PSI - Issue 16

Zinoviy Nazarchuk et al. / Procedia Structural Integrity 16 (2019) 11–18 Zinoviy Nazarchuk, Leonid Muravsky, Dozyslav Kuryliak / Structural Integrity Procedia 00 (2019) 000 – 000

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 T US /4). Excitation of subsurface defects in a studied panel is implemented by US wave frequency scanning in the range from 10 to 150 kHz. Two sequences of elementary SIs that fix maximum and minimum amplitudes of the US wave are accumulated by CCD camera during the chosen frame time T . Obtained two accumulated SIs S n,e ( i , j ) and S n,o ( i , j ) are subtracted in a computer. The resulted fringe pattern S n ( i , j ) allows to visualize the subsurface defect placed under the ROI. Because the contrast of an extracted defect is low, we summed the fringe patterns S n ( i , j ) and obtain the resultant fringe pattern S  ( i , j ). Real subsurface defects located in multilayer composite panels at depths from 0.1 to 8 mm can be detected by this system. Results of real subsurface defects detection in a fibre-reinforced layered composite are shown in Fig. 4. We have to mention that the considered subtractive synchronized ESPI technique, as well as the other mentioned above ESPI techniques, possesses several disadvantages. In particular, very high sensitivity to vibrations and thermal flows and complicated hardware of interferometric systems realizing these techniques constrain or even prevent their use for control of composite constructional elements in working conditions. We offer some new technique for subsurface defects detection, which is based on estimation of the dynamic speckle motion generated by harmonic US wave excitation on the surface areas of a studied composite structure. We propose to generate the dynamic speckles by harmonic US excitation of a studied structure. If a testing surface area of a composite panel containing the ROI is rough, and its roughness is comparable with wavelength of a laser source illuminating this area, a fully developed speckle pattern is produced on the given distance from the surface and is recorded by a digital camera. When the ROI placed directly above the defect begins to oscillate with a resonant frequency, the dynamic speckle motion is more intense within the ROI and less intense or generally absent outside of it. The speckle motion leads to speckle blurring and to changes of temporal and spatial speckle contrast in the ROI. Therefore, recorded speckle patterns of the studied surface including the ROI contain the region of blurred speckles within the ROI and the region of stabile or stiff speckles outside the ROI, which are blurred significantly less than the ROI speckles. The record mode of N pair sequence of speckle patterns during harmonic US excitation of the studied composite panel is similar to the record mode of SIs sequences implemented by the subtractive synchronized ESPI technique. Each n th pair of speckle patterns contains a current n th odd and even frames with intensity distributions I n,o ( i , j ) and I n,e ( i , j ) respectively ( n = 1, 2, ..., N ). Each n th odd and even frame is produced by accumulation of K elementary speckle patterns with intensity distributions I k,n 1 ( i , j ) and I k,n 2 ( i , j ) respectively that are recorded by the same camera for the same time gap  ( k = 1, 2,…, K ). However, the proposed technique anticipates recording of K elementary speckle patterns I k,n 1 ( i , j ) and I k,n 2 ( i , j ) within the time gap  , which start begins from a given time delay  t within the half of the US wave period T US . The first elementary SP I k,n 1 ( i , j ) from each n th pair is recorded during the time gap  4. Subsurface defect detection by estimation the dynamic speckle motion of a composite surface

Fig. 4. Fringe patterns detecting subsurface defects in a fibre-reinforced layered composite.