Practical experiences of interpreting time-average hologram fringe patterns from three-dimensional objects

Abstract

In industrial situations, vibration tends to be measured using accelerometers. Interest in the measurement of the modal behaviour of test objects, as a means for determining ways of controlling their vibratory response, led to the use of holographic interferometry and thence to the comparative exercise which was the basis for this paper. To establish the bona fides of the interpretation procedures for the analysis of the optical fringe patterns, the behaviour of a complex three-dimensional test object was determined using both holography and multiple accelerometers. The purpose of this paper is to review some of the problems encountered when endeavouring to interpret the fringe patterns and to discuss methods which may be used to surmount them. The problem lies in the determination of the local vector of object motion. Since this motion can have six degrees of freedom, it is of the utmost importance to understand exactly what displacement is indicated by the fringe pattern. The second stage is to ascertain exactly the order of each fringe. In simple cases this is quite easy but when the object shape contains discontinuities, such direct identification methods are no longer valid. The most expedient method then is to employ an auxiliary reference to which the test position can be coupled by flexible tape; fringe counting along the tape providing the requisite order clarification. In the paper we shall present an explanation of the means for determining object displacements from interferograms and indicate the agreement that can be achieved with measurement data obtained using accelerometers. We shall discuss the problems to be expected when attempting to interpret fringe patterns on real engineering test pieces of complex shape and will summarise the methods we have now adopted to achieve successful application of holographic interferometry to this type of task