Yuji Nakasone

A Theory of Fatigue Crack Initiation in Solids

It is proposed a theory that is based upon the concept of Gibbs free energy change from a state of dislocation dipole accumulation along a layer to a state of crack initiation. When the Gibbs free energy change is plotted against cyclic numbers of loading, it takes a maximum value at a critical value of the cyclic number which is defined as the crack initiation cycle number

Numerical equivalent inclusion method: a new computational method for analyzing stress fields in and around inclusions of various shapes

Abstract The present study has attempted to develop a new computational method for the elastic stress analysis of inclusions based on the equivalent inclusion method. The proposed method can avoid the complexity of mathematics required for the analysis of non-uniform eigenstrain distributions within inclusions having various shapes. The paper is focused on the formulation for two-dimensional case. The fundamental integral equation is shown first to have a kernel with the 1/ r -singularity. The two-dimensional equations are then discretized by using the triangle polar coordinates. It is shown that the adoption of this coordinate system can eliminate the singularity. Eigenstrain distributions within inclusions having various shapes were calculated by the present method in order to obtain stress distributions within them as well as those in the vicinity of the matrix–inclusion interfaces. The shapes of the inclusions described here are ellipse, circle, triangle and rectangle. The results obtained by the present method were compared and showed good agreements with those obtained by the theories and/or by the FEM analyses except for the sharp corner points of the triangular inclusions where the outside normal vectors can not be determined uniquely and thus the stress becomes singular.

Development of 3D CAD/FEM Analysis System for Natural Teeth and Jaw Bone Constructed from X-Ray CT Images.

A three-dimensional finite element model of the lower first premolar, with the three layers of enamel, dentin, and pulp, and the mandible, with the two layers of cortical and cancellous bones, was directly constructed from noninvasively acquired CT images. This model was used to develop a system to analyze the stresses on the teeth and supporting bone structure during occlusion based on the finite element method and to examine the possibility of mechanical simulation.

Corrosion behavior of Ni-base superalloys at 1373 K in simulated HTGR impure helium gas environment

Abstract The present paper describes the corrosion behavior of recently developed Ni-base superalloys: an oxide dispersion strengthened (ODS) alloy, two kinds of Ni-Cr-W alloys and Hastelloy XR at 1373 K in a simulated HTGR impure helium gas. Corrosion tests were carried out using a transparent quartz retort at 1373 K for up to 3.6 Ms in circulating helium gas which contained a small amount of H2, CH4, CO, CO2 and H2O. The mass change versus exposure time relation revealed that the ODS alloy exhibited excellent oxidation resistance. One Ni-Cr-W alloy (113MA) and Hastelloy XR showed exfoliation of the oxide films after the tests. All the alloys were decarburized from the early exposure times. These results are discussed on the basis of phase stability diagrams for constituent elements in the alloys.

Effects of Gage Diameter and Strain Rate on Tensile Deformation Behavior of 32Mn-7Cr Steel at 4 K

The cryogenic deformation behavior under uniaxial tensile stress was investigated for 32Mn-7Cr high manganese steel using a diametral extensometer. The tensile deformation and serration behavior were observed for varying gage diameters and strain rates (< 5 × 10-4s-1). A significant effect of gage diameter and strain rate on strengths and total elongation was not seen at 4 K. As the gage diameter became smaller or the strain rate lower, the strain to initiate serration decreased. The geometrical changes in the specimen during serrated yielding are also discussed in relation to the internal specimen heating.

Characteristics of the Tooth in the Initial Movement: The Influence of the Restraint Site to the Periodontal Ligament and the Alveolar Bone

It is critical to clarify orthodontic load transfer mechanism from tooth to alveolar bone, and to determine the influence of applied orthodontic force on tooth behaviour. In this study, two dimensional (2-D) finite element (FE) models were constructed to simulate to mechanical behaviour observed during the initial movement of periodontal ligament (PDL) deformation, and to evaluate the effects of the presence of PDL and various restraint sites on tooth behaviour. A 2-D solid FE model of the tooth-PDL-alveolar bone system was constructed and investigated into stress distribution pattern and displacement. The first analysis was carried out with combinations of FE model with and without PDL. The second analysis was compared with three different sites restraint of alveolar bone. By incorporating PDL in FE models, excessively large stress values and deformation generated in a tooth and alveolar bone were relieved. Since restraint conditions did not affect a tooth and PDL, but had an effect on alveolar bone, orthodontic force necessary for tooth displacement was transmitted correctly. The results of this study revealed that inclusion of PDL in FE models is indispensable to transmit orthodontic force appropriately when investigating tooth behaviour for orthodontic treatment. Restrained sites affected stress distribution in alveolar bone.

Applicability of creep damage rules to a nickel-base heat-resistant alloy Hastelloy XR

Abstract A series of constant load and temperature creep rupture tests and varying load and/or temperature creep rupture tests was carried out on a nickel-base heat-resistant alloy Hastelloy XR, which was developed for applications in the High-Temperature Engineering Test Reactor, at temperatures ranging from 850 to 1000°C in order to examine the applicability of the conventional creep damage rules, i.e., the life fraction, the strain fraction and their mixed rules. The life fraction rule showed the best applicability of these three criteria. The good applicability of the rule was considered to result from the fact that the creep strength of Hastelloy XR was not strongly affected by the change of the chemical composition and/or the microstructure during exposure to the high-temperature simulated HTGR helium environment. In conclusion the life fraction rule is applicable in engineering design of high-temperature components made of Hastelloy XR.

Effect of carbon content and helium gas environment on creep crack growth properties of Ni-26 pct Cr-17 pct W-0.5 pct Mo alloy at 1273 K

Creep crack growth tests were conducted on Ni-26 pct Cr-17 pct W-0.5 pct Mo alloys with different carbon contents in air and in helium gas environment at 1273 K using the compact-type (CT) specimen, and the effects of carbon content and environment on creep crack growth rate are discussed. Creep crack growth rateda/dt is evaluated by theC* parameter. Theda/dt is faster in higher-carbon alloys than in lower-carbon alloys in each environment. This effect of carbon content is attributed to the lower creep ductility due to the increase of fine trans-granular carbides in higher-carbon alloys. The environmental effect on theda/dt vs C* relations is scarcely observed in higher-carbon alloys. In the 0.003 pct C alloy, however,da/dt is much lower in the He gas environment than in air. Carburization is observed ahead of the crack tip in the He gas environment at 1273 K. The intergranular carbides precipitated due to carburi-zation have a granular configuration and are considered to prevent the grain boundary sliding in lower-carbon alloys.

Creep Properties under Varying Stress/Temperature Conditions on Nickel-Base Heat-Resistant Alloy Hastelloy XR

Abstract A series of constant load & temperature creep tests and varying load and/or temperature creep tests was carried out on a nickel-base heat-resistant alloy Hastelloy XR, which was developed for applications in the High-Temperature Engineering Test Reactor, at temperatures ranging from 850 to 1,000°C in order to examine the creep behaviors of the alloy under varying load and/or temperature conditions. The minimum creep rate was not affected by the prior exposure to the load and temperature environment. The life fraction rule showed a good applicability among various prediction rules. Above-mentioned two items, i.e. no change of the minimum creep rate and the good applicability of the life fraction rule were considered to show that the change of the chemical composition and/or the microstructure during exposure to the high-temperature simulated HTGR helium gas environment was not appreciable to affect the creep strength of Hastelloy XR.

Weldability and mechanical properties of age-hardened Fe-Ni-Cr-Mn-Ti austenitic alloy for cryogenic use

The electron beam weldability and the low temperature mechanical and magnetic properties of Mn-modified iron-base superalloys have been investigated. Mn addition to an Fe-30Ni-13Cr-2.4Ti alloy suppressed the occurrence of ferromagnetism at 4.2K and did not cause any deleterious effects on the mechanical properties of the alloy and its weldability, i.e., the alloys containing from 3 to 9 mass% Mn were welded without any fusion zone hot cracking and HAZ micro-fissuring. The post-weld heat-treatment which consists of solutionizing followed by aging was very effective to diminish the strength mismatch between base metal and weld metal regions. The absorbed energy of impact specimens of weldments for both the as-welded and heat-treated conditions showed higher values than those of the base materials.