PSI - Issue 18

Matilde Scurria et al. / Procedia Structural Integrity 18 (2019) 586–593 Matilde Scurria, Benjamin Möller, Rainer Wagener, Thilo Bein/ Structural Integrity Procedia 00 (2019) 000–000

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a temperature of 965 °C for 1h and cooled down through 2 stages: at 720 °C for 8h and at 620 °C for an additional 8h, then cooled in air. All the specimens are finally blasted with zirconia with an average particle size of 250 µm before being tested.

a c Fig. 1. (a) specimen geometry; (b) specimen configurations; (c) transition area between support structures and specimen ( Scurria et al. (2019)) The production of three specimens for each configuration is necessary for the analysis at three different maximum strain amplitudes of  a, t = 0.4%,  a, t = 0.6% and  a, t = 0.8%. The experimental campaign is carried out using an E-cylinder test rig, developed at the Fraunhofer LBF, which enables strain-controlled fatigue testing using an electric motor. A load cell measures the force, while the strain is controlled by an extensometer. An anti-buckling device is used to avoid buckling when a strain ratio of R  < 0 is applied. The test rig and the set-up are shown in Figure 2. The specimens are subjected to Incremental Step Tests (IST), where a defined load sequence with decreasing and increasing strain amplitude (Figure 2c), at a strain ratio R  = -1, is imposed, until the failure occurs. The cycles between two maxima belong to one block. The failure corresponds to the n th - block, in which the maximum force decreased by 10% in relation to n /2, which is called the stabilised block or stabilised state . The reversal points of the stabilised block are used to evaluate the cyclic stress strain curve by regression of the Ramberg-Osgood equation (see Ramberg and Osgood (1943)). b

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Fig. 2. (a) E-cylinder test rig, developed at the Fraunhofer LBF; (b) test set-up; (c) example of load sequence used for IST.

2.2. Analysis of the Incremental Step Test results The results of the ISTs have been grouped based on the build direction (Z or XZ) and the maximal strain amplitude  a,t . In Figure 3, the stabilised blocks for each build direction XZ and Z, as well as maximum strain amplitudes of  a,t = 0.4%,  a,t = 0.6% and  a,t = 0.8% for the applied heat treatments ‘A’ (650°C), ‘B’ (760°C) and ‘C’ (965°C), are represented. For  a,t = 0.4%, the hystereses are narrow and therefore only the reversal points are plotted. However, they enlarge as the total strain amplitude, and therefore the plastic strain portion, increases. From the diagrams in Figure 3, essential considerations about the effect of different build orientations and heat treatments can be drawn. What can be underlined in a first analysis is that the combined effect of build orientation and subsequent heat treatment affects the cyclic material behaviour. This has to be taken into account during the