Issue 78

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

with controlled parameters of mechanical impact on the Instron 5982 electromechanical testing system in conjunction with an acoustic emission signal recording system [20]. Diagrams of the application of simulated damage are shown in Fig. 1. For all indenters used to apply simulated external damage to the surface of the polymer composite, ranges of load and indentation displacement values were established that either did not result in any damage to the specimen at all, or where the damage obtained had no effect on the load-bearing capacity of the fiberglass. Ranges of critical load values were also established, which, during the indentation process, led to the appearance of visible macro-defects (through-cracks, rupture of surface layers, etc.).

а c Figure 1: Schematic diagrams showing the creation of simulated defects: a dent using a cylindrical indenter (a), a spherical indenter (b), and a scratch using a steel blade (c): 1 – specimen, 2 – indenter, 3 – acoustic emission sensor b

Figure 2: Force–displacement curves for a 4 mm thick fiberglass specimen with simulated defects created using indenters of the following shapes: sphere – blue solid lines (load level 0.9), blue dotted line – until failure; cylinder – red solid lines: load level 0.9; red dotted line – until failure It has been established that, when a critical indentation is made by a spherical indenter, brittle fracture of the matrix occurs, followed by delamination through the entire thickness of the material in the region of the defect. An indentation deeper than 35% of the specimen thickness leads to macroscopic fracture of the material (Fig. 3). C ompression tests were then carried out on groups of samples cut along the warp and weft, including samples without defects, samples with a cylinder-induced indentation, samples with a sphere-induced indentation, and samples with a scratch. The compression tests were carried out in accordance with ASTM D 3410, with a loading rate of 1 mm/min. During the compression tests, as well as during the preliminary application of defects, acoustic emission signals were recorded using the AMSY-6 Vallen System. A broadband sensor with an operating frequency range of 100–500 kHz and a preamplifier with a gain of 34 dB were used. The sensor was calibrated before testing; the sampling frequency was 10 MHz, and the threshold was set at 40 dB. The sensor was mounted on the specimen with silicone lubricant applied to ensure better adhesion, and was additionally secured using clamps. The choice of a sensor frequency range of 100–500 kHz and a sampling frequency of 10 MHz is based on standard recommendations for acoustic emission testing of polymer composite materials. This range ensures the recording of signals from the main damage mechanisms, from matrix cracking to fiber failure. The selected sampling frequency ensures accurate

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