Constant Stress Conditions Research Articles

Osmosis-induced swelling of Eurobitum bituminized radioactive waste in constant total stress conditions

In geological disposal conditions, contact of Eurobitum bituminized radioactive waste, which contains high amounts of the hygroscopic and highly soluble NaNO 3, with groundwater will result in water uptake and swelling of the waste, and in subsequent leaching of the embedded NaNO 3 and radionuclides. The swelling of and the NaNO 3 leaching from non-radioactive Eurobitum samples, comprised between two stainless steel filters and in contact with 0.1 M KOH, was studied in restricted (semi-confined) swelling conditions, i.e. under a constant total stress, or counterpressure, of 2.2, 3.3, or 4.4 MPa ( i.e. oedometer conditions). Four tests were stopped after hydration times between 800 and 1500 days, and the samples were analyzed by micro-focus X-ray Computer Tomography (μCT) and by Environmental Scanning Electron Microscopy (ESEM). The complete set of data enabled a consistent interpretation of the observations and lead to an improved understanding of the phenomenology of the water uptake, swelling, and NaNO 3 leaching in restricted swelling conditions. Under the studied conditions, the bituminous matrix surrounding the NaNO 3 crystals and pores with NaNO 3 solution behaved as a highly efficient semi-permeable membrane, i.e. osmotic processes occurred. In the main part of the leached layers, a high average NaNO 3 concentration and related to this a high osmotic pressure prevailed, explaining why in the studied range the swelling was not measurably affected by the counterpressure. At the interface with the stainless steel filters, a low permeable re-compressed bitumen layer was formed, contributing to the slow release of NaNO 3 compared to the water uptake rate. A fully coupled Chemo-Hydro-Mechanical (CHM) constitutive model has been developed that integrates the key processes involved and that reproduces satisfactorily the results; this is presented in another work. Combination of the experimental and the modelling study allow to conclude that under semi-confined conditions the swelling of the bituminized waste, and its evolution with time, is the result of several transient processes (salts dissolution, diffusion of salts and water, advection, creep, involving a low permeability material with evolving thickness and properties) that moreover are non-linear and strongly coupled.

Stimuli‐Sensitive Polymers

Stimuli‐Sensitive Polymers

Biaxial Ratcheting Response of SS 316 Steel

Ratcheting is one of the challenging phenomena that needs to be investigated for the Fast breeder reactor (FBRs), to arrive at the optimum structural dimensions that are safe and yet do not have undue redundancy. Austenitic stainless steel is the principal structural material for Indian FBR. Preliminary assessment indicates that there is a need to demonstrate that the main load carrying vessel made of this material can provide sufficient safety margin against ratcheting under biaxial loading conditions. This exercise calls for carrying out many simulated experiments, particularly with biaxial tension torsion specimens to generate adequate data for developing robust constitutive models to predict ratcheting. Accordingly, many biaxial tension-torsion experiments for austenitic stainless steel pipes were conducted and the best results have been reported here. The mechanical behavior of this material has been reported for a given axial tensile stress superimposed with a given range of cyclic shear stress for many cycles of loading. Rectangular rosette is used for capturing the biaxial response. Important material responses like cyclic hardening and biaxial ratcheting have been experimentally observed. Maximum accumulation of 2700 μ axial strain has been observed for a loading condition of constant axial stress of 102 MPa super imposed with a cyclic variation of shear stress amplitude of 120 MPa over 2450 cycles. The amount of progressive accumulation of axial strain was found to be directly dependent on the number of cycles. The observed rate of axial strain accumulation found decreased with increase in number of cycles. All these results are presented in detail in this paper and important conclusions that are useful in modeling the observed behavior are discussed.

Creep, strength and other characteristics of Hawaiian volcanic soils

Strength, creep and other properties of four volcanic soils from Hawaii were investigated under reconstituted triaxial conditions. The selected soils are of different geological origin and have contrasting index properties. Results from triaxial tests suggest that as clayey volcanic soils weather toward a siltier texture, their response to loading changes accordingly, with dilation at larger strains leading to an increase in strength. The frictional strength of two of these soils, a high-plasticity clay from Manoa Valley in Honolulu and a young ash soil from the Hilo area, does not correlate well with conventional empirical relations. In terms of creep behaviour, the response under constant stress conditions can be described, at least to a first order, with a simple rate process equation. A constant that is a measure of strain rate attenuation with time is observed to be stress level dependent at lower deviatoric stresses, but relatively constant at higher stress levels. This constant may be related to plasticity index, although such a correspondence is tenuous at best. Strength gain is observed in post-creep undrained tests, and is attributed in part to prior volume changes and in part to ageing. In terms of changes to the short-term failure envelope, the effect of a sustained creep phase of 100 days or longer is to shift the failure line upward, albeit at a constant angle.

An Experimental Study of the Effect of Temperature and Stress on the Creep of Rocks, with a Discussion of Earth Tide Damping, Isostasy and Mantle Convection

Summary This paper is based in part on a paper by the author and A. K. Misra and published in Geophys. J. R. astr. Soc. (1965), 9, 509–535, where the experimental and theoretical results are fully described. Measurements have been made of the creep of a number of different rocks at temperatures up to 750 °C under conditions of constant compressive stress. The results show that at temperatures below about 0.2 Tm (where Tm is the absolute temperature of melting) the creep strain is proportional to the logarithm of the time under load, and is approximately proportional to the stress and to the absolute temperature. At higher temperatures the creep rate falls offless rapidly with time, and the creep strain is proportional to a fractional power of the time, with the exponent increasing as the temperature increases and reaching a value of 3/1 at temperatures of about 0.5Tm. At these temperatures the creep increases approximately as the square of the stress and possibly exponentially and it increases with temperature (T) as exp(- U/kT), where U is an activation energy and k is Boltzmann's constant. These results are strikingly similar to those obtained in measurements on metals, and it is thought that they can be explained in an exactly similar way in terms of competing processes of strain hardening and thermally assisted processes of plastic deformation and recovery from strain hardening. The results suggest that creep is unlikely to be of importance in the upper part of the Earth's crust but that it will be important in the lower crust and upper mantle, particularly in connection with the damping of Earth tides and with the phenomenon of isostasy. An estimate is made of the rate of steady state creep of peridotite, which is found to be within the lower limit postulated in convection theories of continental drift. Although hydrostatic pressure is known to decrease creep rates (through its influence on thermal recovery) its exact influence on the creep of rocks is still a major unknown factor.

High Temperature Creep Behavior in Al-Mg(Zn)-Fe Alloys

Creep tests were conducted under a condition of constant stress on two aluminum-based alloys containing particles: Al-5% Mg-0.25% Fe and Al-5% Zn-0.22% Fe. The role of grain boundary sliding was examined in the plane of the surface using a square grid printed on the surface by carbon deposition and perpendicular to the surface using two-beam interferometry. Estimates of the contribution of grain boundary sliding to the total strain, <TEX>$\varepsilon_{gbs}/\varepsilon_t$</TEX> reveal two trends; (i) the sliding contribution is consistently higher in the Al-Mg-Fe alloy, and (ii) the sliding contribution is essentially independent of strain in the Al-Mg-Fe alloy, but it shows a significant decrease with increasing strain in the Al-Zn-Fe alloy. Sliding is inhibited by the presence of particles and its contributions to the total strain are low. This inhibition is attributed to the interaction between the grain boundary dislocations responsible for sliding and particles in the boundaries.

Stress exponent and threshold stress of in situ TiCp/LD7Al matrix composite in creep

The elevated temperature creep behavior of in situ 20vol%TiCP/LD7Al matrix composite and LD7Al alloy was investigated under the condition of constant compressive stress at 523 K，573 K and 623 K，respectively. The stress exponents of the composite material and the matrix aluminium alloy were obtained by fitting the creep rate and the applied stress in log-log plot，respectively. By introducing an effective stress （σ-σ0） to the power equation and extroplating the linearly-regressed experemental data to zero creep rate，the threshold stresses were obtained. The results showed that both the stress exponents and the threshold stresses of the composite were higher than those of LD7Al，indicating that the presence of TiC particles significantly improved the elevated temperature creep property of LD7Al. By introducing a threshold stress，the high-temperature creep behavior of the composite can be explained by the cooperative effect of the dislocation-climbing and the particle hindrance to the matrix deformation.

Stochastic Analysis of Microscopic Stress in Fiber Reinforced Composites Considering Uncertainty in a Microscopic Elastic Property

This paper describes stochastic analysis of microscopic stresses in a fiber reinforced composite material caused by a random variation of an elastic property of a component material. The microscopic stress takes an influence of not only the microscopic random variation but also a macroscopic condition such as constant stress or strain. From this reason, the influence of the microscopic random variation on the microscopic stresses must be analyzed with using a multiscale analysis method. In order to investigate the stochastic characteristics of the stresses, the homogenization method and the Monte-Carlo simulation method are employed in this study. With numerical results, the stochastic characteristics of the microscopic stress caused by a random variation of an elastic property in component materials of a unidirectional fiber reinforced component material under constant macroscopic stress or strain condition are discussed.

Correlated transformation of the microrelief of the stressed surface of an amorphous alloy

Samples of Fe70Cr15B15 and Fe77Ni1Si9B13 amorphous alloy foils are loaded at tensile stresses of 0.2–1.2 GPa under constant strain or constant stress conditions. The submicroscopic profiles of the surface are recorded on a scanning tunneling profilometer. The “instantaneous” or extended (in time) transformation of the surface relief is statistically described by calculating the autocorrelation functions of the change in the relief height along two-dimensional scanning paths parallel to the loading axis. The autocorrelation functions thus constructed allow one to estimate the correlation radii of the surface profile, i.e., typical scales of relief components (holes, protuberances, flat regions). The performed analysis has revealed a considerable spatial correlation (∼2 × 103 nm) between the relief components in both initial (unloaded) foils. The application of the tensile load leads to an immediate increase in the roughness of the relief and a sharp decrease in the correlation radius. Subsequently, the relief does not remain stable and changes as a result of competing processes, such as microcrack growth, leveling of bends, and nucleation of kink bands. The behavior of the autocorrelation functions adequately reflects the transformation of the surface profile.

Deformation of lower-mantle ferropericlase (Mg,Fe)O across the electronic spin transition

Recent high-pressure studies have shown that an electronic spin transition of iron in ferropericlase, an expected major phase of Earth's lower mantle, results in changes in its properties, including density, incompress- ibility, radiative thermal conductivity, electrical conduc- tivity, and sound velocities. To understand the rheology of ferropericlase across the spin transition, we have used in situ radial X-ray diffraction techniques to examine ferro- periclase, (Mg0.83,Fe0.17)O, deformed non-hydrostatically in a diamond cell up to 81 GPa at room temperature. Compared with recent quasi-hydrostatic studies, the range of the spin transition is shifted by approximately 20 GPa as a result of the presence of large differential stress in the sample. We also observed a reduction in incompressibility and in the unit cell volume of 3% across the spin transition. Our radial X-ray diffraction results show that the {0 0 1} texture is the dominant lattice preferred orientation in ferropericlase across the spin transition and in the low-spin state. Viscoplastic self-consistent polycrystal plasticity simulations suggest that this preferred orientation pattern is produced by {1 1 0}\1-10( slip. Analyzing our radial X-ray diffraction patterns using lattice strain theory, we evaluated the lattice d-spacings of ferropericlase and Mo as a function of the w angle between the compression direc- tion and the diffracting plane normal. These analyses give the ratio between the uniaxial stress component (t) and the shear modulus (G) under constant stress condition, which represents a proxy for the supported differential stress and elastic strength. This ratio in the mixed-spin and low-spin states is lower than what is expected from previous studies of high-spin ferropericlase, indicating that the spin transi- tion results in a reduced differential stress and elastic strength along with the volume reduction. The influence of the spin transition on the differential stress and strength of ferropericlase is expected to be less dominant across the wide spin transition zone at high pressure-temperature conditions relevant to the lower mantle.

Correlation of Charge Buildup and Stress-Induced Leakage Current in Cerium Oxide Films Grown on Ge (100) Substrates

High-kappa films are currently deposited on Ge substrates to compensate the mobility loss, as Ge offers higher mobility compared with that of silicon. This paper deals with the reliability characteristics of cerium oxide films grown by molecular beam deposition on n-type Ge (100) substrates. MOS capacitors with Pt gate electrodes were subjected to constant voltage stress conditions at accumulation. The correlation of the charge-trapping characteristics and the stress-induced leakage current (SILC) to the applied field is observed and analyzed. The results suggest that one major problem for the potential use of rare earth oxides in future MOS technology is the existence of relaxation effects. The cross-sectional value of the bulk oxide traps is on the order of 10-18 cm2 , thus indicating neutral defects. Direct comparison to reported results on high- kappa/Si and SiO2/Si structures shows that SILC properties are related to the quality of the dielectric layers; the semiconductor substrate is immaterial.

Indium oxides by reactive ion beam assisted evaporation: From material study to device application

Indium oxides were deposited by reactive ion beam assisted e-beam evaporation at room temperature. A material study was conducted through a variety of material characterization including crystal structure, electrical properties, optical properties, and chemical composition, along with an investigation of material properties as a function of primary deposition parameters such as ion flux and deposition rate. Implementing the developed semiconducting indium oxide as a channel material, the authors further demonstrated high-performance indium oxide thin-film transistors (TFTs) with conventional silicon dioxide gate dielectric derived by plasma-enhanced chemical vapor deposition (PECVD). The n-channel TFT has a threshold voltage of ∼2.0 V, a field-effect mobility of 33 cm2/V s at a gate bias of 20 V, an ON/OFF current ratio of 108, and a subthreshold slope of 2.0 V/decade. The stability study displays a small threshold voltage shift of ∼0.6 V under a 60 h constant current stress condition. The TFT reported here has one of the best performance characteristics in terms of field-effect mobility, ON/OFF current ratio, OFF current and device stability, using conventional and large-area foundry-compatible PECVD gate dielectrics. The device performance coupled with PECVD dielectrics makes ion beam assisted e-beam evaporation derived indium oxide TFT a promising candidate for active matrix flat-panel displays.

OS0418 繊維強化複合材料の微視的な弾性定数のばらつきに対するミクロ応力の確率応答解析(先端材料システムの力学とメゾスケールモデリング,オーガナイズドセッション)

This paper describes a numerical result of stochastic analysis on a random variation of microscopic stress caused by a random variation in a material property of component materials. The stochastic properties such as the expectation, variance or coefficient of variance are computed with the Monte-Carlo simulation. The microscopic stress is analyzed with using the homogenization theory based finite element analysis. As an example, a microscopic stress in a unidirectional fiber reinforced composite under a uni-axial macroscopic strain is taken into account. The macroscopic strain may change caused by a random variation in a material property of a component material, the stochastic characteristic of the microscopic stress under both constant strain and stress conditions are investigated. From numerical results, it is confirmed that the stochastic characteristics of the microscopic stress depend on both a kind of microscopic random variation and macroscopic condition on the strain field.

Study of stress-induced leakage current (SILC) in HfO2/Dy2O3 high-κ gate stacks on germanium

In the present work we study reliability issues of Pt/HfO2/Dy2O3/n-Ge MOS structures under various stress conditions. The electrical characteristics of the micro-capacitors are very good probably due to the presence of a rare earth oxide as interfacial layer. It is shown that the injected charge (Qinj) at high constant voltage stress (CVS) conditions induces stress-induced leakage current (SILC) that obeys a power-law. We also observe a correlation between the trapped oxide charge and SILC, which is, at low stress field, charge build-up and no SILC, while at high stress field SILC but few trapped charges. Results show that the present bilayer oxides combination can lead to Ge based MOS devices that show acceptable degradation of electrical properties of MOS structures and improved reliability characteristics.

Creep behavior of in situ TiC P/2618 aluminum matrix composite

The creep behavior of 2618 aluminum alloy and in situ TiC P/2618 aluminum composites (with 15% and 20% volume fraction of TiC particles, respectively) was investigated under a condition of constant compressive stress at 523, 573 and 623 K, respectively. The results showed that both the stress exponent and the apparent activation energy of the composite were higher than these of 2618 aluminum alloy, indicating that the existence of TiC particles significantly improved the high-temperature creep property of 2618 aluminum alloy. Both the calculation based on the Orowan dislocation model through the introduction of the threshold stress concept and the normalization of the creep data showed that the high-temperature creep behavior of the composites could be explained by the dislocation climb (lattice diffusion) mechanism.

Constitutive rules of cyclic behavior of interface between structure and gravelly soil

We investigated the constitutive rules of cyclic behavior of the interface between a structure and gravelly soil using macro- and micro-observation results of a series of large-scale tests. Based on new understandings of the deformation mechanism, five constitutive rules of the cyclic behavior were described along with corresponding experimental proofs: (1) shear strength is proportional to normal stress. (2) Shear stress increases toward a stable value with increasing tangential displacement during the application of a monotonic stress path under constant normal stress condition. (3) Volumetric change due to dilatancy comprises reversible and irreversible dilatancy components. The reversible dilatancy component exhibits significant aeolotropical behavior. Only part of the tangential displacement contributes to the volumetric change due to dilatancy. (4) Compressibility decreases with increasing normal stress. (5) The evolution of the physical state, including particle-crushing and compression of the soil near the structure, causes and governs the evolution of the behavior of the interface and the evolution of the macroscopic stress–displacement relationship response of the interface.

On non-local and non-homogeneous elastic continua

A thermodynamic framework endowed with the concept of non-locality residual is adopted to derive non-local models of integral-type for non-homogeneous linear elastic materials. Two expressions of the free energy are considered: the former yields a one-component non-local stress, the latter leads to a two-component local–non-local stress since the stress is expressed as the sum of the classical local stress and of a non-local component identically vanishing in the case of constant strains. The attenuation effects are accounted for by a symmetric space weight function which guarantees the constant strain requirement as well as the dual constant stress condition everywhere in the body. The non-local and non-homogeneous elastic structural boundary-value problem under quasi-static loads is addressed in a geometrically linear range. The complete set of variational formulations for the structural problem is then provided in a unitary framework. The solution uniqueness of the non-local structural model is proved and the non-local FEM is addressed starting from the non-local counterpart of the total potential energy. Numerical applications are provided with reference to a non-homogeneous bar in tension using the Fredholm integral equation and the non-local FEM. The solutions show no pathological features such as numerical instability and mesh sensitivity for degraded bar conditions.

Effect of granulation scale-up on the strength of granules

Granulation is commonly used to enlarge particle size to impart desirable characteristics and functionality to the granules. In this work, the effect of operating scale of the granulator on the physical properties of granules is analysed. Three scaling up rules of constant tip speed, constant shear stress and constant Froude number have been evaluated using 1, 5 and 50 l Cyclomix high shear mixer granulators, calcium carbonate powder and Polyethylene glycol (PEG) as binder. The strength of granules produced in different granulator scales is analysed by side crushing test. The results indicate that the condition of constant tip speed for scale-up produces granules with relatively similar strength. When the condition of constant shear stress is used to scale-up the granulator, the granules produced in 5 and 50 l have relatively similar strength, but 1 l produces much weaker granules than those of 5 and 50 l. Work has also been carried out to analyse the flow field of granules in the granulator to provide a better insight into the velocity and stress profiles as a function of equipment scale. Positron Emission Particle Tracking (PEPT) analysis shows that under constant shear stress condition macroscopic flow field of 1 l and 5 l granulators are different, a feature which could affect the final structure of the agglomerates. The PEPT results are used to describe the velocity gradient in the Distinct Element Method (DEM) simulation of the agglomerate deformation under bulk shearing. DEM modelling of microscopic interactions in agglomerate behaviour within a shearing bed shows that flow conditions of 1 l granulator make the agglomerate with a higher elongation factor and lower packing fraction, indicating that the agglomerate would be weaker. This feature has also been observed experimentally.

Anomalous charge trapping dynamics in cerium oxide grown on germanium substrate

We have observed charge trapping phenomena in thin films of cerium oxide on n-type germanium (Ge) substrate under constant voltage stress (CVS) condition. The measured shift of the flatband voltage of a high frequency C-V curve immediately after each CVS cycle, was utilized as a method to study the capture dynamics of both preexisting and stress induced oxide defects. At low stress electric field, it is the creation of new interface traps that dominates the trapping characteristics of the corresponding metal-oxide semiconductor capacitors. At higher stress electric field, negative charges are trapped on preexisting traps uniformly located in the bulk of the oxide. From data analysis, the capture cross section of the traps is estimated to be around 1×10−19cm2 which indicates neutral traps possibly related to H+ species and/or oxygen vacancies.

Brittle-fracture criterion and durability of materials under thermomechanical action

Consideration has been give to the thermoelasticity problem for a plane containing a rectilinear crack under the conditions of constant tensile stress and a nonuniform stationary temperature field. The system’s thermodynamic potential from whose stationary condition the criterion of brittle fracture of a material has been established has been computed by solution of the problem indicated. The average durability of the material under thermomechanical action has been evaluated.