PSI - Issue 41

2

Author name / Structural Integrity Procedia 00 (2019) 000–000

760 © 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: laminated carbon fiber, delamination, Staged destruction, fatigue failure, Digital Image Correlation (DIC), acoustic emission, infrared imaging. 1. Introduction A distinctive feature of the mechanism of fatigue failure of a layered polymer composite material in comparison with metals is that damage in their structure appears already at the initial stages of cyclic loading and manifests itself in the violation of adhesion between the fiber and the matrix and the appearance of microcracks in the matrix. Further, as loading occurs, macroscopic adhesion failures between the layers of the laminate - delamination - appear and grow. Upon transition to the final stage of the exhaustion of the resource – fragmentation, a decrease in the load capacity occurs abruptly with following destruction of the specimen. However, it should be noted that all stages of damage are accompanied by a drop in the stiffness of the material [1]. It should be understood that in view of the variety of structures of composite materials, the mechanisms of their destruction must be investigated separately for layered [2-6], 2-D and 3-D woven [7,8,9-11] materials, as well as for materials reinforced with short randomly directed fiber [12-15]. Nowadays the damage assessment with the X-ray and computer micro tomography methods has become widespread [13, 16, 17], but at this stage of development of these technologies, their application is impossible directly in the testing process. Therefore, the development of an engineering approach without the use of expensive equipment can be a good solution for indirect assessment of the damage of polymer composite materials during fatigue testing. In this paper, a set of experimental methods that can be used to determine the moment of crack formation in layered composite materials and to evaluate the process of their development during fatigue tests is proposed. M.Sh. Nikhamkin et al. / Procedia Structural Integrity 41 (2022) 759–765

Fig. 1. Specimen loading scheme

2. Experimental method The purpose of the experiment is to assess the resistance to destruction, as well as to identify indirect signs associated with the appearance of macroscopic damage to the studied in layered polymer composite materials. The study was conducted on specimens of rectangular section 15x25 mm made from carbon fiber. Specimens were clamped on both sides along narrow (see Fig. 1) faces in the direction normal to the woven carbon fiber layers. The study was conducted on the basis of up to 1 million cycles. For the studied specimens, the tests were carried out under “tensile-tensile” conditions with an asymmetry coefficient of the loading cycle R = 0.1 at different load levels. The tests were carried out on a electroresonant fatigue testing machine Zwick HFP 5100 (Figure 2). Due to the high frequency of loading, the machine allows high-cycle fatigue testing with up to 100 million loading cycles. During the tests, the dynamic load values, the natural test frequency, acoustic emission data, dynamic stress fields,