PSI - Issue 72

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

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Higher melting temperature was registered for PLA, and the other two observed temperatures, glass transition temperature and degradation temperature, remained approximately the same. According to Alkabbanie et al. (2024), Jamadar (2024) and Pathek (2024), this might be explained by crystallinity enhancement due to CF presence. PLA is a semi-crystalline polymer, but in FDM 3D printing, it often solidifies in an amorphous or low-crystallinity state, reducing its mechanical and thermal performance. Added carbon fibers in PLA act as nucleating agents, meaning that the carbon fibers promote faster and more efficient crystallization during cooling. This leads to increased crystallinity, which enhances thermal stability and mechanical strength. The results of the tensile properties are listed in Table 4: maximum tensile strength (σ max), elongation at maximum force (ε max), and Young`s modulus of elasticity (E).

Table 4. Tensile properties. σ max, MPa

ε max, %

E, GPa

NT (+20°C)

40.97

1.94 2.48 3.09 1.76 3.76 3.00 1.03 4.41 3.35

2.43

XY HT(+50°C)

3.93 5.92

14.60 19.42

LT (- 30°C) NT (+20°C)

37.24

2.12

XZ HT(+50°C)

6.18 3.86

11.66 10.17

LT (- 30°C) NT (+20°C)

19.53

1.85 7.82

YZ HT(+50°C)

7.27 6.25

LT (- 30°C)

12.21

Orientation (X, Y, Z) influences tensile properties, with optimal performance in orientation XY, at all the observed temperatures, compared to the other two orientations. Interesting to note, samples printed in XY direction had lower tensile resistance at +50  C than at -30  C. Stress-strain curves for samples with different printing orientation, tested at 20  C, are presented in Figure 4.

Fig. 4. Stress – strain curves comparison, +20°C.

The appearance of the samples after the tensile test and impact test are depicted in Figure 5. Due to the presence of CF, these fractured surfaces appear to have gone through a brittle fracture.