[in Japanese]

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[in Japanese]

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Transformation plasticity is one of the typical phenomena during phase transformation of steel, and it considerably affects the distortion of the products. Therefore, precise evaluation of the strain induced as well as residual stress distribution is necessary. Phenomenological constitutive equations considering this phenomena have been developed, while complete formulation has not yet been constructed, since the mechanism has not fully clarified. This study is motivated to model the transformation plasticity phenomena using phase field model, which can treat the interfacial behavior of two phases and is considered to provide better understanding of the phenomena. As the first step of our study, a function representing the mechanical properties depressing at the interfacial region is suggested. A simplified simulation considering the depressing hardening function exhibited large induced strain in the transformation temperature range.

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In the previous paper, dependence of the driving force of the deformation localization on the physical mechanisms of the elastic stiffness degradation caused by the plastic deformation, which is closely related to changes of ultrasonic wave velocities due to the plastic deformation, was examined in detail. Successively, the new model for ductile fracture initiation was proposed based on the new concept, i.e.. ductile fracture initiation is caused by the jump of wave velocity along the intersected crossing line between two different stationary discontinuity bands characterized by vanishing velocity of an acceleration wave. The micro-crack nucleation caused by the jump of the velocity along intersected crossing line between the two discontinuity bands was verified theoretically by the shock wave theory and analyzed numerically using the theory of ultrasonic wave velocity under the plastic deformation proposed by the author. In the present paper, ductile fracture progress of FCC single crystals characterized by four categories of damage levels, i.e., yielding, localized deformation, micro-crack nucleation and macro-crack and/or void nucleation, is simulated using the finite element crystal plasticity model (FEPM) with built in proposed criteria.

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A three-dimensional level set method (LSM) was proposed for dislocation dynamics of which the dislocation curves show extremely successful evolutions, especially when topological changes, i.e.. merging and breaking. occur. The intersections of the zero levels of two vector valued level set functions are used to represent the dislocation lines in three dimensions. Fast Marching Method (FMM) creating Signed Distance Function (SDF) from the given interface can incorporate the existing different shapes into one level set function. Therefore, the dislocation lines can be represented by one set of level set functions. With some specific restrictions, (use FMM to create a smooth constrained plane) we succeeded to demonstrate some behaviors like cross-slip, double cross-slip, and dislocation interactions on cross-slip plane.

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Recently. Ohno-wang model and Chaboche model that described mechanical ratcheting deformation have been developed. But these models don't take into consideration any distortion and rotation of subsequent yield surface. Subsequent yield surfaces, strain path and stress-strain curves have been examined experimentally for the initially isotropic 6-4 Brass subjected to mechanical ratcheting deformation. In this work, some simulation analysis were carried out based on the flow rule with yield surface including some parameters on two surface hardening rule considering both distortion and rotation of subsequent yield surface. The newly developed plastic constitutive model was presented and the reliability of the model was verified by comparing analytical results with experimental results on mechanical ratcheting deformation.

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A hypothesis of plastic adaptation has been proposed and modeled as one idea that is available to link micros- and macro-scopic approaches to fatigue problems. Based on the hypothesis, an equivalent cyclic stress ratio has been formulated as a useful parameter for correspondence between cyclic stress conditions of notched and unnotched specimens. In the present study, the rationality of the hypothesis and its application to multi-axial fatigue problems are discussed from a viewpoint of micro- and macro-mechanics.

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Due to low thermal activation, modified 9Cr-Mo steel can maintain steady-state deformation during long-term creep. The creep strength of its weldment is lower than that of the base metal. Because of different creep strain rate in the solidified weld metal and heat-affected zone (HAZ), creep deformation in HAZ can be lowered but stress augmented in fine-grained HAZ to cause detrimental Type IV cracks in this zone. In this study, FEM analyses were made on long-term creep of over 60 years in double U groove welded modified 9Cr-Mo steel with typical microstructures in the welded region of the steel taken into consideration. The typical welded region of the steel consists of solidified weld, coarse grain HAZ, fine grain HAZ, interfacial HAZ and base metal. These microstructures were modeled as polycrystal structures having slip systems approximated by rectilinearly anisotropic deformation properties.

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