High-chromium ferritic resisting steels have been commonly used for high temperature components of USC boilers. However, it is well known that in the welded joints TYPE IV damage occurs in the fine-grained HAZ during long-term use under elevated temperature, due to the multiaxial stress at the fine-grained HAZ region. TYPE IV damage develops by the creep void nucleation at grain boundaries, following the void growth and coalescence. In order to measure the amount of creep damage, the distribution of voids on two-dimensional cross section is normally observed. However, the damage evaluation has a limitation from the point of accuracy because the void grows three-dimensionally. Recently, X-ray CT has been applied to observe damage distribution in three dimensions, and it is known that X-ray µ-CT in SPring-8, which is synchrotron radiation facility, can detect a void of about 1 µm diameter. In this paper, the TYPE IV damage is observed with X-ray µ-CT. The amount of void nucleation and the size of voids in the fine-grained HAZ region with time are quantitatively evaluated. Comparison between the creep damage evaluations based on two dimensional cross section (average diameter, area fraction and number density) and three dimensional void observations is conducted. It appears that the evaluation based on the area fraction on cross section is closest to that in three dimensions and therefore the most suitable evaluation.
The stress and strain fields formed in the heat-affected zone (HAZ) of the weldment involved in pipes in thermal power plant have recently attracted attention as key plant management issue. The microstructure in the HAZ changes locally as a result of the thermal history imposed by the welding procedure. Hence, these changes manifest in time-dependent deformation, unlike those of the base material. In this study, the creep parameter involved in the constitutive equations of Norton’s law was identified for the base metal, HAZ, and weld metal in the weldment of a high-Cr ferritic heat-resistance steel by conducting a high temperature indentation creep test, which is useful for the in-situ identification of the creep parameter of aged components on site. The steel, which was treated here, is known to be a candidate material for the steam generating tube of the supercritical boiler. Finite element (FE) analysis was used to analyze some important engineering problems, such as the welded plate and tube subjected to creep loading by using the estimated creep parameter. As a result, the FE analysis results from the welded plate revealed that the creep strain localized at the HAZ relaxed as the angle between the weld line and loading direction increased, which indicated that the groove shape of the weld part should be sharp. From the results of the welded tube, the maximum stress component is occurred at the interface between base metal and HAZ and hydrostatic stress is also occurred at the interface in the HAZ at a deep position far from the surface of the pipe, which means that the creep void initiates in the HAZ and then grows near the interface between HAZ and base metal.
Nickel based alloy HR6W is one of candidate materials for high-temperature components in an advanced ultra-super critical power plant in Japan. In application to piping, it is necessary to clarify progress of damage and life property under creep and creep-fatigue loading conditions. In this study, fatigue, creep and creep-fatigue with up to 24 hours strain hold time per cycle tests were performed on HR6W at 750°C, and damaged specimens were produced by interrupting the tests. Cyclic failure life with 1hour hold time reduced to about 1/5 of the fatigue life, and reduction of the failure life increases with increasing the hold time due to accumulation of intergranular creep damage. Failure lives of the creep-fatigue tests were predicted by using the ductility exhaustion method. Creep voids were observed at lower damaged specimens both under creep and creep-fatigue loading, and lengths of the voids increase with increasing the damage resulting in formation of micro cracks at higher damage. For the creep and creep-fatigue damaged specimens, crystal misorientation represented by such as KAM (Kernel Average Misorientation) and GROD (Grain Reference Orientation Deviation) values were measured by using EBSD (Electron Backscatter Diffraction) analysis. KAM average values increase with the damage monotonously, in which the difference between the creep and the creep-fatigue tests was not clearly seen. On the other hand, GROD average values of the creep damage specimens is larger than those of the creep-fatigue damage specimens at the same damage indicating that creep and creep-damage of the HR6W may be detected by measuring GROD average.
Borated stainless steel, B-SUS304P-1, has been used for storage and transportation metal casks in the nuclear industry. Assuming various accidents, it is important to know the effect of strain rate on the fracture toughness of B-SUS304P-1. In the present study, image processing is used to evaluate the fracture toughness, because it is able to evaluate identically under various strain rate. Displacement field around a crack tip is obtained by a digital image correlation (DIC) method. J-integral and crack growth increment are determined by the DIC method and the fracture resistance curve is obtained. This method was applied for the fracture toughness test based on the ASTM Standard E-1820, and fracture resistance curves of the standard and the DIC method were compared. The results reveal that the DIC method is unable to evaluate the plane strain fracture toughness, however, it is possible to investigate the influence of the strain rate on the fracture toughness. It is suggested that the effect of the strain rate on the fracture toughness of B-SUS304P-1 is small by comparing the fracture resistance curves obtained by the DIC method under various strain rate conditions.
The application of Fiber Reinforced Thermoplastics (FRTP), which have the advantage of high specific strength and high specific stiffness, is expected to reduce the weight of automobiles. As thermoplastic resin has difficulty in impregnation into fabrics of continuous fibers due to its high viscosity, FRTP products with complicated shapes are usually manufactured by secondary processing of FRTP laminates. As this method uses high-cost FRTP laminates, in which thermoplastic resin is pre-impregnated into fabrics of reinforcing fibers, it is necessary to develop a low-cost FRTP manufacturing process. In previous study, the molding method, by which FRTP can be molded by compression after injection of melted thermoplastic resin into fabrics, was developed. However, due to the high viscosity of thermoplastic resin, this method is limited to fabrics of discontinuous fibers. As the mechanical properties of FRTP depend on the length of the reinforcing fiber, it is necessary to use continuous fiber for superior materials. In this study, Melted Thermoplastic-Resin Transfer Molding (MT-RTM) process, by which FRTP using continuous fiber can be molded at affordable cost, was developed. The effects of molding condition in MT-RTM on the impregnation properties of injected thermoplastic resin into fabrics of continuous fibers were clarified and tensile tests of MT-RTM products were conducted. In MT-RTM, FRTP of complicated shapes with ribs and bosses can be molded integrally by press while keeping the low viscosity of injected resin at mold temperature near the melting point. GFRTP molded by MT-RTM showed good impregnation property and tensile strength equal to that of GF/PA6 laminates molded by press molding.
In the press and injection hybrid molding of CFRTP, thermoplastics are injected on the preheated continuous fiber-reinforced thermoplastics. Since the press and injection hybrid molded products fracture at the interface between the continuous fiber-reinforced thermoplastics and the injection material, improving methods of the interfacial properties are required. In the press and injection hybrid molding, when the continuous fiber-reinforced thermoplastics are preheated, the resin disappears by thermal decomposition and the carbon fibers are frequently exposed on the surface of the continuous fiber-reinforced thermoplastics. We reported that the interfacial strength between the fiber and the matrix is increased by grafting CNT on the carbon fiber. Therefore, by supplying the CNT grafted plain woven carbon fiber fabric into the interface between the continuous fiber-reinforced thermoplastics and the injection material, the improvement of the interfacial properties can be expected. In this study, to clarify the effects of the CNT grafted plain woven carbon fiber fabric on the mechanical properties of the press and injection hybrid molded products, resin impregnation of CFRTP laminates was evaluated and, the tensile test and the tensile shear test at rib roots were conducted. Grafting CNT on the plain woven carbon fiber fabric improves the resin impregnation due to the better wettability of CNT grafted plain woven carbon fiber fabric. Supplying the CNT grafted plain woven carbon fiber fabric into the interface between the continuous fiber-reinforced thermoplastics and the injection material results in the improvement of the tensile strength and the tensile shear strength at rib roots.
Chemical shift tensor of cellulose Iα was investigated via line-shape simulations of solid-state 13 C NMR spectra obtained for uniaxially oriented cellulose nanocrystals (CNCs). The cellulose film composed of highly oriented CNCs was obtained by shearing of a CNC/water suspension. The solid-state 13C NMR measurements were performed at different angles (βLs), which are defined as angles between the static magnetic field and the direction of uniaxial orientation of CNC. The shape of the obtained NMR spectra varied depending on the βL. The line-shape simulations for the series of the solid-state 13C NMR spectra indicate that the directions of δ33 axis of C2-C6 locate almost parallel to the directions of C-O bonds.
Uniform mixing of cellulose acetate (CA) of DS (degree of substitution) = 2.18 with poly(vinyl alcohol) (PVA) was attained, by taking into consideration the hydrogen-bonding formability of both components. Blend miscibility of CA/PVA binary blends was examined by differential scanning calorimetry (DSC) as a function of the degree of saponification (Sp) of PVA. Although as-cast films of the CA/PVA blends were mostly transparent to the naked eye (i.e., optically compatible), the DSC analysis showed that the CA of DS = 2.18 was immiscible with PVA, irrespective of the Sp value. On the other hand, poly(N-vinyl pyrrolidone) (PVP), capable of forming a miscible monophase with CAs of DS < 2.8, was judged to be miscible with the PVAs of Sp = 19-100 mol%. Therefore, an attempt was made to compatibilize the immiscible polymer pair of CA and PVA by employing PVP as an additional component. Miscibility of the CA/PVA/PVP ternary blend was assessed by comparing the glass transition temperature (Tg) estimated by DSC with the ideal Tg value calculated from a Kwei equation. It was observed that PVP acted as a good compatibilizer for the polymer mixture of CA and PVA. Particularly, when the attractive interaction between the CA and PVA components and that between the PVA and PVP components were balanced with each other in frequency, all the three components were able to mix uniformly (i.e., totally miscible). In addition, a PVP sample of a higher molecular weight (PVP-H, Mw = 360 000) was found to be more effective as the compatibilizer relative to that of lower molecular weight (PVP-L, Mw = 24 500), due to the higher probability of intermolecular interaction of the former (PVP-H) with both CA and PVA per one polymer chain.