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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stresses occurred in a translaminar manner. Conversely, when there were more areas with uneven fiber distribution, the less reinforced areas were more susceptible to interlayer delamination. Figure 7 shows a case in which the destruction took the form of a translaminar crack, starting in the middle of the sample from the tension side. At moment ‘1’, based on DIC, it can be concluded that in the location where the crack was supposed to start, the strain value along the fibers was 1.6%. This is a slightly lower value than in the case of a 2 mm high sample, but at the same time, a non-negligible shear strain was observed, which led to the destruction of the sample. At time ‘3’, in the lower part of the sample, clearly torn fibers from the structure could be observed. The second presented case of a sample with a height of 10 mm and a spacing of 120 mm was the most common case, i.e., its destruction due to delamination (shown in Fig. 8). Characteristically, after a moment ‘1’ the force on the bending diagram dropped at a very slow rate, and not suddenly, as in most other cases. Correlating this observation with the behavior of the sample, it can be concluded that this is related to the fact that the sample delaminated only locally, not along its entire length. The ends of the sample remained intact up to a certain point. Only after the complete delamination of the material did a sudden drop in the force diagram occur. The last variant analyzed was a case in which two types of destruction occurred within milliseconds. First, a translaminar crack (‘1a’) was observed. Still, in this case, the crack path started in the middle of the sample length but from the compressed side, which is a common phenomenon because composite materials, due to fiber buckling, are less resistant to compression than tension, and additionally, local compressive stresses from the loading pin transferring the load act at this location. Then, delamination (1b) was observed, which is already visible in the view of the sample at time ‘2’. After further application of the loading force, both cracks became even more prominent, which can be seen in the view of the sample at time ‘3’. 4. Summary and conclusions The most important general conclusion from the conducted analysis is that digital image correlation reveals cracks that are not visible with an unarmed eye, enabling prediction of them. In the case of materials that crack suddenly, it is necessary to predict the potential crack with a margin. DIC allows for the possibility of analyzing structures, which prevents catastrophes. The tests were carried out for five types of samples, giving five different ratios of maximum stress to shear stress, resulting in various cracks. Detailed observations for each type of sample are as follows:  Three-point bending: h = 2 mm, l = 60 mm ( σ max / τ max ≈ 60): Sudden fracture with 70% force drop, fiber break, fibers partially torn out from the matrix, at a location with maximum strain along fibers.  Three-point bending: h = 20 mm, l = 60 mm ( σ max / τ max ≈ 6): First cracks occur due to shear stresses (and shear strain), ideally in the middle of height, where only shear strain exists, no normal strain, followed by increased force due to compression.  Three-point bending: h = 10 mm, l = 160 mm ( σ max / τ max ≈ 24): Different types of failure are probably connected with uneven distribution of fibers, a mixture of normal and shear strain leading to fracture.  Four-point bending: h = 10 mm, l=240 mm, l’ = 160 mm ( σ max / τ max ≈ 16): Delamination in the middle of the sample thickness, clear separation of these layers on both sides after fracture.  Four-point bending: h = 20 mm, l=240 mm, l’ = 160 mm ( σ max / τ max 8): Similar to above, but after the first destruction, each half behaves as a new sample, dividing into subsequent halves. References [1] Khashaba U. A., In-plane shear properties of cross-ply composite laminates with different off-axis angles, Composite Structures 65 (2004) 167– 177. [2] Wang J et al., Exploring the influence of specimen types on the intralaminar fracture behavior of fiber-reinforced polymer matrix composites, Theoretical and Applied Fracture Mechanics Volume 134, Part A , December 2024, 104684 [3] Monticeli F. M., Fuga F. R., Donadon M. V., A systematic review on translaminar fracture damage propagation in fiber-reinforced polymer composites, Thin-Walled Structures, Volume 187, June 2023, 110742 [4] Maqsood N., Rimasauskas M., Delamination observation occurred during the flexural bending in additively manufactured PLA-short carbon fiber filament reinforced with continuous carbon fiber composite, Results in Engineering Volume 11 , September 2021, 100246