Issue 78

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

Direction\Group of specimens

no defect

cylinder dent

sphere dent

scratch 199±7 186±5

206±4 181±7

208±8 188±4

165±7 160±6

Weft Warp

Table 1: Average compressive strength (MPa) values

Figure 6: Histogram showing changes in compressive strength values as a function of the defects introduced

Fig.7 shows the fracture surfaces of the fiberglass samples with induced defects following compression testing. Failure of specimens without and with a "spherical" dent defect occurs due to loss of stability and macrofracture of the layers with direct fiber separation (red areas, Fig. 7) and subsequent macrofracture (white areas, Fig. 7). Failure of specimens with a "cylinder" dent and scratch defect occurs due to localized delamination on the side where the defect was applied. Next, bending occurs, which leads to rupture of the material layers at an angle of 45-60 ᵒ (yellow areas, Fig. 7) and local delamination near the separation (blue areas, Fig. 7). To assess the reliability of the results obtained and to determine the extent to which various factors influenced them, a two way analysis of variance (ANOVA) was carried out at a significance level of α = 0.05. The results of the ANOVA yielded the following data (Tab. 2).

Source of variation Sampling Columns Interaction

F critical

SS

df

MS

F

P-Value

P>0,05

2016,125 6320,125 460,125

1 3 3

2016,125 2106,708 153,375

54,03354

1,365E-07

4,260 3,009 3,009

Significant Significant Significant

56,461

5,06204E-11 0,017349809

4,111

Inside

895,5

24

37,3125 Table 2: Results of the analysis of variance

Based on the data obtained (Tab. 2), the following key patterns can be identified: there is clear anisotropy in the properties, with statistically significant differences in strength characteristics between the warp and weft directions; the tensile strength of the fiberglass composite along the warp is consistently higher than along the weft for all types of defects considered. The geometry of the damage has a decisive influence on the material’s compressive strength: the most critical reduction in strength properties is caused by a ‘spherical’ dent defect, the reduction in strength resulting from which is significant compared with reference specimens without defects. Furthermore, a clear relationship can be observed between the shape of the defect and the direction of the applied load; in particular, a ‘cylinder’-type defect causes virtually no reduction in the compressive strength when loaded along the warp, whereas its effect is different when loaded along the weft, which is due to the specific nature of stress redistribution within the reinforcing fibers.

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