主要論文概要

  1. Reliability Approach to Fatigue Life Prediction of Degraded Materials during Long Service

    The present study has made statistical investigations on the effects of material degradation on the crack initiation life and on the crack growth behavior in a 2Cr-1Mo (SCMV 4) steel. The steel studied in this paper has been used in a plant having operated for over 60,000 hours in service, being subjected to material degradation due to temper embrittlement. The results obtained for the degraded steel were discussed in comparison with those for the recovered SCMV 4 steel.
    The crack initiation life and crack growth rate in the degraded SCMV 4 steel and in the recovere SCMV 4 steel followed the log-normal distributions. The crack initiation life was shorter in the degraded steel than in the recovered one at a high probability. The crack growth rate da/dN was higher in the degraded steel at higher stress intensity factor range DK. At lower DK levels, on the other hand, the mean of da/dN was a little lower in the degraded steel; nevertheless, a larger variation in da/dN implies that da/dN can become higher in the degraded steel.
    In order to determine the P-S-N diagrams for the degraded and the recovered SCMV 4 steels, a Monte Carlo simulation was made on the basis of the data obtained in the present experiments. The resultant P-S-N diagrams statistically revealed that the fatigue life of the degraded steel was shortened at higher stress levels and that its variation became at lower stress levels. The larger variation in the life of the degraded steel is attributed to shorter fatigue life in the degraded steel than in the recovered steel at a considerably high probability, although the difference of the mean fatigue lives of the two steels became smaller at lower stress levels.

    Key Words: temper embrittlement, crack growth rate, crack initiation life, P-S-N diagram


  2. Theory of Fatigue Crack Initiation in Solids

    The present study proposes a new theoretical model of fatigue crack initiation in metallic materials. The extrusion can be often found in fatigued metallic materials and hence is regarded as one of the possible crack-initiation sites. The extrusion is a type of heterogeneity which causes local energy enhancement to bring about a crack or cracks in a metallic material. In the present theory, the heterogeneity is modeled by a dislocation dipole and analyzed theoretically by the use of the theory of continuously distributed dislocations.
    The resultant governing equation was found a singular integral equation of the first kind with a generalized Cauchy kernel and a bounded Fredholm type kernel. The equation can not be solved theoretically, so that a computational method has been adopted using Erdogan's procedure. It was found that the crack initiation life Ni can be theoretically derived as a critical stress cycle number at which the Gibbs free energy reaches a maximum and where the system of interest becomes unstable. Furthermore, the present theory provides a theoretical interpretation of the fatigue limit as well as that of the microscopic type I crack. The theory can also derive the dependence of the fatigue limit on the yield strength and that of the fatigue strength on the grain size of a material considered.

    Key Words: computational mechanics, fatigue, micromechanics, heterogeneity, dislocation, persistent slip band


  3. Creep Crack Growth Properties of Ni-Base Superalloys at 1,273 K

    The present study investigates the effects of carbon content and environment on creep crack growth behavior in Ni-26%Cr-17%W-0.5%Mo superalloys which have been developed as candidate structural materials for intermediate heat exchangers in Japanese high-temperature gas-cooled reactors. Creep crack growth tests were made on the alloys having different carbon content in air and in helium gas environments at 1,273 K by using CT specimens. Creep crack growth rate da/dt was evaluated by the non-linear fracture mechanics parameter C*. The da/dt was higher in the higher carbon alloys than in the lower carbon ones in each environment. The effect of carbon content on the da/dt vs. C* relationsip can be elucidated by the creep ductility. The environmental effect on the relationship was small in 0.025%C and 0.085%C alloys. In 0.003%C alloy, however, the environmental effect was remarkable; i.e., the da/dt became much lower in helium gas environment than in air. This result is attributed to marked increase in creep ductility this alloy showed in helium gas environment.

    Key Words: creep, crack growth, nonlinear fracture mechanics, creep ductility, nickel-base superalloy, precipitation hardening


  4. Creep Damage in Ni-Base Superalloys at Very High Temperatures

    The present study investigates early-stage creep damage of Hastelloy XR and XR-II alloys, modified versions of Hastelloy X alloy, which have been developed in Japan as most promising candidate structural alloys for Japanese high-temperature gas-cooled reactors (HTGRs). Creep tests were made on Hastelloy XR forging, tube and XR-II tube in a very high temperature range of 1,123 to 1,273 K in a simulated HTGR helium gas environment. The tests were interrupted at different strain levels of up to 5 % in order to evaluate creep damage via intergranular voids. The void sizes along grain boundaries and the A-parameter, the ratio of the number of damaged grain boundaries, on which one or more voids are found, to that of the total grain boundaries observed are used in order to evaluate creep damage. Statistical analyses of the A-parameter as well as the void sizes reveal that the values of the parameter show wide variations and follow the Weibull distribution, reflecting spatial randomness of the voids. The void sizes along grain boundaries, on the other hand, follow the log-normal distribution. The maximum void size dmax and the mean value of the A-parameter Am are calculated and plotted against interruption creep strain eint. The resultant dmax vs. eint and Am vs. eint diagrams show that Hastelloy XR forging had suffered more damage than Hastelloy XR tube; nevertheless, the forging has longer interruption life, or the time to reach a given interruption creep strain. The result indicates that grains may have been deformed more easily in Hastelloy XR in the form of tube than in the form of forging. The diagrams also imply that the addition of boron has suppressed the nucleation as well as the growth of voids and thus has brought about longer interruption life of Hastelloy XR-II.

    Key Words: high-temperature gas-cooled reactors, Hastelloy XR, Ni-base superalloy, impure helium gas, creep damage, material design, material testing, intergranular voids, A-parameter, statistical analysis.


  5. Computational Equivalent Inclusion Method - Similarity of the Equivalent Inclusion Method and the Boundary Element Method -

    The present study investigates the computerization of the equivalent inclusion method. This method was first proposed by J. D. Eshelby, and is considered as one of the most powerful tools for analyzing the mechanics of inhomogeneities often found in advanced materials; e.g., strengthening dispersoids, inclusions, precipitates, etc. The method, however, involves mathematical difficulties especially in the case where one need investigate non-uniform distributions of elastic properties in inhomogeneities. The present method, however, can avoid the difficulties as well as the complexity of the mathematical manipulations by adopting computational techniques with the aid of the finite element discretization scheme.
    The present study is focused upon the formulation for two-dimensional problems, since the three-dimensional formulation is analogous to the 2-D one, and thus is deductible from the 2-D formulation procedure. The governing integral equations were shown to have kernels with the 1/r- and 1/r2-singularities for 2-D and 3-D problems, respectively. The equations for 2-D problems have been discretized by the use of finite elements having the triangular polar co-ordinates. It has been shown that the adoption of the co-ordinate system can eliminate the singularities, and therefore the governing equations have Cauchy principal values. The discretization is found very similar to that employed in the direct boundary element analyses in which body forces, and hence interior points, need be investigated.

    Key words: micromechanics, equivalent inclusion method, Green's function, singular integral equation, boundary element method, computational mechanics

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