PSI - Issue 19

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

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Oxidation test

The plate specimens with 50 mm length, 5 mm width and 1 mm thickness were machined from the material. The specimen length direction was parallel to the solidification direction and within 5 degrees from <001> orientation. The 50 mm × 5 mm planes were parallel within 5 degrees of the (100) crystallographic plane and mechanically polished to mirror surface using alumina powders before the tests in order to remove machining marks. The oxidation tests were carried out at 900, 950 and 1000°C for 10, 30 and 100h. In this study, in order to investigate the effect of the applied stress on the oxidation behavior, the four points bending tests at elevated temperature were also carried out. The bending stress applied at the specimen surface was ±150 MPa or ±200MPa. The environmental condition of the four points bending test was at 900°C for 10h.

3. Results and discussions

3.1. Crack propagation behavior

Typical hysteresis loops measured at the surface during TMF tests are shown in Fig. 4. The mechanical strain, ε mech was evaluated from the difference between the measured total strain during the TMF test and the thermal expansion without loading which was measured prior to the TMF test. As shown in Fig. 4, no inelastic deformation behavior could be observed in the hysteresis loops not only under the lower temperature condition but also under the higher temperature condition. Unfortunately, the change of compliance in the hysteresis loop due to the crack closure couldn't be observed in the hysteresis loops, because the gauge length of the extensometer was larger than the crack size. The stress-strain responses obtained from the FE analysis are also shown in Fig.4. Comparing the FE analysis results with the experimental results, the good correlation between both curves can be observed. Typical examples of small cracks initiated from the mechanical notch under TMF conditions are shown in Fig. 5. All cracks initiated from the corner of the notch and propagated perpendicular to the loading axis. The short cracks which were initiated from the notch corner edge of both sides coalesced together when the half crack length reaching to the almost notch length. After the coalescence, the short crack was propagated with a semi-circular shape. Mukai et al investigated the physically long crack propagation behaviors under the out-of-phase type TMF condition at 200-500°C and 200-800°C by using the CT specimens (Mukai et al. (2014)). The physically long crack propagated on the {111} crystallographic plane at 200-500°C, on the other hand on the plane normal to the loading axis at 200-800°C. The propagation morphologies as shown in Fig. 5 reveal that the short crack propagates by Mode I type and it is difficult to grow with crystallographic propagation behavior. Figure 6 shows the crack propagation rates correlated with the stress intensity factor range, ∆ K . The ∆ K for the corner crack initiated from the notch was evaluated from the FE analysis result from the J-integral obtained from the path integral method. The ∆ K for the semi-circular crack was evaluated by using Newman ’ s equation (Newman et al. (1984)). As shown in Fig. 6 , the good correlation according to Paris’s law is observed in the relationship betw een the crack growth rate and the ∆ K . It is also fined in Fig. 6 that the short crack growth rates under the higher temperature condition are remarkably higher than those under the lower temperature condition. In this study, taking into consideration the crack closure effect and the temperature dependence on the deformation resistance, the crack propagation rates, d a /d N , were correlated with the ∆ K eff / E , where ∆ K eff is the effective stress intensity factor range and E is Young' modulus at the average temperature of TMF test. The crack closure stress was evaluated from the relationship between the stress and the crack tip opening displacement in the FE analysis results by using the compliance method. Figure 7 shows the crack opening ratio, U , as a function of the half crack length. The U under the higher temperature condition is higher than those under the lower temperature condition as the oxidation induced crack closure mechanisms are not taken into consideration. However, it was revealed by the small crack propagation tests under high-cycle fatigue condition at the elevated temperature that the crack opening ration, U , for the small crack was decreased with increasing temperature even if the oxidation became significantly with the increasing test temperature (Okazaki et al. (1996)). The microscopic features (as shown later) suggest that oxide induced crack closure became significant at increasing temperature, resulting in a decreasing of the fatigue crack growth rate with the oxidation of the crack surface. However, if taking into account the oxide induced crack closure effect on the crack propagation, the difference between the crack growth rate under the higher