Keywords: vibration measurement, experimental, full-field, material properties, holography, vibrometry.

Structural analysis and experimental modal analysis, identification of different dynamic material properties, non-destructive testing and the study of vibroacoustic behaviour of different structures impose complex, pointwise and full-field measurements. Among other techniques, acoustical and optical non-contact techniques are today the favourite choice since they do not add any mass to the structure under test. If the range of vibration amplitudes is small, most of the optical techniques are based on interferometric principles. Laser sources and detectors lead to a continuous improvement of vibration measurement techniques. Two classes of optical techniques have emerged: full-field techniques, like speckle interferometry [1], holographic interferometry [2], projection Moiré and digital holography [3] and pointwise techniques such as Doppler velocimetry [4].

The researcher has thus to make a frustrating choice, between spatial resolution and temporal resolution. Another difficult choice is between space bandwidth product and energetic sensitivity of the detector. While the number of pixels of a camera is continuously increasing, the pixel size seems limited at its lower end. Other measurement techniques, like Fresnel digital holography and near-field acoustical holography [5], are characterized by difficulties related to the numerical implementation of wave propagation.

The paper presents a comparative study concerning the vibration amplitude maps measured by some of the most representative full-field and pointwise optical non-contact techniques for vibration measurement and by finite element modelling, as applied at the LMR Mechanics Laboratory of INSA Rouen. The practical case of measurements concerns the free and the forced vibrations of a thick, composite plate used in a study concerning the identification of its complex damping. In the data processing stage, discrete wavelet decomposition has been applied to the experimental data in order to match up the spatial maps of vibration amplitudes. Among other points of interest, experimental evidence is presented about the real behaviour of the boundary conditions during the testing.

1

K. Creath, "Phase shifting speckle interferometry", Appl. Opt., 24, 3053-3058, 1985. doi:10.1364/AO.24.003053

2

R.L. Powell, K.A. Stetson, "Interferometric vibration analysis by wavefront reconstruction", J. Opt. Soc. Am., 55, 1593-1598, 1965. doi:10.1364/JOSA.55.001593

3

U. Schnars, W. Jüptner, "Direct recording of holograms by a CCD-target and numerical reconstruction", Appl. Opt., 33, 179-181, 1994 doi:10.1364/AO.33.000179

4

M. Martarelli, G.M. Revel, "Laser Doppler vibrometry and near-field acoustic holography: Different approaches for surface velocity distribution measurements", Mechanical Systems and Signal Processing, 20(6), 1312-1321, 2006. doi:10.1016/j.ymssp.2005.11.011

5

J.M. Maynard, E.G. Williams, Y. Lee, "Nearfield acoustic holography: I. Theory of generalized holography and the development of NAH", J. Acoust. Soc. Am., 78( 4), 1395-1413, 1985. doi:10.1121/1.392911

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