Papers by Author: Tetsuo Shoji

Abstract: It is well known, that thermally grown oxide (TGO) forms at the interface between the thermal barrier top coating (TBC) and the bond coating during service. In previous work, SEM images showed that the TGO layer contained at least two layers with different oxides. One layer was Al2O3, and the other was a mixed oxide of NiO, CoO, Cr2O3, and their spinels. It was supposed that a reason for macro-crack formation near an interface is due to a decrease in bond strength or to the formation of stress concentration sites caused by the formation of pores in the mixed oxide. In order to improve the bond strength, a modified bond coating material was developed, which is MCrAlY with Ce and Si added. Four- point bending tests were carried out to measure the bond strength of conventional TBC and of the modified TBC with MCrAlYCeSi bond coating. As a result, the TBC with modified bond coating had a bond strength superior to the conventional one. It is likely that the reason for the superior bond strength is due to a notable difference in oxidation behavior.

Abstract: The fracture mechanics characteristics in the critical locations of the wheelset for high-speed train have not been studied enough yet despite of severe conditions due to increase in operating speeds. Moreover, the fracture mechanics characteristics with respect to the aging effects of wheelset materials have not been clearly studied. In the present study, the following fracture mechanics characterization tests were carried out in accordance with various locations on the wheelset for high-speed train: fracture toughness depending on load rate, fatigue crack growth rate and fatigue thresholds. The results show that the fatigue crack growth rates in accordance to the locations on wheelset were not remarkably different, and the fatigue threshold in the region of the bolt-hole is lower than that in other regions. The fracture toughness depending on load rate data shows that once the downward curve from quasi-static values was reached, subsequent values showed a slow increase with respect to the impact velocity. This means that dynamic fracture toughness should be considered in the design code of the wheelset material.

Abstract: The primary objective of this study is to develop a quantitative model to predict the effects of materials, environment and mechanics such as loading configuration on environmentally-assisted cracking (EAC) of stainless steels in high-temperature water. It has basically been accepted that crack propagation in oxygenated high temperature water is controlled by a slip-dissolution and/or deformation-oxidation mechanism. According to this mechanism, the crack-tip strain rate is an extremely important mechanical parameter for determining the crack growth rates. Based upon a formulation obtained by combining Faraday’s equation with an elastic plastic analysis of the strain singularity at a growing crack-tip in work hardening materials, a theoretical formulation of crack-tip strain rate has been derived for plane strain and plane stress conditions. The FEM analysis for 3D crack growth can be compared to the theoretical 2D analysis. In this paper, we first make a CCP (Center Crack Plane) model, and performed a 3-dimensional Finite Element Analysis (3D-FEA) to evaluate the crack-tip stain rate paying attention to the element mesh size and to the loading history. After optimization these parameters, the calculated crack-tip strain distribution, including its logarithmic singularity, was founded to agree well with the theoretical distribution. The significance of the crack-tip strain rate upon the crack-tip strain distribution and crack growth rate was demonstrated. The specimen size effects on crack growth rates were discussed from this point of crack-tip strain distribution. Finally, we focused on the importance of crack-tip strain rate as a unique mechanical parameter that controls the crack growth rate.

Abstract: A System Safety Benchmark Facility (SSBF) was designed and constructed for assessign the crack growth behaviour of welded pipes. Using the SSBF, the effect of flow rate on the SCC susceptibility of 304, 304L and 316L stainless steels was investigated in high temperature water with 2ppm dissolved oxygen. As a result, at a flow rate of 5.5 cm/sec a penetrating crack was found at the Heat Affected Zone (HAZ) of type 304 steel next to the weld metal 182 alloy after 1030 hrs of testing. The maximum crack growth rate was about 6×10^-7mm⋅s^-1. Additionally, the oxide film formed on the inside surface of specimen was examined by Scanning Electron Microscope (SEM), Raman spectroscopy and Electron Spectroscopy for Chemical Analysis (ASCA). A Focus Ion Beam (FIB) technique was used to measure the thickness of the film. The results showed that the composition. morphology, and thickness of the oxide film rate, the types of material, and the existence of HAZ. The implications of the characteristics of the oxide film to SCC susceptibility are discussed.