Stiffness properties of paper materials can readily be characterized in the laboratory using conventional ultrasonic techniques. For on-line inspection on a paper machine, due to the high translation velocity and the somewhat fragile nature of the moving paper web, contact ultrasonic techniques using piezoelectric transducers are of limited use. To overcome this limitation, non-contact laser- based ultrasonic techniques can be used. Due to the rough surface of the paper, the reflected light is composed of many speckles. For efficient detection, the receiver must be able to process as many speckles as possible. Adaptive receivers using the photorefractive or photo-emf effects are characterized by a large etendue, and thus, are well suited for detection on paper and paperboard. Moreover, the translation velocity of the moving web implies that the detection system must adapt extremely quickly to the changing speckle pattern. In this work, a photo-emf receiver was used to detect Lamb waves excited using a pulsed Nd:YAG laser in moving paper. Experiments were performed using a variable-speed web simulator at speeds much higher than 1 m.s-1. Results corresponding to various translation speeds are shown, demonstrating the feasibility of laser- based ultrasound for on-machine inspection of paper and paperboard during production.
KEYWORDS: Interferometers, Ultrasonics, Crystals, Ultrasonography, Laser crystals, Signal to noise ratio, Photodiodes, Beam splitters, Two wave mixing, Laser systems engineering
Laser-Based Ultrasound (LBU) systems are now entering their maturity years by penetrating the factory in both the areas of non-destructive testing and process control. A LBU system can be used for the on-line characterization of a paper web in a paper mill. Compared to contact techniques, LBU is able to generate and detect on a paper web both symmetric and antisymmetric waves with a non-contact tool which is the spot of the generation laser. This provides all at once a rich amount of data about the stiffness properties of the sheet. To demonstrate this concept we made some experiments on static paper first, our ultimate goal being to monitor the stiffness properties of a paper web, on-line, at industrial speeds. A photorefractive interferometer using the two-wave mixing method with a continuous electric field applied to a BSO crystal has been built for ultrasound detection. Results are presented on different paper grades, using a Nd:YAG laser for generation. Both A0 (anti- symmetric) and S0 (symmetric) modes of Lamb waves have been detected with acceptable signal to noise ratio in single laser shot. The dispersive nature of A0 wave is clearly visible as well as the higher frequency content of S0 wave.
The early detection of corrosion in aging structures is probably one of the most important challenges of the aeronautic maintenance services. Laser-ultrasonics offers interesting characteristics to become an industrial technique able to solve this problem. Neverless, to become quantitative, this non-destructive method requires a precise description of the laser-ultrasonic generation. This paper presents a new and original model which takes into account the layered structure which is generally encountered in aeronautic materials subjected to impacts, fatigue and corrosion. This model solved the Christoffel equations in an axisymmetrical configuration over an infinite plate of finite thickness presenting a cylindrical orthotropy. The sample is a flat plate made of two layers of different materials and the laser impinges the sample normally to the surface. The method of resolution used allows fast calculation and observation of the displacements over a long time period. This is very useful in NDT, especially in the case of thick samples. Validations were conducted by comparing the results calculated by this model to the ones obtained with a previous model and with experimental measurement using a Nd:YAG pump laser and an interferometric detection.
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