PSI - Issue 77

5

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

380

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

Heat-treated B/T=1 σ a =28.9 MPa (3)

Untreated B/T=1 σ a =28.9 MPa (2)

Heat-treated B/T=2/3 σ a =28.9 MPa (1)

Crack length, a (mm)

a=b(N) c

0 20000 40000 60000 80000 100000 120000 140000 160000 180000 200000

Number of cycles, N

Fig. 4. Numerically predicted versus experimentally measured: (a) crack initiation sites; and (b) early-stage crack growth paths of the first crack observed in the specimen subjected to combined bending-torsion loading for both the untreated and heat-treated conditions. Experimental measurements fitted using exponential functions (red continuous lines). and heat-treated). Compared to the untreated material, the heat-treated condition exhibited delayed crack initiation and slower fatigue crack growth, suggesting a beneficial effect on fatigue performance under combined bending-torsion loading with pulsating (R=0) conditions. Nevertheless, it should be noted that in some cases, differences were less significant, and in other cases, a detrimental effect was observed. Therefore, further research is required to gain a deeper understanding of the influence of this stress relief heat treatment on the multiaxial fatigue behaviour of L-PBF processed AlSi10Mg aluminium alloy. 4. Conclusions This study examined the fatigue behaviour of notched hollow cylindrical bars manufactured by L-PBF from AlSi10Mg powder subjected to combined bending-torsion loading. The notch was machined by CNC after the manufacturing stage and consisted of a 5 mm-diameter through hole in the cylinder wall. Two material conditions were analysed: untreated and heat-treated. Heat treatment consisted of a low-temperature stress relief executed at 250 °C (i.e., below the first exothermic peak of the tested alloy) for 2 hours followed by water quenching. Regarding the loading scenarios, the combined bending-torsion tests were performed in-phase under pulsating (R=0) loading. Two bending moment to torsion moment ratios (B/T) were considered, more specifically B/T=2/3 and B/T=1, corresponding to normal stress to shear stress ratios ( σ a / τ a ) equal to 4/3 and 2, respectively. The following conclusions can be drawn: • The crack initiation process was characterised by the nucleation of two cracks at diametrically opposite locations around the hole. These initiation sites were affected by the B/T ratio, tending towards Mode-I as the B/T ratio increased, due to a reduction of the shear stress level. Heat treatment did not affect the crack initiation sites; • The early-stage crack growth direction was also affected by the B/T ratio. Similar to the initiation sites, crack paths tended towards Mode-I as the B/T ratio increased, which was attributed to the reduced mixed mode contribution associated with lower shear stress levels. No notable effect of heat treatment was observed on the early-stage crack growth direction; • Crack initiation locations and early-stage crack growth directions were reasonably predicted through the first principal stress field around the hole region. The former was linked to the maximum value of the first