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Fracture mechanics is a key to fatigue assessment in AM metal components. Short fatigue cracks are initiated at defects and pronounced surface roughness intrinsic to AM. The subsequent crack-propagation is strongly influenced by microstructural interactions and the build-up of crack-closure. The aim of the present study is to give an insight into short-crack propagation in AM-metals. Fatigue crack propagation resistance curves were determined experimentally for AISI 316L manufactured by Laser Powder Bed Fusion (L-PBF) which was heat treated at three different temperatures. Differences in the build-up of the fatigue-crack propagation threshold in between the L-PBF specimens and compared to wrought material are due to the residual stress states, a pronounced roughness of the crack-faces in the L-PBF specimens and phase transformation in the vicinity of the crack-tip, resulting in increased crack-closure. This, together with crack-branching found along the crack path, enhances the resistance to the propagation of fatigue cracks.
The topic of the presentation consists in some basic considerations on the application of fracture mechanics to fatigue live and strengh prediction of metallic componends manufatured by additive manufacturing. These are based on an approach developed at BAM which comprises elements such as the elastic-plastic modelling of the cyclic crack driving force, a physically meaningfull determination of the initial crack size and multipile crack initiation and propagation due to variations of the local geometry and material charactaristics. Spezial emphasis is put to spezific aspects of materials composed by selectiv laser melting such as surface roughness, porosity and gradiants in the microstructure.