Issue 53

E. M. Strungar et alii, Frattura ed Integrità Strutturale, 53 (2020) 406-416; DOI: 10.3221/IGF-ESIS.53.31 407 aimed at obtaining new experimental data about the regularities of evolution of displacement and strain fields in the zones of composite elements stress concentration at inelastic behavior by means of digital image correlation method appears relevant. Despite the significant achievements of experimental composite mechanics, the issues that need extra studies include complex theoretical and experimental study of damage accumulation process regularities, defects development, structural and macro-damage of structural polymer composite materials operating under the conditions of non-homogeneous strain fields in the parts with various geometry concentrators. It is noted that the composite materials structure complexity results in the necessity of consideration the defects at various levels, and finding the damage development stages [22-24]. The objective of this work is the experimental study of regularities of inelastic deformation, the processes of damage accumulation and structure polymer composite materials damage under the condition of non-homogeneous strain fields. M ETHODOLOGY AND METHODS OF RESEARCH his work was carried out in Perm National Research Polytechnic University using Unique Scientific Equipment «Complex of testing and diagnostic equipment for studying properties of structural and functional materials under complex thermomechanical loading» . Registration and analysis of evolution of non-homogeneous strain fields and the temperatures on the surface of structurally non-homogeneous material provided the joint use of in-situ optical methods of experimental mechanics: the highly efficient method of digital images correlation (DIC) on the basis of three-dimensional digital optical system Vic-3D (Correlated Solutions) and infra-red tomography method FLIR SC7700M with thermal imaging system with MCT (Mercury-Cadmium Telluride) cooled detector. The quantitative comparison of the images in DIC method is provided by means of computation of image conformity factor according to one the criteria and the search for its extremum. This work uses the criteria of normed sum of squared differences with zero mean [15]. In the process of data processing, we establish the surface points displacement fields, and the transition to deformation is performed by means of finite strain tensor in the sense of Lagrange [15]. The analysis of composite material damage due to macro-fissure development, the study of developmental prototype fracture surfaces was performed with the use of stereomicroscope Carl Zeiss SteREO Discoverj у .V12. The study of inelastic deformation processes, the accumulation of damages and development of defects in composite objects depending of stress conditions and modes provided the performance of mechanical tests at quasi-static loading at multipurpose electromechanical test system Instron 5882 (100 к N), Instron 5989 (600 к N) as well as at cyclic loading with various cycle parameters at servohydraulic two-axis (tensile-and-compression/torsion) test system Instron 8850 (100 к N/1000 Nm) [8]. M ETHODICAL SPECIFICS OF THE USE OF DIGITAL IMAGES CORRELATION AND DIGITAL OPTICAL SYSTEMS METHOD AT COMPOSITE MATERIALS TESTING he paper considers the methodical issues of conducting the experiment with the use of digital optical system for 3D analysis of displacement and strain fields. The method DIC suggests the selection of correlation analysis parameters, in particular, the subarea size  and the step value  . During the correlation processing of digital images the displacement vectors computation is performed not at each individual image point (pixel), but through the study area discretization into small local study areas or sub-areas. The step is another parameter to be set in the algorithm of building the displacement vector fields, its size determines the procedure of search and identification of image sections. The step value sets the distance between the points analyzed in the process of mathematical processing in pixels [15]. In view of the fact that composite materials possess significant structural non-homogeneity, which is not considered in the method DIC, the authors performed a number of calculations of deformation fields on the surface of dimensionally laminated carbon composite for one frame at various values of subarea in the interval of  =9  89 pixels and at the fixed step value  =5. The authors have built a diagram of dependence of the subarea size impact on the results of correlation ratio calculation Fig. 1). The paper takes into account the recommendation of literature sources [15], that the subarea size is acceptable if correlation ration does not exceed σ≤ 0.01 [15]. The selected subarea size 49×49 has sufficient uniqueness of pixels T T