Issue 35

R. Konečná et alii, Frattura ed Integrità Strutturale, 35 (2016) 31-40; DOI: 10.3221/IGF-ESIS.36.04 36 a) polyhedral grains in circles and in columnar grains, LM b) detail of polyhedral grains in the circle, LM c) cuboidal and ellipsoidal particles, LM d) non melted powder particle e) cuboidal particles, SEM f) elongated particles, SEM Figure 7 : Structural details in the x-y plane Figure 8 : Fatigue crack growth data for SLM Inconel 718 alloy Fractography The local crack growth rate is influenced by the local microstructure and the level of applied stress intensity factor. Examples of characteristic fracture surfaces, as observed by SEM, are presented in Figs. 9 and 10. The fatigue fracture surface corresponding to the crack growth rate of 8 x 10 -7 mm/cycle and the stress intensity factor amplitude K a = 2.0 MPa m 1/2 (i.e. near the threshold) is shown in Fig. 9 (note that the macroscopic crack propagation direction is from top to bottom). The macroscopic plane of the fracture surface is parallel to the building direction of SLM process. The crack propagates in a transgranular manner. The fracture surface is characterized by many parallel secondary cracks, which are often markedly opened. The secondary cracks are indicated in Fig. 9a with arrows. The average distance of the parallel secondary cracks is of about 0.5  m, which is a value nearly three orders of magnitude larger than the average crack increment at the corresponding crack growth rate. The variability of the fracture surface in the near threshold region