A developed crack propagation analysis system using the finite element method for components like vessels and pipings in nuclear power plants is described. One of the characteristic features of the system is that its input data include welding residual stresses and the stresses produced by applied forces in the components which have been obtained from previous analyses. In the system, the nodal forces on the crack surface are calculated from these input stresses. The authors extended the Virtual Crack Closure-Integral Method so as to calculate stress intensity factors when nodal forces on crack surface exist and applied the method to the system. In order to set test analysis problems for the system, conditions were found in which an elliptical crack holds the elliptical shape in fatigue crack propagation, and accordingly the crack size changes can be predicted using the theoretical stress intensity factors for an elliptical crack in infinite bodies subjected uniform tensile stresses. Under these conditions, a test analysis was carried out using the system, and the obtained crack size changes were shown to be in good agreement with those obtained from the theoretical stress intensity factors. As an example problem for practical structures, crack propagation due to SCC in a cylinder with a residual stress distribution solved by the system are presented and the results are compared with reliable reference values calculated using stress intensity factor data in a literature.
This paper describes an Eulerian formulation for solid-fluid interaction dynamics. A computational mesh control can be divided into Lagrangian and Eulerian solutions. Although the Lagrangian and Eulerian solution has been generally adopted for deformation of solid and flow of fluid respectively, the highly distorted Lagrangian mesh cannot retain numerical accuracy for flexible solid material. The present approach establishes one governing equation for both solid and fluid models using mixture theory assuming incompressibility in the full Eulerian framework. Hyperelasticity for solid and Newtonian fluid are employed in the constitutive equations. A discretization of the proposed formulation for solid-fluid interaction dynamics is based on an explicit finite element method. The explicit finite element method reduces computational cost, except that the finite different method instead of the finite element method is used to solve Poisson and advective equations. We test the validity of the established formulation in the two repressentative solid-fluid interaction examples including flexible solid.
Passive infrared thermography is an effective technique for detecting delamination in concrete structures. However, the method is dependent on weather conditions, when solar radiation is used. This research investigates the possibility of quantitative estimation of delamination depth in concrete by using the passive infrared thermography under solar radiation. An inverse approach is introduced for the quantitative estimation from transient temperature distribution on concrete surface under various weather conditions. In the inverse method the time series thermographs of observed surface are compared with those obtained by the FEM analyses. The inverse analysis is applied to experimental data obtained by infrared camera. It is found that delamination depth can be quantitatively estimated by using the inverse analysis.
Annealed ultrafine-grained metals contain some grains with extremely low dislocation density, so that the critical resolved shear stress increases at the first stage of deformation due to the exhaustion of dislocation sources in a grain. In this paper, in order to express the increase of critical resolved shear stress, the conventional Bailey-Hirsh's relationship is extended on the basis of physical consideration for grain boundary that plays a role of dislocation source. A triple-scale dislocation-crystal plasticity FE simulation based on the above model, geometrically necessary crystal defects and the homogenization method is carried out for annealed FCC polycrystals with different initial grain size and initial dislocation density. Yield point drop and propagation of Luders bands observed in macroscopic specimen with annealed FCC fine-grains are numerically reproduced. Moreover, macroscopic yielding of specimen and microscopic grain yielding are investigated in detail so as to clarify the initial yield behavior of annealed ultrafine-grained metals. It is also shown that plastic deformation is easy to be localized and the tensile ductility decreases as the grain size reduces.
In this paper, the shear response of three-dimensional lattice structure was investigated based on numerical stress analysis, FEM. In particular, effects of number of unit cells in three directions on the mechanical properties (shear modulus G^* and collapse strength τ^*) of lattice structures were discussed based on theoretical analysis and FEM. It is found that the mechanical properties strongly depend on the number of unit cell in three directions x, y, z, and for a flat structure (N_y=1), the deformation pattern in the structure can be classified into two types. The shear modulus G^* for a flat structure obtained by FEM can be estimated by the elementary beam theory with a good accuracy. Also, for a flat structure with slender struts, the collapse is occurred by elastic buckling, and that with relatively thicker struts, the collapse strength agrees well with the theoretical result. Moreover, the cubic structure having the same number of unit cell in x and z directions (N_x=N_z=N) shows a unit curve for the shear modulus G^*, so that the modulus can be estimated by the curve for various cubic structures.
In this paper, the collapse behaviors of thin plate with corrugated cross-section subjected to three-point bending are studied by using the finite element method. In order to estimate the energy-absorption characteristics of the beam, it is vital to understand the relation of load and displacement. It is found that the load decreases by flattening of the cross-section of the beam, and the flattening shape can be quantitatively expressed by using curvature radius of the plane of the top and bottom in the cross-section. Based on an idea that the external work is mainly expended by the flattening deformation of the cross-section, a new prediction method is proposed for estimating the relation of load and displacement. Its validity is verified by comparing with the numerical results by FEM under various conditions.
In this paper, the equivalent elastic constants for the out-of-plane deformation of three kinds of honeycombs, consisting of respectively triangle, rectangle and hexagon cells, were studied theoretically based on the equivalence of deformation energy. The out-of-plane deformation of honeycomb consists of bending deformation and twisting deformation of cell plates. For the twisting deformation of cell plates, a restraint condition must be satisfied at both ends, because each cell plate is in contact with adjacent plates at both ends. When not taking such a restriction condition into consideration, a large analysis error would arise. The validity of the analysis was checked as compared with results of numerical analysis of FEM.
Soft cellular solids like sponge and muscle have the nonlinear viscoelasticity due to the internal gas or liquid in cells and the behavior of the matrix of the solids. Their stress-time curves are different by the variation of their internal structures, and it causes difficulties in the evaluation of the mechanical characteristics of the solids. In this study, we adopted the multiplication form of power and exponent functions that is able to approximate the nonlinear and fluctuated behavior of soft cellular solids. Then, we examine the adequacy of this method by its application to the experimental results of tensile testings of SBR sponge and biological soft tissue. The results of the examination show that the adopted form of the functions have good ability to fit the complex results of the stress-time curves and it can realize good evaluation of viscoelasticity of soft cellular solid by using three-element solid model.
Although a lot of interface crack problems were previously treated, few solutions are available under arbitrary material combinations. This paper deals with a central interface crack in a bonded infinite plate and finite plate. Then, the effects of material combinations on the stress intensity factors are discussed. A useful method to calculate the stress intensity factors of an interface crack is presented with focusing on the stress at the crack tip calculated by the finite element method. For the central interface crack, it is found that the results of bonded infinite plate under remote uni-axial tension are always depending on the Dundurs' parameters α, β and different from the well-known solution of the central interface crack under internal pressure that is only depending on β. Besides, it is shown that the stress intensity factor F_1 of bonded infinite plate can be estimated from the stress of crack tip in the bonded plate when there is no crack. It is also found that dimensionless stress intensity factor F_1<1 when (α+2β)(α-2β)>0, F_1>1 when (α+2β)(α-2β)<0, and F_1=1 when (α +2β) (α-2β)=0.
The split Hopkinson pressure bar technique has widely been used for impact testing of materials in the strain rate range from 10^2 to 10^4s^<-1>. However, some problems still remain in obtaining precise stress-strain curves of a sample. In these problems, a radial inertia and a friction during the impact test affect a determination of a size in the sample. In this paper, the theory on the basis of the energy conservation, for this technique proposed in the past is examined and some modifications derived from the radial momentum conservation are applied. Then, it is shown that the inertia and friction effects are coupled to each other. A computational simulation by using the commercial FEM code ABA-QUS/Explicit ver. 6.8 is conducted to check a validity of these modifications. Simulations are performed by changing a friction coefficient and a ratio between the diameter and height of the specimen.
The contact stress distributions and permanent sets at the bearing surfaces in bolted joints under initial clamping load are analyzed using an elasto-plastic FEM calculation when hexagon bolts with flanges are used. It is found that the difference of the contact stress distributions between elastic analyses and elasto-plastic analyses is large when the bolt preload is high. The effects of the flange slope angle θ and the flange thickness t in the bolts with flanges on the contact stress distributions at the bearing surfaces are examined. It is seen that the flange slope angle θ and the flange thickness t substantially effect on the contact stress distribution and permanent sets at the bearing surface. In the case where θ is 0.25°, the thicker flange of the bolt head and the slope angle of θ=0.25° cause smaller permanent set. In addition, the equivalent length for the bolt with flange is proposed. Furthermore, in the experiments, hollow cylindrical specimens fabricated with mild steel and aluminum alloy were compressed by the bolts with flanges and the permanent sets at the bearing surfaces were measured. The permanent sets at the bearing surfaces obtained from the FEM calculations are in a fairly good agreement with the measured results. It is shown that the permanent set can be estimated by elasto-plastic FEM calculations. Discussion is made on the critical stress at the bearing surfaces. Taking into account the deformation of bolt head with flange, its equivalent length in the spring constant of the bolt is proposed.
The stop hole is one of the crack arrester systems, which is used to prevent the growth of fatigue cracks. However, the stop hole is not expected to prevent high-speed crack propagation. Because the mechanical condition of fast crack propagation behavior near the stopping hole is not cleared, the criterion of the stop for brittle fracture is not established. In this study, authors focus re-propagation start (restart) condition of crack from the stop hole. To discuss the restart condition of high-speed crack propagation, experiment observation and finite element analysis result are compared. In the numerical simulation, dynamic crack propagation simulations are carried out, using the moving finite elememt method based on Delaunay automatic triangulation. The various parameters are calculated from the numerical results.
In general, cracks are initiated from stress concentration sites: notch, hole and so on. The stress intensity factors of the cracks which are initiated from typical notches were computed numerically in the earlier studies. Then, as for the stress intensity factor of the short crack from the notch root, some simple formulae were proposed by some researchers. However, there are few data on the stress intensity factors of the cracks from the complex notches. In this paper, the stress intensity factors of the cracks from a center double U-shaped notch in a finite plate under a tensile stress were computed by the body force method. Then, the influences of the double notch and the prate edges on the stress intensity factors were examined, and the stress intensity factors were expressed with the simple formulae approximately.
The non-combustible Mg alloy is useful for structural material because of the high specific strength and the high ignition point, but the suitable welding condition was not established for manufacturing floor. In this study, a simple evaluation method for fatigue limit characteristics of welded joint was proposed, using the edge shape of welding bead geometry, the inner defect size and the material characteristics. Moreover, the welding condition ranges to ensure a stable fatigue limit for TIG butt joint of non-combustible Mg alloy were proposed by using this method.
There are several factors of hydrogen gas environment effects on strength of high strength steel SCM435 with a sharp notched specimen. In this paper, tensile tests were carried out in several hydrogen and helium gas environments. The examined factors were the gas pressure, the gas temperature, the crosshead speed and the notch root radius. The result of tensile tests in hydrogen gas environments showed the degradation of tensile strengths at any given environment factors, which were not occurred in helium gas environments. Additionally, as the result of investigating the area of intergranular fracture, it was found that the tensile strength had an inverse proportion with the area of intergranular fracture regardless of several environment factors.
High and low cycle fatigue tests were conducted for extruded magnesium alloys, AZ31 and AZ61. In the high cycle fatigue properties, AZ61 showed higher fatigue strength than AZ31, while both alloys did not show fatigue limits. In this case, the rotating bending tests revealed higher fatigue strength than uniaxial loading tests. In the low cycle fatigue properties, AZ31 and AZ61 revealed almost equal fatigue strength under constant strain amplitude tests. However, the hysteresis loops were very unique, showing anisotropy between tension and compression sides.
In order to investigate the availability of ultrasonic fatigue test for the evaluation of fatigue properties under conventional loading frequency, fatigue tests under ultrasonic frequency and rotating bending were carried out using plain specimens of an age-hardened and extruded Al alloy 7075-T6 in 7 kinds of environments of controlled humidity of 25, 50, 70 and 85%, distilled water oxygen gas and nitrogen gas. Although fatigue strength was decreased by high humidity, the decrease by high humidity was very small when the humidity was lower than about 60%〜70% and fatigue strength was largely decreased above that humidity under both tests. However, the main reason for the decrease in fatigue strength by high humidity was different between rotating bending fatigue and ultrasonic fatigue. That is, the decrease in fatigue strength was mainly the acceleration of crack growth caused by brittle fracture under rotating bending and the transition to shear mode crack accompanied with glide plane decohesion and void formation under ultrasonic loading, respectively.
In transverse fatigue of a bolted joint, althogh the real fatigue limit (the highest nominal stress at the root of the first thread of bolt without generating fatigue failure) is the same for bolts with the same size and property class, the apparent fatigue limits (the highest amplitude of transverse vibration force which can be applied to the bolted joint without generating fatigue failure) vary according to the bolt tightening conditions. Hence, it is necessary to develop a guideline for safety and design of bolted joints under transverse vibration. In this study, the relationship between the apparent fatigue limit and the real fatigue limit has been experimentally revealed. The method to predict the apparent fatigue limit using the real fatigue limit has been developed.
This paper describes fracture behavior evaluation of wall-thinned pipes by image processing strain measurement system. Regular grids with nominal size of 10×10mm were marked on the 100 A carbon steel pipes and the images taken with 6 CCD cameras of 15 million pixels were correlated to realize resolution of 0.3% strain. Strain of the cylinder outer surface was evaluated by 1) modeling the grids as a cylindrical shell, 2) measuring deformation of the grid on a projected plane, and 3) by applying updated Lagrangian method. The results indicate that the method is effective for analyzing fracture modes and fracture criterions.
Mixed mode fatigue tests are conducted using surface cracked specimen. Slant surface cracked specimens are made where crack angle is 15°, 30°, 45° and 60°. It is shown that factory roof is made at deepest point of surface crack due to ΔK_<III>, and crack growth rate decreases by the factory roof. Fatigue crack growth is simulated using S-version FEM (Finite Element Method) using crack growth criteria. It is shown that conventional crack growth criteria are not available to predict fatigue crack growth with factory roof. In this study, modified criterion for the prediction of crack growth rate is proposed. By using this criterion, fatigue crack growth simulation is conducted, and results are compared with those of experiments and discussed.
The authors have been developing a fully automated three-dimensional crack propagation analysis system. Although three-dimensional finite element analyses have become a common tool in the industries to perform design analyses, there still exist many difficulties in performing three-dimensional crack propagation analyses. That is because, although fully automatic mesh generation techniques are available for tetrahedral finite elements, hexahedral elements are commonly used in three-dimensional crack analyses. Furthermore, the analysis models tend to be large in their scales. The key components of present analysis system are the mesh generation software and virtual crack closure-integral method (VCCM) for the second-order tetrahedral finite element. VCCM is an energetic method to compute the stress intensity factors. In this paper, methodologies in automatic mesh generation for crack propagation analysis are described in detail and some numerical examples are presented.
A high-strength and high-hardness steel generally shows a duplex or a stepwise S-N curve due to change in fracture mechanism from surface-inclusion induced failure mode in high stress amplitude level to subsurface-inclusion induced one in low-stress amplitude and very high cycle fatigue (VHCF) regime. GBF area is formed around a subsurface-inclusion at crack origin and its formation mechanism was previously proposed as 'dispersive decohesion of spherical carbide' model by the authors. It is possible to control the appearance of subsurface-inclusion induced failure in VHCF regime by means of the reduction in size and number of spherical carbide particles around an inclusion according to the proposed model. New high speed steel was made experimentally by control the chemical compositions and evaluated with.cantilever-type rotating bending fatigue tests. From the experimental results, fatigue crack initiation mode changed from large carbide in surface layer in low-cycle regime to matrix crack in surface in high-cycle regime, and subsurface-inclusion induced failure never appeared in VHCF regime. This behavior could be caused by decrease in distribution of small MC-carbide particles in the matrix and restriction of the GBF area formation around a subsurface inclusion.
This work aimed to characterize crack growth of stress corrosion cracking (SCC) and corrosion fatigue (CF) for two types of martensitic stainless steel, SUS410 and SUS410S, in air and hydrogen charging environment. The crack growth characteristics were evaluated by fracture mechanics tests which were conducted under stress intensity factor, K-control, and by acoustic emission (AE) monitoring. Crack growth acceleration was observed in the hydrogen charging environment. The SCC characteristics of SUS410S were slower than those of SUS410, while the CF characteristics of both materials were similar. The observations of the fracture surface suggested that the hydrogen embrittlement cracking occurred near the surface of the specimen, where hydrogen diffused more quickly. Based on the AE and electrochemical analyses, the difference in SCC characteristics of both materials was attributed to the penetration of the hydrogen through the passive film for each material. Since the crack growth characteristics of the both materials exhibit the same relation, the passive film is broken by cyclic loading in CF tests.
Liquid impingement erosion is an important issue for the pipe wall thinning due to the ageing of power plant. In this study, liquid impingement erosion tests were carried out on the pipe steels using the test chamber specified in the ASTM G134 standard. It was found that erosion rate increases with the 6th power of impact velocity for S15C and STPA24, and the 7th power for SUS304, and the threshold velocity below which erosion rate is negligible small was found 80m/s for S15C, 90m/s for STPA24 and 120m/s for SUS304. It was clarified that the erosion mechanism by liquid impingement proceeds due to fatigue. That is, asperities appear by plastic deformation at the crystal grain boundaries and the asperities produce high stress concentrations, resulting fatigue crack initiation and material removal.
In order to specify the plasticity of a nanoscale Cu component, we develop a novel experimental method by means of a general purpose transmission electron microscopy (TEM). The characteristic shape of a designed thin specimen, where a Cu film with the thickness of 200nm is sandwiched by rigid materials (Si substrate and SiN layer), enables us to apply a load near Cu/Si interface edge by an indenter, and prevents buckling due to compressive stress. Continuous in-situ TEM images of the Cu film during deformation are successfully obtained, and the generation and expansion of local plastic region of 10nm-30nm are recognized near the Cu/Si interface edge. The yield stress of the plastic region is approximately evaluated to be 200MPa-400MPa, which is close to the one obtained by an inverse analysis under a continuum assumption (345MPa).
To understand the nature of mechanical instabilities of dislocation structures, e.g., veins and persistent slip bands (PSBs), as an origin of fatigue or plastic behavior in metals, it is essential to evaluate a critical mechanical condition where a dislocation structure collapses. In this paper, we developed an analytical method to describe the instability criterion for arbitrary dislocation structures based on the discrete dislocation dynamics (DDD) concept. According to the proposed method, the mechanical instability starts when the minimum eigenvalue of the Hessian matrix of the potential energy reaches zero. The corresponding eigenvector indicates the displacement of dislocations at the instability. We applied the method to veins and dislocation walls with the Taylor-Nabarro lattices under external loading, and it can successfully describe the onset of instability as well as their displacement mode, regardless of difference of their structure and size. This clearly indicates the validity of the proposed method. The success enables us to address mechanical instability issues on more complicated dislocation structures.
When a flaw is detected in stainless steel pipes during in-service inspection, the limit load criterion given in the codes such as JSME Rules on Fitness-for-Service for Nuclear Power Plants or ASME Boiler and Pressure Vessel Code Section XI can be applied to evaluate the integrity of the pipe. However, in these codes, the limit load criterion is only provided for pipes containing a flaw with uniform depth, although many flaws with complicated shape such as stress corrosion cracking have been actually detected in pipes. In order to evaluate the integrity of the flawed pipes for general case, a limit load estimation method has been proposed by authors considering a circumferential surface flaw with arbitrary shape. The plastic collapse bending moment and corresponding stress are obtained by dividing the surface flaw into several segmented sub-flaws. In this paper, the proposed method was verified by comparing with experimental results. Four-point bending experiments were carried out for full scale stainless steel pipes with a symmetrical or non-symmetrical circumferential flaw. Estimated failure bending moments by the proposed method were found to be in good agreement with the experimental results, and the proposed method was confirmed to be effective for evaluating bending failure of pipes with flaw.
The rheological behavior of semi-solid Al-20vol% SiC alloy, i.e., Duralcan F3A. 20S, and the mother Al alloy A356 for the comparison was studied using a self-made parallel-plate drop-forge viscometer. The duration of an experiment can be less than 5ms at shear rates in excess of 10^4s^<-1>. In a typical experiment, the viscosity decreased in the early increasing shear rate stage and subsequently the viscosity increased as the shear rate decreased. Thus, the viscosity takes a minimum around the maximum shear rate. The decrease in viscosity accompanied with the increase in the shear rate depended on both rises in the temperature and the applied force, not the duration of shear. The summarized behavior between the viscosity, μ, and the shear rate, γ, may be described by a power-law model of μ=3.2×10^7γ^<-1.5> for Duralcan F3A. 20S and μ=1.6×10^7γ^<-1.5> for Al alloy A356. The power-law index is the same for both materials while the power-law constant of Duralcan F3A. 20S is around 2 times higher than that of mother alloy depending on a distribution of 20vol% of solid SiC particles. Judging from the results, the effective operating temperature is in the range of 580℃ to 582℃.
The moldability of sub-μm structure was investigated by the injection molding at injection rates of 74, 186 and 3905mm/sec into 4 typed dies; SKD 11, WC/Co, Ni and DLC coated SKD 11. The moldability for SKD 11, WC/Co and Ni dies increased with increasing injection rate. On the contrary, the moldability for the DLC coated die, which is low polar component of the surface energy, slightly decreased with increasing injection rate. It is suggested that polar component of the surface energy influences to the moldability. Focused on the moldability for DLC with COP, which is one of non polar component materials, was significantly larges at the slow injection rate, and it is indicatied that the friction coefficient of DLC was more effected than that of the spreading coefficient to the moldability of DLC.
Diamond films have been synthesized by combustion flame using commercial acetylene-oxygen mixture gas. But, the combustion flame condition was unstable by emitting of the acetylene gas during the synthesis of diamond films. This cause was that purity of commercial acetylene gas was not stable by included impurities in dissolution acetylene. In this study, high purity acetylene gas was used, because the purity of the pure acetylene gas was stable by using high purity special dissolution acetylene. But, diamond films were not synthesized on a Mo substrate by combustion flame using high purity acetylene gas. Therefore, nitrogen gas as diamond promotion ingredient was added to high purity acetylene-oxygen mixture gas to synthesize diamond films. And, to investigate of the synthesis of the diamond film, nitrogen flow rate was changed. Here, to prevent the film delamination, a three-step synthesis method was used. The results show that the synthesized film in the case of the nitrogen flow rate 0.500cm^3/s was a good diamond film.
Particle methods such as Smoothed Particle Hydrodynamics (SPH) or Moving Particle Semiimplicit Method (MPS), etc., are powerful numerical techniques for simulating various physical phenomena, not only those large deformation problems but also nonlinear ones. On the other hand, functional graded materials (FGMs) are studied much for their excellent properties especially under severe thermal loads. In this study, the heat transfer problem is solved by SPH for FGMs in which the thermal conductivity is a function of the spatial coordinates and the temperature, both the steady state and transient cases are discussed, under various boundary conditions. Several calculations are performed to test the validity of the formulation. As a practical use, a problem of FGM cylindrical plates subjected to thermal shock is calculated, in which the thermal conductivity is temperature dependent and the heat transfer coefficient varies in the radial direction. The results are compared with those from other methods, as well as the experimental data.
Compression properties and deformation microstructure of a Mg_<85>Ni_6Y_9 (at. %) cast alloy were investigated. Yield strength of the Mg_<85>Ni_6Y_9 cast alloy was 365MPa which was higher than that of AZ91D. Kink deformation were frequently observed in the long period stacking ordered (LPSO) phase after compression test at room temperature (R.T). Also, the basal plane of the LPSO phase has tendency to orient a compression plane by compression test at R.T. It was considered that the basal texture of the LPSO phase is difficult compare with pure-Mg, due to introduce a kink formation. Although the yield strength of the Mg_<85>Ni_6Y_9 cast alloy decreased with increasing of test temperature, high yield strength above 200MPa was maintained at 573K. The LPSO phases with grain size of about 10μm were observed after compression test at 793K with strain rate of 4.2×10^<-3> S^<-1>. This result indicated that dynamic re-crystallization of the LPSO phase occurred at 793K.
Microstructure and mechanical properties of the Mg-Cu-Y alloy were investigated. Volume fraction of a long period ordered (LPO) phase was increased with the increasing of Cu and Y contents in the Mg_<100-x-y>Cu_xY_y (x=1〜3, y=2〜6 at. %) alloys. Yield strength of Mg-Cu-Y alloy tends to increase with the increasing volume fraction of the LPO phase, while elongation decreases. The Mg_<91>Cu_3Y_6 cast alloy indicated yield strength of 242MPa, and high yield strength above 200 MPa was maintained at 523K. The Mg_<91>Cu_3Y_6 cast alloy exhibited high strength over a wide temperature range compared with commercial Mg alloys (AZ91D or WE54-T6). The Mg_<91>Cu_3Y_6 annealed sheet exhibited yield strength and an elongation of 412MPa and 6%, respectively, at room temperature, and 254MPa and 24%, respectively at 523K. Specific strength of the Mg_<91>Cu_3Y_6 annealed sheet was 198kN・m/kg which is higher than that of commercial Mg alloy (AZ31) or super extra duralumin (A7075-T6).
This paper investigated the effect of high-rate deformation on the change of mechanical properties on the surface of metallic materials during a peening as a surface modification of metallic materials. Especially, we focused on the introduced compressive residual stress and the yield stress increase on the surface, which brings about the improvement of fatigue strength of the materials after peening. For this purpose, we compared the results using two different peening. One is a static indentation with a steel ball, and the other is a dynamic peening with high velocity. The residual stresses were obtained through the comparison with experimental depths using an elastic-viscoplastic finite element model for the peening, and the variation in yield stress was also obtained using indentation tests and numerical analysis of indentation tests. As a result, we clarified the effect of strain-rate on the variation in yield stress and induced residual stress. Especially, the compressive residual stress and the increments of yield stress of the surface layer within 600μm depth after dynamic peening was larger than that after static peening.
The present study proposed an inverse analysis to identify the yield stress of metals using micro-indentation tests with a spherical indenter. The present inverse analysis utilized an updating response surface, relating the mechanical properties of metals including Young's modulus, yield stress and hardening exponent to the indentation depths, which was determined by direct numerical simulations with an elastic-plastic finite element model for an indentation test. First, we selected an optimum parameter for characterizing the load-displacement curve obtained from an indentation test, confirming the convergence of the identified value of yield stress to the actual value. Moreover, we demonstrated that the present method using updating response surface is effective for identifying yield stress in comparison with conventional method utilizing the approximate function for the representative strain during an indentation loading, because the accuracy of response surface can be improved around the actual values of mechanical properties with a small number of numerical simulation data. Finally, we demonstrated that the identified yield stresses from actual indentation tests on the specimens made of stainless steels or aluminum alloys were very close to those obtained by tensile tests, even if the residual stress existed in the material.
The subloading surface model is the only elastoplasticity model which fulfills the smoothness condition and thus describes always the continuous variation of tangent stiffness modulus leading to the smooth stress-strain relation. Then, it does not require to incorporate the algorithms for the judgment of yielding, i.e. the judgment whether or not the stress reaches the yield surface and for pulling back the stress to the yield surface. It is refined so as to describe the unloading-reloading behavior pertinently in this article. Then, based on this model, the constitutive equation of metals is formulated introducing the cyclic stagnation of isotropic hardening with substantial modification based on the concept of subloading surface. The applicability of the present model to the description of real deformation behavior of metals is verified by comparisons with various test data for cyclic loadings.
With combining 3D-EBSD method and SEM images, authors have firstly observed three-dimensional shape of creep voids and geometrical relationship with grain boundaries. The method is applied to a 1Cr-1Mo-0.25V turbine rotor steel after creep rupture test (580℃,180MPa). Also, interrupted creep specimens are prepared to observe the progress of void growth. Forty sections with 0.5μm interval and 100μm×100μm area are measured by mechanical polishing in order to reconstruct three-dimensional shapes. In the result, four types of creep void are observed. One is sphere type whose radius is about 1μm. It is observed in the specimen whose creep life fraction is 25%. In the specimens with 50% and 75% creep damage, prolate and oblate spheroids whose radius is around 2.5μm are observed. Finally, connected voids are occupied in ruptured specimen. As the creep damage is progressed, not only void growth but also void nucleation is observed. Especially, on prior austenite grain boundary which is three-dimensionally perpendicular to stress, creep voids are nucleated and grown in concentrated manner. However, such nucleated small voids do not affect the volume fraction of creep void.
Triphasic theory has been proposed to couple mechanical, chemical and electrical phenomena in the analysis of hydrated charged soft tissues. As a cardiac myocyte, which consists of a solid phase and a charged fluid phase filling its interstices, is this type of tissue, the applicability of the triphasic theory to the modeling of cardiac myocyte was discussed in this research. An efficient finite element formulation for large deformation analyses was newly developed. The concentration of each ion is set as a primary variable to treat major ions, such as sodium, calcium, and potassium, separately. Physiological features of cardiac myocyte, such as activities of ion channels and sarcoplasmic reliculum, were reasonably included into. the new formulation. Even under a simple 2D modeling with limited number of parameters, the simulation qualitatively reproduced various cardiac behaviors such as excitations of cardiac cells induced by current stimuli and field stimuli.
The purpose of this study is to examine the effect of head rotational motion on brain shear strain. A head physical model constructed from individual medical images of a head was used for the experiments. The model consists of the parts reconstructing the skull, falx, cerebrospinal fluid (CSF) and brain, which represents actual human head shape. Rotational impacts were applied around frontal axis of the model under the conditions in the case that maximum angular acceleration and angular velocity were controlled. As the results of experiments, shear strain in brain part correlates strongly with peak change of angular velocity but correlates poorly with maximum angular acceleration which has been often proposed as a head injury criterion. The reason is that brain shear strain is relatively small at the time when angular acceleration reaches the peak value because relative motion of skull and brain occurs due to the structure of the head consisting brain, falx, skull and CSF. Thus, peak change of angular velocity of a head should be included in the injury criterion in the case that rotational motion of a head occurs.
Deformation induced in a shape memory polymer in its rubber state under an applied load may be fixed at a lower degree of temperature than its glass transition temperature when the load is removed. In this study, micromechanical modeling of such a shape fixity effect of the SMP is examined and its magnitude of shape fixity coefficients, which implies the ratio of the residual strain to the initial applied strain in the rubber state, is expressed as a function of the elastic moduli of the SMP. Moreover, a SMP composite material containing many non-shape fixed fillers is modeled by using the eigenstrains for constituents of the composite. It is demonstrated that its macroscopic shape fixity coefficient can be expressed in terms of these eigenstrains successfully. The magnitude of the shape fixity coefficient of the composite material decreases with increase in the aspect ratio of the constitutive sheroidal reinforcement.
This note presents about an idea including the temperature measurement technique for micro material by using photomultiplier combined in scanning electron microscope. Recently, application of micro material is extending to more wide area in engineering field, and the environment which the material is employed is also spreading from low to high temperatures. According to this situation, an importance of the temperature measurement technique for such micro material is increasing. In this study, the simple technique based on the detection of thermal electro radiated from the material surface by photomultiplier in SEM vacuum chamber is considered. Richardson-Dushman relation certifies that the current density by thermal electron increases with temperature. Application of this technique to SUS thin film heated up to 1300K showed that the proposed method was available.
The basket of transport and storage cask must have structural strength, neutron absorption ability and heat dissipation function. Borated stainless steels are suitable for application to baskets in transport and storage casks for spent fuels. In order to use this material for cask basket, it is necessary to be registered to the "Rules on Transport/Storage Packagings for Spent Nuclear Fuel (JSME S FA1-2007)" by the Japan Society of Mechanical Engineers. Therefore, various mechanical properties of B-SUS304P-1 such as tensile strength at elevated temperature, fracture toughness and allowable stress have been evaluated according to the "Rules on Transport/Storage Packagings for Spent Nuclear Fuel (JSME S FA1-2007)".