Applications of pulsed cineholographic interferometry

A new cineholographic system working at 25 Hz with double exposure and double reference beam allows to determine as a function of time the deformation map of a structure under stressing. This set-up includes a double YAG-laser system and a phase stepping device allowing to obtain quantitative results in the reconstruction process. Two types of applications have been investigated with such a system: (1) fast non-destructive testing on site, mainly in the aeronautic field, (2) on-site vibration analysis of structures. It is this last type of application which is the object of this paper. The fact of disposing of a map of the displacements (occurring during a very short space of time) with a high spatial resolution and also as a function of time (25 Hz) allows new very interesting possibilities in the field of on- site vibration analysis of structures: (1) determination of phase and magniture maps for vibration flow measurements. For a complex vibration resulting of the superimposition of n sinusoidal excitation it is possible to know the magnitude and the phase of each frequency by recording 2n double-exposure holograms using the cineholographic system described before (2) calculation of the structural intensity which describes the transfer of vibrational energy by elastic waves and calculation of the divergence of the structural intensity vector to localize mechanical zones and to understand the dynamic behavior of the structure. Structural intensity created by flexural waves in a plate can be expressed using only the normal vibrational velocity component. However, the principal difficulty in obtaining it experimentally arises from the fact that it contains spatial derivatives of this normal velocity up to the third order. Use of double-exposure holographic interferometry is very suitable to calculate this structural intensity due to the fact that the holographic method gives directly the normal vibrational velocity at a very high number of points on the structure under investigation. Therefore, spatial derivatives can be calculated with enough precision.