Recently, the high-efficiency gas turbine combined cycle power generation has become essential in operation as a backup power supply of renewable energy sources. In turbine blades and vane of the gas turbine operated as a backup power supply, there is growing concern about the damage due to thermo-mechanical fatigue (TMF) loading because of expecting increased starting/stopping cycle and frequent load fluctuation. This study investigated the small crack propagation behavior in single crystal Ni-based superalloy under the in-phase TMF loading cycle. The creep-fatigue interaction on the small crack propagation behavior was discussed based on the relationship between the crack growth rate and the creep J-integral, considering the small-scale creep at the crack tip. The experimental results indicated that the crack growth rates under the in-phase TMF loading cycle were significantly higher than those under the out-of-phase TMF condition at a given fatigue J-integral range. In addition, introducing the tension holding at the maximum temperature in the in-phase TMF loading cycle accelerated the crack growth rates. The small-scale creep stress field at the crack tip might dominate the small crack propagation behavior under the in-phase TMF loading with the high-temperature holding as the creep-fatigue interaction.

Ni based superalloy Hastelloy X is used for high temperature components of power generation gas turbines and aircraft jet engines. Creep damage occurs preferentially in stress concentration portions. Therefore, it is important to clarify creep damage evolution process and to develop creep damage detecting method under multiaxial with stress gradient for safety operation. In this study, creep tests have been conducted by using smooth and notch specimens which have multiple round notches with notch tip radius of 0.5 mm (R0.5) and 2.0 mm (R2.0) on a Hastelloy X. Crystal misorientation parameter GROD of damaged specimens was measured by an EBSD method. Creep rupture times of the notch specimens were longer than those of the smooth specimens showing notch strengthen effect. Larger amount of creep voids on grain boundaries was observed in the notch specimens than that in the smooth specimens. Void number density took the maximum value around the notch root in R0.5, and at the specimen center in R2.0 indicating that a distribution pattern depends on notch root shape. Those distribution patterns correspond to the maximum stress distribution thorough the notch root section. From the crystal misorientation measurement, GROD maps show that values of GROD increase with increasing creep damage, and that higher GROD values occurred at around notch root in the notch specimens. Change in GROD average at the notch root section corresponds to axial creep strain distribution obtained from creep analysis of the notch specimens. Eventually, an equation indicating relationship between axial creep strain and the normalized GROD average independent on temperature and stress states was derived.

Nickel-based superalloy, Inconel 718, has high temperature strength effected by γ"(Ni₃Nb) phase at approximately 973 K. It has been found that the creep damage of Inconel 718 was induced by γ"phase disappearance. In the study, we applied the nonlinear ultrasonic method, nonlinear three-wave interaction, for evaluation of the creep damages and the microstructural degradation in γ" phase for Inconel 718 at 973 K, 310MPa. In the nonlinear three-wave interaction method, two intersecting ultrasonic waves produced a scattered wave when the resonance condition was satisfied. The amplitude in resonant scattering wave was measured. We analyzed the relation between the microstructural change and nonlinear acoustic characterizations by the electromagnetic acoustic resonance (EMAR) throughout the creep life, and the nonlinearity corresponded to change of the dislocation density. This was supported with X-ray diffraction, EBSD and SEM observation.

Cr-Mo casting steels are widely used as casings and valves in thermal power plants. Creep fatigue damage gradually proceeds in these high-temperature components during operation resulting in creep voids and micro crack initiation in which repair welding is considered to be applied for life extension. In this study, a temper bead welded joint of Cr-Mo casting steel as well as fine and coarse grain simulated materials were prepared. Creep and fatigue, creep-fatigue tests were performed using specimens taken from base and weld metals of the welded joint, the simulated materials and cross welded specimens. Material constants of creep deformation and cyclic stress-strain properties for constituted materials of the welded joint were obtained from the creep and fatigue tests. Minimum creep strain rate of the base metal at the same stress was the highest than that of other materials. Plastic deformation resistance is higher in the order of the base metal, fine grain, coarse and weld metal. Creep-fatigue life of the welded joint, which failed at the base metal 2mm away from the boundary between the base metal and fine grain, was shorter than that of the uniform base metal specimen indicating that the temper bead welding may reduce the creep-fatigue life. Finite element analysis of the cross welded specimen showed that elastic-plastic and creep strains accumulated at the base metal near the boundary are higher than those at other regions causing life reduction and failure at the base metal. The creep-fatigue life prediction by using the analysis results indicated that the nonlinear damage accumulation model gave more accurate life prediction results than the time fraction and ductility exhaustion rules.

This study evaluated the multiaxial creep properties and creep rupture life of Super304H using a welded miniature cruciform specimen. The material to be evaluated was used only for the central gauge part of the cruciform specimen, and separately prepared parts were welded to the loading griping part to minimize the size of the evaluated material. Deformation in the central gauge part of the weld-type cruciform specimen was larger at the SUS316 gripping area than at the Super304H gripping area. However, there was no significant effect of the different gripping materials on the creep rupture life under the testing conditions of 973 K and 180 MPa equivalent stress. Under the unequal biaxial tensile condition, the creep strain in the direction of minimum principal stress was significantly smaller than the strain in the direction of maximum principal stress. It is suggested that von Mises equivalent stress is effective for creep rupture life correlation. Although the multiaxial creep rupture lives show a tendency to be slightly longer than the uniaxial creep rupture lives, the equivalent stresses indicate that they can be correlated within a factor of 2.

A single particle impact testing system was developed to estimate an absorbed energy in particle perforation process of aero-engine materials. Thin target plates of 0.1-1 mm thickness were impacted by a 3 mm diameter spherical particle. A wide range of impact velocities from 50-375 m/s was covered using a compressed gas gun. First, particle impact tests were conducted on aluminum and copper plates to evaluate the validity of the experimental system. The experimental results show that there is no significant difference in the effect of the particle impact velocity on the absorbed energy of aluminum and copper, and the ballistic limit can be reasonably estimated based on a simple energy balance equation. Second, impact tests were conducted on TiAl alloys and Ni-based alloys. There was no clear ballistic limit for the TiAl alloys, and the absorbed energy tended to increase as the particle impact velocity increased. The absorbed energy of the TiAl alloy was half that of the Ni-based alloy for specimens of the same thickness. Radial and petal-shaped cracks were observed in the Ni-based alloys after impact tests, while brittle cracks, fractures, and scattering of debris were observed at the periphery of the penetration hole in the TiAl alloys.

The dislocations introduced during creep in the hard C14–Mg₂Ca lamellae for the α-Mg/C14–Mg₂Ca eutectic alloy were analyzed using high-resolution transmission electron microscopy. The isolated dislocations are introduced into C14 lamellae from the stress-concentrated α/C14 interface during creep and identified as α dislocation with the Burgers vector of 1/3[1120], supposing that all dislocations in the Mg₂Ca phase split into partial dislocations. The introduced perfect dislocation is an α dislocation on the basal plane, which can dissociate into partial dislocations by the following reaction during creep; 1/3[1120]→1/3[1010]＋SF₍₀₀₀₁₎＋1/3[0110].

Recently, additive manufacturing has been receiving considerable attention as a cost-effective technology compared to traditional manufacturing methods for its capability of fabricating complex parts directly from feedstock materials. In order to investigate possibilities of wire-arc additive manufacturing of commercially pure titanium by metal inert gas welding under atmospheric condition, fundamental experiments mainly focusing on contamination during welding process and its influence on mechanical properties of welded metal were conducted. From such experimental results, it was clarified that nitridation attributes largely to hardening and embrittlement of welded metal. Moreover, it was shown that promoting retention of shielding gas from welding torch or after shielding nozzle may be an effective solution to lower such risk of contamination or embrittlement problem.

The purposes of this study are to examine the applicability of polypropylene fiber reinforced concrete (PPFRC) to new pavements mixed with fly ash expecting a long-term improvement of durability and a reduction of CO₂ emissions. In the field of concrete pavement, there are many studies on steel fiber reinforced concrete pavements in which steel fibers are mixed into concrete for the purpose of improving the mechanical properties. Polypropylene fibers also enable to improve the bending toughness of concrete. On the other hand, in the construction sector, a large amount of CO₂ is emitted during the production of cement. The reduction of CO₂ emissions is an urgent task in the world. It can be reduced by replacing a part of cement with fly ash as one of the alternative materials. In this study, first, the fly ash substitution for concrete pavement was evaluated. Second, the compressive strength and flexural strength tests were carried out on PPFRCs. Using the bending test results, the tension softening curves of PPFRC were inversely calculated and determined. Finally, the pavement thickness was calculated taking into account the residual flexural strength and the size effect, and the reduction of CO₂ emissions was also estimated. Consequently, the PPFRC is able to use for the concrete pavement on the mechanical and environmental benefits.

In recent years, heat management by a temperature reduction effect, such as a water sprinkling effect, is strongly desired because of suppression of global warming. In this study, in order to evaluate the sustainability of the water sprinkling effect of pavement blocks, we attempted to clarify the evaporation performance from the inside of the blocks by the viewpoint of wettability. Specifically, the wettabilities were analyzed by measurement of static and dynamic contact angles and of the pavement blocks with different surface porous spaces. Water evaporation properties were measured from the fully water-containing block. As a result of sustainability of water evaporation, the ceramic block, which has a small porous space and a slow water absorption rate, was capable of transport water from the wetting sand to the inside, and had the ability to transport the inside water to the evaporation surface. In order to maintain the water sprinkling effect of the paving block, the water diffusion behavior for maintaining the sustainability of water evaporation on the surface layer is important, and the water inside the block is diffused to the surface. It was found from these results that it is necessary to absorb moisture from the external environment on the block surface.