PSI - Issue 19

Rainer Wagener et al. / Procedia Structural Integrity 19 (2019) 380–387 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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The service related data describes the loading and environmental conditions of the component usage. In case of the design process from scratch, the service loads have to be assumed on basis of empirical knowledge. Depending on the component usage the required service life and depending on the experiences the maximum allowable load have to be defined. Fig. 1 represents the microstructure of a notch. This structure has been built from the bottom to the top. First, two different types of microstructure can be determined, one for the core and one of the rim. These different microstructures are the result of the different sets of scan parameters for the core and the contour. Furthermore, the microstructure of the lower half seems to be sound material. However, the contour of the upper half especially in the down skin area has many and huge pores. These pores are the result of the built conditions and the used scan parameters for the down skin contour. The discussion whether the used scan parameter are suitable or whether they have to be optimized is not subject of this paper.

Figure 1: Notched specimen, etched surface

For the fatigue approach, it is more interesting, that another set of scan parameters is used for a specific area. In order to improve the fatigue approach this process related information must be available. From this point on, a digital twin, which starts living with the first sketch and dies at the end of the recycling process, comes in the focus of interest, because one task of the digital twin is just collection data over the entire life cycle. The digital twin builds the bridge between the design data like geometry, the powder, the process parameters and the service loading conditions. Due to the inhomogeneous microstructure, a local fatigue approach concept should be the first choice. Therefore, the cyclic stress-strain behavior and a strain-life relation are required. The test campaign includes stress-controlled fatigue tests in order to derive the high cycle fatigue behavior and strain-controlled fatigue tests with constant amplitudes to derive the strain-life curve as well as Incremental Step Tests in order to analyze the cyclic stress-strain behavior related to variable amplitude loading conditions. With respect to the build time when using the Laser Powder Bed Fusion technology a small size flat specimen geometry, Fig. 2, was used. The specimen geometry was optimized and contains an 11mm long homogeneous cross section, which is necessary for the usage of a clip-on strain gauge. Furthermore, the transition zone from the test section to the clamping section enables the printing of the specimens without support structure as long as the built direction is parallel to the loading direction. In order to describe the specimen orientation, the coordination system according to Fig. 3 was used. In case of Z specimens the built direction is parallel to the load direction, whereas in case of X direction the built direction is perpendicular to the load direction. 2. Test campaign