Summary of Major Papers Published

  1. Plastic Behavior of Nuclear Stractural Metals After Cyclic Prestraining

    The plastic behavior of metals (SUS 304 stainless steel, 2Cr-1Mo steel and Al-alloy) after cyclic prestraining is systematically and experimentally investigated in a state of plane stress that consists of axial and torsional stress (strain) components. A practical biaxial extensometer for use in cyclic and subsequently large strain tests is also developed.
    The strass level in reloading after cyclic prestraining is never below the corresponding initial stress-strain curve, regardless of the loading direction, even after cyclic softening. Initial plastic anisotropy does not disappear or become small due to cyclic prestraining, and may become strong, depending on the prestraindirection. The larger the cyclic strain hardening, the smaller the work hardening coeffcient in reloading.
    This suggests initial stress-strain relations should and can be used as basic data on materials when analyzing the strength and safety of a structure undergoing normal cyclic loading followed by abrupt overloading such as that occurring during an accident or earthquake.


  2. Expermental Study on Viscoplastic Bahavior of SUS 304 Stainless Steel After Axial Cyclic Preloading

    Structural materials used in such products as airplane and pressure vessels in nuclear reactors are usually subjected to cyclic loading and to abrupt accidental overloadings. An accurate estimate of strength properties of cyclic prestraind materials under accidental overloading can be very useful for the purpose of design safety and cost reduction. In this report, viscoplastic behavior of Type 304 stainless steel after axial cyclic preloading of various types was investigated experimentally at rom temperature. The difference between flow stress levels depending on strain rate decreases considerably after cyclic preloading. Howeverm the relaxation stress drop increases rather than decreasing with an increase of flow stress even after cyclic preloading. No consistency in viscoplastic behavior, which is shown by the customary viscoplastic theories, can be found. Plastic modulus Ep depends on the flow stress level for the case of comparatively small cyclic preloading. Ep becames much smaller and constant after strong cyclicpreloading. Thus much larger plastic strain may occur when an abrupt overload is applied. A very strong linear relationship between Ep and plastic work per cycle is found.
    Key Words:Viscoplasticity, Stress Relaxation, Tensile Properties, SUS 304 Stainless Steel, Cyclic Preloading, Plastic Modulus, Stress-strain Relations


  3. High-Temperature Strength of TiC-Coated SUS316 S.S

    Some ceramic-coated metals are nominated as first-wall materials. TiC-coated type-316 stainless steel is expected to be superior to other materials in high-temperature strengh and in endurance properties under heavy irradiation. Delamination between the ceramics layer and base metal is considered to be one of the most important problems when such ceramic-coated metals are used in a gradient temperature field such as the first wall. In this report, the high-temperature strength of TiC-coated type-316 stainless steel, which is compared to the first wall of the fusion reactor, is investigated experimentally and computationally. A simple and precise thermal stress testing system is developed. The effects of surface roughness as wall as the thermal stress and the residual stress on bond strength are investigated. Experimental and numerical results on residual stress distribution are compared mutually for confirming the reliability of the inelastic analysis of the finite-element mathod. It is expected that the suitable surface roughness decreases the residual stress in the inelastic FEM analysis for the heating condition used in the experments.
    Key Words:Delamination, Ceramic-Coated Metal, Bond Strength, Inelastic Analysis, Finite-Element Method, Thermal Stres, Residual Stress, First-Wall Material, Gradient Temperature Field


  4. Plastic Constitutive Model for the Anisotropic Hardening Metals

    A simplified anisotropic plastic constitutive equations for an initially anisotropic metals is developed. Various plastic behavior of the initially anisotropic metals after multiple-prestrainings have been investigated experimentally by the author. The new constitutuve model is based on the numerous experimental facts and it contains the new evo;utional rules of outer/bounding surface based on the two-surface concept. Benchmark tests are performed to estimate the applicability of the new model comparing with the two-surface constitutive models of isotropic/mixed hardening types under combined axial with torsional stresses, The new model is expressed in the five dimensional deviatoric stress space and the directions of the principal axes of the anisotropic hardening outer surface are fixed. So, the model is not available for a
    precise discussion on general anisotropies and on these evolutional change including rotations of the principal axes, but, is comparatively simple and easy to apply and to discuss evolutional changes in the anisotropies due to multiple preloadings.


  5. Effect of Prestrain and Strain Rate on Viscoplastic Properties and Strain Ageing of Metals

    Although many studies on viscoplasticity of metals have been reported, these do not necessarily succeed in unitive interpretation of viscoplastic behavior, for example, strain rate inverse-dependency of plastic flow stress and ununitive properties in both flow stress and stress relaxation.
    In this study, authors have succeeded to interpret unitively viscoplastic behavior of metals including both creep and stress relaxation behavior by investigating strain ageing properties under plastic flow of metals quantitatively. In the previous report, an experimental method is successfully presented to get both a viscosity dependency and a strain ageing dependency of viscoplastic flow stress of metals separately.
    In this report, both an increasing strain ageing during stress relaxation in which strain rate decreases gradually, and temporal over-hardening in reloading are estimated quantitatively. It is shown that the proposed viscoplastic constitutive model including strain ageing effect has a good applicability to an inelastic behavior having a stress-relaxation.
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