Abstract: In this study, to determine incremental, perturbed displacement fields in periodic
elastoplastic solids, an incremental homogenization problem is fully implicitly formulated, and an
algorithm is developed to solve the homogenization problem. It is shown that the homogenization
problem can be iteratively solved with quadratic convergences by successively updating strain
increments in unit cells, and that the present formulation allows versatility in the initial setting of
strain increments in contrast to previous studies. The homogenization algorithm developed is then
examined by analyzing a holed plate, with an elastoplastic micro-structure, subjected to tensile
loading. It is thus demonstrated that the convergence in iteratively solving the homogenization
problem strongly depends on the initial setting of strain increments in unit cells, and that quick
convergences can be attained if the initial setting of strain increments is appropriate.
Abstract: As one of the activities carried out by our group of IMS-VHT (Virtual Heat Treatment tool
for monitoring and optimising HT process), results of a benchmark project on the simulation of
carburized quenching process is summarized. Several programs available for taking into account the
metallo-thermo-mechanical coupling had been employed for the simulation for a cylinder, ring as
well as a helical gear by use of common data of material properties and cooling characteristics.
Comparison of the simulated values of distortion, residual stresses and profile of induced phases with
the experimental data is made with some discussions.
Abstract: The dependence of the transformation rate on the multiaxial stress state corresponding to
different loading paths is investigated. On the basis of the single shear, uniaxial tension, plane strain
and equibiaxial stretching tests, the influence of stress state on stability of retained austenite is
analyzed and an equation of transformation kinetics is developed. In sheet metal forming, the material
undergoes complicated deformation, the prediction of the volume fraction of retained austenite
during forming process is essential to estimate the contribution of the TRIP effect to improving
formability. To this aim, the volume fraction of retained austenite in the part’s different regions has
been calculated using finite element method. The calculated results were compared with the
experimental data measured in deep drawing experiments.
Abstract: We have developed a phase-field model which can simulate the growth process of
self-assembled SiGe/Si islands during deposition. The novel feature of this model is that it can
reproduce the morphological transitions of islands, i.e., from single-faceted pyramid to multifaceted
dome and from dome to barn, by taking a high anisotropy and a sixteen-fold anisotropy of surface
energy into account. Two-dimensional simulations have been performed on a large computational
model. As a result, island nucleation on the surface of a wetting layer, island morphological change
and Ostwald ripening due to an interaction between two neighbor islands were well reproduced. The
bimodal distribution of island size, which is a very important phenomenon in self-assembled quantum
dots, could also be generated. Furthermore, it has been clarified that the bimodal distributions are
largely affected by island morphological change from pyramid to dome. Furthermore, in order to
discuss the mechanism of island growth, a simulation of single-island growth has been conducted and
the variations of island size and energies have been estimated in detail. As a result, it is concluded that
the island morphological transitions occur so as to reduce the elastic strain energy.
Abstract: In this paper, the mechanical behavior of a PMMA used as the windshield of aircraft was
tested. The experiments were finished under two quasi-static strain rates and a high strain rate with
the testing temperature from 299K to 373K. The results show that the mechanical property of this
PMMA depends heavily on the testing temperature. The Young’s modulus and flow stress were
found to decrease with increasing temperature at low strain rate. At the strain rate of 10-1 1/s, strain
softening was observed under all experiment temperatures. At high strain rate, with the temperature
increasing, the flow stress decreases remarkably while the failure strain increases, and the strain
soften was also observed at the temperature above 333K. Comparing the experiments results at
same temperature, it was found the flow stress increases with the rising strain rate. The predictions
of the mechanical behavior using the ZWT theoretical model have a good agreement with
experimental results in the strain range of 8%.
Abstract: In this study, the thermal analysis FTIR Micro-Spectroscopy method was used to observe
the energy changes of the functional groups of Poly(ethylene terephthalate) (PET) during
manufacturing progress. The conformational apparent enthalpy H in the melting process has been
calculated by the van’t Hoff equation at the constant pressure and then plotted with temperature. The
structure and properties of PET/PTT (Polytrimethylene Terephthalate) blends with different blending
ratio had also been studied by using DSC and DMA analysis.
Abstract: The uniaxial tensile creep of a commercial grade Poly(methyl methacrylate) was
measured for 4000 seconds under various temperatures and stress levels ranging from 14 oC to
26 oC and 6 MPa to 32 MPa. The resultant creep compliance curves depart from each other for
stresses beyond a critical value which varies with temperature, indicating nonlinear viscoelastic
behavior. The time-temperature-stress superposition principle (TTSSP) was used to construct a
smooth master compliance curve with a much longer time-scale interval from the short-term tests at
higher stresses and temperatures. It is shown that the master curve covers a period of over 290 days,
which is nearly 3.9 decades longer than the test duration. Moreover, it is verified that the
time-temperature shift factors are dependent on stresses at which the shifts are applied, and that the
time-stress shift factors are dependent on reference temperatures.
Abstract: A viscoplastic constitutive equation based on the kinematic hardening creep theory of
Malinin-Khadjinsky and the nonlinear kinematic hardening rule of Armstrong-Frederick is
formulated to describe the inelastic behavior of high-density polyethylene under various loading. The
gentle progress of back stress by the introduction of loading surface in the viscoplastic strain space
and smaller material constant under unloading can be expressed. Material constants are identified by
various stress-strain curves under compression at constant strain rate and creep curves under
compression at constant stress. The viscoplastic model can describe stress-strain curve under
compression with change in strain rate and shear stress-strain curve including unloading. The model
can qualitatively describe stress-strain curves under compression with changed strain rate including
unloading, but it is quantitatively insufficient.
Abstract: Finite element analysis of V-bending process of polypropylene was performed using
hydrostatic-dependent elastic-plastic constitutive equations proposed by the present authors.
Kinematic and isotropic hardening rule was employed for the plastic constitutive equations. The
kinematic hardening rule was more suitable for the expression of the stress reversal in uniaxial stress
- strain relation than the isotropic hardening. For the result of the finite element analysis of V-bending,
the kinematic hardening rule was able to predict the experimental behavior of springback more
properly than the isotropic hardening. Moreover, the effects of hydrostatic pressure-dependence were
revealed by examining the calculated distribution of bending plastic strain, bending stress and the
width of the bent specimen.