The fatigue performance is determined by the microstructure, defects and surface roughness in additive manufacturing (AM) processes. Testing strategies with novel measurement techniques and fracture mechanics models enable high precision monitoring und prediction of the influence of micro- and defect-structure or surface integrity on fatigue deformation and damage evolution.
Within the studies, intermittent fatigue testing of steel and aluminum alloys revealed the interaction between local porosity, microstructure and fatigue crack initiation in the high (HCF) and very high cycle fatigue (VHCF) regions. For different materials like AlSi10Mg, Ti-Al alloys and 316L steel the effect of defects, building direction, structure, stress ratio and cyclic stress-strain behavior on the fatigue damage tolerance was investigated. The fatigue strength could be correlated with the hardness and the effective defect or pore size relative to the load direction using Murakami concept including the light metal extension of Noguchi. A significant influence of the texture could be quantified. By elastic-plastic modification of Murakami-Noguchi approach by J integral of Fischer including surface and internal defects, the effect of microstructure on cyclic stress-strain behavior and fatigue damage tolerance could be proofed for Al-Si alloys by comparison of AM and cast materials. The deformation and damage evolution could be further monitored by alternating current potential drop (ACPD) method, hysteresis and resonant frequency analysis. By non-destructive, intermittent CT damage analysis, the changes of condition monitoring parameters could be correlated with crack initiation and propagation.
These studies are presented to reveal the potential of modern fracture mechanics models for a reliable and unified fatigue lifetime and damage tolerance of AM metals and structures based on comprehensive micro-/defect-structure-deformation-lifetime relationships including a fatigue damage tolerant design.

Acknowledgements The authors gratefully acknowledge the funding by the German Research Foundation (DFG) for the projects “Identification and modeling of fatigue damage mechanisms in Al-Si-Mg cast alloys under HCF and VHCF loading” (No. 282318703), “Mechanism-based assessment of the influence of powder production and process parameters on the microstructure and the deformation behavior of SLM-compacted C+N steels in air and in corrosive environments” (No. 372290567) and “Microstructure and defect controlled additive manufacturing of gamma titanium aluminides for function-based control of local materials properties” (No. 404665753).