Time-resolved full-field imaging of ultrasonic Lamb waves using deflectometry

Abstract

This pioneering experimental work is a proof of concept in which ultrasonic flexural waves have been imaged in a spatially and temporally resolved manner. Thin vibrating plates made of mirror glass and carbon/epoxy composite have been used in the experiments. Results obtained via a standard approach (scanning laser Doppler vibrometry) and the novel methodology based on deflectometry have been compared with a multi-physics finite element simulation. There is a very good correlation between the two experimental techniques. The numerical model provides insight into the experiments, but differs in its detailed structure due to uncertainties over material properties. The extreme slope resolution of deflectometry allows the measurement of peak-to-peak deflections of a few tens of nanometres in one shot. The use of an ultra-high speed camera allows for both space and time resolved measurements of Lamb waves which, to the best knowledge of the authors, has never been reported before. The limitations of the technique arise from the need for a flat specularly reflective surface. However, coating is possible for non-reflective materials and extension to moderately curved surfaces is possible in the future.

Infrared Deflectometry for Slope Deformation Measurements

“…In terms of slope, the best resolution for the MB camera is 3.6 μrad (or mm.km −1 ). Again, this is comparable to what has been obtained in the visible range, though larger as the imaging distance is only 560 mm while 1 m was used in [34] for instance. For the 1 m imaging distance used for the bending test, this slope resolution will be less that 2 μrad, which is typical for this kind of experiment.…”

“…One can see that even though the strains are quite small, they are two orders of magnitude higher than the measurement resolution which is typically on the order of 1 μstrain for this configuration [34]. This explains why the random noise only moderately affects the data, even though no spatial smoothing has been employed.…”

“…Currently, visible light cameras can provide frame rates up to millions of frames per second [31]. This allows for timeresolved deflectometry even in the ultrasound range [34]. Bolometer-based infrared cameras are limited by their response time, here, 8 ms, and the time for the active sensor element to cool down before another image can be taken.…”

Time‐resolved identification of mechanical loadings on plates using the virtual fields method and deflectometry measurements

“…The physical limit of this increase is given by the magnification and quality of the camera lens used. []…”

“…The physical limit of this increase is given by the magnification and quality of the camera lens used. [34] An important consideration when performing grid image measurements is ensuring an integer number of pixels N per grid period in the image. This is a tuning requirement for the phase shifting algorithm used to extract the phase values in the x and y directions.…”