PSI - Issue 19

Yasuhiro Yamazaki et al. / Procedia Structural Integrity 19 (2019) 538–547 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

540

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The out-of-phase type thermomechanical fatigue tests were conducted to investigate the fatigue crack propagation. All tests were performed under the load-control condition. The test conditions are summarized in Table 1. The test conditions were selected based on the results of the strain-controlled type TMF test in the previous study (Okazaki, et al. (2003)). The crack propagation tests were carried out under the higher and lower temperature conditions of which the temperature ranges between 500 and 900°C or 200 and 600°C, respectively. All tests were performed under load-controlled conditions by means of the TMF testing machine. A triangle waveform was used for both mechanical and thermal cycling. The hysteresis loop during the test was measured by using the extensometer with 5 mm gauge length which was located across the notch.

2 μm

Fig. 1. Microstructure of the tested material.

Fig. 2. Geometry of specimen used for crack propagation test; unit in mm.

In this study, the finite elemental (FE) analysis was performed to investigate the crack tip opening behavior and to obtain the stress intensity factor range for the corner crack initiated from the notch by using ANSYS® Workbench R18.0. The example of FE model is shown in Fig. 3. The number of nodes and elements of the model used in the FE analysis were ranged between from 62,000 to 96,000 and from 27,000 to 36,000, respectively. The FE analysis was carried out with the inelastic constitutive equation obtained from the tensile test at several test temperature (Sengupta at al. (1994)) with taking into account Norton’s creep law (Koizumi et al. (2006)). Because the cyclic stress-strain responses obtained from the FE analysis were saturated after 5 cycles, the stress intensity factor and the crack closure were determined from the cyclic stress-strain responses at 5 cycle. The crack opening displacement, CTOD, was evaluated according to Rice (1986) and Shih (1981) methods, with using the distance between the representative nodes as shown in Fig. 3.

Table 1. Crack propagation test conditions.

Test type

H: high temp. condition Max. 900, Min. 500

L: low temp. condition Max. 600, Min. 200

Temperature [°C]

Stress range, ∆ σ [MPa]

950

1190

Stress ratio, R σ Frequency [Hz]

-0.73 1/400

Phase angle between temperature and stress cycles

-180 degree (Out-of-phase)

Fig. 3. Model used for the FE analysis.