Fatigue Strength Verification of Additively Manufactured Structures Considering the Local Loading Conditions and Microstructure (LBM-Fatigue)

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Additive Manufacturing (AM), especially the powder-based Laser Beam Melting (LBM) process, enables a high potential to manufacture structural components in a lightweight design. Therefore, a valid fatigue strength verification concept is indispensable. Although established design concepts allow to consider AM specific influencing factors (e.g. anisotropy, “as-built” (AB) surface, etc.), this often results in a high deviation to the actual lifetime of parts. However, the current state of research shows that microstructure, defect characteristic, which comprises the size, number, position and type of process-induced defects, as well as mechanical properties can vary within an additively manufactured component. Thereby, relevant research works further demonstrate that the defect tolerance of an AM material highly influences the fatigue lifetime. These aspects must be considered, but cannot be integrated readily in existing verification concepts yet. Moreover, the influence of the loading condition and its interrelation with the influencing factors described above, must be included into an extended design concept. Consequently, the main goal of this research project is the elaboration of a valid fatigue strength verification concept, which is based on local concepts and/or fracture mechanics, and which considers the AM specific influencing factors described above. This concept will be elaborated over 3 stages, which are represented by 3 working packages (AP), by using specimens made of AISI 316L and manufactured via LBM.In AP1, the influence of the AB surface, building direction and their interrelation will be analyzed and quantified based on uniaxial push-pull fatigue tests. Additionally, the defect tolerance will be determined for the different building directions by using the √area-approach and instrumented cyclic indentation tests (CIT), complemented by an analysis of the local defect characteristic and microstructure. Subsequently, these relations will be quantified and integrated in a first fatigue strength verification concept.Based on this, the effect of loading gradients, achieved by rotational bending and torsion, as well as their interrelations with the influencing factors analyzed in AP1 will be elaborated in AP2. In this context, a special focus will be on the defect tolerance. These findings will be integrated in the verification concept, which will be prevalidated in biaxial fatigue tests.Finally, the concept developed will be validated in AP3 by means of fatigue tests at a demonstrator, which has to be developed and exhibits multiaxial graduated loadings caused by a complex geometry. To enable the transferability of the approaches elaborated in AP1 and AP2, the defect characteristics, the microstructure, the surface topography as well as the mechanical properties of the critical areas will be analyzed. Moreover, advises for a durable design of additively manufactured components will be drawn based on the verification concept.