Due to the strain-induced martensitic transformation (SIMT), the strength, ductility and toughness of TRIP steel are enhanced. TRIP steel possesses such favorable macroscopic mechanical properties as a result of the appropriate combination between this SIMT behavior and the deformation behaviors of the austenitic and martensitic phases at a scale of crystal grain. Recently, a texture evolution by martensitic transformation is being focused on and it is possible that the control of such favorable mechanical properties of TRIP steels will be realized by the control of the texture evolution by SIMT. Therefore, not only the appropriate constitutive model at the scale of a single crystal but also a suitable model for a formation of a microstructure are necessary to predict this transformation texture evolution. Here, the constitutive equation for a single crystal TRIP steel including transformation strain on each variant is formulated based on the continuum crystal plasticity theory. Then, the deformation behavior with patterning process of martensitic phase is simulated under plane strain condition by introducing formulated constitutive equation to FEM with cellular automata approach.
Based on the well known fact for the transformation plasticity that the plastic deformation takes placee in mother phase due to the progressing new phase when applying small stress during transformation, an simple phenomenological mechanism on pearlite and martensite transformation from austenite is discussed by considering the difference in thermal expansion coefficients : Numerical analysis of both phases connected each other shows that stress in mother phase increases to the yield level under small applied stress even lower than the yield stress, which gives the background of theory of unified transformation-thermoplastic constitutive law proposed by the present author. As an application of the theory, thermoplastic behavior for a fire resistant steel is simulated for complicated variation of stress and temperature, which shows fairly good agreement with experimental data. This may motivate to provide key knowledge for the decrease in strength of steel structure due to fire sometimes occurring after earthquake.
Stress distribution and the residual stress in the solidified materials are investigated by the numerical analyses based on the phase field method. The governing equations considering the coupling effects among phase transformation, temperature and stress/strain, including elasto-plastic constitutive relation, are formulated using the phase field model, and they are numerically solved by the finite element method. As a typical case of the microstructure formation, the dendritic growth processes are studied. In spite of several assumptions employed for the material properties of liquid, the complicated stress distributions in dendrites are exhibited : Strong stresses are distributed at the bottom of the side branches. Due to the plastic behavior, residual stress distributions are obtained when the solidification and cooling are completed in the whole region. When a microstructure constructed with two dendrites is formed, high stresses are generated in the regions where the liquid is remained till the very last stage of the solidification. Subsequently, two types of the morphology of microstructures are investigated, and it reveals that the residual stress distribution and the maximum value are strongly affected by the microstructure.
Methane hydrates 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. Geotechnical survey has been conducted around Nankai trough in order to obtain background information of the Methane hydrate layers. In the present study, an elaso-viscoplastic model for soil is applied to the triaxial test results of the samples from Nankai trough. In the model, softening due to structural degradation of the soil, and kinematic hardening are considered. Simulation results shows the applicability of the model to the deep seabed soil.
To elucidate the deformation behavior of rubber blended semi-crystalline polymers, the micro- to mesoscopic structure and its mechanical behavior was modeled by using large-deformation finite element homogenization method. In this paper, the effect of direction and strain rate of tensile loading, volume fraction of rubber particle and distance between rubber particles was investigated by numerical simulations of uniaxial tension of semi-crystalline polymer with uniformly distributed rubber particles. A series of computational result clarified the followings. The stress in earlier stage of the deformation, where the stress increases linearly with the strain, is high when the volume fraction of rubber particles is small, when distance between rubber particles is small, and when the load is applied parallel to the nearest neighbor direction. That in following stage, where the stress increases non-linearly with the strain, is high when the volume fraction of rubber particles is small and when tensile strain rate is high. Furthermore, change in the local strain rate and mean stress distribution with the increase in the global tensile strain rate is closely related to the distance between rubber particles and tensile direction.
A micro indentation testing has been used to investigate the local mechanical properties of engineering materials. Since the size of indentation becomes much smaller than the average grain size of ordinary metallic materials by that testing, the results must be influenced more or less by the characteristics of each grain in which the indentation is imposed. However, the influences of crystal orientation of the grain upon the indentation results have scarcely been investigated for polycrystalline metals. In the present study, the micro indentation tests were performed on the grains of commercially pure titanium, of which the crystal orientations were evaluated beforehand by an electron back scattering diffraction technique. The relationships between the crystal orientation and the resultant indentation shapes as well as the indentation hardness values were then experimentally investigated. The results showed clear orientation dependences of the indentation shape and indentation hardness for the titanium grains. The anisotropy and the distortion of indentation profile tended to increase, while the obtained indentation hardness tended to decrease with increase in the angle between <0001> direction and the surface normal direction. The distortion of the indentation shape was found to appear due to the pile-up of materials in the limited area around the indentation by the restricted plastic flow with regard to the crystal orientation. These results are supposed to indicate the possibility to use an indentation made on grain's surface for the identification of its crystal orientation in polycrystalline titanium.
The influences of hydrogen on nano-indentation of stainless steels JIS-SUS304 and JIS-SUS316L with electrochemically-polished surfaces before and after hydrogen exposure were investigated. The “pop-in” behavior, which was presumed to correspond to the onset of catastrophic initial dislocation emissions, was obtained in the nano-indentation force-depth curves of both SUS304 and SUS316L. The force at pop-in was lower in the hydrogen-charged specimens than in the uncharged specimens of both SUS304 and SUS316L. In SUS304, the maximum penetration depth in the hydrogen-charged specimens was shallower than that in the uncharged specimens as a given maximum force, although the Vickers hardness was not changed before and after hydrogen-charging. In the hydrogen-charged SUS304, slip bands were less and pile-up on the specimen surface around the indentation was lower compared with the uncharged SUS304. This implies that slip localization by hydrogen leads to the decrease in the penetration depth. In the SUS316L, there was no pronounced difference in the maximum penetration depth and slip bands morphology between in the hydrogen-charged specimen and in the uncharged specimen. These results suggest that hydrogen facilitates dislocation motions and promotes planar slip, and that the slip planarity is more sensitive to hydrogen in SUS304 with lower austenitic stability than in SUS316L with relatively higher one.
It will be common understanding that soft material is worn off under wear test with hard material. However, there is possibility that hard material can be wear-damaged by soft material under particular circumstances. In the present study, wear damage behavior of aluminum alloy with rubber contact pad was investigated in air, water and 10% CaCl2 solution. The hard aluminum alloy A6061 was not damaged in the wear test with the soft rubber under air and water. However, the aluminum alloy A6061 was significantly worn in the wear test with rubber contact pad under 10% CaCl2 solution, while the cast aluminum alloy AC4C was not seriously worn. The significant wear in A6061 aluminum alloy would be caused by high Cu content and low Si content compared to the cast AC4C alloy. High Cu and low Si contents resulted in grain boundary corrosion under 10% CaCl2 solution and sliding contact of rubber will enhanced corrosion wear.
Bond plays a critical role in strengthening and upgrading of concrete structures using FRP bonding technology. Since the interfacial surface may be heterogeneous and bond failure is typically brittle, the size effect can be significant, making the bond parameters obtained with small specimens unsuitable to full-scale applications. This paper is to examine the size effect on the bond properties in concrete structures strengthened externally with bonded FRP sheets. A unique apparatus was developed for large-sized specimens with 350mm width. As suggested by Lu et.al., increasing a ratio between FRP sheets width (bf) and concrete width (bc) decreased Gf values. However, it was found that a function of stiffness of interface between FRP sheets and concrete could be larger than that suggested by Lu et.al. as a result of experiments. Moreover, effective transfer length of FRP sheets is not affected by the size effect. Gf of specimens anchored with U-shape FRP sheets are no larger than that of no-anchored specimens, but the ductility in the final debonding becomes larger.
In the conventional reliability-based design method of structures, the reliability of a partially damaged structure that is not satisfied with the demand performance has not been considered. In performance-based design, however, the intermediate state such as partially damaged state that is between two distinguished states (perfect functioning and complete failure) should be considered. Especially, in an emergency, it is important to know if structures that break down partially can function at the level. If a structure is considered to be MSS (Multi-State System), the reliability-based design of this structure can be performed with considering the partially damaged state. In this study, a reinforced concrete short column is treated as a MSS structural example, the three reliability indices such as the availability, the mean performance and the expected performance deficiency are calculated and compared with the reliability index evaluated by a conventional reliability evaluation method, and then some advantages of this proposed reliability evaluation method using MSS is verified.
In this paper, damage assessment for TBC sprayed combustor in a land-based gas turbine is shown based on analytical method. Firstly, TBC damage examination for 20000hours-used combustor was conducted with a visible inspection and SEM micro-observation. The visible examination indicated that TBC damages such as crack and delamination were not occured. However, SEM observation showed that a lot of micro-cracks parallel to the coating interface were distributed in the liner and duct especially. TBC damage assessment method in consideration with thermal-stress based damage such as crack initiation and ceramic coating spalling was proposed. The proposed method was applied to evaluate TBC remaining life in the 20000 hours-used combustor. It was shown that current TBC has more long life and will keep good condition in-service in spite of including micro-cracks in ceramic coating layer.
Advanced Pore Free SiC (APF-SiC) developed by newly improved reaction sintering method has excellent property such as super-high strength and damage tolerance ability by preceding failure of uniformly dispersing residual metallic silicon particles. In order to clarify effects of damage tolerance on strength property of this material, we have researched by carrying out 4-point bending tests at loading rates. As the result, at above loading rate of 0.5mm/min, it is found that the fracture strengths increase slightly with increasing loading rate. It is considered that delay fracture depended on the time from starting of slip of inclusion to fracture occurs in this material. One the other hand, it is found that the fracture strengths also increase at below loading rate of that. Generally, strength of the residual metallic silicones inviting damage tolerance decreases with decreasing loading rate since slow crack growth occurs. Then a lot of internal energy accumulated by loading is released with decreasing loading rate in this material. Therefore it is considered that resistance toward slip of inclusions increases and this behavior causes damage tolerance in this material, where can be theory explained by effects loading time. Moreover, it is suggested that unified strength estimation method proposed by Okabe et al.10) would be effectively to estimates strength property of material with damage tolerance ability.
In order to characterize piezoelectric thin films fabricated on silica substrate, the relation between displacement response and piezoelectric strain constants have been analyzed by finite element method. At first, the effect of substrate dimensions on deformation was investigated when an electric load was applied to piezoelectric thin film by disc-type electrode. And then, a displacement measuring condition to reduce size dependence was searched by using two parameters, which are the thickness of piezoelectric thin film and the radius of top electrode. Computational results indicated that piezoelectric strain constant d33 of thin film has good correlation and its value can be identified with displacement response under the appropriate measuring condition.
A newly developed high-strength reaction-sintered silicon carbide (SiC), which has two or three times higher strength than the normal sintered SiC, is one of the most promising candidates such as the lightweight substrate of optical mirror, because of fully dense, small sintering shrinkage (±1%), good shape capability and low sintering temperature. In this paper, in order to improve the performance of newly developed reaction-sintered SiC, the microstructure was investigated by paying attention to the crystal structures and the interface of each crystals using the observation of transmission electron microscope and X-ray diffraction analysis. As a result, it made clear that the finer-scale microstructure could be observed as consisting of large particles (∼1μm in diameter) of starting powder α–SiC and small particles (<1μm in diameter) of β–SiC formed during the reaction (Si+C→SiC) with the residual silicon filling the remaining porosity. Also, it was identified that the β–SiC formed during the reaction referred to the cubic (3C) polytype and the α–SiC of starting powder referred to the hexagonal (6H) polytype.