PSI - Issue 19

Kai Schnabel et al. / Procedia Structural Integrity 19 (2019) 442–451 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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Figure 5 Tensile test results on stress-relieved heat-treated AlSi10Mg specimens built orthogonal to the building platform without support structure

For fatigue assessment according to the FKM guideline, the specimen are categorized as not welded components. Because AlSi10Mg is an aluminium alloy that is typically used for casting, the constants for these type of alloys are used. All necessary input parameters are given in Table 1. The surface roughness and the stress gradients are also taken into account. The surface roughness is measured on different sides of the specimens and the highest value is used to represent a worst-case scenario. Although the FKM guideline is not designed for AM materials, a comparison of the calculations is made with experimental derived W ÖHLER -lines of specimen in the as-built state, Figure 6 left. As expected, the sharper notched ‘as - built’ geometries assessed with the FKM guideline lead to a lower fatigue strength. The factor between the stress amplitudes at = 1 ∙ 10 6 load cycles is for both geometries approximately ≈ 1.4 . For a standard fatigue calculation, which is mostly performed on the nominal geometry, a large misjudgment of the critical stresses can result.

Table 1 Used parameter set for the fatigue assessment using the FKM guideline

Parameter

K t = 1.7, nominal geo.

K t = 2.5, nominal geo.

K t = 1.7, as-built geo.

K t = 2.5, as- built geo.

Tensile strength R m [MPa]

360 1.33 1.60 1.70 120 2.94

360 3.46 0.60 2.47 120 3.23

360 4.14 0.51

360

Related stress gradient G σ [1/mm]

10.19

Notch radius r [mm]

0.23 3.87 120 3.65

Stress concentration factor K t (calculated) [-]

2.6

Roughness R z [µm]

120 3.29

K t -K f ratio according to Stieler n σ (calculated) [-]

Scattering T σ (presumed) 1.30 Because the FKM guideline was not developed for AM components, it does not take AM specific properties and characteristics into account. The FKM guideline pretends a slope of = 5 before and a slope of ∗ = 15 after the knee point at = 1 ∙ 10 6 load cycles. The AM specific properties are reflected by lower fatigue strength of the experimentally results for ‘as - built’ geometries in high cycle fatigue, with a deviating slope of = 7 of the W ÖHLER - 1.30 1.30 1.30