PSI - Issue 17

Radomila Konečná et al. / Procedia Structural Integrity 17 (2019) 138 – 145 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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Type A+ and Type A- specimens are strongly affected by the layer-wise fabrication (i.e. stair stepping, see Gebhard et al. (2014)). In addition, the surface of Type A- notches is classified as down-skin while the surface of Type A+ notches is up-skin. It is well known that down-skin surfaces are of lower quality (i.e. higher roughness poor geometrical accuracy) than up-skin surfaces. Applied stress in both Type A specimens is parallel to the layers. This qualitative description of the interaction of SLM fabrication and surface quality of the notches suggest that the fatigue behavior will depend on specimen directions.

3. Results and discussion

3.1. Directional notch fatigue behavior

Fig. 3 shows the directional notch fatigue effect obtained in the four sets of the present mildly notched miniature specimens of SLM Inconel 718 with as-built surfaces. The independent variable is the maximum nominal stress of the bending load cycle with R = 0 and the dependent variable is number of cycles to failure. Four different and well defined S/N plots are determined for the different directions of fabrication. Considering Fig. 1, the Type B specimens shows the best fatigue performance while the Type A specimens the worst fatigue performance. Type C specimens determine an intermediate behavior. As Inconel 718 is face-centered cubic it may not exhibit a true fatigue limit, Reed (2006), a nominal fatigue strength at 2x10 6 cycles can be considered for comparison. So the fatigue strength varies from a maximum value of 360 MPa for Type B notched specimens to a minimum of 200 MPa for the Type A- specimens with a down-skin notch. These data will be assessed against limited published data, i.e. Witkin et al. (2018), in a later section. However, the relatively large differences in the respective S/N plots suggest that the fatigue behavior is not primarily controlled by the notch geometry (i.e. theoretically by K t ) but rather by the as-built surface quality, which on the other hand depend on process parameters and surface orientation. .

Fig. 3. Directional notch fatigue data for as-built SLM Inconel 718 (K t = 1.63).

3.2. Directional quality of notch surfaces

This section presents the characterization of the notch surfaces of the four specimen types of Fig. 2 using magnified optical microscopy. The aim is the qualitative comparison of the actual notch geometry from the theoretical semicircular geometry of 2-mm-radius. Not the entire profile is however relevant for the notch fatigue