PSI - Issue 38

W. Radlof et al. / Procedia Structural Integrity 38 (2022) 50–59 W. Radlof et al. / Structural Integrity Procedia 00 (2021) 000 – 000

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3.2.2. Torsion loading The strain evolution of three specific surface points (location of P1-P3 is shown in Fig 6b-I) of a cyclic torsion loaded 50% porosity sample is shown in Fig. 6a in comparison to the measured potential difference as well as the stiffness loss. Up to 75% fatigue life the potential difference increases continuously, whereas the stiffness decreases continuously, too. The local strains increase as well, so that at 70% fatigue life strain peaks at the edges of the unit cells are visible (Fig. 6b-I). First staircases occur in the potential curve at approximately 75% fatigue life and in the stiffness curve at 85%. From the strain evolution of the points P1-P3 (Fig. 6a) it becomes clear that the strains increase up to approximately 75-85% fatigue life until the correlation fails in the DIC (red points in Fig. 6a). The loss of correlation can be explained by formation of surface cracks and is visible in pictures II, III and IV in Fig. 6b (corresponding to 80%, 90% and 100% fatigue life). Based on the results, the occurrence of macroscopic cracks can be associated with the increase in the potential difference and was also observed in all porosity designs tested.

3.3. Temperature field measurement The temperature at the lattice structure surfaces was measured and analyzed with the software IRBIS 3.1. With the aim of identifying local damage by means of a temperature rise, the temperature progressions on selected struts were analyzed. Therefore, regions of interests were defined on the lattice structures. In detail, lines ( ) were defined on struts which have failed by the end of the test and two control points (P1 and P2) were placed on the solid part of the specimens, which can be seen in Fig. 7c-I and Fig. 8b-I. The temperature development of the struts ( ∆ ) were calculated according to Eq. 1 with the maximum temperature at the lines ( , ) and the average temperature of the control points ( 1 , 2 ). Fig. 6: Fatigue behavior of an exemplarily 50% porosity lattice structure under torsion lading. (a) Cyclic deformation curves based on the change of stiffness, potential and local strains determined by DIC and (b) DIC images of the tested lattice structure at different states of the fatigue life. The red points at the end of the strain curves highlight the loss of correlation.