PSI - Issue 19

Akifumi Niwa et al. / Procedia Structural Integrity 19 (2019) 106–112 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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has a gap of 0.1 mm above and below the test piece. The stress conditions applied in the cross section (A), (B) and (C) of the test piece in Fig. 2 are as shown in the schematic images in Fig. 3 when the test piece is assumed to be an elastic body. Since the axial tensile stress is added to it, the stress ratio differs depending on the position in the thickness direction even in the pulsating fatigue test. That is, tension-tension loading is repeated with stress ratio > 0 in tension side (A), constant stress due to axial stress is loaded in the central part of thickness direction (B), tension compression loading is repeated with stress ratio < 0 in compression side (C). Note that, as mentioned above, Fig. 3 shows the stress state assuming that the test piece is an elastic body, it is necessary to be aware that different stress conditions will occur as plastic deformation occurs depending on the test conditions. The waveform used for the test is a sine wave with displacement control, and the test frequency is f = 5 Hz, 12.5 Hz and 20 Hz. And an electric furnace with a molybdenum disilicide heating element was installed at the center of the test piece, the center of the notch was controlled to be 1400 °C, and the test was performed in the air atmosphere.

Fig. 2. Schematic image of bending fatigue test with axial stress.

Fig. 3. Theoretical images of applied stress in the specimen. (a) Applied stress at position (A), (b) Applied stress at position (B), (c) Applied stress at position (C).

2.3. FEM analysis

In order to evaluate the distribution of stress and strain generated in the notch during the test, analysis by the finite element method was performed using ABAQUS. The analysis uses an 8-node hexahedron non-conforming element, and it is a 1/2 model of 5220 nodes and 3680 elements. Moreover, based on the temperature distribution of the test piece measured as shown in Fig. 4 (a), the analysis was performed in consideration of the temperature dependency of the physical properties. Next, Fig. 4 (b) shows the constraint conditions. Regarding the constraint conditions, the part corresponding to Jig B in Fig. 2 is completely constrained, and a rigid body is provided with a gap of 0.1 mm above and below the test piece as in the actual test jig A. As a result, Jig A side was restricted only in the width direction but not in the axial direction.