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

15 5

SIs recorded by the same camera for the same time gap  at minimum amplitude of US wave during each period T US . Therefore, these sequences contain N pairs of SIs, and each n th pair ( n = 1, 2, ..., N ) consists of odd and even SI, which registration during the time gap  is synchronized respectively with maximum and minimum amplitudes of the US harmonic wave. As a rule, the time gap  is very short and it can’t be provided by the digital camera shutter. Therefore, a pulsed laser or an optical shutter such as an acousto-optical deflector or electro-optical modulator should be used to generate such short time gaps. In order to detect the ROI, several dozen of SI pairs should be recorded. Recording procedure of speckle patterns can be expressed in the simplest case as a two consecutive series of pulsed rectangle optical shutter functions, that is

1 K  

1 K  

1 [( ( 1 4) ) ] t k T  

1 1 [( ( 1 4) ) ], t k f   

,0 r t n

( )

   

   

(8)

US

k

k

0

0





1 K  

1 K  

1 [( ( 3 4) ) ] t k T  

1 1 [( ( 3 4) ) ], t k f   

, n e r t

( )

   

   

(9)

US

k

k

0

0





where k = 1, 2, …, K ; f = 1/ T US is the US wave frequency. An intensity spatial distribution S n ( i , j ) of n th correlation fringe pattern of the studied panel surface deformation is obtained by real-time absolute subtraction between intensity distributions S n,e ( i , j ) and S n,o ( i , j ) of a current n th even frame and the previous n th odd frame respectively, that is

S i j

( , ) S i j S i j  

( , )

( , ) ,

(10)

n

, n o

, n e

where i , j is the pixel number in digital patterns. Due to subtraction of two SIs, the correlation fringes are most contrast in that places of a surface area where surface deformations are the largest. If the observed surface area contains a ROI and the resonant frequency of a defect located under the ROI coincides with current frequency of the exciting US wave, correlation fringes appear against the background noise surrounding the ROI. To enhance the contrast and intensity of the correlation fringes within the ROI and suppress the background surrounding the ROI, all the obtained N correlation fringe patterns S n ( i , j ) are summed. The accumulated resultant fringe pattern can be expressed as

N 

1 N S i j  

S i j 

( , ) ( ) 

( , ),

(11)

n

n

1



where  ( N – 1    1) is the factor allowing to choose the optimum intensity level of the fringe pattern S  ( i , j ). The subtractive synchronized ESPI technique has several common features with synchronized reference-updating ESPI technique operating in a real-time modulation mode, which was developed by Pouet et al. (1993). However, practical results of the synchronized reference-updating ESPI technique realization have shown that correlation fringes within the ROI for every n th fringe pattern S n ( i , j ) are fuzzy. So, it is difficult to distinguish them against the background of ambient noise by technical or visual means. To suppress the random spatial noise surrounding the ROI and enhance the fringe pattern within the ROI, which is a useful signal, we raise the signal-to-noise ratio by summing of all N correlation fringe patterns S n ( i , j ) (see Eq. (11)). In order to realize the proposed technique, a hybrid interferometric system (HIS) breadboard based on a Twyman-Green interferometer architecture was created. Laminate, sandwich and honeycomb composite panels with thickness ranging from 1 mm and more can be tested by this system. The system is similar to one described by Muravsky et al. (2017). In the HIS the excited composite panel states are recorded in the Twyman-Green interferometer as SIs of a surface region containing the ROI. Time gaps  allowing recording extreme thickening and thinning of the ROI are provided by an acoustooptic deflector (AOD) working as an optical shutter. The time gap  is synchronized with generated US frequency f . It is equal about one fourth of the US wave period T US (i.e. 