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The structural engineer's interest in vibration can generally be summarised as a desire to know the modes of vibration which an engineering structure can assume, the resonant frequencies, the sharpness of the resonances (related to the damping forces in and on the structure) and their amplitudes under given driving forces. Most of all he is interested in the non-resonant vibration of the structure under the influence of a random driving force, and he would like to determine the direction (in three dimensional space), as well as amplitude, of the motions involved. In industries in which exceptionally high levels of structural integrity are required through long periods of continuous or near continuous operation, such os the aeronautical or nuclear industries, accurate vibration analysis is an essential first step towards an assessment of the fatigue life of the structure. In this case the most important factor is the dynamic stress in the structural material. Measurement tools available to the engineer, in order to obtain the information he needs, are numerous, varied in character, and generally unable to meet all the needs outlined above. They may be contacting (e.g. accelerometers or straingauges) or non contacting (for example holographic interferometry, ESPI or SPATE). They may provide data continuous in space (holographic interferometry) , with limited spatial resolution (ESPI), or discrete point measurements (accelerometers, laser vibrometers). Whilst accelerometers provide data continuous in time, and ESPI has limited time resolution, holographic interferometry is in general severely limited in its usefulness because it can only provide data for a given moment in time, and successive holograms are either limited in number or difficult to relate in time. On the other hand it is now possible to derive the full direction of motion from holographic records, whereas a single accelerometer is usually only able to measure acceleration in one direction. The final choice of measurement technique is a compromise based on an evaluation of the cost, the reliability, the sensitivity, the accessibility of the structure to contact techniques, and the importance of obtaining a full and accurate analysis. Because of the great importance attached, in the nuclear industry, to obtaining the fullest possible analysis of structural vibrations, all available methods of vibration analysis are constantly being assessed, and, where appropriate, utilised. This paper describes the use of pulsed lasers to analyse vibrations of parts of operating nuclear reactors. The techniques used to allow these sophisticated systems to operate remotely under difficult site conditions are described and the results obtained presented together with an analysis of their value and limitations. In order to fully derive the motion vector it is desirable to take more than one hologram simultaneously, and this is best achieved using optical fibres. Some results of experiments with optical fibres are presented. A more general use of holographic interferometry for vibration analysis will only occur when well engineered robust equipment is available which can be operated by non-specialist staff, and when the techniques are developed for analysing results obtained under non-resonant conditions. There is some discussion of these points.
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