Toshiyasu Sueyoshi

Detecting Overload from Strain Information during Fatigue Crack Propagation under Negative Stress Ratio

In the present study, a detection method of an overload application during stress cycles under constant amplitude was investigated. Also, the effect of the tensile overload was shown at three stress ratios: R = 0, -1, and -1.5, to understand the effects of R on crack propagation after an overload. At the baseline of R = 0, after the overload, retardation in the crack propagation was observed, and the crack growth rate decreased. However, in the case of R = -1.5, the fatigue crack growth rate actually accelerated after the tensile overload. The detection of that crack propagation behavior was attempted through the information of the strain waveform h; h = ey + 1.2λx, where ex and ey are the local strains at the specimen axis, and λ is the strain range ratio Δey/Δex. The waveform shape of h was changed after the overloading. Also, the application of the overload could be detected by the variation of the strain range ratio λ. Especially, the present method is useful for cases of the crack propagation stage under negative R conditions.

Thermal stress analysis of thin films in the context of generalised thermoelasticity

This study deals with the three-dimensional generalised thermoelasticity based on Lord and Shulmans theory and Green and Lindsays theory. The fundamental equations which include both theories, are used. The thermoelastic problems for a homogeneous and isotropic film whose surfaces are traction free and subjected to a partial heating are analysed by means of the Laplace and Fourier transforms. The state space approach is used in the transformed domains. The inversion are carried out numerically. The numerical calculations for temperature and stresses are carried out. The influence of the finite velocity of the thermal wave grows as the film thins.

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.

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”.