Key Engineering Materials Vols. 340-341

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.