The elasto-viscoplastic-creep constitutive equation based on the concept of continuum damage mechanics is formulated for the gray cast iron as used in diesel engine parts. It employs the viscoplastic strain given by Chaboche-Rousselier and the creep strain given by Perrin and Hayhurst. A part of the material parameters contained in the creep constitutive equation which was previously identified by Oka and Toi are assumed to be temperature-dependent in the present study. The identified model is applied to the creep fatigue life prediction of the cylinder head used in diesel engine parts which is made of gray cast iron. Although there are some points to be improved, the validity of the proposed model is illustrated by comparing the calculated results with the experimental results for the crack initiation locations and the crack propagation directions.

View full abstract

Diamond films have been synthesized by flame combustion using a mixture of high-purity acetylene and oxygen gas. To obtain good quality diamond films and to achieve good adhesion, diamond films were synthesized by stable flame combustion using the mixture of high-purity acetylene and oxygen gas with nitrogen gas added as the diamond promotion agent; nitrogen flow rate was carefully varied. In previous study, the optimal nitrogen flow rate was 0.500 cm³/s, and good quality diamond films were synthesized. In this study, to investigate effects of nitrogen addition flow rate on the bonding strength of synthesized films, the scratch test by indenter of Rockwell to synthesized delamination-free diamond films was performed, and the bonding strength was discussed. In results, the bonding strength was increased with increasing the nitrogen flow rate from the nitrogen flow rate 0.000 cm³/s, and it was concluded that diamond films synthesized by nitrogen flow rate 0.500 cm³/s had the highest bonding strength. And, the bonding strength was decreased with increasing the nitrogen flow rate from the nitrogen flow rate 0.500 cm³/s. The bonding strength of synthesized diamond films was changed by varying the nitrogen flow rate. Thus, nitrogen addition to high-purity acetylene affects bonding strength. The reason for this is that the nitrogen affects the crystallite morphology of synthesized diamond films. The nitrogen flow rate 0.500 cm³/s is good condition for the bonding strength.

View full abstract

In order to investigate the effectiveness of laser quenching for ceramic coated steels, 3 kinds of ceramic coated specimens of CrN, CrAlN and TiAlN were prepared, and the laser quenching experiments under various irradiation conditions were carried out. The influence of laser irradiation on the substrate hardness, film hardness and adhesive strength were investigated. Because of the high heat absorption of CrAlN, TiAlN and CrN compared to TiN, it was possible to quench the substrate effectively without any absorbent material for these specimens, although an absorbent was required for TiN coated specimen. The quenched area on the cross section of the substrate became larger in the order, CrN coated specimen, TiAlN coated specimen, CrAlN coated specimen. The difference of the quenched area could be explained by the difference of the heat absorption of these films. It was also possible to improve the adhesive strength of these films by laser irradiation. Although the film hardness decreased considerably by furnace quenching for ceramic coated steels, film hardness did not decrease by laser irradiation. It was concluded that the improvement of the adhesive strength and substrate hardness without the decrease of film hardness was achieved by laser quenching for CrN, CrAlN and TiAlN coated specimens.

View full abstract

In the present paper, three-dimensional Green's functions due to a point in piezo anisotropic-isotropic elastic bimaterials are derived by using Stroh's formalism and two-dimensional Fourier transforms. Green's functions for piezo-piezo anisotropic materials were derived until now, however, the functions cannot be applied to the analysis for piezo anisotropic-isotropic bimaterials, because the boundary condition at the interface of piezo-piezo anisotropic bimaterials cannot be satisfied with the boundary conditions for insulator and conductor. In the present paper, a general expression of Green's function for piezo anisotropic-isotropic bimaterials is firstly derived, and a specified Green's function for piezo transversely isotropic -isotropic materials is secondly derived in a closed form. Numerical example is given to demonstrate the validity of the present formulation of three-dimensional point-force Green's functions.

View full abstract

Finite element analyses of brain are effective for investigation of traumatic brain injury mechanism. In past analyses, brain parenchyma has been generally modeled as isotropic materials with viscoelastic properties, and damage in axon, which is one of main causes of higher brain dysfunction, has not been modeled. In this study, we developed a new constitutive model representing the mechanical properties of the brain parenchyma and the evolution equation for the functional damage of the axon. A free energy function is described as the sum of volumetric elastic, isochoric elastic and isochoric viscoelastic parts. The isochoric viscoelastic part is modeled as a three-dimensional model constructed by extending the one-dimensional generalized Maxwell model. The functional damage of the axon was defined as the reduction of the conduction function of an axial fiber and modeled to reproduce the reduction of the action potential area under tensile tests. The proposed model well reproduced the properties reported by experimental researchers: anisotropy in cerebral white matter, strain rate dependency, tension-compression asymmetry, and functional damage of axon. The proposed model has the potential for better understanding of mechanism of traumatic brain injuries.

View full abstract

A solution for thin and moderately thick anisotropic cylinders under torsion, axial compression, and combined loads is derived that includes the effects of layup anisotropy and transverse shear deformation. Partial differential equations of equilibrium and boundary conditions are obtained by using the strain-displacement relation in cylindrical coordinates with first-order shear deformation theory and the principle of virtual work. The equations are shown to coincide with Flügge's equation when neglecting transverse shear deformation and layup anisotropy. The solutions satisfy both the partial differential equations of equilibrium and the boundary conditions. In addition, a closed-form particular solution is obtained, and it shows that torsional deformation under axial loading and out-of-plane and axial deformations under torsional loading occur when there is layup anisotropy. Numerical results for a honeycomb-sandwich cylinder with carbon fiber reinforced plastic (CFRP) face sheets and a CFRP laminated monocoque cylinder show that layup anisotropy and transverse shear deformation affect the deformations and stresses of the cylinders complexly and they cannot be predicted by Donnell's shallow shell theory.

View full abstract

To evaluate effect of a relative scale of microstructure to macrostructure, a simulation procedure using second-order homogenization based finite element method was proposed. In this method, a microscopic characteristic displacement function for macroscopic strain gradient was added to the conventional first order homogenization method. Then, a procedure to solve a macroscopic boundary problem was established based on the principle of virtual work in macroscopic scale represented by the microscopic characteristic displacement function. To validate the proposed second-order homogenization method, computational simulations of deformation behavior of cavitated rubber (void) blended amorphous polymer were performed using the proposed second-order homogenization. From the result of bending deformation where tension or compression was given to upper side or lower side of the macroscopic model, the material containing larger void required a larger energy for the bending of the model. With decrease in the void size, the energy converged to that predicted by first-order homogenization method. Basically, the deformation behavior predicted by proposed homogenization model was qualitatively and quantitatively similar to that predicted by full scale model. The proposed model is expected to be applied for computational prediction of the scale-dependent deformation in various cases because the model does not limit the form of constitutive equation, shape of the unit cell and deformation mechanisms and structure of the material.

View full abstract

For the purpose of removal of ¹³⁷Cs from highly densified contaminate soil produced after contaminated soil washing process, reclamation soil techniques of contaminated soil through use of structural instability of mordenite contained in contaminated soil under high temperature heating was testified and the following results were obtained;(1)Structural instability of Cs typed typical native mordinite yielding in Tohoku area in Japan was observed and dissociation of ¹³⁷Cs from this mordenite was found after heat annealing at 1173K. (2) Remarkable dissociation rate in the early stages was observed and then was slowed down afterword. Dissociation rate of ¹³⁷Cs reached up to 65% after 1173K×1hr annealing. (3) 100% recovery of Cs from vaporized Cs gas was achieved by technology combination of electric precipitation FP filter newly developed and cold trap.

View full abstract

It has been shown that susceptibility and growth rate of stress corrosion cracking depend on the material strength such as yield strength. However, the material strength is not uniform near weld joint, where the stress corrosion cracking has been found in nuclear power plants. In this study, the digital image correlation (DIC) technique was applied for identifying the inhomogeneous material strength distribution near weld joint of a pipe. A specimen was taken from a butt welded joint and subjected to tensile test. The DIC technique enabled to measure the displacement field on the specimen surface during the tensile test. The local stress-strain curves were successfully measured and the inhomogeneous distribution of 0.2% proof strength was identified. It was confirmed that the stress triaxiality caused by inhomogeneous deformation had little influence on the identification of the proof strengths and local stress-strain curves. Furthermore, it was shown that the change in cross-sectional area of the specimen during the tensile test could be measured by the DIC technique, and then, the local stress (true stress) considering the change in the cross-sectional area was estimated.

View full abstract

The number of cycles to failure of specimens in fatigue tests can be estimated by predicting crack growth. Under a cyclic thermal stress caused by fluctuation of fluid temperature, due to the stress gradient in the thickness direction, the estimated fatigue life differs from that estimated for mechanical fatigue tests. In this paper, the influence of crack growth under cyclic thermal loading on the fatigue life was investigated. First, the thermal stress was derived by superposing analytical solutions, and then, the stress intensity factor was obtained by the weight function method. It was shown that the thermal stress depended not on the rate of the fluid temperature change but on the rise time, and the magnitude of the stress was increased as the rise time was decreased. The stress intensity factor under the cyclic thermal stress was smaller than that under the uniform stress distribution. The change in the stress intensity factor with the crack depth was almost the same regardless of the rise time. The estimated fatigue life under the cyclic thermal loading could be 1.6 times longer than that under the uniform stress distribution. The critical size for the fatigue life determination was assumed to be 3 mm for fatigue test specimens of 10 mm diameter. By evaluating the critical size by structural integrity analyses, the fatigue life was increased and the effect of the critical size on the fatigue life was more pronounced for the cyclic thermal stress.

View full abstract

High cycle fatigue tests under push-pull axial loading were carried out on SNCM439 in high vacuum and air environments to clarify the mechanism of sub-surface fatigue fractures. To reveal the effects of vacuum environment on defect size dependency of fatigue limit σ_w, specimens with different sizes of small artificial defects were tested followed by fracture surface observations. The main results obtained in this study are summarized as follows: (1)High vacuum environment increased the fatigue lives and lowered the fatigue limits compared with air environment. (2)Regardless of defect sizes, S-N curves in high vacuum did not show clear endurance limit although those in air indicated a clear knee point with a level line. (3)Defect size dependency of σ_w is considered different between high vacuum and air environments. (4)Sub-surface fractures initiating from Al₂O₃ can be simulated by vacuum fatigue tests using specimens with an artificial small defect.

View full abstract

When the defects are detected in the nuclear components in Japan, structural integrity assessment should be performed for the technical judgment on continuous service based on the Rules on Fitness-for-Service for Nuclear Power Plants of the Japan Society of Mechanical Engineers Code (JSME FFS Code). Fatigue crack growth analysis is required when the cyclic loading would be applied for the components. Recently, fatigue crack growth rate curve in air environment for Nickel-base alloys weld metal used in BWR was proposed by the authors and it was adopted as a code case of JSME FFS Code to evaluate the embedded flaw. In this study, fatigue crack growth behavior for heat-affected zone (HAZ) of Nickel-base alloys in air was investigated. And a unified fatigue crack growth rate curve in air for HAZ and weld metal of Nickel-base alloys used in BWR was evaluated. As a result, it was found that the curve for weld metal could be applied as a curve for both HAZ and weld metal since moderately conservative assessment of fatigue crack growth rate of HAZ is possible by the curve for weld metal in the Paris region. And the threshold value of stress intensity factor range (ΔK_th) is determined to 3.0 MPa√m based on the fatigue crack growth rate of HAZ.

View full abstract