PSI - Issue 41

A.M. Ignatova et al. / Procedia Structural Integrity 41 (2022) 550–556 2 Ignatova A.M., Balakhnin A.N., Bannikov M.V., Kuper K.E., Nikitiuk A.S., Naimark O.B./ Structural Integrity Procedia 00 (2019) 000–000 551

Based on the processing results, a dependence was obtained that characterizes the ratio of the size of the local region of the structure with a characteristic density value to the distance between these regions, that is, the desired integral characteristic from the conditional density range of the analyzed regions. © 2022 The Authors. Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of the MedFract2Guest Editors. Keywords: X-ray microtomography; image analysis; defects; cracks; composite material; carbon composite; nondestructive testing; resource evaluation; Introduction. Structures exposed to intense mechanical (e.g., dynamic) loads during operation are often produced with the use of carbon-carbon composite materials (CCCM) (Murashov, 2013). Regardless of the grade and composition, CCCMs are a combination of polycrystalline carbon matrix and carbon fiber reinforcing framework; quite frequently, woven carbon fiber framework. The use of the fiber shaped as woven materials provides for a three dimensional orthogonally ordered macrostructure of reinforcing elements in the composite structure. The woven framework of reinforcing fibers ensures uniform transfer and distribution of stress in the matrix. Despite the apparent advantages of the woven framework, manufacturing CCCMs and products made of them inevitably entails certain manufacturing defects. According to the classification presented in (Demidov et al., 2021), such defects are divided into three categories: micro, mini, and macro defects. In terms of size, micro defects are close to the fiber diameter; they include matrix defects in the spaces between the elementary fibers (carbon filament elements), defects at the fiber-matrix interface, cracks, inclusions and voids. Minidefects are an order of magnitude larger than micro defects, they include length-wise deviations of the geometric shape of the fiber caused by a wrong twist, improper filament orientation, grooves, scratches, dents, breakage of individual filaments, etc. Macro defects are the most extensive discontinuities in the material, such as cracks extending to the matrix and the framework. In the process of CCCM products operation, defects aggravate and transition to larger categories; so, micro defects evolve into minidefects, while the latter evolve into macro defects. The risk of transformation of defects is the reason why their detection using the methods of nondestructive testing is relevant, beyond ensuring CCCM quality and forecasting their operating behavior, for evaluation of structural changes in the process of deformation. According to (Masaji Kato et al., 2013), (Gunyaev and Gofin, 2013), and (Tolbin et al., 2012), in the course of CCCM production, the most likely defect is porosity, as a rule, oriented along the fibers of the woven framework. The experience of using composite materials proves a direct correlation between porosity and mechanical properties such as compressive, tensile and interlayer shear strength. Besides, CCCM response to the mechanical loads is determined by the change in the porosity parameters and the change in the distribution of density over the volume (Dolgodvorov and Dokuchaev, 2015). There are many methods to control the porosity of composite materials, varying in their degree of accuracy; in the meantime, there are next to no methods that would help assess the uneven distribution of density, except for X-ray microtomography. Microtomography also enables a sufficiently detailed study of the material's porosity, making it a universal means to assess the macrostructure of composite materials. The existing approaches to the analysis of X-ray microtomography data are based on the commonly accepted approaches to image analysis; in particular, image segmentation (Limanova and Ataev, 2018) is quite popular. The most commonly used is the method of threshold segmentation (Demidov et al., 2021). Such an approach enables the identification of morphometric characteristics of each individual region of the structure with a different density, be it a continuity defect or a piece of fiber. The aggregate volume of regions with a specific density value and morphometric parameters of individual regions are insufficient to generalize the results of the tomographic study. To evaluate the resource and shock-wave sensitivity and for prediction purposes, an integral characteristic of the structure has to be determined. In (Naimark, 2003), the proposed integral characteristic of the structure is the ratio between the size of the structure's local region with the characteristic density value and the distance between these regions. 2 © 2022 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of the MedFract2Guest Editors.