Tsuneaki Sakaki

Role of internal stress for continuous yielding of dual-phase steels

Abstract Role of internal stress for the continuous yielding of dual-phase steels is discussed. (1) The distributions of the plastic strain in the ferrite surrounding martensite islands and of the internal stress set up in the ferrite and martensite due to the martensite transformation are calculated based on the continuum mechanics. (2) There are many preferentially yielding zones around the martensite islands, and initial yielding begins simultaneously from these zones under a very low applied stress compared to the normal yield strength of the ferrite because the internal stress assists the initial yielding. Thus, the steels yield continuously. (3) The effect of the internal stress on low proof stress and aging is also discussed.

Creep rupture strength and grain-boundary sliding in austenitic 21 Cr-4Ni-9Mn steels with serrated grain boundaries

The effect of grain-boundary strengthening on the creep-rupture strength by modification of the grain-boundary configuration is studied using austenitic 21 Cr-4Ni-9Mn steel in the temperature range from 600 to 1000° C in air. Grain-boundary sliding is also examined on a steel with serrated grain boundaries during creep at 700° C. The improvement of creep-rupture strength by the strengthening of grain boundaries is observed at high temperatures above 600° C. The 1000 h rupture strength of steels with serrated grain boundaries is considerably higher than that of steels with straight grain boundaries, especially at 700 and 800° C. The strengthening by serrated grain boundaries is effective in retarding both the crack initiation and the crack propagation at 700° C, while it does not improve the life to crack initiation at 900° C. Grain-boundary sliding is considerably inhibited by the strengthening of grain boundaries at 700° C. The amount of it in steels with serrated grain boundaries is less than about one-third of that of steels with straight grain boundaries at the same creep strain. The stress dependence of grain-boundary sliding rate in the steady-state regime is also examined from the steels with these two types of grain-boundary configuration.

Plastic anisotropy of dual-phase steels

Abstract A continuum mechanics model on yielding and plastic deformation of dual-phase steels consisting of a large number of spherical second-phase islands dispersed in a matrix has been developed. The model is based on the Hills theory on orthogonal anisotropy and the mean internal stress arising from the differences in strengths and plastic anisotropies of second phase and matrix with different elastic constants. The model can describe r value of dual-phase steels from r values and flow stresses of second phase and matrix. For the dual-phase steels with high yield stress ratio and flow stress ratio between second phase and matrix, the method of correction for the flow curves has been proposed. Laws of mixture on r value and flow stress of dual-phase steels have been proposed. The law of mixture on r value is available under some restricted conditions.

The effects of orientation and thickness on the notch-tensile creep strength of single crystals of a nickel-base superalloy

The effects of crystallographic orientation and thickness of specimen on the notch-tensile creep strength of single crystals of a nickel-base superalloy UDIMET∗520 has been examined at 700°, 850°, and 900 °C. It was found that the notch-tensile creep strength of thin specimens depended on the crystallographic orientations not only in the tensile direction but also in the normal direction of the specimens, and that the creep strength was superior in the thin specimens with the [011] tensile and the [011] normal orientations or the [001] tensile and the [110] normal orientations. The thick-notched specimens exhibited great creep resistance regardless of the crystallographic orientations.

Creep deformation of ductile two-phase alloys

Abstract A continuum mechanics model is developed to explain the creep deformation of ductile two-phase alloys. The model predicts that the transient creep is caused by the internal stresses in second phase and matrix resulting from the difference in creep strain between two phases induced by the strength difference, even if the inherent transient creep in both phases is not taken into account. The difference in creep strain between two phases in steady-state creep is analytically obtained for the alloys in which both second phase and matrix exhibit the exponential law, the power-law or the hyperbolic sine law creep. The continuum mechanics model gives the same values of steady-state creep rate as the constant creep rate model by McDanels and co-workers. The results of analysis based on the continuum mechanics model are compared with the experimental results.

Plastic anisotropy of clad sheets

Abstract A continuum mechanics model dealing with yielding and plastic deformation of clad sheets with a sandwich structure has been developed. Surface layers and core of clad sheets may have independently the Hills orthogonal plastic anisotropies and different elastic constants. The model can compute r value and flow stress of clad sheets from those of surface layers and core by the step by step method. Simple laws of mixture on r value and flow stress have also been derived. The applicability of laws of mixture have been discussed.

Creep of a Hollow Sphere

Using internal stress arising from a spherically symmetric, finite plastic strain, creep of a hollow sphere subjected to inner and outer pressures, and also thermal stress, is discussed. If computer-aided numerical calculation method is used, creep is easily followed up to a finite plastic strain range including initial transient creep, whatever type of creep law is employed. If assumed in a steady state, creep rate, stress, small plastic strain leading to a stress state in steady creep, and another small plastic strain relaxing thermal stress are analytically obtained. Numerical method is also applicable to creep relaxation. Further, the origin of residual stress after unloading is clarified.

Plastic anisotropy of sheets with continuously varying anisotropic parameters and flow stress

A continuum mechanics model has been developed on the basis of Hills theory of orthogonal anisotropy for predicting global mechanical properties of sheets with a through-thickness texture gradient and strength gradient. By the present model, the globalr value and yield and flow stresses of the entire sheet can be predicted from the local anisotropic parameters, yield and flow stresses which are given as arbitrary functions of the through-thickness position of the sheet.

CREEP DEFORMATION AND FRACTURE OF DUCTILE TWO-PHASE ALLOYS

ABSTRACT A continuum mechanics model is developed to explain the creep deformation of ductile two-phase alloys. The model predicts that the transient creep is caused by the internal stresses in second phase and matrix resulting from the strength difference between two phases, if the inherent transient creep in both phases is not taken into account. The continuum mechanics model gives the same values of steady-state creep rate as the constant creep rate model proposed by McDanels and co-workers. Further, the fracture of ductile two-phase alloys is also discussed.

Internal Stress Arising From a Spherically Symmetric, Finite Plastic Strain and Yielding of a Hollow Sphere

Internal stress is obtained on the basis of the Eshelbys theory on inclusions with stress-free strain. Using the internal stress, yielding of a thick-walled hollow sphere is discussed