PSI - Issue 66

Karolina Głowacka et al. / Procedia Structural Integrity 66 (2024) 108 – 121 Author name / Structural Integrity Procedia 00 (2025) 000–000

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with thermoplastic matrix, caused uneven fiber distribution in the material. Each layer was densely reinforced with fibers in the middle and much less frequently on the edges, which is why the material had areas with less and more volume percentage of fiber reinforcement.

Figure 1. Diagram of the fiber arrangement in the tested material

The basic material data of the tested composite are:  Material density

ρ = 0.91 g/cm3;

V f = 47%.

Volume percentage of fibers Single layer thickness: Single fiber diameter:

  

th layer = 188 µm;

d = 18 µm. As part of the research, three-point and four-point bending tests were performed. In this way, samples with five different dimensions were obtained, which resulted in 5 different ratios of maximum normal to shear stresses (and strains):  Three-point bending, h=2 mm, b=15 mm, l=60 mm, σ max / τ max ≈ 60;  Three-point bending, h=10 mm, b=15 mm, l=120 mm, σ max / τ max ≈ 24;  Four-point bending, h=10 mm, b=15 mm, l=240 mm, l’=160 mm, σ max / τ max ≈ 16;  Four-point bending, h=20 mm, b=25 mm, l=240 mm, l’=160 mm, σ max / τ max ≈ 8;  Three-point bending, h=20 mm, b=25 mm, l=60 mm, σ max / τ max ≈ 6. By testing samples with different ratios of maximum normal to shear stresses, various types of fracture (interlaminar/translaminar) were obtained. In the case of a high value of the ratio of normal to shear stresses, the stresses causing destruction were normal stresses, resulting in translaminar cracking. When this ratio was small, the decisive stresses were shear stresses, resulting in delamination. In the case of intermediate values, it was expected that both types of stresses could be destructive. Still, the issue to be analyzed was to what extent and how to predict potential destruction before it occurs. 5 specimens of each type were tested. In all cases, the bending test was carried out with 2 mm/min constant displacement. During the tests, an image of the sample was recorded, and by applying an appropriate pattern to the sample surface, it was possible to use digital image correlation (DIC) at the stage of measurement analysis. Due to the fact that during bending, the forces causing destruction occur only in the bending plane, it was assumed that it would be sufficient to analyze the image only in this plane (defined by the length and height of the sample). However, by supplementing the tests with DIC, not only theoretical information was obtained, which could be determined using basic mathematical formulas, but also experimental information, taking into account local strains generated by the loading pin and supports and not only those resulting from bending. The research analyzed 2 stages of failure analysis. First, the material was examined until the first crack. It was supported by digital image correlation, which revealed the strain values in the samples and allowed localization of the beginning of the crack. The view of the sample was analyzed just before and just after destruction. Then, further analysis was carried out until the complete destruction of the material. Still, at this stage, digital image correlation was no longer applied because its results would already be distorted.