PSI - Issue 56

Cristina Vălean et al. / Procedia Structural Integrity 56 (2024) 97– 104 Author name / Structural Integrity Procedia 00 (2023) 000 – 000

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3.2 Flexural results

From Figure 4a it can be seen that the flexural stress-strain of printed samples shows a very different behavior depending on the type of filament used. All the curves show a linear-elastic zone, followed by the yield of the material and finally its fracture. The samples from PLA+BP show a settlement zone (up to around 0.4% strain), while the other types of configurations (PLA, PLA+GF and PLA+CF) do not highlight such a zone. It seems that the low stiffness of the samples induces this rather obvious and pronounced area. Moreover, the PLA+BP samples also present the shortest linear-elastic region, at the opposite pole are the PLA+CF samples with the most extensive area. Regarding the fracture zone, the samples from PLA+CF show a quasi-brittle behavior, the other samples a ductile behavior. In this sense, due to the lack of filament reinforcements and the defects induced by them (gaps or cracks in the matrix), the samples made of unreinforced PLA show a pronounced deformation until fracture. The presence of reinforcements (GF, CF and BP) in the polymer matrix considerably accelerates the fracture of the samples at low levels of strains. Significant differences between the four types of samples are also observed in the flexural energy absorption-strain curves (Figure 4b). Due to the pronounced plastic deformation (Figure 4a), unreinforced PLA samples absorb the largest amount of energy. The PLA+BP samples are identified at the opposite pole. It was found that in the area of small deformations, below 1%, the curves almost overlap.

Fig. 4. Flexural stress-strain (a) and energy-strain (b) curves of printed samples

The highest flexural modulus values are recorded for the PLA+CF samples, and the lowest for the PLA+BP samples (Figure 5a). Thus, in this case differences of over 56% are obtained between the two types of samples. Smaller but still considerable differences are also obtained between the first and second material. Thus, PLA+CF shows flexural modulus values over 43% higher than PLA+GF. Between PLA+GF and PLA samples, the difference is only 10% in favor of PLA+GF. The CFs in the matrix provide stiffness to the sample when this is loaded perpendicular to the direction of deposition. The CFs are relatively longer and stiffer than GFs and therefore the elastic properties are superior for this filler. The strength properties follow somewhat the same pattern as the elastic ones, with the observation that in this case the order of stress values is reversed between the PLA+GF and PLA samples (Figure 5b). This time, the differences between the first three materials are much smaller than in the case of flexural modulus. Thus, differences in terms of flexural strength of 5.9% are obtained between PLA+CF and PLA samples, respectively of 10.6% between PLA+CF and PLA+GF. Also, the PLA samples reinforced with BPs show the lowest flexural strength values, over 64% lower than the PLA+CF samples. Due to the different fracture mode (quasi-brittle or ductile), the biggest differences between flexural strength and stress at break are obtained for PLA samples (50.2%), and the smallest differences for PLA+CF samples (4.7%).