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

17 7

with the time delay  t and is synchronized with the first half-period of the period T US , and the second SP I k,n 2 ( i , j ) from this pair is recorded during the same time gap  with the same time delay  t and is synchronized with the second half-period of T US (see Fig. 5). So, the spatial intensity distributions of a current n th odd and even frames I n,o ( i , j ) and I n,e ( i , j ) of the n th pair sequence can be expressed as

K

K

1    k

1    k

, n o I i j

, 1 I i j k n

, n e I i j

, 2 I i j k n

( , )

( , ),

( , )

( , ).

(12)

Evaluation of the speckle motion influence on differences of speckles statistical characteristics within the ROI and outside its limits can be implemented using pointwise intensity-based estimates proposed by Stoykova et al. (2017) and biospeckle signal descriptors analized, for example, by Dai Pra et al. (2016). Application of such estimates and descriptors can be very promising for NDT of composite structures. To detect the subsurface defects, we suggest using descriptor that takes into account the record mode of N pair speckle patterns sequence and the specific conditions of operation of the proposed technique. Utilizing the fact that the panel under study is excited by harmonic US wave, and taking into account the mentioned above procedure for producing of the accumulated resultant fringe pattern S  ( i , j ) by the subtractive synchronized ESPI technique, the descriptor of the dynamic speckle motion of composite surface areas can be expressed as where  ( N – 1    1) is the factor for choosing the optimum intensity level of the resultant speckle pattern I  ( i , j ). We elaborated the experimental breadboard including an optical-digital system (ODS) for implementation of the proposed technique. The operating mode of the ODS is similar to the operating mode of the HIS. However the optical interferometer in the ODS is absent. So the ODS records speckle patterns instead SIs. Therefore, the ODS is much simpler and more reliable than the HIS. In particular, it is less sensitive to vibrations and is able to analyze a larger surface area. The functional diagram of the ODS breadboard is represented in Fig. 6a. Results of test subsurface defects detection in a fiberglass panel containing blind circular holes with diameters of 4 mm and 12 mm are shown in Fig. 6b. The panel thickness is 6 mm, and the holes are located at a depth of 1 mm from the observed surface. The resultant speckle patterns I  ( i , j ) containing four blind holes of 4 mm diameter and one blind hole with the diameter of 12 mm were obtained by accumulating of 46 and 79 pairs of frames I n,o ( i , j ) and I n,e ( i , j ), respectively, using the proposed descriptor of the dynamic speckle motion expressed by Eq. (13). 5. Conclusion The possibility of excitation of high-Q resonance transverse vibrations in the main mode for the rigid joint of a layer with interface in the presence of an interface crack/delamination is established. We have found that the imaginary part of the complex resonance frequency decreases with increase in the ratio between the crack length and layer thickness. We have obtained an estimate for the lower bound of the ratio between the crack length and layer thickness for which one can diagnose it on the basis of the excitation of a resonance response. To implement the detection of subsurface defects in multilayer composites, the subtractive synchronized ESPI technique and the technique for subsurface defect detection based on estimation of the dynamic speckle motion of composite surface areas are developed. The experimental breadboards of two systems that realize these techniques are created. In the HIS defects are detected due to a correlation fringe pattern produced within the ROI. The ODS possesses to detect subsurface defects using the proposed speckle descriptor and producing the speckle pattern within the ROI with such speckle contrast that differs from the speckle contrast of a surface area outside the ROI. The created ODS is simpler than the HIS. It is not so sensitive to vibrations and other external influences as the HIS. Therefore, modifications and improved versions of the ODS experimental breadboard can be used for NDT of composite panels in working conditions. 1 N I i j I i j    | ( , ) ( , ) |, , n o , n e 1 ( , ) ( )  N  n I i j   (13)