PSI - Issue 83

Davide D’Andrea et al. / Procedia Structural Integrity 83 (2026) 256–264

262

(a)

(b)

Figure 5. RTM’s results for a) traditionally manufactured and b) AM specimens

Table 3 reports the results obtained for each test. In addition to the differences in fatigue limit, it is worth noting that the slope and Energy Parameters characterizing the AM material are orders of magnitude lower, highlighting that the specimen develops lower temperatures for the same stress level and releases less energy during the fatigue process. On the other hand, the coefficient C 1 is higher for the AM material, highlighting that crack initiation and propagation phenomena occur more easily in the Laser Powder Bed Fusion (LPBF) material. Another discussion concerns the ratio between the area below the third phase’s temperature ( ɸ cr ) and the total Energy Parameter ϕ . The results show that, for the traditionally produced steel, the third phase accounts for about 40% of the total fatigue life of the specimens, while lower ratios have been estimated for the AM counterpart, suggesting that once the crack initiates, it propagates faster than what happens in traditional material (Chakotay et al., (2024)).

Table 3. Results summary for traditional and AM AISI 316L specimens’ batch.

ID

Step_AISI316L_trad01

Step_AISI316L_trad02

Step_AISI316L_AM01

Step_AISI316L_AM02

଴,ோ் ெ [MPa] m [°C/MPa 2 ] Φ [°C ⋅ cycles] ϕ cr [°C ⋅ cycles] ϕ cr / ϕ [-] C 1 [mm 2 /°C]

304

303

287

263

6.620E-03 6,69E-05

6.541E-03 5,93E-05

1.393E-04 2,52E-03

1.035E-04 3,61E-03

2547012 1006065

2644342 1074760

281437

270289

41120

28590

0.39

0.41

0.15

0.11

Table 4 reports the number of cycles associated with the third phase, as calculated using the proposed methodology. It is worth noting that, at the same stress level, the traditionally manufactured specimens exhibited a longer third phase, indicating a longer crack propagation duration. Finally, Figure 6-a shows results concerning plastic work rate calculated by temperature’s data. Every curve has been normalized with respect to the maximum coefficient β p , which was calculated for traditionally manufactured steel. It can be observed that each test is characterized by an increase in plastic work with increasing stress. At low stress levels, low percentages of plastic work are observed, while an increasing trend occurs for both batches of specimens. It should also be noted that the AM material is characterized by lower plastic work rates due to the weaker interparticle bonds typical of L-PBF processes, whereas the traditionally manufactured steel exhibits stronger metallurgical bonding (Haghdadi et al., (2020); Joshi et al., (2023)). Figure 6-b shows the S–N curves derived from Risitano’s Thermographic Method. An experimental constant-amplitude (CA) campaign was carried out to verify their validity for both batches of specimens. The two S–N curves highlight the differences between the specimen batches: