The directed growth of precipitates proceeds in their parent phases at low transformation temperatures. The growth needs the activation energy. The activation energy is usually supplied in the form of thermal energy. However, at low transformation temperatures, the thermal energy is not enough to surmount the activation barrier, and so the strain energy developed in the parent phase assists surmounting the barrier, resulting in the directed growth of precipitates. The strain energy can result from a difference in density between the nucleus and matrix and a lattice mismatch along the nucleus:matrix interface. The fundamental concept of the model is that the maximum growth rate of precipitate is along the direction that generates the maximum strain energy and minimizes the interface energy. In this paper, orientation relationships between ferrite precipitate and parent austenite, between orthorhombic cementite precipitate and parent austenite, between cementite precipitate and parent ferrite, and between hexagonal Mo₂C precipitate and parent ferrite have been discussed based on the directed growth model.

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We developed a method for evaluating the life of solder joints for an electronic package with non-solder-mask-defined (NSMD) solder pads. Generally, the thermal-cycling life of a package with NSMD pads is longer than that of a package with solder-mask-defined (SMD) pads, which are usually used in packages. However, the fracture mechanism of joints on NSMD pads has not been sufficiently investigated. Therefore, we conducted mechanical fatigue tests on the joints of NSMD pads. We found that the crack propagation behavior in NSMD joints is different from that in SMD, and that this difference in behavior causes the life span of NSMD joints to be longer than that of SMD. In addition, we found that the relationship between the strain and life in NSMD joints is different from that in SMD, but the life of a package with NSMD pads can be predicted by relating the strain in the solder bumps, analyzed using a finite element method, to life span data from mechanical tests.

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A simple generalized shear deformation theory was developed to accurately analyze the bending behavior of shear deformable beams composed of inhomogeneous materials such as functionally graded materials. The generalized theory expresses the axial displacement in terms of the transverse shear strain derived from the equilibrium equations of stress in the axial direction. To confirm the validity of the generalized shear deformation theory, we analyzed simply supported beams subjected to a transverse load. The transverse shear strain and stress given by the generalized theory matched the ones by the solutions based on the two-dimensional elasticity. The generalized theory is thus accurate enough for any inhomogeneous beam.

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The fatigue properties of the 5056 aluminum alloy, which is the highest Mg-rich alloy among the commercial Al-Mg alloys and exhibits high work hardening and strain aging, was investigated. Rotating-bending tests using smooth specimens were performed at room temperature with the stress ratio R=-1 and frequency of stress cycle f =55 Hz. The experimental results showed that a distinct fatigue limit as in the case of steel existed on the S-N curve. Surface observations using the replica method revealed a wavy pattern around the second phase particle for N=10⁶-10⁷, but no non-propagation cracks were observed. It was speculated that the observed fatigue limit appeared as a result of high work hardening rate and strain aging in the heavily deformed zone, and the fatigue cracks did not initiate from this zone up to the maximum number of stress cycle, N=3.5×10⁷, in the present study.

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The effects of sintering and heat treatment conditions on the mechanical properties of martensitic stainless steel fabricated by metal injection molding (MIM) process were investigated. The specimens were made by injecting the mixture of gas-atomized powders of 5 µm and 10 µm in mean particle diameter and a polymer binder into a metallic mold. The injection molded compacts were debound in air at various temperatures between 533 K and 593 K for 7.2 ks. They were sintered in vacuum at various temperatures between 1273 K and 1623 K for 7.2 ks. And the specimens were tempered at various temperatures between 373 K and 973 K after vacuum hardening. The density of the as-sintered compact of 5 µm powder was higher than that of the as-sintered compact of 10 µm powder. In case of the as-sintered compact of 5 µm powder, the tensile strength depended on the change in density, and the tensile strength of the compact sintered at 1373 K was 1600 MPa. On the other hand, in the case of the as-sintered compact of 10 µm powder, the tensile strength was rather lower than that of the as-sintered compact of 5 µm powder because of coarsening of the grain size. The tensile strength and elongation of the heat-treated compact of 5 µm powder were 1800 MPa and 12 %, respectively. The tensile strength and elongation of the heat-treated compact of 10 µm powder were 1680 MPa and 10 %, respectively. Thus, the mechanical properties of the compacts were approximately equal to those of the wrought material.

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An analysis of dynamic behavior is carried out in the case when dynamic load with central symmetry is applied on the inner surface of the cavity with central symmetry of a thick-walled elastic-plastic-viscoplastic sphere. Formulas are derived of the propagation speeds of the elastic-plastic-viscoplastic spherical waves. The theoretical formulas are analytically proved to be strain rate and stress rate dependent. Ordinary differential equations among physical quantities are derived along characteristic curves. The propagation theory of spherical wave based on the elastic-plastic-viscoplastic constitutive equation is shown to contain that based on the elastic-plastic constitutive equation. Calculated examples are shown on the basis of the elastic-plastic-viscoplastic theory. A comparison is done of calculated results based on the elastic-plastic-viscoplastic theory with those based on the elastic-plastic theory.

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Equivalent stiffness of clamped plates should be prescribed not only to evaluate the strength of bolted joints by the scheme of “joint diagram” but also to make structural analyses for practical structures with many bolted joints. We estimated the axial stiffness and bending stiffness of clamped plates by using Finite Element (FE) analyses while taking the contact condition on bearing surfaces and between the plates into account. The FE models were constructed for bolted joints tightened with M8, 10, 12 and 16 bolts and plate thicknesses of 3.2, 4.5, 6.0 and 9.0 mm, and the axial and bending compliances were precisely evaluated. These compliances of clamped plates were compared with those from VDI 2230 (2003) code, in which the equivalent conical compressive stress field in the plate has been assumed. The code gives larger axial stiffness for 11% and larger bending stiffness for 22%, and it cannot apply to the clamped plates with different thickness. Thus the code shall give lower bolt stress (unsafe estimation). We modified the vertical angle tangent, tanφ, of the equivalent conical by adding a term of the logarithm of thickness ratio t₁/t₂ and by fitting to the analysis results. The modified tanφ can estimate the axial compliance with the error from -1.5% to 6.8% and the bending compliance with the error from -6.5% to 10%. Furthermore, the modified tanφ can take the thickness difference into consideration.

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