Issue 78

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

Destruction mechanism Breakdown of the matrix Onset of delamination Loss of adhesion, fiber friction

Frequency range

Cluster (weft)

Cluster (warp)

57–84 kHz

1

1 2 3 4

101–276 kHz 277–432 kHz

2 + 3

4

Fiber cracking

>450 kHz

—

Table 4: Results of the clustering of signal peak frequencies. The connection between AE clusters and failure mechanisms is supported by references to studies where AE signal parameters were correlated with damage types in polymer composites [18, 19]. The approach used for clustering by amplitude, duration, rise time, and peak frequencies is consistent with the methodology described in [9, 14, 20]. The attribution of low-frequency signals to matrix failure, and intermediate and medium-frequency signals to delaminations, does not contradict the data previously obtained by the authors for fiberglass and carbon fiber reinforced plastics [21, 22, 24]. The obtained results are consistent with the data of independent studies. The conclusion that a dent from a spherical indenter poses the greatest danger among the types of defects considered is confirmed in [13], where it was found that damage with a pronounced crumple zone and internal delaminations cause a more significant reduction in compressive strength compared to surface notches. The predominance of signals from matrix failure and delamination, recorded in samples with defects, corresponds to the findings of [6], according to which impact damage to laminated plastics under compression initiates a localized loss of stability of the sublayers. The critical indenter penetration depth, which amounted to ~35% of the sample thickness and led to macroscopic failure, correlates with the results of [15], where the threshold of irreversible damage for polymer composites was recorded at comparable relative depths. Differentiating defects by severity can be used to develop threshold criteria for inspecting polymer composite structures. Fracture images (Figs. 3, 7) demonstrate differences in damage patterns and can be used as reference material for identifying defect types. Acoustic emission characteristics corresponding to different failure mechanisms can be used in signal classification algorithms for continuous monitoring (SHM) systems, consistent with approaches described in [4, 8]. Recording signals from delamination clusters at early stages of loading can serve as an indicator of impending failure and the basis for determining the timing and scope of repair measures. C ONCLUSIONS est results have shown that the most dangerous defect is a spherical indentation, which leads to a 20% reduction in compressive strength for weft-oriented samples and a 12% reduction for warp-oriented samples. It was also found that a spherical indenter penetration of 35% of the material thickness leads to macroscopic failure of the material and a reduction in static strength of 95–99%. Scratch and cylindrical indentation defects had virtually no effect on the compressive strength values at the selected defect application level, and the difference in values can be explained by statistical variation. Acoustic emission signals were also recorded and analyzed during the compression test. Thus, different types of fillers exhibit approximately the same patterns in material behavior. In two specimens, a direct strong dependence is observed between the impulse force and its decay time, with a proportionality in the temporal characteristics being evident: a relationship between the total signal duration and its rise time. The frequency characteristics show an inverse (negative) relationship with amplitude, duration and rise time. Data clustering was performed based on correlation analysis. Four clusters were identified for the signal parameters under investigation. Characteristics of the signal shape in each cluster were noted. Cluster analysis was also carried out for the peak frequencies of the signals, where the boundaries of the frequency ranges were refined and regularities in the failure mechanisms were noted. The obtained results have practical significance for assessing the residual service life of polymer composite structures with operational surface damage. The developed method for controlled defect introduction in combination with acoustic emission monitoring can be used to create reference damage and classify defects by severity during non-destructive testing and assessment of the technical condition of structural elements. T

217