In high chromium steel weldments used in major high-temperature components of thermal power plants, Type IV cracking is known to occur by accumulation of creep-fatigue damage. In this study, creep fatigue life data of P91 steel weldments were obtained by conducting strain-controlled creep fatigue tests. The inelastic finite element analysis was performed to clarify the stress distribution in P91 weldment/base metal under the creep-fatigue loading. Consequently, a highly accurate creep-fatigue life evaluation method considering the stress multiaxiality in the weldment was proposed based on the ductility exhaustion rule and nonlinear finite element analysis. As a result of this study, it was confirmed that Type IV cracks occur at the heat-affected zone (HAZ) in these creep fatigue tests, and the creep fatigue life of P91 weldment can be predicted within a factor of two by the modified ductility exhaustion method modified by the stress multiaxiality.

Long-term creep rupture behaviors of nickel-based Alloy 617 base and similar welded joint were investigated. Creep tests were conducted at 650°C, 700°C and 750°C using smooth bar specimens, and the creep deformation and fracture were discussed in relation to the changing microstructure. It was found that fracture ductility of base metal decreased with decreasing the temperature and was less than 10% at 650°C. There was little difference in time to creep rupture of the base metal due to product geometry. Times to creep rupture of welded joint were shorter than those of base metal at 700°C but were comparable at 750°C, and most of the welded joints were ruptured at weld metal with significant reduction of ductility in comparison with base metal. γ' in grain and carbides (M₂₃C, M₆C) at grain boundaries were formed in both base and weld metals during creep. Especially M₆C precipitated in the grains and coarsened after a long time and the amount in the weld metal was greatly higher than that in the base metal. Strain concentrated near the grain boundary and voids or microcracks were formed at the grain boundaries or interface between the carbides and matrix. The local strain near the boundary at fracture in the weld metal was smaller than that in the base metal observed from EBSD.

The crack propagation analysis was carried out using the finite element method to investigate the causes of the characteristic crack shape formation and crack propagation behaviors observed in the SBF (small bulge fatigue) test with Type 316 steel. Specifically, several crack models simulating the SBF crack propagation process were created to investigate the effects of crack shape and branch angle on the stress intensity factor and the energy release rate. The analytical results showed that the energy release rate, which had been calculated by the stress intensity factor obtained by the analysis, increased with increasing branch angle, and this increase in energy release rate seemed to be a driving force behind the branching of SBF crack. It could be also explained by the maximum tangential stress theory that the crack propagated with opening angle of 90° and the SBF specimen was macroscopically divided into four pieces after the test. In addition, the experimentally measured crack propagation rate, da/dN, was relatively well correlated with the analytically obtained maximum stress intensity factor, K_max.

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.

Alloy 617 is an important candidate material for the boilers and turbines in the advanced ultra-super critical (A-USC) energy system. Here, the understanding of thermo-mechanical fatigue failures is a critical issue to be concerned. In this program the TMF lives were investigated, in comparison with the isothermal fatigue lives. A special attention was paid to the effect of strain/temperature phase angle. It was found that the TMF lives under the out-of-phase condition were significantly shorter than those under both the in-phase TMF and the isothermal low-cycle fatigue lives. This behavior was discussed considering the understanding of crack propagation process, where a new simple energy density parameter was proposed based on non-linear fracture mechanics analysis. The analysis strongly suggested that the phase angle effect would be attributed to the level of mean stress that depended on the TMF test conditions. The similar approach was applied to the TMF lives of a single crystal Ni-based superalloy, showing the similar conclusion.

Single crystal Ni-based superalloys have been used as a material for turbine blades and vanes in gas turbine power generation because of their superior high temperature strength. The turbine blades are exposed to the thermomechanical fatigue (TMF) loading, which is superposition of temperature and mechanical loading. The wall thickness of the superalloy blade with the cooling flow path is thin as some millimeters. Therefore, understanding the small fatigue crack propagation by the TMF loading is essential to prevent the failure. This study investigated the small crack propagation behavior in a single crystal Ni-based superalloy under the counter-clockwise diamond (CCD) typed TMF loading cycle and with the holding at the maximum temperature for 3 min and 10 min. The experimental results indicated that the introducing hold time at the maximum temperature accelerated the crack growth rates even if the mechanical loading was almost zero. The oxide film and γ’ depleted zone at the crack tip might dominate the small crack propagation behavior under CCD TMF loading with the high temperature holdings. In addition, observations of microstructure around the crack tip under the out-of-phase (OP), the In-phase (IP), and the CCD conditions revealed that the composition of the oxide film and the degree of influence on crack propagation behavior depends on the phase angle.

In this study, time-dependent behavior of thermal strain in composite materials which composed of elastic and viscoelastic solids is investigated. First, homogenized thermal expansion behavior of a unidirectional continuous CFRP is numerically analyzed using mathematical homogenization method. It is assumed to be orthotropic thermo-elasticity for carbon fiber. On the other hand, isotropic thermo-viscoelasticity based on generalized Maxwell model is assumed for base resin. It is shown to exhibit time-dependent thermal strain because of stress relaxation of base resin in the micro-scale (unit cell) of the CFRP. Second, the behavior of thermal strain is formulated by assuming that thermal strain is additively decomposed into equilibrium and non-equilibrium parts such as stresses in generalized Maxwell model. Non- equilibrium part of thermal strain is assumed to be dominated relaxation time of each Maxwell elements of the base resin. The resulting formulated thermal strain is shown to be perfectly successful for simulating the relaxation behavior of the unidirectional continuous CFRP.

We investigated the evolution of nonlinear acoustic characteristics, specifically the mixed-frequency response, with electromagnetic acoustic resonance (EMAR) in a nickel-based superalloy, Inconel718, under creep and thermal aging conditions. We clarified the relationship between these evolutions and the corresponding microstructural changes. EMAR is a combination of the resonant acoustic technique and a non-contact electromagnetic acoustic transducer (EMAT). We used bulk-shear-wave EMAT, which transmits and receives shear-waves propagating in the thickness direction of a plate specimen. Creep tests were conducted at 973 K, 310 MPa and 1033 K, 220 MPa, with several specimens interrupted at different time steps for analysis. The nonlinear ultrasonic properties during creep increased from the first stage to approximately 20% of the total life, after which they remained almost constant, and then increased rapidly from about 60% of the life until rupture. In contrast, the nonlinear properties of thermally aged material changed monotonically over time, with relatively small variations. These phenomena were interpreted in terms of dislocation recovery and microstructural change caused by the transformation of the precipitation-strengthened γ" phase, as supported by scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) observations. This non-contact resonance EMAT measurement technique can effectively monitor the evolution of the bulk-shear-wave nonlinearity throughout the creep life, offering potential for assessing damage evolution and predicting the creep life of metallic metals.

Creep rupture properties of weld joints and ultra miniature specimen of KA-STPA28 steel, Japanese standard of Gr. 91 steel for high temperature piping, aged material was investigated. Time dependence of creep rate of each test were compared and the results were discussed with scanning electron microscopy (SEM) observation of fracture surfaces and cross section of specimens. The welded joint creep specimens tested at high stress fractured in weld metal, and the specimens tested at low stress fractured in fine grain HAZ. It was suggested that the fine grain HAZ scattered in weld metal has inferior high temperature strength to those of base metal. Creep rate of UMC specimens at the initial stage of tests were about 5 times higher than those of round bar specimens. By comparing relationship between modified minimum creep rate and rupture time of each tests, in case of UMC specimen, excessive creep strain corresponded to those at 3% of rupture time was considered to be accumulated during initial stage of tests. Duration time between when specimen showed corresponded to minimum creep rate and rupture was relatively longer in case of UMC specimen because of higher ductility showed at fracture. Specimen surface oxidation was well suppressed by employing appropriate Ar gas flow rate.

Recently, an ultra-miniature creep (UMC) test by using a small plate specimen has been applied to assessment of the remaining creep rupture life of in-service power plant components in Japan. However, such miniature creep tests have an essential problem of the size effect on creep rupture life. Therefore, in this study, using the UMC specimens with different thickness (0.5 and 1 mm) of 1Cr-0.5Mo steel, the creep test was conducted in an argon gas environment for comparison in creep and rupture properties to the standard round bar specimen (φ6) tested in air. The size effect was discussed from the viewpoint of 3 factors having an influence on the size effect (i.e., number of grains on the cross section of a specimen, oxidation thinning and shape effect). As a result, number of grains had no size effect in the material tested because of small grain size of about 50 µm. Also, a simplified procedure was proposed for evaluating life reduction due to the oxidation thinning. Furthermore, using a shape effect index L₀/√A₀ (L₀: parallel length, A₀ : cross section area), the shape effect was found to be observed in the index of 4.5 or less.

This study discusses the design of small-sized cruciform creep specimens with an external dimension of up to 20 mm. One of the reliable methods for assessing remaining life is destructive testing. It is desirable to reduce the size of test specimens which limits the damaged area affected by sampling. Cruciform specimens, which allow for multiaxial creep tests over a wide range of principal stress ratios, have been studied for miniaturization, and a 50 mm-sized specimen has been developed. Further miniaturization will require consideration using FEM. In this study, the effect of the structural dimensions of the cruciform creep specimen, particularly the dimensions of each part such as the arm length and slits, on the generated stress was examined using 63 patterns of elastic analysis. The dimensions of the cruciform creep test specimens were determined using the knowledge gained from the elastic analysis, and the validity of the designed 20×20×4 mm-sized specimens was confirmed by the elastic-creep analysis. The gauge section achieved plane stress condition and resulted in a von Mises equivalent stress of approximately 200 MPa with an applied load of 1.25 kN. The volume of the specimen could be reduced to 1/680 of that of conventional bulk specimens.