Issue 78

N. Naboulsi et alii, Fracture and Structural Integrity, 78 (2026) 74-95; DOI: 10.3221/IGF-ESIS.78.05

comparison between orientations was supported by the combined interpretation of the Weibull parameters, the linear fitting quality, and the corresponding survival-failure probability curves, providing a consistent evaluation of strength dispersion and characteristic mechanical behavior. The model for linear regression is expressed as follows:

  

   

 

  

1

  mln mln x 

  



Ln ln 

(1)

 F x





1  

;m,

with F(x; m, η ) represent the experimental ultimate stress and strain values ranked in ascending order, and the corresponding empirical failure probabilities were subsequently calculated for the Weibull analysis.

R ESULTS AND DISCUSSION

Effect of infill densities ensile tests were performed on all printed samples prepared in compliance with ISO 527-2 Type 5A at different infill densities (20%, 40%, 60%, 80%, and 100%) and printing orientations (0°, 45°, and 90°). The data acquisition system connected to the tensile testing machine recorded the force measured by the force sensor and the displacement captured by the extensometers until the samples failure. Strain-stress curves (%- MPa) were generated for all samples from this raw data. These curves were then used to extract the relevant mechanical properties in order to analyze the influence of PLA-CB infill density at each print orientation [32]. This analysis is particularly important for low infill densities, where the printed structure contains larger internal gaps and fewer continuous load-bearing paths, which may increase the sensitivity of the specimens to local printing non-uniformities and contribute to the dispersion of the tensile results. For Orientation 0°: Fig. 7 shows the stress-strain curves reflecting the tensile behavior of PLA-CB and pure PLA for a layer orientation of 0°. The mechanical characteristics derived from these curves are shown in Tab. 3. The pure PLA with 100% filling density has the highest maximum stress, reaching 58.181 MPa, with an elongation at break of 14.332%, reflecting good stiffness combined with a certain degree of ductility. The comparison with the curves for the PLA-CB confirms that the composites present a lower mechanical performance overall, confirming the brittle effect of CB within the PLA matrix. However, the infilling density also has a significant effect on the results: for low densities between 20% and 40%, the maximum stress is limited to 25.306 and 26.163 MPa respectively, with early failure at approximately 4.474 to 4.740% deformation. This decrease can be explained by the presence of internal voids which decrease the effective surface for the stress transmission and promote the initiation of cracks. The tensile strength slightly increased from 28.582 to 29.608 MPa with the increase of density from 60 to 80% and the elongation was about 5.052 to 5.432%. The increase in material content therefore improves the compactness of the structure and interlayer cohesion, which slightly delays failure. However, the embrittlement induced by CB remains apparent. Finally, at 100% density, PLA-CB shows a clear improvement, with a maximum stress of 49.680 MPa and an elongation of approximately 6.555%. Although this value is still lower than the value for pure PLA, it demonstrates the positive effect of a fully dense structure, which reduces internal defects and allows for better load transmission along the tensile axis. We note that the increase in filling density partially offsets the loss in mechanical performance caused by the conductive filler. This confirms that filling density is a key parameter for optimizing the mechanical properties of printed parts. Although pure PLA still exhibits higher strength and ductility, fully dense PLA-CB represents an interesting compromise, combining relatively high mechanical strength with the functional potential associated with conductivity. For Orientation 90°: Fig. 8 presents the effect of infill density on the tensile properties of PLA-CB samples with a layer orientation of 90°. Maximum stresses are generally much lower than at 0°, which is a proof of the vulnerability of parts with filaments oriented perpendicularly to the tensile axis. The ultimate stress is still limited to about 23.481 to 24.415 MPa with early failure at about 5.626 to 5.727% strain at low densities of 20 to 40%. These values indicate a high fragility due to the larger internal voids and the stress pattern that is adverse to interlayer bonding. With increase in density from 60 to 80%, there is a slight increase in strength to 25.650 to 32.671 MPa for elongation of around 5.931 to 6.706%. This shows the improvement in internal cohesion. T

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