In the aeronautic sector, the technique is now being used for inspecting parts and structures made of composite materials and is being developed for the detection of corrosion in aging aircraft and the characterization of thermal barriers coatings. The presence of large areas to be inspected, contoured shapes, the introduction of advanced materials and the aging of current aircraft require further development of these techniques and innovative solutions. Among those, laser-ultrasonics that uses lasers for the generation and detection of ultrasound at a distance presents several advantages for the inspection of aeronautic parts and materials. As shown in Figure 1, when the two lasers, one for generation and the other one for detection, impinge at the same location on the surface of the curved part a train of ultrasonic echoes is generated and then detected.
Generation produces essentially an ultrasonic wave propagating normal to the surface.
When the two laser beams are scanned across the surface by using a rotating mirror, the ultrasonic echoes are still observed, even when the optical beams are far off the normal to the surface. Since there is no need to determine the shape of the inspected part to follow its contour and no need of complex fixturing, laser-ultrasonics has the capability to be much faster than any conventional mechanical scanning system. We describe first below the system that has been developed, then present the results obtained on the inspection of composite parts, on the detection corrosion in airframes and on the measurement of the thickness of thermal barrier coatings. The two arrows indicate the generation and detection laser beams, which are shown at two locations, determined by the optical scanner. The inserts indicate schematically the ultrasonic echoes, which can, for example, be observed on an oscilloscope.