Misao Itoh

Macro and Microscopic Deformation Behaviors of Materials with Artificial Inclusions.

The influence of artificial inclusions on tensile properties of iron-base sintered material with artificial inclusion is sudied experimentally and numerically. Stress and strain around inclusion have been examined by FEM. The microscopic observation of the initiation and growth of voids leading to macroscopic necking and subsequent ductile fracture has been carried out. The ultimate tensile strength depends on the interparticle spacing;which is function of volume fraction and particle size of inclusion. Initiation of a void at the interface between matrix and inclusion depends on the sort and size of inclusion. The onset of numerically predicted microscopic necking, internal necking, prior to the experimentally observed macrosoopic necking relates to the distance between inclusions which is depend on both of the volume fraction and grain size of inclusion.

Cyclic Plasticity Behavior of Mild Steel with Special Emphasis on Yield-Point Phenomena.

Annealed steels exhibit a sharp yield point and the subsequent abrupt yield drop followed by the yield plateau (Luders deformation) in their stress-strain curve of uniaxial tension. This paper provides some typical examples of experimental data of cyclic plasticity with special emphasis on the yield-point phenomena for an annealed mild steel, i.e., uniaxial tension at several crosshead speeds;cyclic straining;stress- and strain-controlled ratchetting. In order to describe the stress-strain response characteristics such as the yield-point phenomena, cyclic hardening/softening, the Baus-chinger effect and rate-dependent ratchetting, constitutive modelling of cyclic elasto-viscoplasticity is discussed on the premise that the sharp yield point and the subsequent abrupt yield drop result from the rapid dislocation multiplication and the stress-dependence of dislocation velocity.

Effect of Hydrostatic Stress on Ductile-to-Brittle Transition Behavior of A Sintered Chromium

ABSTRACT The effect of hydrostatic stress on the ductile-to-brittle transition behavior of a sintered chromium has been studied at elevated temperatures under the tension and the torsion with superimposed axial compression. Both the tension and the torsion tests revealed minimums in the fracture strain at 873K. This temperature of the ductility minimums corresponded to the temperature at which dynamic strain aging appeared. The superposition of a compressive stress of more than 50 % of the yield stress resulted in a very significant increase in the fracture ductility and a decrease in the ductile-to-brittle transition temperature.

Yielding of Mild Steel after Hydrostatic Pressurization

A sharp yield point of mild steel is absent or substantially reduced in specimens subjected to high hydrostatic pressure prior to tension. The effect of high hydrostatic pressurization on the yielding behavior in the subsequent uniaxial tension is investigated by means of FEM simulation. A unit-cell model of an elastic-viscoplastic matrix with an elastic inclusion is used for the analysis. This analysis shows that the yield stress reduces markedly, only when the matrix has a sharp yield point and abrupt yield-drop characteristics, and there exists an elastic inhomogeneity between the matrix and the inclusion.