The thermo-elastoplastic analysis considering the phase transformation has been widely used to predict the residual stress and distortion going with the heat treatment. In the heat treatment simulation, transformation plasticity should be treated implicitly, because which is considered to have relevance to current stress field. Furthermore, complex mechanical response caused by temperature change and phase transformation often leads to stress reversals, which possibly requires kinematic hardening models in the heat treatment simulation. However, few previous studies consider both fully-implicit formulation and kinematic hardening model in the thermo-elastoplasitic analysis including transformation plasticity. In this paper, therefore, thermo-elastoplastic constitutive relations considering the transformation plasticity and kinematic hardening are formulated with the return mapping algorithm, and heat treatment simulations of the steel cylinder are performed based on the proposed formulation. The calculated residual stresses show good agreement with the XRD measurements, and the capability of the fully implicit stress integration with the kinematic hardening model is demonstrated.
During tempering process of steel materials, a plastic strain occurs along loading direction even if the macro-scopic stress is less than yield stress. The authors name this phenomenon “tempering plasticity”. The tempering plasticity may occur by not only external force but also residual stress. It can be expected, therefore, to improve the precision of prediction of residual stress by taking account of the tempering plasticity. In this paper the plastic strain is experimentally investigated by tempering of quenched specimen under tensile or compressive stress and a mathematical model of tempering plastic strain rate is formulated by using an analogy between transformation plasticity and tempering plasticity. The newly developed formulation of tempering plasticity is implemented in commercial FE code and is applied to evaluate the residual stress after tempering in the locally quenched steel block. It is confirmed that the new model predicts the residual stress in high precision.
The influences of austenitization temperature and holding time on transformation plasticity behaviors were experimentally investigated in a three-point bending test, wherein the test specimen is simply supported from one end and fixed at the opposite end. The specimens were fabricated from S45C steel and heated to different austenitization temperatures (850, 900, and 950℃). The temperatures of the specimens in the austenite phase were kept constant for 5 or 20 min, and then naturally cooled to room temperature. During the cooling process, the specimens were subjected to the maximum bending stress (less than the yield stress) before the transformation start point (around 800℃). Under the bending stress, the transformation plasticity due to austenite to pearlite occurred. From the measured maximum transformation plasticity deflections, the transformation plasticity coefficients were determined. These coefficients were found to strongly depend on the austenitization temperature and holding time.
Sliding wear tests were conducted on Fe-20%Cr alloy single crystals having (001), (110) and (111) surfaces, using the ball-on-disk type friction machine under the condition of 10N vertical load and 30m wear distance. After the wear tests, the single crystals were cut to investigate microstructures developed below the worn surface. The cross-sectional observation revealed that subsurface regions of all the worn single crystals were composed of fine grains which were generated by severe plastic deformation introduced by the sliding wear process. It was confirmed from EBSD analyses that, below the fine-grained regions, all the single crystals rotated around the axis which is almost perpendicular to the wear direction. Such lattice rotation was expected to be a preceding event for the grain boundary formation in the fine-grained region. The lattice rotations were evaluated by the misorientation angles from the single crystal matrices. Extent of the lattice rotation depended on crystallographic orientation of the single crystals. The depth where the misorientation angle is 10° increased in ascending order of (001), (110) and (111) single crystals, suggesting that the lattice rotation was promoted favorably at the (111) single crystal. In order to understand the orientation dependence of the lattice rotation, we considered three slip deformation models (horizontal shear, vertical shear and resolved shear stress models) which incorporated the geometrical relationship between a slip system and wear direction.
In this study, mechanical properties of grain boundary are evaluated by indentation tests at grain boundaries and grains consisting of the grain boundary. We find that maximum displacement of indenter at the grain boundary is different from the average of maximum displacement of the two grains. Moreover, we perform three-dimensional crystal plasticity FEM analyses on the indentation test and we discuss factors and mechanism on mechanical properties of grain boundary. Dislocation information is introduced into a hardening law of crystal in order to investigate the effect of dislocation behavior on the grain boundary. The load-displacement curves obtained by the crystal plasticity simulation predict that the maximum displacement of indenter at the grain boundary depends on crystal orientation and dislocation behavior.
In this study, two-dimensional FE simulations for a single crystal of a Mg-based LPSO phase are performed to computationally reproduce a deformation kink in the LPSO phase using a dislocation-based crystal plasticity model for HCP crystals developed in our previous work. We take account of activities of only basal slip systems of which CRSSes are set to be consistent with the experimental results on the LPSO phase. In addition, the characteristic shape of the LPSO crystal caused by preferential growth along the a-axis is introduced into the specimen. Since we need to consider some initial imperfection to numerically express a buckling phenomenon such as the deformation kink, we assume a small inclination of the basal plane for the loading direction as the initial crystal orientation. Moreover, we apply rmin method for the slip increment to the analysis for stability of calculation. As a result of this simulation, we predict the kink band formation computationally through the accumulation of GN dislocation, disclination and the crystal lattice rotation. The validity of the present model is discussed by comparing obtained results with a well-known explanation for kink formation based on dislocation behaviors qualitatively. Then we conclude that the kink band is rapidly formed by localized basal slips.
In order to describe the mechanical response of metal foams, a phenomenological elastoplastic constitutive model is proposed based on the Cosserat continuum theory. Since metal foams involve complex structures in geometry, simple models based on the conventional scheme are of no use in the sense that the models do not capture the specific features of the internal structures. Following a series of experimental and analytical investigations on metals foams, basic equations required for a generalized continuum theory are stated, and then an elastoplastic constitutive model is formulated. The model features are as follows; simple linear elasticity models both for stress and couple stress, and a single yield function accounting for material compressibility and length-scale dependence. The constitutive model naturally approaches to the conventional one when the couple stress terms diminish, and there are only three material parameters particular to the couple stress component. Applying the mixed-type element discretization technique to the virtual work principle, we obtain the finite element equation, which is regarded as one of the simplest extensions from the conventional scheme. A bending process simulation is carried out to demonstrate the effect of couple stress on the mechanical responses.
Coarse-grained molecular dynamics simulations are conducted, for the purpose of the study on the microscopic structure change of crystalline polymer due to degradation. Interlamellar structure changes are observed during the degradation, decreasing loop and bridge chains, increasing short tail chains. From the elongation simulation of lamellar structure, craze strength is dropped during the degradation, because of decreasing oriented interlamellar chains and drawing of the molecular chains from the crystal part. Multi-scale modeling is conducted to demonstrate the relationship between microscopic structure changes and macroscopic mechanical properties. As a result, macroscopic fracture strain is reduced by the degradation, because craze growth rate becomes faster with the degradation.
Methane hydrates are ice-like materials composed of methane and water which naturally exist in the permafrost sediments and the deep seabed ground. They are presently viewed as a potential energy resource for the 21st century because an amount of methane gas is trapped in hydrate reservoirs around Nankai trough and Japan sea. Recently, the world’s first offshore production test from methane hydrates bearing sediments by the de-pressurization method have been conducted off the coasts of Atsumi and Shima peninsula in Japan. In the present study, an elasto-viscoplastic model for soil is applied to the triaxial test results of the hydrate-bearing sandy soils as well as the pure hydrate samples. In the model, softening due to structural degradation and hydrate bonding effect are taken into consideration. Simulation results show the applicability of the model to the hydrate-bearing soil as well as the pure methane hydrate.
To reduce the weight of connecting structures, welding is usually used. In the case of welding thin plates, fatigue cracks often propagate in the heat affected zone (HAZ) rather than base metals. Therefore, it is important to investigate the extension behavior of fatigue crack in HAZ of welding joint. In this study, four-point bending fatigue tests for two high strength steels, an ordinary shipbuilding steel K36A and a shipbuilding steel K36 with low carbon contents by TMCP, were carried out under the stress ratios (R) of 0.1 and 0.8. The specimen with HAZ introduced by thermal cycle simulating the temperature history of HAZ was prepared. The crack extension rate of K36A HAZ specimen is lower than that of the corresponding base metals, especially at small repeating range of stress intensity factor. For R = 0.8 tests imitating tensile residual stress, the crack extension rate much greater than that of R = 0.1.