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Usually, structural vibrations are investigated using accelerometers or laser-vibrometers, but both provide point-wise measurements and, accelerometers can be also considered as intrusive. An alternative way is proposed with optical digital holography. This optical technique is able to provide contact-less and full-field measurements with a very high spatial resolution. Historically, only stationary vibroacoustics problems have been analyzed with digital holography under a stroboscopic regime. However, such an approach is not adapted to investigate transient acoustic problems, since vibration of structures under non stationary operational functioning conditions requiring an analysis in the time domain. The vibroacoustics problems can be addressed in the time domain by using high speed CMOS sensor.
In this paper, we aim at demonstrating the capabilities of digital holography method to provide a full-field analysis. The goal of this new approach is to give a high resolution measurement of transient vibrations in both the spatial and temporal domains. Such high resolution can be used to analyze structural mechanical properties and composite materials or to detect default by using vibratory inverse problem, or other.
So as to pave the way for a real-time-full-field vibroacoustics optical probe, a thorough analysis of experimental parameters must be achieved. To do this, the parameters constraining the design of the proposed method were established and a parametric study from a numerical simulation of the full holographic process was carried out. The optical parameters that are of primary of interest are: the optical wavelength, the laser power and the lightening properties. The sensor parameters are: the pixel sensor dimensions, the number of pixels, their sensitivity and the full width of the sensor. Concerning the vibroacoustics parameters, the size of the analyzed structure, the amplitude of the vibrations, its temporal frequency bandwidth have the main influence on the design of the experimental method. The simulation results allow defining the optimal operating rules. These rules provide both indicators on the noise level and the error in the reconstruction of the displacement field.
After having designed the experimental setup by using the numerical simulation, an experimental bench is implemented to compare this new metrological approach to the classical one. The experimental results show a very good agreement between those obtained from a laser-vibrometer and those given by the high-speed digital holography.
An application of the optical method to the study of an ABH (Acoustic Black Hole) is proposed. The ABH is a passive method of reducing vibrations of panels; it consists in a local reduction of the panel thickness, associated to a thin viscoelastic layer placed at the center. Such a pit acts as a trap for flexural waves. This type of structure induces a complex vibration field, exhibiting large variation in the spatial domain and important amplitude dynamic. It can be investigated only by non intrusive techniques.
As a conclusion, the analysis of complex structure such an ABH highlights the advantage of this new contact less metrological approach based on very high-speed digital holography.
