Kobe Material Testing Laboratory (Japan)

Abstract This study presents the development of a piezo actuator-based fatigue testing machine specifically for miniature specimens. The machine’s specifications include a maximum axial load of 1 kN, a maximum frequency of 15 Hz, and overall dimensions of 1,174 mm × 370 mm × 270 mm (height × length × width). Finite element analysis was employed to design two types of specimens with dimensions of 22 mm length, 8 mm width, and 2.2 mm thickness, each having a gauge length of 4 mm and diameters of 1.6 and 1.8 mm. Preliminary testing indicated that fractures initiated in the R-section of the specimen with a 1.8 mm diameter, leading to the selection of the 1.6 mm diameter specimen. To examine the influence of size on fatigue behavior, validation tests were conducted using AISI 304 and AISI 310S. Results indicated that, in the low-cycle fatigue region, size effects were negligible. However, as applied stress amplitude decreased, the fatigue life or fatigue limit of the miniature specimens surpassed that of bulk specimens.

An applicability of pneumatic drive small disk bending test to fatigue strength assessment of resin materials was examined in this study. The effects of specimen thickness and air pressure on deformation behavior of epoxy resin were investigated using a finite element method to determine specimen geometry and test conditions. Then, the pneumatic drive bending fatigue testing apparatus with small disk-type specimen (8 mm in diameter) was designed. Fatigue test was carried out at a variety of maximum pressures of 0.25-0.73 MPa and frequency of 2 Hz using epoxy resin. The experimental results showed that the stable air pressure was alternately applied to both specimen surfaces. The change in the strain measured at the center of specimen surface using a strain gauge, followed well to the change in pressure. It was found the fatigue life, which was defined as the number of cycles to 10% drop in air pressure, decreased by attaching the strain gauge, because crack was initiated in the vicinity of strain gauge. It was revealed that there was a relatively good correlation between the applied pressure and the fatigue life.

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Thin films of barium tantalum oxynitride (BaTaO2N) with thicknesses of 150–680 nm are grown on TaN/Ta substrates via the interfacial reaction of BaCO3/Ta3N5 layers in a N2 atmosphere. A TaN thin film is first deposited on a Ta substrate by the Ar/N2 ion sputtering of a Ta target, and then covered with a TaOx layer by Ar/O2 sputtering. The TaOx layer is subsequently nitrided in an NH3 flow to a Ta3N5 layer. Finally, Ba metal is deposited on the Ta3N5/TaN/Ta substrate under vacuum, oxidized, and carbonized to produce BaCO3. The interfacial reaction between the BaCO3 and Ta3N5 layers is conducted in a N2 atmosphere at high temperatures. X‐ray diffraction patterns confirm BaTaO2N and Ta2N phases on the Ta substrates, while cross‐sectional scanning electron microscopy images show a three‐layer BaTaO2N/Ta2N/Ta structure. Cobalt oxide (CoOx)‐deposited BaTaO2N/Ta2N/Ta photoanodes for water oxidation provide a high photocurrent of 4.6 mA cm−2 at 1.23 V (vs RHE) and an incident photon‐to‐current conversion efficiency of 9% at 600 nm under simulated AM1.5G irradiation. The efficiency of this photoanode is remarkably improved compared with previously reported photoanode thin films. Interestingly, a BaTaO2N/Ta2N/Ta electrode with similar photoanode performance is also obtained via the interfacial reaction between TaN and BaCO3 layers.

The primary purpose of this research is to examine the stress corrosion cracking (SCC) resistance of Alloy 800NG in pressurized water reactor (PWR) primary water and pressurized heavy water reactor (PHWR) primary water. Rates of SCC growth of 20% cold-worked (CW) Alloy 800NG measured over the temperature range between 270°C and 360°C were compared with previously reported results for 20% CW Alloy TT690 and 20% CW Alloy 600 in order to consider which material is the most SCC resistant among materials presently being used for steam generator (SG) tubing worldwide. The secondary purpose is to examine the effect of chromium addition on SCC growth in PWR primary water of a series of alloys based on the Alloy 800 composition. SCC growth measurements were performed in PWR primary water over the chromium concentration range from 16% to 27% to obtain fundamental knowledge useful for considering a future alternative SCC-resistant material for SG tubing in extended life PWRs and PHWRs. The third objective is to examine the rate of cavity formation of 20% CW Alloy 800NG to obtain basic knowledge of one possible mechanism for SCC initiation after long-term operation. Measured rates of cavity formation in 20% CW Alloy 800NG were compared with previously reported results of 20% CW Alloy TT690 to compare the rate of SCC initiation caused by cavity formation. Four important patterns were observed. First, excellent SCC growth resistance was observed for 20% CW Alloy 800NG compared to 20% CW Alloy TT690 at 320°C, 340°C, and 360°C. Second, an inverse temperature dependence on SCC growth was observed in Alloy 800NG. The rate of SCC growth increased with decreasing temperature which was completely different from the trend for Alloy 600. Third, a significant beneficial effect by chromium addition in 800 series alloys on SCC growth resistance was observed in PWR primary water in the operating temperature range of PWRs and PHWRs. The rate of SCC growth decreased with increasing chromium concentration in the chromium concentration range between 16% and 27% chromium at 270°C, 290°C, and 320°C. However, no beneficial effect of chromium addition in these alloys was observed at 340°C and 360°C. Finally, a more than 10 times slower rate of cavity formation was observed in 20% CW Alloy 800NG than for 20% CW Alloy TT690. Results suggested that because of cavity formation, a more than 10-fold faster crack initiation occurred in Alloy TT690 than in Alloy 800NG. Further, carbide coverage and grain size significantly affected the rate of cavity formation. Detailed and comprehensive studies of long-term SCC initiation are necessary to ensure the future reliability of life-extended PWRs and PHWRs.