PSI - Issue 83

Ahmad Issmail et al. / Procedia Structural Integrity 83 (2026) 229–238

235

however, is still smaller than considering only the sloping part of the curve, through a merely statistical analysis of the failed specimens.

300 (a)

S|45° m|45°

300 (b)

S|45° S|V

MLE (50%)

45° k = 4.10 A = 14.14 T σ = 1.22

Standard k = 4.27 A = 14.55 T σ = 1.24

S max [MPa]

S max [MPa]

ASTME739 (50%)

30

30

1.E+04

1.E+05

1.E+06

1.E+04

1.E+05

1.E+06

Cycles to Failure, N f

Cycles to Failure, N f

300 (c)

300 (d)

m|45° m|H

S|V S|45° m|45° m|H

Miniature k = 4.78 A = 15.72 T σ = 1.16

Collective k = 4.50 A = 15.07 T σ = 1.19

S max [MPa]

S max [MPa]

30

30

1.E+04

1.E+05

1.E+06

1.E+4

1.E+5

1.E+6

Cycles to Failure, N f

Cycles to Failure, N f

Fig. 4: S-N curves of: (a) the 45° orientation; (b) standard specimens; (c) miniature specimens; and (d) collective S-N curve of all four categories. The reported values of the inverse slope (k), Basquin intercept (A), and scatter index (T σ ) refer to a collective ASTM-E39 statistical analysis of all specimen categories reported in each graph. In addition to the collective ASTM-E739 regression line and scatter bands, the S–N curve determined with the Maximum Likelihood Estimation (MLE) at 50% probability of failure is also illustrated (in red). Table 3 : Fatigue behaviour of tested PBF–LB AlSi10Mg specimens; (*) indicates a statistical ASTM-E739 fatigue limit lower than the highest nominal stress corresponding to a runout (RO) and is thereby substituted by the latter. Specimen Category 45° Standard Miniature Collective ASTM E739 fatigue limit (%50) at 2×10 6 cycles [MPa] 81.6* (116.2 RO ) 85.5* (98.6 RO ) 93.1* (116.2 RO ) 88.8* (116.2 RO ) MLE fatigue limit (%50) at 2×10 6 cycles [MPa] 94.7 113.7 119.2 109.1 Inverse slope of the S–N curve, k 4.10 4.27 4.78 4.5 Intercept of the S–N curve, A 14.14 14.55 15.72 15.07 Scatter index, T σ 1.22 1.24 1.16 1.19 A comparison of the fracture surfaces of specimens tested at a nominal stress level of 150 MPa across the different categories is presented in Fig. 5. The fractography shows that all fatigue cracks initiated from surface defects (Fig. 5(a1–d1)). Specifically, S|V mostly failed due to shallow defects at the corners (Fig. 5(b2)). On the other hand, S|45°, m|45°, and m|H underwent multi-crack initiation from multiple dominant surface defects at the rough downward surface (Fig. 5(a2,c2,d2)). The multiple propagation of independent cracks and their coalescence often contribute to fatigue life reduction [8, 28], which, in addition to the higher irregularity and sharpness of its profile height distribution, is a further reason for the reduced fatigue performance of S|45° compared to S|V. Regarding the fracture behaviour, three different zones could be identified for all specimen categories (Fig. 5(e– f)). The early crack propagation manifested ductile features such as fatigue striations (Fig. 5(e)), while most of the fracture surface was dominated by rough morphology with ductile features such as dimples (Fig. 5(f)), which increased in size and depth at the final fracture zone (Fig. 5(g)). The microstructural homogenizing effect of the T6 HT is reflected by the absence of traces of laser scan tracks on the fracture surfaces that are often observed for as-built and stress-relieved specimens [16].