Kiyohiro Miyagi

A Study of Joining of Different Melting Point Materials by Charging With Electromagnetic Energy

An electrical resistance welding method was applied under atmospheric conditions by using one of metal powder medium or media mixture which was sandwiched in the space between the two solid metal bars of specimen (i.e., solid specimen material), and was compressed longitudinally by oil pressure servo control electrodes (upper and bottom) and simultaneously current was conducted to generate Joule thermal heat. In the joining experiments, a solid aluminum specimen material was used as a basis material, and was joined to another solid aluminum specimen material or one of four other solid specimen materials with different melting points by using resistance-welding apparatus. Some fundamental data on the mechanical properties of the joint were obtained by material testing. In the experiments, the specimen used as solid specimen materials in this study were pure aluminum, copper, stainless steel, carbon steel and titanium bars of solid specimen, and the powder media were aluminum, nickel and silicon powder. Proper mixed ratios of total amount of the powder media were determined for reliable joining, and material testing was prepared for mechanical properties. The obtained data were examined with the intent of optimizing the method using metal powder media between a pair of specimen materials and were compared with that of the solid specimen material, in terms of tensile strength, Vickers hardness, bending U-shape flexure stiffness. On the tensile strength and Vickers hardness, they were found to be reliable, but on bending U-shape flexure stiffness, they were not definite enough.Copyright

A Study of Brinell Hardness Test of Porous Materials.

The Finite Element Method is applied to the analysis of Brinell hardness test of porous materials. The numerical calculation is performed using four different plastic constitutive rules such as Gursons rule, Tvergaards modification of Gursons rule, Goya-Nagaki-Sowerbys rule and a stereology-based rule that is a modification of Goya-Nagaki-Sowerbys rule. For the investigation of the validity of the rules, the numerical results are compared with experimental data for the porous materials produced by Spark Plasma Sintering method which can product porous materials of higher porosity. From the comparison it is concluded that the numerical results obtained using Tvergaards modification of Gursons rule or the stereology-based modification can well predict the experimental results. However, the numerical results deviate from the experimental data for the porous material of higher porosity such as fo=0.3. This deviation is attributed to the fact that the shape of pores in the porous material of fo=0.3 are quite different from the sphere that is a fundamental assumption in developing constitutive rules.

Mechanism of Ductile Fracture Originating in Surface Defect and Evaluation of Fracture Ductility. Case of Specimen Having a Single Hole.

Tensile fracture tests of specimens having a single blind hole were carried out in order to investigate the mechanism of ductile fracture originating in a surface defect. Although a crack was initiated at the hole edge, the true stress-true strain curve of the holed specimen approximately coincided with that of the plain specimen with no hole untill the true stress reached a maximum value. In the deformation stage of the specimen after this stress value was reached, the growth behavior of the crack was not affected by the deformation of the hole. On the basis of these phenomena, parameter fac was defined by the ratio of crack area to minimum cross section of the specimen at which the true stress reached the maximum value, and the effect of hole size on the fracture ductility was examined using the parameter fac. The ture strain ec at the point of maximum true stress showed good correlation with the parameter fac, and the shear mode growth of the crack started unstably at this point. The relation between the fracture ductility ef and the parameter fαc was predicted from the relation between ec and fac and from the growth behavior of the crack. This relation coincided with the experimental results, and it was found that the fracture ductility ef of the present experiment is determined mainly by the starting conditions of unstable fracture and the growth behavior of the crack.

Validity of Finite Element Unit Cell Model for Studying Yield Condition of Isotropic Porous Materials.

Assuming a spherical void in an infinite rigid plastic material, Gurson proposed a yield function for isotropic porous solids. It is, however, well known that the Gurson model gives harder response than those predicted by experimentation on actual porous solids. In numerical studies to check the validity of Gursons model, most of the past researchers have introduced a cubic unit cell model, in which a spherical void is placed at the center of the cube. The cubic model can be a good approximation of a porous solid, if the void volume fraction is very small. The cubic model, however, may be not appropriate for the study of porous solids with high ratio of void volume fraction since the model automatically introduces an orthotropy effect due to the geometrically repetitive distribution of voids in three orthogonal axis directions. This research will propose a new unit cell model which is appropriate for the study of the yield functions for isotropic porous materials. The model is also favorable for the study of the anisotropic effect due to the void shape because the unit cell includes less of the anisotropy based on the distribution than the cubic model does.

Comparison Between Numerical and Analytical Predictions of Shear Localization of Sheets Subjected to Biaxial Tension

The classical J2-Flow(J2F) rule results in unrealistic critical loads when applied to plastic bifurcation problems in which stress paths change their directions abruptly at critical points. This unrealistic response is due to the property of the J2F rule that the direction of plastic strain increments is independent of the direction of stress increments. Most of constitutive rules proposed in the past are given through an attempt to loosen the severe restriction on the direction of plastic strain increments in the J2F rule[1,2].

F. E. M. Analysis of Shear Localization using a New Constitutive Equation for Isotropic Porous Materials

In sheet metal forming, the failure is caused by the necking deformation called “shear localization” or by the wrinkling. It is well known that the shear localization of sheets subjected to biaxial tension is not observed in Hill’s theoretical frame work based on the use of the classical J2-Flow rule which limits the direction of plastic strain increments to the same one of corresponding total deviatoric stresses: the classical J2-Flow theory gives unrealistic critical values when it is applied to bifurcation problems where the stress increments change their directions at critical points. Several modifications have been introduced to the J2F rule in trying to make the constitutive rule applicable to the bifurcation problems. In the past researches, the modification have been discussed mainly in terms of two different constitutive properties such as “stress increment directional dependence” and “dilatational effects of voids”.

Determination of constitutive parameters by forming-limit tests

Abstract Goya and Ito have developed a constitutive expression of plastic materials introducing two essential parameters μ(α) and β(α) which denote the magnitude and directional angle of the plastic strain increment, respectively, where α denotes the directional angle of the stress increment in the deviatoric stress space. In this report, the authors investigate specific forms of μ(α) and β(α), and determine the values of material constants through the application of the linear-comparison solid theory to the localized necking problem of a rigid-plastic plate subjected to biaxial stretching. The specified forms of the transition parameters are described as follows: β(α)=(β max /α max )∗φ( Θ )α , μ(α)={ cos (α−α)} 1+0.9φ , φ( Θ )=(1−2K 0 )∗(1− Θ ) r +K 0 , where γ and K0 are new material constants, and Θ denotes the deviation of the current stress state from the uniaxial stress state.