Issue 78

D. S. Lobanov et alii, Fracture and Structural Integrity, 78 (2026) 207-219; DOI: 10.3221/IGF-ESIS.78.12

I NTRODUCTION t is inevitable that products and structures made from polymer laminated composites will sustain external surface damage during service or transport. Consequently, it has become essential to assess the severity of such damage and to investigate the influence of surface service defects on deformation behavior, mechanical performance and residual strength properties [1–3]. Matrix microcracks and interlaminar delamination are the main types of damage that occur in laminated composite materials under compression [4–6]. Matrix microcracks can develop along the thickness of the layer and create stress concentrations at the layer interfaces. Cracks will ultimately lead to an increase in interlaminar stress and reach a level that initiates delamination. This then propagates along the layer boundary. The presence of delamination significantly degrades the structural strength and gradually leads to the failure of composite structures [7]. An effective approach to preventing catastrophic structural failure is the timely performance of maintenance based on monitoring results. However, these two types of damage can occur in any layer, leading to damage that is not visible during visual inspection. Non-destructive testing methods, such as the acoustic emission (AE) method, have the potential to address this problem [8]. Acoustic emission is a method for recording and converting elastic waves in a material under load. Each acoustic emission signal has several informative parameters, the analysis of which allows one to assess the fracture processes in the material. As a rule, research employs the analysis of several AE parameters [9–14]. In this context, the selected parameters must correlate with one another so that the identified patterns are valid. In the mechanics of composite materials, rank correlation serves as a preliminary analysis tool that helps to identify the most effective parameters for subsequent cluster analysis [15]; this method is also used to assess the quality of various composite materials [16] and to determine their mechanical properties. Spearman’s rank correlation is well suited for processing data obtained using AE, due to its robustness to outliers and its ability to identify monotonic relationships when the distribution deviates from the normal distribution [17]. Since the parameters of acoustic emission signals vary depending on the material, loading conditions and pre-loads, difficulties arise with classification based on statistical methods. It is impossible to precisely define the initial parameters, ranges and characteristics by which the signals could be classified. In such cases, researchers employ cluster analysis, which allows the data structure to be identified without additional input parameters. Thus, cluster analysis based on several signal characteristics enables a significant improvement in the identification of damage mechanisms within the material [18–20]. This work continues a series of studies on the influence of internal manufacturing and external service defects on the mechanical behavior and changes in the residual strength and deformation properties of polymer composites, previously published by the authors [21–25]. During operation, polymer composite structures are subject to various loads. Therefore, it is necessary to conduct studies not only under tension, but also under compression, shear, bending, and various combined effects. Thus, the aim of this work was to test methods for introducing external service defects onto the surfaces of polymer composites under controlled mechanical loading conditions, as well as to assess the influence of defects on the deformation behavior and residual strength properties of the composite under compression, including based on data obtained by an acoustic emission signal recording system. The novelty of this work in relation to previously published works consists of the method of mechanical testing (compression), the use of different types of dents, and the use of the Spearman method for cluster analysis of data obtained by the acoustic emission signal recording system. M ATERIAL AND METHODS he work was carried out at Perm National Research Polytechnic University at the Centre for Shared Use (CSRF) “Centre of Experimental Mechanics” using the unique scientific facility (USF). (This work was carried out at Perm National Research Polytechnic University in the Core Shared Research Facility (CSRF) “Centre of Experimental Mechanics” using the Unique Scientific Equipment “A complex of testing and diagnostic equipment to study the properties of structural and functional materials under complex thermomechanical loading conditions. The material used in this study is fiberglass plastic STEF. This is a laminated reinforced fiberglass plastic obtained by hot pressing of glass fabrics impregnated with a thermosetting binder based on combined epoxy and phenol-formaldehyde resins, with a lay-up scheme of [0/90] with sufficiently high mechanical characteristics. Fiberglass plastic is used in insulation systems of large mechanisms and machines, hydro- and turbo generators, and parts of electrical engineering. A series of experimental studies was conducted to develop methods for applying simulated external damage to the surface of fiberglass I T

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