Issue 46

V. E. Wildemann et alii, Frattura ed Integrità Strutturale, 46 (2018) 295-305; DOI: 10.3221/IGF-ESIS.46.27 296 A need to timely reveal operational damages and technological defects requires a prompt control of states of structures and their consequent repairs aimed at service life extension [4-10]. The use of embedded optical fibers with Bragg gratings for the analysis of mechanical behaviour and evaluation of material damage is of interest at this time in the field of monitoring the state of structural elements made from composite materials [11-17]. In order to provide reliability of the monitoring system during operations of real products, it is required to develop the evaluation methods of signal’s sufficiency received from fiber-optic sensors under complex loadings [18]. To solve this problem, the methods were developed which aim at mechanical testing of composite specimens with prior introduced technological defects with a joint use of monitoring systems of inhomogeneous deformation fields and temperatures to detect damages with advanced technologies and methods of diagnostics [19-24]. In this paper, we consider the application of the correlation methods of digital images and infrared thermography to analyze the mechanisms of deformation and fracture of specimens with an internal defect under tension with torsion. The obtained experimental data make it possible to estimate the inhomogeneity of the deformation process under complex stress state. EXPERIMENTAL PROCEDURE Material and specimens with embedded defect lat stripe specimen were prepared from the pre-preg of the serial VKU-39 carbon fiber with prior introduced technological defects for the experimental studies of the mechanisms of deformation and fracture of specimens of layer composite materials with prior introduced technological defects that correspond to possible inappropriate compacting and inappropriate bonding of the material layers at a set restricted domain. Autoclave moulding from prepregs is used to produce specimens of composite plates, which is the most common technology used in the aviation industry. The production process of specimens with introduced technological defects of the “delamination type" is based on the use of a fluoroplastic film, which is laid according to the scheme of defects arrangement (Fig. 1, a). The work considers two variants of laying the technological defects: 1 is a package of two defects in the form of a square sized 20×20 mm 2 , 2 is the package of eight defects in the form of a square sized 10×10 mm 2 (Tab. 1). The total surface area of the delamination is 800 def S  mm 2 in both cases. Then the technological package was assembled to carry out the polymerization process in the autoclave complex. To identify the location of the technological defects, non- destructive testing of the workpiece was carried out using ultrasonic method. Type of laying defects Number of defects Defect size, mm 2 Scheme of laying defects Type 1 8 10×10 1 and 2 lay, 2 and 3 lay, 3 and 4 lay, 7 and 8 lay, 8 and 9 lay, 12 and 13 lay, 13 and 14 lay, 14 and 15 lay Type 2 4 20×20 2 and 3 lay, 3 and 4 lay Table 1 : Type of laying embedded defects in the form of the artificial delamination in composite specimens. Geometric parameters of the specimens correspond to the parameters of large-sized shell structures, which are used in aircraft engines. The specimens are made using Zund G3-L2500 plotter according to the drawing shown in Fig. 1 (a). An image of the specimen made from a composite material with an internal defect is shown in Fig. 1 (b). It is worth mentioning that the defect is not visible on the specimen’s surface. Testing equipment and testing program Mechanical tests on tension with torsion were carried out using the biaxial (tension / torsion) servo-hydraulic test system Instron 8850 (100 kN / 1000 N·m). During the tests, the dependences between the change of “load-displacement” and “torque-torsion angle” were recorded using the built-in sensors of the test system. F