Nano-polycrystalline material that consists of many minute (nanometer order) grains shows some interesting mechanical properties;e.g.opposite Hall-Petch relationship, superplasticity, and elastic modulus softening. To clarify the origin of these properties, we study the behavior of 3-dimensional nano-polycrystalline model under uniaxial tensile loading. The model consists of 48 aluminum grains and the dynamics of total 551011 atoms are simulated by using a large scale parallel molecular dynamics (MD). First, it is shown that the elastic moduli is located on intermediate value between the overall properties estimated by Voigt and Reuss models. Then, it is shown that crystalline slips caused by dislocations emitted from grain boundaries are one of main mechanisms of its deformation. Finally, the technique of the stereographic projection which is used to investigate the rotation of each grains in usual polycrystalline material, is applied to this atomistic simulation to elucidate a mechanism of slip. As the result, it is concluded that the crystal slip and grain rotation which are often observed in usual polycrystalline material appear during the atomistic deformation and they play an important role of the mechanism of inelastic deformation in nano-polycrystalline material.

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Ingot annealing is indispensable process for GaAs single crystal to improve its electric characteristics. One of the technical problems of GaAs ingot annealing is the increase of dislocation density during its annealing process which affects the performance of electronic devices. A computer code was developed for dislocation density evaluation of a single crystal ingot during annealing process. In this computer code, the temperatures in a single crystal ingot are used as input data, which were obtained from transient heat conduction analysis of an ingot. A dislocation kinetics model called Haasen-Sumino model was used as the consitutive equation. In this model, creep strain rate is related to the dislocation density, and this model extended to the multiaxial stress state was incorporated into a finite element elastic creep analysis cede for axisymmetric bodies. Dislocation density analyses were performed using this computer code for GaAs ingots of 4- or 6-inches diameter, and time variations of dislocation density and equivalent shear stress and residual stress after the ingot annealing were obtained by this computer code.

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Recently, the crystalline plasticity based finite element technology has revealed the great progress in the field of mesomaterial mechanics to study the strength and formability of sheet metal. For this phenomenological mesomaterial modeling, the parameter identification is required by employing the experimental results of single and polycrystal material deformations. In this paper, a hardening-softening evolution equation is newly proposed and embedded in the elastic/crystalline viscoplastic constitutive equation. The uni-axial tension tests of pure aluminum single crystal under the condition of different directions between the crystal orientation [010] and the tensile axis, have provided the stress-strain relationships, slip lines and strain localization patterns. It is demonstrated that numerical results of deformation and strain localization of single crystal sheet tension show good agreements with the experimental observations, which elucidate the anisotropic effect on the deformation clearly.

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The blending with 20 to 40% volume fraction of inclusions having sizes in the range of 0.1 up to 10 μm is commonly used to control the mechanical properties of polymer. To clarify the mechanical aspect and provide quantitative picture associated with the improving mechanism of mechanical characteristics of the blended materials, the computatinal methods have been establisched with newly developed constitutive equation for glassy polymer that can reproduces the experimentally observed deformation behavior under large strain. The parametric study has been performed through series of simulation for the plane strain glassy polymer unit cell containing the different volume fraction and sized cylindical inclusions with different spacing and pattern of distribution. The results clarified the effect of these parameters on the characteristic feature of the onset and propagation of neck/shearband that essentially affect the macroscopic mechanical characteristics of the blended materials. Especially, it should be noted that the spacing of inclusions has key effect on the resulting mechanical properties of the blended materials.

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The time-dependent change of stress profiles resulting from matrix viscosity in unidirectional fibrous composites was studied using Raman spectroscopy, which enables us to determine fiber stress on the basis of the stress dependence of Raman band peak positions. Three kinds of model composites, in which single carbon fibers were embedded in three different resins, were tested by pulling the fibers at constant stresses for 500 to 1000 hours. The resins were normal epoxy, flexible epoxy and uv-curable acrylic. It was observed that a carbon fiber/normal epoxy composite exhibited negligible increase of stress transfer length with time whereas flexible epoxy and uv-curable acrylic allowed, respectively, considerable and significant changes of fiber stress profiles with time. These observations were shown to be consistent with the creep and stress relaxation test results of the matrix materials. It was however found that the time-dependent change of fiber stress profiles was much slower in the experiments than in the prediction based perfect bonding at fiber/matrix interface. It was then shown that the sliding at fiber/matrix interface, which was responsible for the slow change of fiber stress profiles, ocurred in the model composites tested.

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In this paper, a variational method is developed for analyzing the matrix creep induced change of fiber stress profiles in unidirectional composites. A functional of admissible profiles of fiber stress is first presented by supposing a broken fiber in matrix as well as a fiber pulled out from matrix. The functional is shown to have the stationary function satisfying a rate-type differential equation based on a shear lag theory. Then, the stationary function is determined approximately at every moment by assuming bilinear profiles of fiber stress and a power law of matrix creep, leading to an analytical solution for the time-dependent increase of a stress transfer length. The analytical solution is compared with finite difference computations based on the shear lag model and with an experiment performed by the present authors using a single carbon fiber/acryl model composite. Thus, the effects of matrix creep, matrix shear rigidity, and interfacial slip on the stress transfer length are discussed, and it is shown that the solution has a good agreement with the experiment if interfacial slip is taken into account.

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A creep-rupture simulation of continuous fiber-reinforced composites was proposed, based on a shear-lag model. This simulation model is composed of tension-carrying elastic fibers and shear-carrying viscoelastic matrix including the effect of stress history. Simultaneous integro-differential equations obtained from their force equilibrium were semi-discretized with respect to space coordinate and solved analytically with respect to time. The result showed that the unrecovered stress region and stress concentration region around a broked fiber were increased with time, similarly to a conventional anlaytical model. On the other hand, since the above equations are changed with each fiber breakage, it is uncertain which initial condition of the equations should be used in the simulation. To solve this problem, the break-influence superposition (BIS) method was applied to the simulation, in which independent solutions with different fiber breakage points are superposed with weight coefficients. It is proved that the BIS method can reasonably predict a complicated viscoelastic behaviour caused by initially and later broken fibers. Finally, the simulation was carried out for the composites reinforced by the fibers without time-dependent breakage to predict a possibility of creep-rupture.

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Characteristics of fatigue behavior in GFRP pipes under cyclic compressive loading was investigated at both room and cryogenic temperature. Experimental work was carried out to clarify an effect of the temperature rise of the specimen on the dynamic mechanical properties for both environmental conditions. It was found that surface temperature of the specimen rose in the room temperature tests and that the variation of the temperature was dependent on the frequency of the fatigue test. Although, there was not a large change is temperature at 77K. Variation in the stiffness and the energy dissipation caused by the static friction were investigated experimentally. And a viscoelastic-plastic model based on the Damortheology was also used to describe the hysteresis loops and separate the energy dissipation into the each of static frictional element and viscous element. The variation of the static frictional element with cyclic loading was a good agreement with the experimental data.

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The influence of strain rate on the pseudoelastic behavior associated with the martensitic transformation was investigated by tensile tests for TiNi shape memory alloy. The results were summarized as follows. The temperature rise due to the stress-induced martensitic transformation measured by a non-contact method is an order of magnitude larger than that measured through a thermocouple. The increase in the transformation stress with an increase in strain rete appears due to the temperature rise. The pseudoelastic behavior depending on strain rate is well expressed by the proposed constitutive equation considering the temperature variation. The proposed equation is useful for evaluation of pseudoelastic properties.

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A nonlinear thermomechanical constitutive model of shape memory polymer is developed by modifying a linear model. Coefficients in the model are expressed by a single exponential function of temperature in order to describe the variation in mechanical properties due to the glass transition. The proposed theory expreses well the thermomechanical properties of polyurethane-shape memory polymer, such as shape fixity, shape recovery and recovery stress. The proposed model is useful for design of shape-memory polymer elements.

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The advantage of TiNi shape memory alloy (SMA) film as an actuator material compared with SMA wire is that the heat transfer is carried out faster because the rate of the surface area to the volume is larger. For example, when the SMA film is used with electrical heating, its response speed is better due to the fast heat transfer. On the other hand, SMA film suffers inhomogeneous phase transformation, which influences the behavior of the actuator. In this paper, experiments are carried out to investigate the effect of inhomogeneous phase transformation to the function of SMA film actuator. A simple model for SMA film actuator (Sublayer Model) is proposed, taking into account the heat conduction to the transverse direction of each layer. The theoretical stress-strain curve is found to exhibit a similar tendency to the experimental result. On the basis of the theoretical model, it can be concluded that the stress and the temperature are distributed to the transverse direction of the SMA film, and that the proposed sublayer model can describe the behavior of the SMA film actuator in electrical heating.

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A multiscale modeling method based on the mathematical theory of homogenization is introduced for studying the micro-macro coupled mechanical behaviors. After describing a general nature of two-scale boundary value problem for nonlinear elasticity in terms of the generalized variational principle, the multiscale mathematical modeling for several microscopically defined mechanical behaviors are provided by using the generalized convergence theorems in the homogenization theory. With some numerical examples for each modeling, the mathematical structure of this modeling is interpreted in mechanics' points of view. That is, the macrostructure refers its material response to the corresponding microscopic mechanical behavior which defines microscopic boundary value problem. Due to the generality of this modeling method and the numerical algorithm, the micro-macro coupling effects of heterogeneous media can be analyzed to clarify the highly complex mechanisms in nonlinear or inelastic responses.

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The homogenization method is applied to evaluate the effective elastic moduli of damaged materials containing periodically distributed micro elliptical cracks. The microscopic equations of the homogenization method are solved by the superposition method. First part of this paper presents the development of the superposition method with finite element solution of an uncracked solid and the VNA analytical solution for an elliptical crack subject to arbitrary crack-face loading. The superposition method makes it possible to solve elliptical crack problems by using relatively coarse regular mesh pattern which does not describe the shapes of the elliptical cracks. Second part presents the adaption of the superposition method to the homogenization method. By using the developed method, we can evaluate not only macroscopic effective elastic moduli of the damaged materials but also microscopic behavior such as microscopic stress fields and stress intensity factors of the elliptical cracks. The developed method was used to solve the various types of damaged materials.

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The intent of this paper is to investigate mixed mode interface fracture toughness Jc. Many workers have shown by experiments that the interface fracture toughness depends on the mode mixity. however, the definition of the interface stress intensity factor is not clarified and therefore the mode mixity varies with the characteristic length L of the interface stress intensity factor. To evaluate the strength of an interface, it is important to model the fracture behavior of the interface. The cohesive force model is a direct way to represent the interface nonlinear properties. Based on the internal variable theory of thermodynamics, a continuum interface constitutive relation between interface traction and interface separation has been developed. In this paper, we apply this constitutive model to the interface crack. The interface fracture toughness is analyzed for a wide range of bimaterial constant by Finite Element Method. The results show that the fracture boundary curve is elliptical shape and changes with the ratio of the Young's modulus. The interface fracture toughness is strongly influenced by the mode mixity. However, if the characteristic length L of the interface stress intensity factor is chosen suitably, the intrinsic fracture criterion can be derived.

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A hybrid experimental-numerical investigation was used to determine the effectiveness of a tear strap in the presence of MSD. The 2024-T 3 aluminum alloy biaxial fracture specimens, 0.81 mm thick, were loaded to failure with loads simulating the hoop stress and the differential axial load generated by an unsymmetrical flap opening. MSD was simulated by s side groove along the crack path. The results showed that a previously developed mixed mode crack kinking criterion effectively predicted crack kinking, or lack thereof, of a running crack approaching the tear strap. The crack propagated through the tear strap in the presence of continuous MSD and kinked when MSD terminated at the tear strap.

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Interaction of multiple axial pre-cracks in a pressurized fuselage was investigated using a small scale model of an idealized fuselage. Stain between multiple cracks under rapidly cracks propagation was measured. The recorded crack velocities justified the use of a successive finite element analysis with large shell deformation. The experimental results of a rupturing model fuselage represented the variations in the mixed mode stress intensity factors, K_I and K_IIand the remote stress components σ_oxwith cracks extension. The crack kinking location, kinking angles and the off-axis crack trajectories are predicted by numerical analysis, and the validity of the calculated results is discussed in comparison with the experimental data.

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Inelastic deformation of 316 FR stainless steel are experimentally investigated under the cyclic nonproportional loading conditions, with a combination of cyclic push-pull and cyclic torsion, at 923 K. The magnitude of the cyclic hardening under nonproportional loading is quite larger than that observed under uniaxial push-pull or proportional loading (Additional Hardening) and it was found that this additional hardening strongly depends on the strain rates. The viscoplasticity theory based on overstress (VBO) by Krempl which was extended to the uniaxial cyclic deformation of the present material at 923 K by one of the authors is extended to reproduce these interesting behaviors observed under biaxial nonproportional loading. Good agreement is found between the simulation by the present model and the experimental results.

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In this paper the authors intend to expand the multiple strata plasticity model for describing various cyclie plastic deformations. Recently, some applications of the previous model have suggested that any plastic deformations can be described as the macroscopic behaviors caused by the activation state of the slip systems in a polycrystalline metal. An effect of back stress acting on the dislocation source on a slip plane is introduced in order to consider a reciprocating motion of slip system. A new modified model has been proposed by adopting a simple rule expressing formation and disappearance of back stress under cyclic loading. It has been clarified that each biaxial ratchetting behavior can be explained as a cyclic deformation caused by an activation state of the slip systems under nonproportional loading

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Annealed steels exhibit a sharp yield point and the subsequent abrupt yield drop followed by the yield plateau (Luders deformation) in their stress-strain curve of uniaxial tension. This paper provides some typical examples of experimental data of cyclic plasticity with special emphasis on the yield-point phenomena for an annealed mild steel, i.e., uniaxial tension at several crosshead speeds;cyclic straining;stress- and strain-controlled ratchetting. In order to describe the stress-strain response characteristics such as the yield-point phenomena, cyclic hardening/softening, the Baus-chinger effect and rate-dependent ratchetting, constitutive modelling of cyclic elasto-viscoplasticity is discussed on the premise that the sharp yield point and the subsequent abrupt yield drop result from the rapid dislocation multiplication and the stress-dependence of dislocation velocity.

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Cyclic loading tests are performed on thin-wall tubes of SUS 304 stainless steel at low temperature of liquid nitrogen (77K) under constant plastic strain range. Cyclic Stress-strain curves for constant plastic strain range are experimentally investigated under tensile or torsional cyclic loading. Saturation properties of the cyclic stress-strain curves are examined by different plastic strain ranges of cyclic loading. Stress-strain curves are saturated with increase of cyclic number. A large plastic strain range of cyclic loading results in large plastic hardening. Ratchetting behavior is also experimentally investigated with torsional cyclic loading under different constant axial stresses. The rachet deformation depends on the constant axial stress. The rachet strains by torsional cyclic loading is produced even without the constant axial stress and is not produced under a certain compressive axial stress.

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In the first part of this paper, a series of systematic experiments of 5083 Al alloy are carried out at 833 K to elucidate the characteristic features of superplastic deformation under multiaxial loading conditions. Proportional deformation tests at constant strain-rates show that the initial yield stresses obey the Prager-Drucker criterion. It is also shown that under constant strain-rate tests the material hardens under tension, while it softens under compression. Significant strain-rate dependence of the flow stresses is elucidated for tension, compression and torsion loadings. In the second part of the paper, the constitutive model of superplasticity formulated in the previous paper is improved to obtain better description of the drop of the flow stress due to cavity growth under tension loading. For this purpose, the concept of effective stress and the strain equivalence principle of continuum damage mechanics are introduced into the model, and the relevant evolution equations are modified. Comparison of the results of the proposed model with those of the corresponding experiments shows that the proposed model gives better predictions than that previously proposed by the authors.

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This paper studies the creep and creep rupture properties of 63 Sn-37 Pb solder. Creep and creep rupture tests were carried out using 63 Sn-37 Pb solder specimens at 313 K, 353 K and 398 K to develop a creep constitutive equation and to obtain rupture time. Short primary and long tertiary creep stages were found but there was no clear steady creep stage at the three temperatures. A creep constitutive equation was proposed as a function of stress and temperature. The proposed equation predicted the experimental creep strain within a factor of 1.25. A new equation for predicting creep rupture time was also proposed. The equation predicted the experimental rupture time within a factor of two.

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Damage field around an extending mode I creep crack in Oxigen Free High Conductivity (OFHC) copper is quantified by means of micrographical observation of grain-boundary cavities and by applying the quantitative stereology. Namely, by performing a series of uniaxial creep tests at 250°C for thin-walled OFHC copper tubular specimens with a pair of perforated holes, the nucleation of creep crack and the development of garin-boundary cavities around the crack-tip are first observed and photographed by the use of optical microscope at several stages of creep crack growth. Then, the distribution of the microcrack density is calculated, by applying the theory of the quantitative stereology to the results of these observations, and the damage field is identified by the density of microcracks. Finally, the resulting damage field is compared with the analytical expression discussed in the former papers of the authors.

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Elaboration of the asymptotic stress and damage fields of a mode I creep crack in steady-state growth are analyzed by employing the continuum damage mechanics and semi-inverse method. The damage evolution equation are expressed as a power function of the equivalent stress, the maximum principal stress and the hydrostatic stress. Analytical relations among the exponent p of the stress singularity of the asymptotic stress field and the exponents n, m and q of the power creep constitutive law and the power creep damage law are obtained for plane strain state. Based on the results of the analysis, the conditions for the damage evolution equation required to obtain a non-singular crack -tip stress were discussed. For Kachanov-type damage evolution law, more precise numerical results are derived for both the plane stress and plane strain states. The effects fo the shape of damage distribution on the stress singularity are also discussed.

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Cylindrical shells are utilized as structural elements of nuclear power plants, heat exchangers or pressure vessels, which are operated under elevated temperature. Creep buckling is one of the failure modes of structures at elevated temperature. In some experiments conducted by other authors, axially compressive cylindrical shells with a large ratio of radius to thickness were observed to buckle with circumferential waves. We reported that the circumferential waves occur due to bifurcation buckling. But, the critical time and the minimum loading for bifurcation buckling obtained from calculations of finite element analyses are not very good agreement with those of the experiments. One of the reasons for the disagreement is considered to be that the creep constitutive equations employed in many previous analyses represent steady creep. The creep phenomena usually have primary creep period, steady creep one and tertiary creep one. A creep strain-time relation through the three periods can be simulated by using a constitutive equation based on creep damage meachanics. In the present paper, we analyze bifurcation creep buckling of circular cylindrical shells subjected to axial compression by the use of the finite element method taking account of the creep damage machanics of Kachanov-Rabotnov.

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Continuum damage mechanics is applied to the prediction of the initiation of cracks in structral members under hot-dip galvanization. The constitutive equation based on Lemaitre's theory has been implemented in the 3-d finite element program. The critical value of the damage variable has been determined by the comparison of the finite element solution with the bending test result in molten zinc in order to consider the effect of zinc-embrittlement. Numerical studies for pylon members have been conducted to show the validity of the present computational mechanics approach.

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Plate impact tests were repeated up to three times on carbon steel. The spall damage within the target plate was evaluated with a low frequency scanning acoustic microscope as well as ultrasonic velocity, attenuation (amplitude change of B 2 to B 1 echo) and backscattering intensity. The spall damage generated by the first impact have increased in its density and sizes by the second impact of which stress is higher or lower than the first impact stress. Small cracks once generated have been extended by lower impact stress in successive impacts. The change in B- and C-scan images under repeated impact are well correlated with the change in ultrasonic velocity, attenuation and backs-cattering intensity. These methods give us an advanced means to evaluate nondestructively spall damage and to make clear the mechanisms of the spall damage growth under repeated impacts.

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Prediction methods of macroscopic and local creep behaviors of perforated plates are examined in order to apply these methods to the structural design of perforated structures such as heat exchangers used in elevated temperatures. Both primary and secondary creeps are considered for predicting macroscopic and local creep behaviors of perforated plates which are made of actual structural materials. Both uniaxial and multiaxial loading of perforated plates are taken into consideration. The concept of effective stress is applied to the prediction of macroscopic creep behaviors of perforated plates, and the predicted results are compared with the numerical results by FEM for the unit section of perforated plates under creep, in order to confirm the propriety of the proposed method. Based on the idea that stress exponents in creep equations govern the stress distribution of perforated plates, a modified Neuber's rule is used for predicting local stress and strain concentrations. The propriety of this prediction method is shown through a comparison of the prediction with the numerical results by FEM for the unit section of perforated plates under creep, and experimental results by the Moire method.

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Elevated temperature structural design codes pay attention to strain concentration at structural discontinuities due to creep and plasticity, since it causes to enlarge creep-fatigue damage of materials. One of the difficulties to predict strain concentration is its dependency on loading, constitutive equations, and relaxation time. This study investigated fundamental mechanism of strain concentration and its main factors. The results revealed that strain concentration was caused from strain redistribution between elastic and inelastic regions, which can be quantified by the characteristics of structural compliance. Characteristic of compliance is controlled by elastic region in structures and is insensitive to constitutive equations. It means that inelastic analysis is easily applied to get compliance characteristics. Paying attention to this fact, simplified inelastic analysis method was proposed based on characteristics of compliance change for prediction of strain concentration.

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