Issue 78

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

Without defects

Scratch

Dent (cylinder) Dent (spherical) Figure 7: Side surfaces of the fracture surfaces of fiberglass samples—one without defects and others with applied scratch and dent defects—following compression testing. In addition, images of fracture surfaces were used. Fig. 3 shows a specimen fracture after the introduction of a spherical indenter to failure: multiple interlayer cracks (yellow areas) and matrix crumpling zones (red areas) are observed. Fig. 7 shows the fracture lateral surfaces of specimens without a defect, with a scratch, and with an indent after compression. Specimens with a spherical indentation exhibit areas of preliminary delamination and multiple crack formation in the matrix, consistent with the predominance of signals from clusters 1–3. Specimens with a scratch defect have no zones of preliminary delamination, and the fracture is localized. Acoustic emission signals were also recorded during the quasi-static compression tests. The plots of the cumulative energy of AE signals versus time (Fig. 8) show that when defects are introduced using a flat and spherical indenter, the maximum values of cumulative energy do not differ significantly for samples cut along the warp and weft. Whereas in compression tests of samples without defects and with a scratch, the cumulative energy values for samples cut along the weft are 3–6 times higher than for samples cut along the warp. The histograms showing the distribution of peak frequencies in the AE signals (Fig. 9) reveal two main frequency ranges (from 20 kHz to 130 kHz and from 250 kHz to 350 kHz) and one intermediate range (from 150 kHz to 220 kHz). The main ranges – low-frequency and medium-frequency – indicate matrix cracking and delamination in the material under compression. Signals in the intermediate range are present only in defect-free specimens and in specimens with a defect induced by a flat indenter. No signals were recorded in the high-frequency range. In the RA-AF plots (Fig. 10), a significant number of signals can be identified along the AF axis at low RA values; at the same time, signals are present in sector I, indicating delamination in the material. Signals with high RA values are practically absent in samples with defects. The distribution of signals along the RA-AF axes in defect-free samples and samples with

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