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

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such direct identification methods are no longer
valid.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.