data from the instrument are presented to the user in two formats: an instantaneous A- scan of the time-domain pulsed eddy-current signal and a C- scan image of the scanned area. The C-scan image can be time-gated to provide images representing signals form various depths and to discriminate against signals from fasteners or other interfering features. We describe the instrument in detail and provide examples of its use to detect both corrosion and second-layer cracks.