Von Mises equivalent strain and stress have no negative values so that they have a difficulty to express strain and stress ranges even in a simple push-pull loading. Tresca equivalent values, on the other hand, have negative values but they also have a difficulty to put a sign to the shear strain and stress in cyclic loading. In non-proportional multiaxial fatigue, principal directions of strains and stresses change during a cycle. In such a case, strain and stress ranges and mean strain and stress cannot be easily determined. Developing an appropriate method of determining the ranges and mean values are needed for the design of pressure vessels and so on. This study proposes a simple method of defining the maximum strain and stress ranges and the mean strain and stress under proportional and non-proportional loading.
This paper proposes a method of inelastic structural analysis for YH61 nickel base single crystal superalloy. Hill's anisotropic constitutive equation was applied to describe the elastic-plastic deformation of the superalloy. The equation well described the static tensile curves of the solid bar specimen with <100> and <110> axes at 1173K. The equation also well expressed the torsion curves of hollow cylinder specimens with <100> axis at 1173K. The Hill's equation was applied to simulate the creep deformation of the superalloy. The equation gave satisfactory results for predicting the creep deformation of solid bars having <100> and <110> axes. The Hill's equation was used to predict the creep-fatigue deformation of a cruciform specimen in biaxial loading, resulting in a good agreement with the experimental deformation.
In order to establish a reliable remaining life assessment method for high temperature components in thermal power plants, it is necessary to understand the internal damage mechanism in heat resistant alloys and establish the damage prediction method. In the case of commercial heat resistant alloy used for thermal plants, nucleation, growth and coalescence of creep voids are dominant damage mechanism. So far, the void growth mechanism is well investigated and various growth models are suggested. However, rational modeling of void nucleation was remained to be difficult subject because basic mechanism has been unclear. Therefore, in this study, void nucleation behavior of a turbine rotor material under creep condition was investigated by a scanning electron microscope and development of void nucleation models was examined. Void nucleation model was developed based on the idea that creep voids nucleate by vacancy condensation at the interface between precipitated carbides and matrix on the grain boundary. In addition, consideration of creep property was included in the model via a relationship between strain and vacancy density obtained from the molecular mechanics simulation. In addition, model was expanded to predict the void number density, by a statistical treatment of the different physical property related to void nucleation in the every grain boundary. It was confirmed that the present void nucleation model can reproduce evolution of void number density.
The creep-fatigue life has been evaluated by the hysteresis energy in 316FR stainless steel with low carbon and medium nitrogen, which is a candidate for structural material in Fast Breeder Reactor (FBR) plant with the design life of 60 years. The creep-fatigue is a main damage mode to prevent. The hysteresis energy rate is considered as the parameter to predict the life time. It is clear that the relationship between this parameter and the time to failure can be approximately expressed by the power-law function. The function depends on the ratio of plastic strain to total strain. Total fracture energy for creep-fatigue loading intends to be independent of the ratio of plastic strain to total strain in long-term test condition. The value is related to grain boundary strength for creep-fatigue loading because fracture mode in long-term test condition is intergranular fracture. The life could be predicted by the function in the case of no significant change of fracture energy. Coarse precipitation, for example sigma phase, might be considered as a factor to change the fracture energy. It is important to predict the precipitation formation. The result of life prediction by the hysteresis energy rate is compared with that of the time fraction rule based on “Demonstration Reactor Design Standard (Draft)”. The predicted lives by both methods for long-term region are comparable and independent of the ratio of plastic strain to total strain.
11Cr-0.4Mo-2W-CuVNb steel (ASME Gr.122) has been used for boiler components in ultra-supercritical (USC) thermal power plants. Creep strength of high Cr steel welds decreases due to the formation of Type-IV creep damage in heat-affected zone (HAZ) during long-term use at high temperatures. In the present paper, the creep strength and microstructures of simulated HAZs and a welded joint of Gr.122 steel were investigated. In order to elucidate the processes of Type-IV creep damage, creep tests using thick welded joint specimens were interrupted at several time steps, and the initiation, evolution and distribution of creep voids were measured. It was found that Type-IV creep voids grew after 0.5 of life and a crack formed after 0.9 of life. Creep damage was observed mostly at 1/3 below the surface of the plate. Differences of the Type-IV damage processes between Gr.91 and Gr.122 steel weld were discussed. Experimental creep damage distributions were compared with computed versions using finite element method and damage mechanics analysis.
In order to investigate the possibility of electrochemical material characterization method as a procedure for remaining-life prediction of 12%Cr ferritic steels, this method was applied to three kinds of 12%Cr ferritic steels used in boiler and steam turbine and the changes in their anodic polarization behavior due to creep were measured. Experimental results revealed that the peak current densities “Ip1” and “Ip2” appeared at specific potentials during anodic polarization curve measurement in 1M-KOH solution. These peak current densities corresponded to the preferential dissolutions of several kinds of precipitates, respectively. The Ip2, which corresponded to the selective dissolution of M23C6 and Laves phase, increased more significantly with creep than thermal aging in all three steels. On the other hand, the effect of applied stress was not clearly reflected on the change in Ip1 corresponding to the selective dissolution of copper precipitate in the boiler steel, although it increased with thermal aging and creep as well as the Ip2. Therefore, it was expected that these peak current densities were useful indicators for estimating a temperature and applied stress working in high-temperature components of 12%Cr ferritic steels. Additionally, it was also revealed that the ΔIp2, which was increment of Ip2 from the as-tempered, increased with increasing creep life fraction and its variation showed almost no steel dependence.
We studied the evolution of microstructure in a Cr-Mo-V (JIS-SNB16) during creep by three kinds of nondestructive evaluations (NDEs). After a series of creep samples with various strains was obtained under a tensile stress of 25 and 35MPa at 923K, small samples were removed from the creep samples. Round-robin NDEs were carried out with the small samples. Internal friction with electromagnetic acoustic resonance (EMAR), parameters in magnetic minor hysteresis loops and thermoelectric power (TEP) were measured. Furthermore, the evolution of microstructure was observed with electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). The results by these NDE techniques have good correlations with microstructural observations. The combination of these techniques would utilize the advantages of the different methods and compensate for the weaknesses. This study suggests the importance of the combined usage in establishing reliable NDE procedures for the determination of creep damage.
Modification of small punch (SP) creep test was investigated to develop a new procedure for evaluating the creep property and damage of welded joint of high Cr ferritic steels by a miniaturized specimen. Disk and square-types specimens are usually clamped between the upper and lower dies in the isotropic manner in the conventional SP creep test, whereas the specimen used in this study was rectangular (10 × 6 × 0.50t mm) and was clamped at both ends of the specimen. This newly proposed SP creep test was applied to the welded joint of P122 steel pipe, which had been actually used in a fossil power plant for a long time. The specimens were removed from the mid section of the thickness of pipe so that the HAZ was located on the center of specimen perpendicular and parallel to the longitudinal direction of specimen respectively. In addition, the rectangular specimen consisting of only base metal and the conventional SP test specimen (10 × 10 × 0.50t mm) with the HAZ were also subjected to the SP creep test for comparisons. Experimental results revealed that, by arranging the HAZ perpendicular to the longitudinal direction, the creep rupture strength decreased significantly reflecting the lower strength of the fine grain HAZ (FGHAZ). This decrease in creep strength was attributable to the acceleration of initiation and growth of cavity and/or microcrack at the FGHAZ. Consequently, it is indicated that the present modified SP creep testing technique has a high potential as a tool for evaluating the susceptibility to Type IV cracking and/or the creep damage of welded joints.
The effects of various factors such as the surface finish condition of specimens, lubricant and argon gas flow rate on both the creep deformation behavior and time to rupture for 2.25Cr-1Mo low alloy steel used for boilers by small punch testing were studied. There was little difference in both the small punch creep deformation behavior and time to rupture due to the difference in surface roughness between #1200 polished specimens and those in mirror polished condition. The application of BN lubricant to the specimen surface resulted in the decrease in time to rupture, due to the lowering of friction coefficient. Furthermore, the time to rupture increased according to the increase of argon gas flow rate. The depth analysis by μ-ESCA for surface oxide film formed in an argon gas atmosphere demonstrated that the formation of Fe-rich oxide film with a low friction coefficient produced a decrease in time to rupture under a low argon gas flow rate condition. The formation of Cr-rich oxide film with high friction coefficient produced the increase in time to rupture under high argon gas flow rate condition. This suggests that the reliable control of argon gas flow rate is very important in order to evaluate the life of material by small punch creep testing.
Thermal barrier coatings (TBCs) are applied to high-temperature components of land-based gas turbine (GT). In TBC deposited by plasma-spray process, spallation damage has been often observed in the component, because of subjecting to cyclic thermal stress due to start-steady-stop operation of GT. Thus, it is necessary to grasp the stress generated in the coating during service, in order to predict precisely life of TBC spallation. The coating stress is composed of residual stress formed by plasma-spray process and thermo-mechanical stress generated during the operation. The residual stress is especially known to take different value with dependence on plasma-spray condition (powder velocity and substrate temperature). This paper presents influence of the spray condition on the residual stress of TBC. Then, the residual stress evaluation method, which had been already proposed, is improved based on particle deposition modeling plasma-spray process. As obtained results, it was found that the residual stress measured by strain gage method increased compressively with a higher particle velocity. Influence of substrate temperature on the residual stress was very few. It was considered with the particle deposition model that the residual stress increased compressively with elastic modulus of TBC due to stronger splat-to-splat cohesion by high-speed impact of particles. The residual stress evaluation method was improved in consideration of the elastic modulus changing by particle velocity. The result estimated method agreed with one measured by the strain gage method.
Cold spray is a new technique that can be used instead of the conventional thermal spray technique to provide a dense and low-oxide-content metallic coating. Conventional thermal-sprayed MCrAlY coatings are widely used for land-based gas turbine applications against high-temperature oxidation and hot corrosion. Recently, thick MCrAlY coatings were successfully obtained with cold spray process. The cold-sprayed MCrAlY coatings are denser than the low-pressure plasma-sprayed MCrAlY coatings. In order to evaluate the high-temperature oxidation behavior of cold-sprayed MCrAlY coatings, high-temperature exposure tests of cold-sprayed and low-pressure plasma-sprayed MCrAlY coatings were performed. Moreover hardness-changing behavior within high temperature exposure tests were investigated. From the results of exposure tests in an atmospheric environment of 1100°C, it can be observed that the cold-sprayed MCrAlY coatings have a lower oxidation ratio than that of the low-pressure plasma-sprayed coatings in this environment since the cold-sprayed coatings are denser than the low-pressure plasma-sprayed coatings. Moreover, cold-sprayed MCrAlY coatings have similar hardness changing tendency as low-pressure plasma-sprayed MCrAlY coatings. Therefore, the cold-sprayed MCrAlY coatings may have possibility to apply the gas turbine applications.
This paper presents the results of tensile properties of four kinds of polyamide thin films with different contents of filler used in electronic devices. Tensile tests were performed to determine Young's modulus, proportional limit, yield stress, ultimate tensile strength and elongation of polyamide thin films. The Young's modulus of polyamide films increased with increasing the contents of filler, and the trend of the data was satisfactory simulated by the Kerner model. The polyamide film with 42% filler showed the largest proportional limit, yield stress and ultimate tensile strength. The elongation decreased with increasing the contents of filler. The most suitable filler content was around 40% from considering the coefficient of thermal expansion and mechanical properties of the films.