PSI - Issue 72

Miloš Vorkapić et al. / Procedia Structural Integrity 72 (2025) 470 – 478

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Fig. 5. HT specimens after the tensile test (a) and the impact test (b).

From the summarized tensile test results we may observe some general trends: the highest tensile strength is observed in the XY orientation at room temperature (NT, +20°C), reaching ~40 MPa. Strength drops significantly at both high and low temperatures. The highest strain values occur at higher temperatures, indicating increased ductility. Regarding the values of elastic modulus, at -30°C the material is stiffer (higher modulus), while at +50°C it becomes much more compliant (lower modulus). A dramatic reduction in tensile strength and stiffness (E) is observed at +50°C for all printing orientations. This is due to approaching the glass transition temperature (Tg) of PLA, where the polymer matrix softens, leading to weakened intermolecular forces, thus lower strength. Also, this is due to higher polymer chain mobility followed by increased strain before break. At -30°C, the material becomes stiffer and slightly stronger, due to the PLA matrix entering a more rigid, glassy state, when elastic modulus increases in all orientations due to reduced molecular motion and higher resistance to deformation. However, σ max still drops significantly compared to room temperature, indicating increased brittleness and potential micro-cracking at lower temperatures. The increase in strain at break in some cases suggests slight ductility improvement due to crack blunting effects at low temperatures. For instance, in the XY orientation at -30°C, we see a notable increase in modulus (19.42 GPa vs. 2.12 GPa at 20°C), while strength drops to 5.92 MPa. This suggests high stiffness but increased brittleness. These experimental results are in agreement with existing literature, highlighting the critical influence of printing orientation and temperature on the tensile properties of PLA/CF composites, in similar research conducted recently by Chalgham et al. (2021), Jin et al. (2024), El-Deeb et al (2024), Magri et al. (2021) and Alkabbanie et al. (2024). Impact toughness of the examined 3D print specimens of PLA/CF, assessed by the Charpy impact test, are summarized in Table 5, where mean values from 3 measurements with standard deviations are given.

Table 5. Impact toughness.

Impact toughness

Ut, kJ/m 2 ± St.Dev.

Orientation/ temperature

+20°C

+50°C

-30°C

XY XZ YZ

8.27 ± 0.31

7.33 ± 0.29

6.84 ± 0.30 6.36 ± 0.13 4.21 ± 0.14

6.22 ± 0.12 6.94 ± 0.40 4.20 ± 0.41 4.63 ± 0.29

The obtained results illustrate the material’s ability to absorb energy upon sudden impact. PLA/CF exhibits the highest impact toughness in XY orientation of 3D printing, and the best results were obtained for XY samples at higher temperatures. In the XY orientation, the printed layers are parallel to the direction of impact. This means that the impact force is absorbed primarily by the continuous polymer matrix and carbon fibers, rather than being transmitted across weaker interlayer regions. In contrast, in XZ or YZ orientations, the impact force is applied perpendicular to the printed layers, making the samples more prone to delaminating and layer separation, leading to lower toughness. At higher temperatures, PLA becomes more ductile, meaning it can deform more before fracturing. This helps the composite material absorb more energy before breaking. Carbon fibers do not soften significantly with temperature, but they can still improve energy dissipation by reinforcing the matrix and preventing crack propagation.