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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defects and delaminations in multilayer structures. On the other hand, the hybrid techniques combining optical and acoustical approaches are also perspective for detecting of subsurface defects. For example, Pouet and Krishnaswamy (1994) and Fomitchov et al. (1997) have developed additive-subtractive electronic speckle pattern interferometry (ESPI)/shearography techniques and Pouet et al. (1993) have proposed synchronized reference updating ESPI technique. These techniques combined with US excitation of a studied composite specimen with variable frequencies from tens to hundreds kHz allow to detect and localize disbonds and delaminations in multilayer composites. Time-average digital holography technique for recognition of subsurface defects in honeycomb sandwich panels through their vibration square wave excitation was proposed by Thomas et al. (2017). Such a technique provides time-consuming frequency sweep and is faster than mentioned above ESPI/shearography approaches. However, the mentioned above techniques are very sensitive to external vibrations as well as air and heat fluxes. In addition, the interferometric systems that implement these methods have complex equipment that complicates and even prevents their use in practice. Therefore, development of more simple methods allowing detecting and localizing subsurface defects in composite materials and capable to carry out NDT of composites in natural conditions is the actual problem. Besides, the resonance properties of the local subsurface defects in composite structures are not sufficiently studied up to now. Such circumstance complicates the design of ultrasound generators compatible with optical systems of speckle interferometry and digital holography, as well as the search for resonant frequencies for various types of defects. In this paper, we consider new approach for evaluation of crack/delamination US resonant frequencies. In order to study the resonance properties of the local subsurface defects let us consider the problem of the elastic SH-wave diffraction from a crack/delamination formed on the boundary of a perfectly rigid joint of a layer with a half-space. We apply the solution to determine the complex eigenfrequencies and natural vibrations of the structure and to estimate the effectiveness of the resonance regime sounding. The subtractive synchronized ESPI technique with fringe patterns accumulation and the technique for estimation the dynamic speckle motion of composite surface areas combined with harmonic US excitation of a multilayer composite were developed for detection of subsurface defects using the developed approach. Experimental verification of these techniques was implemented on basis of developed experimental breadboards of hybrid interferometric and optical-digital systems. The last ones were used for detection of test and real subsurface defects in multilayer composites. Let us consider a crack/delamination on the plane boundary of the perfectly rigid joint of a homogeneous elastic layer :{ ( , ), ( ,0)} P x y d      and a plane interface :{ ( , ), S x    0, ( , )} y z     , where Oxyz is a Cartesian coordinate system. Let us assume that the crack/delamination on the boundary of the joint of the layer P with interface occupies a domain :{ ( ,0), 0, ( , )} x L y z        (see Fig. 1). This structure is irradiated by a normal waveguide mode propagating in the layer at its perfectly rigid joint with the half-space in the absence of defects. We consider monochromatic irradiation as i t e   . The diffraction processes in this system are determined by a single scalar function ( , ) u u x y  connected with the displacement field ( , ) z e u x y  u . Then we formulate the boundary value problem as follows: 2. Resonance properties of the local subsurface defects

2 0 u k u    ,

(1)

( i y u u y d x         , ) 0, ; ( , )

(2)

Fig. 1. Problem geometry.