NONDESTRUCTIVE TESTING: Turnkey laser system inspects aircraft parts

The US Air Force Air Logistics Command at McClellan Air Force Base (Sacramento, CA) has begun using a laser-based ultrasonic inspection system (LUIS) to image both large composite and aluminum aircraft parts up to about 12 × 40 ft in size. While inspecting parts structure, LUIS also provides a three-dimensional map that displays any flaws and their location. The technology was developed by the National Research Council in Canada, Industrial Materials Institute (IMI) and licensed to UltraOptec (both in Boucherville, Québec, Canada). Ultrasonics group leader at IMI Jean-Pierre Monchalin emphasizes the production nature of the system, which the Canadian Forces as well as civil aviation facilities are expected to adopt (see figure).

The system uses two lasers operating with collinear beams. A TEA CO₂ laser generates 10.6-µm output pulses of 2 to 120 ns at a 100-Hz repetition rate. This wavelength achieves significant penetration in a composite epoxy matrix—typically to 20 µm—and even deeper in paint coatings. Maximum energy per pulse is 150 mJ. The relatively deep penetration, coupled with a small (2.5 to 5 mm) spot size, produces a strong thermally induced ultrasonic emission in the test part; the emission is normal to the surface of the part and independent of beam orientation to the surface. Any defects present change the time-of-flight of reflected ultrasound waves through the sample; these waves are detected by Doppler-frequency shift of a long-pulse (about 50 µs), Nd:YAG laser beam coupled to a demodulation interferometer to reduce speckle effects.

Maturing technology

Laser-based ultrasonic testing development started at IMI about 10 years ago. Inspecting the increasing number of composite aircraft components could not be done completely with x-ray and neutron techniques, which are unable to find debonding and delamination between layers and other features. The ultrasound method, though, can detect defects in conventional aluminum honeycomb-and-skin structures as well as composites.

McClellan LUIS program manager Wes Frazier notes that laser-based inspection is useful for parts with complex shapes. Conventional ultrasound systems use a narrow stream of water to conduct ultrasound radiation to the material under test. The stream, which must be oriented within a few degrees of vertical to the surface at each point, cannot produce enough energy to penetrate thick parts and needs a "detector" water stream on the opposite side of the part. The laser, however, produces sound directly at the test surface, thereby increasing part penetration, and can inspect and map complex shapes because it works from only one side of the surface while being flexible in orientation to the surface.

Future laser-ultrasound inspection at McClellan AFB will involve imaging an entire Marine Harrier aircraft wing, which is completely fabricated from composites. Frazier adds that the method has to be optimized for aluminum structures because unpainted metals have too much reflectivity. The solution may be to inspect degreased parts before their paint is stripped off or to apply peelable or washable paint for the inspection process.