PSI - Issue 77

R. Branco et al. / Procedia Structural Integrity 77 (2026) 376–381 Branco et al. / Structural Integrity Procedia 00 (2026) 000–000

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Fig. 1. Specimen geometry used in the proportional bending-torsion tests manufactured via L-PBF from AlSi10Mg powder.

loading levels were considered. A stress ratio (R) equal to 0 was used. Crack initiation and growth were monitored using a high-resolution digital camera. Image acquisition was manually performed at intervals ranging from 2500 to 5000 cycles throughout the tests. 3. Results and discussion Figure 2 shows typical examples of crack initiation sites and early-stage crack growth directions observed in the combined bending-torsion tests under B/T=2/3 (Fig. 2(a)) and B/T=2 (Fig. 2(b)) for the untreated state. Overall, the crack initiation process was characterised by the nucleation of two cracks in diametrically opposite sides of the hole. The crack initiation, as can be observed in the figure, was influenced by the B/T ratio. Higher normal stress to shear stress ratios ( σ / τ ) led to higher β angles (i.e., the angles between the continuous white and dashed white lines). This behaviour is expected, as higher σ / τ ratios correspond to loading conditions closer to Mode-I, thereby progressively rotating the red line toward a horizontal orientation. A comparison between these two exemplary cases shows that the β angles of the first crack observed in the specimens ( β 1 ) increased from 54º under B/T=2/3 to 76º under B/T=1. In general, the β angles of the second crack observed in the specimens ( β 2 ) were close to those of β 1 . Τ hese differences were more significant when the second crack initiated at a later stage, which affected the specimen’s stiffness and, in turn, influenced the stress state around the hole and, consequently, the crack initiation location. The early-stage crack growth directions were also affected by the B/T ratio. These directions were evaluated through the α angles (i.e., the angles between the crack direction and the X-axis). Similarly to crack initiation sites, the early-stage crack growth directions were close to Mode-I as the B/T ratio increases. As can be seen in Fig. 2, the higher is the B/T ratio, the lower is the α angle of the first crack observed in the specimen ( α 1 ). Specifically for the two exhibited cases, it reduces from 21º under B/T=2/3 (Fig. 2(a)) to 17º under B/T=1 (Fig. 2(b)). This can be attributed to the higher shear stress levels associated with lower B/T ratios, which contributes to a more significant mixed-mode condition, progressively deviating the crack path from the horizontal direction. In a few cases, it was also noted some differences in the α 1 angle relative to the α angle of the second crack observed in the specimen ( α 2 ) . As explained above, this difference was more evident when the second crack initiated at a later stage, which changed the specimen’s stiffness and, consequently, the stress state at the hole region. The different initiation times of the two cracks observed

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Fig. 2. Examples of crack initiation sites and early-stage crack growth directions observed in the combined bending-torsion tests: (a) B/T=2/3; and (b) B/T=2- The Y-axis represents the longitudinal axis of the specimen, while the X-axis represents the circumferential direction around the hole (see Fig. 1 for reference).