PSI - Issue 66

Vladimír Chmelko et al. / Procedia Structural Integrity 66 (2024) 426–432 Vladimír Chmelko at al ./ Structural Integrity Procedia 00 (2025) 000–000

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3.4. Different slope of the S-N curve in tension and torsion The relative ratio of the yield stress in tension and in torsion is the basis for strength assessment under static loading. For most conventionally produced metals, this ratio is maintained throughout the S-N curve, i.e. in both the low and high cycle region. This behaviour changes for AM metallic materials. Some materials retain the approximate parallelism of the S-N curve in the tension-compression regime with the curve in the alternating torsion regime. However, the more common case is the non-parallelism of these curves, as documented by the plots in Fig. 5. The change from the parallelism of the curves  a =f(N f ) a  a =f(N f ) for the conventionally fabricated material to the non parallelism for the AM material is documented on the AlSi10Mg material.

Fig. 5. Relative position of curves  a =f(N f ) a  a =f(N f ) for the materials AlSi10Mg, 316L and MS1.

The relative position of the torsion and tension-pressure curves shows that in the so-called low-cycle region, the fatigue strength in cyclic tension is higher than the fatigue strength in cyclic torsion. In the so-called high-cycle fatigue region, this ratio is reversed and AM materials have higher fatigue strength in torsion than in cyclic tension at lifetimes from 6.10 4 cycles. This unexpected phenomenon is probably due to the existence of defects and the characteristic mode of crack propagation for the material. 4. Conclusions The large number of experiments of AM materials under cyclic loading allows to generalize the specificities in fatigue properties brought by this new manufacturing technology. Compared to conventional metal forging, AM has resulted in the following specificities: ‐ Anisotropy of mechanical properties as a consequence of the direction of layer addition ‐ Internal and surface defects