PSI - Issue 76

Davide D’Andrea et al. / Procedia Structural Integrity 76 (2026) 151–158

154

obtained from each test. In Table 1 the average mechanical properties and corresponding standard deviations are presented for specimens printed in the X and Y directions separately, as well as for the combined dataset, given the absence of significant differences in their mechanical response. The mechanical properties in the X, Y, and combined directions show minor variations, reflecting near-isotropic behaviour. The Young’s Modulus , the yield and ultimate stress of the Y direction set are approximately 1.5%, 3.7% and 6.8% higher than X direction set, respectively, while its yield strain and ultimate strain are 3% and 33.8% lower than X direction set. These percent differences suggest that the material is nearly isotropic in terms of stiffness and critical stress properties, while strain properties show moderate anisotropy, likely due to microstructural alignment or processing effects. In Figure 2, it can be observed that the linear portions of the curves overlap until the yield stress is reached, while a slight divergence appears in the plastic region.

Figure 2. Stress-strain curves for PA12 printed in X and Y direction.

Table 1. PA12’s m echanical properties.

Yield Strain [//]

Ultimate Strain [//]

Yield

Ultimate stress [MPa]

Young’s

Stress

Modulus

[MPa]

E [MPa]

1482 ± 74 1505 ± 143 1493 ± 102

43.1 ± 0.3 0.169 ± 0.001 44.7 ± 0.3 0.164 ± 0.009 43.9 ± 0.9 0.167 ± 0.006

41.3 ± 0.3 0.273 ± 0.051 44.1 ± 0.4 0.204 ± 0.014 42.7 ± 1.6 0.238 ± 0.051

X direction Y direction

Both direction

During the quasi-static tensile tests, the evolution of the superficial temperature of the specimens has been monitored with an IR camera considering the maximum temperature value recorded over time of a rectangular measurement area placed on the specimen surface. Figure 3 reports the stress vs. temperature trends. It is possible to observe as the applied stress increased, the temperature decreases in a linear way up to t= 33 s, according to Phase I of the STM; then it continues to decrease but with a different slope until it reaches a minimum value according to Phase II of the STM. The transition point between Phase I and II can be linked to the onset of irreversible damage within the material. The correspondent macroscopic stress level can be indicated as the limit stress, σ lim , according to STM. Indeed, if this limit stress is applied under fatigue regime it will cause the material to fail. To assess the limit stress, D’Andrea et al., (2025) proposed an iterative algorithm. The first subset used for linear regression is represented with red pentagons, while the second is represented with blues squares. Data excluded from calculations are represented with “x” and “y” markers. The inflection point, calculated as the intersection between Phase I and Phase II occurred at 33.1 s; in this instant a limit stress of 26.5 MPa is estimated in correspondence of a temperature variation