Effects Of Self-stress Research Articles

Mechanical performance of self-stressing CFST columns under uniaxial compression

Applying the expansive concrete in concrete-filled steel tube (CFST) columns can prevent the debonding between the steel tube and internal concrete. Self-stresses are on the steel tube and internal concrete before an external load is applied because of the concrete expansion. Mechanical performance of self-stressing CFST columns under the uniaxial compression were studied in this paper by tests and finite element analysis. A calculation method was proposed to calculate the self-stress based on the free deformation of concrete. Ten CFST columns with different amounts of expansive agent, tube thicknesses, and curing ages were tested under the uniaxial compression. The results indicated that the axial load bearing capacity of CFST columns improved significantly after adding the expansive agent to the internal concrete. Besides, the improving degree was related to the initial self-stress. Subsequently, a finite element analysis was carried out on self-stressing CFST columns to investigate the interaction mechanism between the steel tube and internal concrete further. The simulated results agreed well with the experimental results through some response comparisons. According to the simulated results, contact stresses were observed between the steel tube and internal concrete of self-stressing CFST columns throughout the loading process. Furthermore, a parameter analysis was carried out, which indicated increasing the compressive strength of concrete, yielding strength of steel tube, and thickness of steel tube increased the axial load bearing capacity of self-stressing CFST columns and the corresponding load shared by concrete. Finally, a calculation formula considering the effect of self-stress was proposed to predict the axial load bearing capacity for short self-stressing CFST columns with reasonable accuracy.

Effect of rebar self-stress on behavior of autoclaved aerated simply supported R/C thin beams subject to uniform transverse dead load

Autoclaved aerated concrete (AAC) beams typically incur counter-arch deflection with stored stress induced by the high-temperature and pressure steam curing. This study investigates the effects of the resulting self-stress on the flexural performance of steel rebar reinforced AAC beams. Accordingly, a simplified experimental model was designed to decouple the bar self-stress from other load mechanisms that exist in full-scale structures. Surcharge loading was applied, and the self-stress of steel rebar was measured by the releasing method. The cracking pattern of the AAC test beams and the associated load-deflection curves were analyzed. The values of self-stress in different locations of the beam were obtained, and the influence of steel bar self-stress on the cracking load was calculated. The test results show that the cracking moment resisted by the self-stress accounted for 65.7% of the theoretical cracking moment. Numerical simulation results confirmed that asymmetric reinforced AAC beams incur a counter-arch phenomenon, and further quantified the corresponding effect of self-stress on the cracking load. Therefore, accurately accounting for the rebar self-stress can more accurately define the cracking load capacity of AAC beams and should be considered in design, which should make AAC beams a stronger contender in diverse field applications.

Behavior of steel fibers reinforced self-stressing and self-compacting concrete-filled steel tube subjected to bending

This paper described an experimental work for steel fibers reinforced self-stressing and self-compacting concrete-filled steel tube (FSSCFST) specimens subjected to bending. The study aimed to evaluate the effect of self-stress and steel fibers on the behavior of self-compacting concrete filled steel tube (SCCFST) specimens. Twenty-seven SCCFST specimens reinforced with steel fibers, self-stress or their couple were tested under bending. The variables considered in the test were self-stress, thickness of steel tube, concrete strength and steel fiber volume percentage. Experimental results showed that the FSSCFST specimens behaved in a relatively ductile manner, but steel fibers and self-stress failed to change the failure mode of CFST specimens. Self-stress can slightly increase the flexural capacity and lead to a higher flexural rigidity. Steel fibers can lengthen the elastic stage and improve the flexural capacity of FSSCFST specimens. This beneficial effect of steel fibers decreased with the increasing of steel tube thickness. A formula was proposed to predict the flexural capacity of FSSCFST specimens, and the comparison between predictions and experiment results proved the accuracy of the formula.

Anisotropy of hydrogen diffusion in nickel single crystals: the effects of self-stress and hydrogen concentration on diffusion

Hydrogen diffusion has an important role in solute-dependent hydrogen embrittlement in metals and metallic alloys. In spite of extensive studies, the complexity of hydrogen diffusion in solids remains a phenomenon that needs to be clarified. In this paper, we investigate the anisotropy of hydrogen diffusion in pure nickel single crystals using both an experimental approach and a thermodynamic development. As a first approximation, experimental data from electrochemical permeation and thermal desorption spectroscopy are described using the classical Fick’s laws and an apparent diffusion tensor. Within a thermodynamic framework, the diffusion equation can be derived from Fick’s laws with an apparent diffusion coefficient which contains an added solute content dependent term β. This term is due to the elastic strain field associated with the insertion of solute atoms. For nickel crystals, the dependence of β on the crystallographic orientation arises from the elastic anisotropy. Additionally, our results elucidate the discrepancies between the thermodynamic model and experimental observations of the effect of the solute concentration on the diffusion process. Moreover, this highlights the importance of the impact of hydrogen on vacancy formation and the subsequent consequences on the anisotropy of the apparent diffusion coefficient.

Effects of self-stress on hydrogen diffusion in Pd membranes in the coexistence of α and β phases

We analyze and compare the influence of lattice strain gradients on hydrogen absorption in tubular and planar membranes in the coexistence of α and β phases. We show, by means of numerical simulations and analytical arguments, that self-stresses strongly influence the dynamics of the propagation of the β phase within a planar membrane. Conversely, the α–β phase transition is only slightly influenced by lattice strain gradients in a tubular membrane. The occurrence of “uphill-diffusion” phenomena at the early stages of the permeation process is also addressed.

Effects of Self Stress on the Transport of Guest Species in Solids: Transport of Hydrogen in Metals

Influence of self stress on the transport of a guest in a host matrix is studied within the process of hydrogen permeation through a large thin membrane having properties similar to those of palladium or Pd 8 1 Pt 1 9 alloy. The equilibrium in the system is perturbed at one side of the membrane by a sinusoidal signal of hydrogen concentration. At the opposite side, the hydrogen concentration is maintained at its initial value, and the flux of hydrogen is the response. The transport equations are solved numerically for a wide range of frequencies and amplitudes of the perturbing signal. At the periodic steady state, the larger the signal amplitude, the more important become the zero and higher harmonics of the flux, accompanying the fundamental harmonics. The presence of the zero harmonics causes a permanent pumping of hydrogen throughout the membrane. The fundamental harmonics closely reproduces the results obtained by solving the linearized transport equations. This means that, up to the largest amplitudes, the frequency spectrum of the fundamental harmonics allows for obtaining reliable values of the diffusion coefficient of hydrogen and the bulk elastic modulus of the metal matrix.

Concentration transfer function of hydrogen diffusion in self-stressed metals

The effects of self-stress on the concentration transfer function of hydrogen diffusion in a continuous elastic solid-metal matrix were analyzed. A large thin-plate metal specimen saturated with hydrogen is the essential part of the system. The equilibrium is perturbed by a small-magnitude sine-wave input signal of hydrogen concentration applied at one surface of the specimen. In response, oscillations of hydrogen concentration appear at the opposite surface. The concentration transfer function is defined as the ratio of the stationary response to input signals. The derived diffusion equations are non-linear. They are linearized, and then solved analytically. The resulting transfer function is discussed in terms of hydrogen permeation through a specimen of properties similar to palladium and Pd 81Pt 19 alloy, in wide ranges of hydrogen concentrations in the metal matrix and of frequencies of the signal. At relatively high frequencies, the system is highly sensitive to non-Fickian diffusion, resulting from the non-local effect of self-stress. Transfer function spectroscopy seems to be a more powerful tool for studying the hydrogen transport in self-stressed metals than the commonly used transient break-through method. In particular, it should allow the dependence of the hydrogen diffusion coefficient and of the elastic modulus of metal–hydrogen solids on the hydrogen concentration to be studied.

Diffusion of hydrogen in self-stressed metals — transfer function spectroscopy approach

The transfer function spectroscopy approach to the diffusion of hydrogen in a self-stressed isotropic elastic metal matrix is proposed. The system should be close to equilibrium. It is perturbed by a sine-wave input signal applied at one surface of the thin-plate specimen. The magnitude of this signal, of varied frequency, is small enough to treat the system as linear. The response signal is measured at the opposite surface of the specimen. The transfer function is the ratio of steady-state response to input signals. The hydrogen concentration and hydrogen flux are the input and output signals, respectively. The diffusion equations are derived, and they are solved analytically. The resulting transfer function is discussed in terms of hydrogen permeation through a specimen of properties similar to palladium and Pd 81Pt 19 alloy, in a wide range of hydrogen concentrations in the metal matrix. It is demonstrated that at relatively high frequencies the transfer function is highly sensitive to the non-Fickian diffusion, resulting from the non-local effect of self-stress. In contrast, at infinitesimally low frequency, i.e. at steady-state, both local and non-local effects compensate. Hence, the self-stress is absent. Under the proposed experimental conditions the transfer function spectroscopy is more appropriate for studying the diffusion of hydrogen in self-stressed metals than the commonly used transient break-through method. It should allow the study of the diffusion coefficient of hydrogen in metals, and, moreover, of the elastic modulus of metal–hydrogen solids, both these quantities as function of hydrogen concentration in isotropic matrixes.

Modeling of Dislocation Mobility in Metals: Effect of Obstacles and Thermal Processes

ABSTRACTThermally activated dislocation glide velocity through weak point obstacle arrays is studied analytically and computationally. Thermal activation rate is estimated using the modified Friedel relations under the weak obstacle approximation. The average flight velocity after an activation event as a function of stress and temperature is obtained by the discrete dislocation dynamics (DD). This numerical calculation includes the effect of self-stress, interaction with electrons and phonons, and the inertial effect. These results are implemented into a phenomenological model to study the dislocation velocity under various conditions. The model can reproduce both obstacle control and drag control motion in low and high velocity regions, and a flow stress anomaly at transient regions.

Effects of self-stress on the hydrogen absorption into palladium hydride electrodes of plate form under galvanostatic conditions

Effects of diffusion induced stress on the hydrogen absorption into plate form electrodes of b-phase PdHx are discussed numerically based on the Volmer‐Tafel route of the hydrogen evolution reaction, and thermodynamic considerations involving stress fields and non-ideal interactions of hydrogen in the electrode. It is found that the self-induced stresses enhance the absorption rate and may exceed the yield stress, especially when the thickness of the plate and:or charging current (or negative potential) increase. On the other hand, a plate with both sides exposed to electrolyte absorbs hydrogen more rapidly than that with only one side exposed to electrolyte under the same equivalent thickness and other conditions. Of course, the stresses developed in the former plate are always greater than those of the latter. © 1999 Elsevier Science S.A. All rights reserved.

Effect of Self Stress on Flexural Strength of Gypsum-Polymer Composites

Self stress generated in polymer impregnated gypsum (referred as GPC) when it is composed is estimated, and its influence on flexural strength is discussed. The estimation of the self stress is based on measured values of shrinkage caused by polymerization of impregnated monomer and elastic modulus of dried gypsum base just before impregnation. The effect of this self stress on flexural strength of GPC is investigated. It was found that the following equation is valid to predict flexural strength of GPC (б b ) in terms of the self stress as a variable: б b = б gb + V p (б p − б sp ), where б gb = flexural strength of gypsum base, V p = specific volume of polymer, б p = tensile strength of polymer, and б sp = self stress generated in polymer phase. If extremely low water-gypsum ratio is adopted to prepare gypsum base, cracking is observed just after polymerization preceding any flexural loading. For somewhat higher water-gypsum ratio, specimens are not cracked, but their flexural strength is decreased after polymer impregnation. The self stress corresponding to this case turns out to be higher than the tensile strength of polymethyl methacryrate used for the impregnation. Since prediction of б sp in the equation is based on tri-axial compressive strain of gypsum base that is within its elastic region, б sp in polymer phase should positively exist. Even for this condition, the validity of the equation seems to be maintained, although the value in the parenthesis of the equation becomes negative. Based on this fact, an unstable physical state where one phase of a composite material is stressed beyond its macroscopic strength as an individual material owing to the crack arresting effect of the other phase (gypsum in this case) has been postulated. This state is designated as a “superstressed” state, taking its resemblance with supercooling or supersaturation into consideration.

The effect of self-stress on diffusion in solids

We demonstrate that the local diffusion flux in solids depends on stresses arising from compositional inhomogeneities (and even the shape) of the entire specimen. This dependence of local flux on distant inhomogeneity is considered to be a failure of Fickt's law. From an eluation relating stress and composition field, we can find special cases in which a diffusion law resembling Fickt's law occurs, but the anisotropy of the apparent diffusion coefficient reveals that non-local factors are still present. le also examine the case of diffusion through a thin plate for the functional dependence of diffusion on the composition field and predict much deeper diffusional penetration and accumulation on the far side.