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In laser-ultrasonics for inspecting composites, the beams are usually directed onto the part with a computer controlled scanning mirror. This approach has sensitivity limitations when the surface is very shiny (mold facing surfaces). This limitation is eliminated by controlling the direction of the laser beams with an articulated robot, its trajectory being determined from the CAD of the part or its surface mapping from a 3D laser scanner. The scanning mirror is eliminated. We are reporting here successful implementation on a 6-axis robot. [ABSTRACT FROM AUTHOR]

Many engineering structures are composite and include for example a protective coating or a bonded layer. A novel technique, close to laser-ultrasonics but significantly different, has been developed for the detection of disbonds between the coating or the bonded layer and the substrate. It is also applicable to the detection of core unbonds in honeycomb structures. The technique is based on the thermoelastic excitation by a pulsed laser of the top layer or top skin which is driven into vibration if it is detached from the substrate underneath. This vibration is then detected by a second laser coupled to a photorefractive interferometer. The technique can be made very flexible by using optical fiber coupling. One foresees its application to the in-service inspection of aerospace structures for the detection of core unbonds in honeycombs or near surface delaminations. Examples of application to honeycombs and to various coatings are presented. © 2007 American Institute of Physics [ABSTRACT FROM AUTHOR]

The best detection limit of an optical system for the detection of ultrasound is obtained for a system limited by the shot noise. However, typical laser oscillators are not free of intensity and phase fluctuations, and such laser noise exceeds the shot-noise level above a given laser light power. Moreover, since typical industrial surfaces are optically rough and absorbing, laser amplifiers are frequently used to increase the light power collected back from the surface. Such amplifiers add both intensity and phase fluctuations. While intensity fluctuations can be eliminated by a simple differential scheme when using a confocal Fabry-Perot, no simple solution has been given to reduce the phase noise. In this paper, differential schemes that reduce both laser intensity and phase noises are proposed for the transmission and reflection configurations and experimental results on the noise reduction reached are presented. The advantages of this differential approach to image variable reflectivity surface parts and to relax detection laser requirements are then discussed. © 2007 American Institute of Physics [ABSTRACT FROM AUTHOR]

Laser-ultrasonics has been used with success for the microstructure evaluation of metals by measuring ultrasonic attenuation and velocity. Extension of this technique to fiber reinforced polymer composites could provide useful information such as fiber contents and porosity. Ultrasonic generation in composite materials is usually performed by using a pulsed TEA-CO2 laser operated on several transverse modes. The intensity profile of this laser could change from shot-to-shot, thus affecting the ultrasonic generation pattern and the accuracy of attenuation measurement. Two solutions are possible to overcome this problem. The first reported approach consists in numerically correcting for the diffraction effect by using a Monte Carlo integration after simultaneous acquisition of the ultrasonic signal and the laser mode pattern. The second reported approach consists in modifying the laser resonator to get a single transverse mode while maintaining high energy per pulse, and to make the diffraction correction much simpler. Both approaches have been tested on a bare carbon epoxy plate with laser generation in the thermoelastic regime. The results obtained from using either approaches show a significant improvement and are compared to those obtained by conventional ultrasonics. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

Two-wave mixing based interferometry has been demonstrated to be a powerful technique for non-contact, broadband and speckle insensitive measurements of the small surface displacements produced by ultrasonic waves propagating in an object. When the object is in rapid motion along the line-of-sight of the probing laser or when the laser beam is rapidly scanned on a wavy surface, the two-wave mixing photorefractive interferometer loses sensitivity to the point it could become useless. To circumvent the Doppler frequency-shift produced by this relative motion, we propose a dynamic compensation scheme. We report a particularly simple scheme to implement this concept by monitoring the low-frequency output signal of a balanced two-wave mixing demodulator whose output is proportional to the frequency difference between the pump and signal beams, and feeding this signal back to the acousto-optic shifter. With this new concept, the two-wave mixing interferometer can operate on objects in rapid motion while maintaining its sensitivity to low frequency ultrasound. © 2003 American Institute of Physics [ABSTRACT FROM AUTHOR]