Full‐Field Vibration Measurement by Time‐Average Speckle Interferometry and by Doppler Vibrometry – A Comparison

Abstract

Identification of dynamic material properties, non‐destructive testing and study of vibroacoustic behaviour of different structures require the use of complex, pointwise and full‐field measurements, which are capable of providing data for experimental modal analysis or model updating. Nowadays, among other techniques, optical non‐contact techniques represent the favourite choice as they do not add mass, stiffness or damping to the structure under test. When the range of vibration amplitudes allows it, most of these techniques are based on interferometric principles. Development of laser sources and detectors leads to a continuous improvement of vibration measurement techniques. However, a hard choice still has to be made 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. The paper presents a comparative study of the vibration amplitude fields as measured by two full‐field non‐contact techniques, speckle interferometry and laser Doppler vibrometry, and predicted by finite‐element model. The measurements concern the free and the forced vibrations of a thick, composite plate with free boundaries.

A new Method for Processing Time Averaged Vibration Patterns: Linear Regression

“…The sign of the Bessel function can be retrieved when a set of steady state measurements, taken under the same conditions as the dynamic one, is available [10,11]. When this is the case, another set of Equations (3), with J 0 (φ d ) = 1, can be used to solve the unknowns I B , I M and φ.…”

“…Furthermore the results at zero crossings of the Bessel function would degrade when applying an averaging filter, because the derivative of its absolute value is not continuous there. Considerable improvements have been reached [8][9][10], also by involving measurements taken under stationary conditions, eliminating most of the noise and producing zero crossings and skeleton lines of a large number of Bessel-type fringes, allowing local inversion of the Bessel function. Regional inversion requires reliable skeleton indexing in order to be able to identify the correct fringe order for skeleton pairs.…”

“…Furthermore, discontinuities in the vibration field may adversely affect indexing causing inversion errors to propagate. Phase changes between the static and dynamic states are handled by level-shifting the phase iteratively, supported by histogram analysis [8][9][10]. In these references, phase differences within the range of Àπ to +π are reported but no specific details on the level shifts are given.…”

“…Time-averaged vibration patterns may have much lower quality than the ones presented in Refs. [8] to [10], giving rise to the question whether patterns obtained outside the more or less stable conditions of the laboratory could render indexed skeletons that are reliable enough for regional inversion. In view of this we propose a method that produces reliable results even when the data sets have lower quality.…”

Vibration Measurement by Speckle Interferometry between High Spatial and High Temporal Resolution

“…The spatial resolution of speckle interferometry is in most cases high enough for the needs of the measurements, and the vibration amplitude maps found by this technique are used, as in (Moreau, 2008) to check and validate the results produced by other techniques (Laser Doppler Velocimetry, Near-Field Acoustical Holography). The direction into which speckle interferometry should make the bigger steps is improving its temporal resolution, so as to be able to characterize the relative vibration phase between different points, the vibration waveform, or to cope with non-stationary phenomena.…”