PSI - Issue 53

Rainer Wagener et al. / Procedia Structural Integrity 53 (2024) 161–171 Author name / Structural Integrity Procedia 00 (2019) 000–000

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derive the cyclic material behavior experimentally. In this way the influence of the rim zone, that means surface topology and microstructural gradients are excluded and have to be considered by an additional set of material properties in case of each type of microstructures or be included in the microgeometry of the CAD-model. Finally, the main disadvantage of the local strain concept is the transfer from the material properties derived with sound material to the component-related material behavior, which is characterized by imperfections like pores and property gradients and technical surfaces. In order to close this gap within a numerical fatigue approach transfer concepts are used. Accounting for property gradients, size effects etc. are used to manipulate the stress-strain and strain-life curve. 2. Representative Structural Elements Keeping the heterogenous microstructure of additively manufactured components in mind, structural elements, which are correlated to the exposure strategy, can be a promising alternative to properties of sound materials and influence factors. Therefore, three different orders of Representative Structural Elements (RSE) has been introduced by Wagener (2019) and specified by Wagener and different co-workers (2020 and 2021): - 1 st order: Structural elements of the first order describe the sound material behavior. So far, they can be assumed equivalent to the conventional cyclic material behavior. - 2 nd order: Structural elements of the second order consider the influence of a single feature like a pore or surface roughness. - 3 rd order: Beside the consideration of a single defect, the structural elements of the third order contain the interaction between different microstructures including defect formations maybe caused by different scanning strategies or cooling conditions. Therefore, they are suitable to consider the different microstructures of the rim zone and the core material. Testing additively manufactured specimens means investigating the Representative Structural Element’s behavior of the third order.

Fig. 1: Strain-life curves and stress-strain curves and strain-life curves for Representative Structural Elements and sound material

In order to describe the cyclic structural behavior, the cyclic stress-strain curve and a strain-life curve is required. For the second, the Fatigue Life Curve, according to Wagener and Melz (2017), is suitable since it describes the strain amplitude-fatigue life correlation continuously from the Low Cycle Fatigue to the Very High Cycle Fatigue regime. Depending on the order the stress-strain behavior as well as the strain-fatigue life correlation is shifted to lower values of stress respective strain at a given strain or number of cycles to failure, Fig. 1.