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Weibull stress criterion based on the “Local Approach” is applied for rational brittle fracture assessment of steel components taking account of the plastic constraint effect on fracture resistance. Once the cumulative distribution of critical Weibull stress is identified for the material itself, that is Weibull shape parameter m as a material constant independent of size/shape of components, loading mode and temperature, fracture toughness can be corrected to the fracture resistance of structural components on the basis of the Weibull stress criterion. On the other hand, a miniature fracture toughness specimen is useful for evaluating the toughness of local area of the component and limited material such as irradiated test sample. Then, this study focuses on developing a method for identification of shape parameter m of critical Weibull stress distribution using miniature specimens. Two types of miniature 3PB (3-point bend) specimen with thickness B=1.65mm that has different crack depth are used to identify the m-value. The m-value determined from the two types of miniature specimen is found to be almost the same as that obtained from normal size specimens with thickness B=15mm, whereas both testing are conducted at different temperature. It is demonstrated that the fracture toughness of the steel at the other temperature can be precisely corrected to the fracture resistance of the cracked component with lower crack-tip plastic constraint by means of the m-value identified from test results for two types of miniature specimen.

This study assesses the applicability of the Weibull stress for the assessment of the crack-tip plastic constraint effect as well as the mixed mode I and II loading effect on brittle fracture resistance of a ferritic steel. Brittle fracture toughness under different crack-tip plastic constraint conditions and different loading mode conditions was respectively obtained by conducting 3-point bend (3PB) test for single edge cracked specimens with different crack depth subjected to mode I load, and 4-point shear (4PS) test for a single edge cracked specimen with deep crack depth subjected to mixed mode I and II load. By using the results of 3PB tests, the critical Weibull stresses distribution independent of crack-tip plastic constraint was identified, and the critical Weibull stresses obtained by 4PS tests provided significantly smaller distribution than that for 3PB specimen. The mixed mode loading provides the different combined stress field around the crack-tip from that under mode I loading, where the 4PS specimen showed the lower distribution of σ2/σ1 and σ3/σ1 than the 3PB specimens. The lower principal stress ratio to be enlarge the fracture driving force was found to have no or much less influence to linear energy release rate for micro-crack, that was employed in the conventional derivation of the Weibull stress. Consequently, the reason why the conventional Weibull stress could not evaluate the effect of loading mode on fracture resistance could be that the linear energy release rate could not exactly take into account the local fracture driving force enlarged due to the combined stress field under mixed mode loading.

True stress-true strain curves are fundamental for designing and analyzing structures such as fusion reactors. These curves are typically obtained by conducting tensile tests on round bar specimens. However, due to material dimension limitations, plate specimens are sometimes used instead of round bar specimens. Obtaining true stress-true strain curves experimentally from plate specimens can be challenging. To address this challenge, this study aims to obtain true stress-true strain curves of miniature plate specimens using both analytical and experimental methods. The analytical method involved inverse finite element method (FEM), while the experimental method utilized real-time measurement of the minimum cross-sectional area and radius of curvature of a miniature plate specimen with a high-speed laser profiler. Comparing the true stresses obtained from the analytical and experimental methods, we found that the difference was typically within 5%. These findings suggest that inverse FEM and laser profilometry are effective methods for determining the true stress-true strain of miniature plate specimens.

低放射化フェライト鋼の高温低サイクル疲労に伴う水素放出特性の変化を調べ，核融合炉ブランケット第一壁の疲労寿命評価への水素昇温脱離分析法の適用性について検討し、その成果について発表を行う。, 日本材料学会九州支部 第７回学術講演会・総会

Reduced activation ferritic/martensitic (RAFM) steels are assumed as structural material of blanket design activities. Determining the design limit is essential for the RAFM steels to qualify their use in fusion blankets. This work investigates fatigue properties of a RAFM steel, F82H at elevated temperature up to 823 K in the air and vacuum condition. The tests were carried out with axial strain controlled condition using cylindrical specimen, and the results were summarized using the Manson’s modified universal slope method, which can describe fatigue lifetime as a function of tensile properties. Although fatigue lifetime of F82H decreased with temperature, the fatigue lifetime up to 723 K fell into factor of 2 of the lifetime at ambient temperature. The lifetime at vacuum condition was successfully described using universal slope method. On the other hand, the lifetime in the air was 1/3 or shorter than that in vacuum condition. It is notable that a heat of F82H prepared for IEA round robin tests demonstrated 1/5 of shorter lifetime than latest F82H. The difference is discussed with inclusion morphologies affected by steel manufacturing process and so on. This report has been prepared as an account of work assigned to the Japanese Implementing Agency within the ‘‘Broader Approach Agreement’’ between the Government of Japan and the European Atomic Energy Community.

This study assesses the applicability of the Weibull stress for the assessment of the crack-tip plastic constraint effect as well as the mixed mode I and II loading effect on brittle fracture resistance of a ferritic steel. Brittle fracture toughness under different crack-tip plastic constraint conditions and different loading mode conditions was respectively obtained by conducting 3-point bend (3 PB) test for single edge cracked specimens with different crack depth subjected to mode I load, and 4-point shear (4 PS) test for a single edge cracked specimen with deep crack depth subjected to mixed mode I and II load. By using the results of 3 PB tests, the critical Weibull stresses distribution independent of crack-tip plastic constraint was identified, and the critical Weibull stresses obtained by 4 PS tests provided significantly smaller distribution than that for 3 PB specimen. The mixed mode loading provides the different combined stress field around the crack-tip from that under mode I loading, where the 4 PS specimen showed the lower distribution of σ2/σ1 and σ3/σ1 than the 3 PB specimens. The lower principal stress ratio to be enlarge the fracture driving force was found to have no or much less influence to linear energy release rate for micro-crack, that was employed in the conventional derivation of the Weibull stress. Consequently, the reason why the conventional Weibull stress could not evaluate the effect of loading mode on fracture resistance could be that the linear energy release rate could not exactly take into account the local fracture driving force enlarged due to the combined stress field under mixed mode loading.

The paper summarizes the current status of the materials property handbook for a structural design using Japanese reduced activation ferritic/martensitic steel F82H. Specifically, the key structural parameters, e.g., time-independent/dependent design stresses and fatigue design curves, were determined following the French structural design code RCC-MRx. Besides, under the Japan-U.S. collaboration, tensile data were newly added to the benchmark heavy irradiation data up to 80 dpa, as critical input information in the intermediate check and review in Japan. Furthermore, the status of structural material data and the near-term and long-term issues were clarified by the evaluation using the attribute guides. In parallel, the structural design approaches which were newly introduced and extended to cope with the structural design issues under the complex environmental conditions peculiar to the DEMO reactor were noted with the initial R&D results. Of many design issues, the multi-axial loading condition due to the complexity of the DEMO reactor as well as the coolant compatibility and the irradiation effect is mentioned. For example in the paper, the multi-axial fatigue-creep testing and evaluation using the modified universal slope method and the brittle/ductile fracture testing and evaluation by the local approach are explained toward DEMO.

A case study on a fusion blanket design such as DEMO indicated that there could be some sections with high stress triaxiality, a parameter to evaluate the magnitude of plastic constraint, in the case of plasma disruption or coolant loss accident. Therefore, it is necessary to accurately understand the ductility loss limit of structural material in order to conduct the structural design assessment of the irradiated and embrittled fusion reactor blanket. Tensile tests were conducted by using three kinds of tensile specimen shapes to investigate of the plastic deformability of F82H. From the results, the fracture ductility is lower as the stress triaxiality is higher. Voids of the interrupted RB1 specimen were observed along grain boundaries and expanded parallel to the tensile axis. That of interrupted R0.2 specimen was rounded shape compared with those of RB1. The fracture surface of RB1 and R0.2 specimens were observed the elongated dimples and the equiaxed dimples without so much elongation, respectively. It is considered that the decrease of plastic deformability for the notched specimen was caused by the process of voids formation and crack growth due to the effect of plastic constraint of the notch.

Reduced activation ferritic/martensitic (RAFM) steels are promising candidate for structural material of tritium breeding blanket in a fusion reactor. Database accumulation and definition of design criteria for RAFM have been intensively studied together with the progress of blanket design activities. As a part of database accumulation for fusion blanket, a RAFM steel, F82H was fatigue-tested at 573 and 673 K in the air. Axial strain-controlled fatigue tests were carried out with a cylindrical specimen with 8 mm of diameter with -1 of strain ratio condition in accordance with Japanese Industrial Standard, JIS Z 2279, “Method of high-temperature low cycle fatigue testing for metallic materials.” For high cycle tests, the maximum test cycles exceeded 106 cycles. Fatigue lifetime of F82H at temperatures ranging from room temperature to 673 K fell into the factor of 2 of the empirical fitting curve for 673 K. It was studied using experimental results that fatigue-related design limit based on RCC-MRx, such as fatigue design curves, half-life cyclic curves, and related coefficients.

Non-irradiated and irradiated material properties of a reduced activation ferritic/martensitic steel, F82H, have been extensively accumulated through international collaborative researches. F82H then demonstrates material properties applicable to the structural material of breeding blanket module. At present, material strength standard needs to be developed for the design activity of blanket module. Material strength standard generally gives average, minimum, and design values of mechanical property and their equations complying with the definition in construction code. In this work, we summarized and analyzed the experimental data of tensile, creep, and fatigue properties, and developed the material strength standard for RCC-MRx. Obtained values were then compared to those of Eurofer. Though some minor differences were observed in minimum and average values of tensile and creep properties, no significant differences were observed in their design values. As for fatigue property, F82H showed conservative design curves.

The small specimen tensile test is commonly used as the basic test to evaluate the irradiation effect on the strength of fusion structural materials after irradiation, and a lot of attempts were made to qualify the data obtained by small size specimen, but the specific attention on the evaluation of strain were not satisfactory given. This study therefore aims to propose a novel non-contact strain measurement technique, based on digital image correlation (DIC) method, for reduced-activation ferritic/martensitic (RAFM) steel, e.g., F82H, as the leading candidate structural material of fusion in-vessel components in Japan. A flat rectangular miniature specimen, e.g., SS-J3 type, was adopted to evaluate non-irradiated and neutron-irradiated tensile properties. In the proposed DIC, very tiny surface machining flaws were recognized by the DIC computation program and utilized to set several pairs of the gauge end points with a fixed gauge length. In case of the room-temperature test, it was clearly demonstrated that this procedure could provide more precise and reliable data compared with the previous approach of the single-scan measurement under floated gauge length. Superior linearity was achieved and recognized DIC points of concern were traceable during the entire period of the test. By applying this technique, superior high-dose stability of elastic properties of F82H was first demonstrated.

Reduced activation ferritic/martensitic (RAFM) steels are promising candidate for structural material of tritium breeding blanket in a fusion reactor. Database accumulation and definition of design criteria for RAFM have been intensively studied together with the progress of blanket design activities. As a part of database accumulation for fusion blanket, a RAFM steel, F82H was fatigue-tested at 573 and 673 K in the air. Axial strain-controlled fatigue tests were carried out with a cylindrical specimen with 8 mm of diameter with -1 of strain ratio condition in accordance with Japanese Industrial Standard, JIS Z 2279, “Method of high-temperature low cycle fatigue testing for metallic materials.” For high cycle tests, the maximum test cycles exceeded 10^6 cycles. Fatigue lifetime of F82H at temperatures ranging from room temperature to 673 K fell into the factor of 2 of the empirical fitting curve for 673 K. It was studied using experimental results that fatigue-related design limit based on RCC-MRx, such as fatigue design curves, half-life cyclic curves, and related coefficients.

The small specimen tensile test is commonly used as the basic test to evaluate the irradiation effect on the strength of fusion structural materials after irradiation, and a lot of attempts were made to qualify the data obtained by small size specimen, but the specific attention on the evaluation of strain were not satisfactory given. This study therefore aims to propose a novel non-contact strain measurement technique, based on digital image correlation (DIC) method, for reduced-activation ferritic/martensitic (RAFM) steel, e.g., F82H, as the leading candidate structural material of fusion in-vessel components in Japan. A flat rectangular miniature specimen, e.g., SS-J3 type, was adopted to evaluate non-irradiated and neutron-irradiated tensile properties. In the proposed DIC, very tiny surface machining flaws were recognized by the DIC computation program and utilized to set several pairs of the gauge end points with a fixed gauge length. In case of the room-temperature test, it was clearly demonstrated that this procedure could provide more precise and reliable data compared with the previous approach of the single-scan measurement under floated gauge length. Superior linearity was achieved and recognized DIC points of concern were traceable during the entire period of the test. By applying this technique, superior high-dose stability of elastic properties of F82H was first demonstrated.

The small punch (SP) creep test using a TEM disk-type specimen (ϕ3 × t0.25 mm) was applied to the fusion zone (FZ), the coarsened grain heat-affected zone (CGHAZ), the fine grain HAZ (FGHAZ), the tempered HAZ (THAZ) and the base metal (BM), respectively, which were removed from the joint of 8Cr–2W–VTa steel welded by an electron beam welding, in order to investigate the creep properties of such local regimes. Experimental results revealed that the SP creep rupture of the FGHAZ was accompanied by creep void formation even if the rupture time was relatively short and its creep rupture strength was inferior to those of the other regimes. This reduction in rupture strength seemed to be attributable to the microstructural changes during the welding thermal cycles, which were represented by the annihilation of clear lath martensitic structure and the change in precipitate distribution.

The small-punch (SP) test and the SP creep test were carried out by using a further miniaturized plate-type specimen (3 mm diameter × 0.25 mm thick). Those tests were applied to the electron-beam welded joint of reduced-activation ferritic steel for evaluating the distribution of high-temperature strength in the joint. The experimental results revealed that the distribution of the maximum load measured by the SP test at 873 K was very similar to that obtained from Vickers hardness, and the fusion zone was relatively devoid of high-temperature ductility. In addition, the ratio of the load of SP creep test to the stress of standard uniaxial creep test was calculated so that both the creep rupture curves were overlapped to convert the results of SP creep test into those of standard test. As a result, the ratio was determined to be 0.43 for the base metal and this load-stress conversion coefficient is in good agreement with that obtained from finite element analysis.

Type IV creep damage has recently been a worldwide issue for high Cr ferritic steels. The small punch (SP) creep test has been successfully applied to evaluate this damage of low alloy ferritic steel by the author’s group. However, the heat affected zone (HAZ) of fusion reactor material welded by electron-beam (EB) welding is so thin that it is not easy to evaluate its mechanical properties by the conventional SP test. In this study, the SP test using a further miniaturized specimen was applied to the EB welded joint of reduced activation ferritic steel (RAFs), for evaluating high-temperature tensile properties of the HAZs. As the result, the σy and σB of the tempered HAZ at 873 K were estimated to be as low as 275–300 MPa and 325–340 MPa, respectively.

We discuss the magnetoconductance oscillations in small junctions of a ballistic semiconductor and a superconductor. The conductance is calculated both numerically and analytically based on the Bogoliubov-de Gennes equation. The conductance oscillations appear when the Andreev reflection is not perfect at the interface and the diameter of the cyclotron motion of a quasiparticle is comparable to the width of the junction. An interplay between the cyclotron motion of a quasiparticle near the interface and the phase shift by the magnetic field is the origin of the conductance oscillations.

The small punch (SP) creep test was carried out at the temperatures of 823∼923 K by using a further miniaturized specimen, namely, TEM disk-type specimen (φ 3.0×t0.25 mm). The tests were applied to the fine grain heat affected zone (FGHAZ), tempered HAZ (THAZ) and base metal (BM), respectively, which were removed from the joint of the reduced activation ferritic steel welded by an electron beam welding, in order to investigate the creep properties of such local regimes. The results obtained from the SP creep test were correlated with those of uniaxial creep tests using the base metal (BM) and welded joint (WJ). Experimental results revealed that there were no large differences between the SP creep rupture strengths of the FGHAZ and THAZ and that of the BM at the relatively high load levels. This result was in good agreement with the fact that the uniaxial creep strength of the WJ was almost coincident with that of the BM at the relatively high stress levels. In addition, the ratio of load (P) to stress (σ), which gave same rupture time, was calculated by using the creep rupture data of the BMs. As a result, the ratio was determined to be 0.43, resulting in the following equation; P = 0.43 σ.Copyright © 2008 by ASME