Uniaxial Direction Research Articles

Effect of embedded steel on the compressive damage behavior of cement mortar

Steel embedded in cement mortar in the uniaxial compressive stress direction was found to decrease by 80% the compressive longitudinal strain at a given stress and increase by 300% the tensile transverse strain at a given longitudinal strain. The former effect is due to longitudinal confinement of mortar by steel. The latter is due to debonding at the steel-mortar interface.

Phase coexistence of neutral polymer gels under mechanical constraint

We have measured the equilibrium diameter of cylindrical poly(N-isopropylacrylamide) (NIPA) gels with submillimeter diameters under a mechanical constraint along the uniaxial direction. The linear swelling curve, diameter vs temperature, was obtained in the vicinity of the volume phase transition temperature under isometric constraint: both ends of the gel were fixed at rest (stress was zero) in a swollen state at 30 °C (a few degrees below the transition temperature). Thereafter, the temperature was gradually changed by keeping the uniaxial length constant (to the fixed length). It was demonstrated that the gel could take a coexistent state at the transition point and remain stable for several days. This time was much longer than the characteristic relaxation time of the phase transition in this tiny gel. The phase coexistence observed here was caused by stress inhomogeneity along the uniaxial direction due to the mechanical constraint. A collapsed to swollen phase transition induced by uniaxial stress at a fixed temperature was also presented. It was shown that the ratio of the swollen portion to the total length could be controlled by the degree of elongation: when the elongation was slightly increased or decreased, the phase boundary between the swollen and collapsed phases was accordingly shifted to increase or decrease the swollen state, respectively. The ratio of the swollen to the collapsed phase in the case of stress-induced coexistence is discussed herein in terms of a phase diagram (diameter vs elongation), and a simple phase selection rule is presented.

Evaluation of Tribological Properties of Diamond-Like Carbon Thin Film Prepared by the Hot Cathode PIG Discharge Type Plasma-Enhanced CVD Method on Experiment and Molecular Dynamics Simulation.

This paper describes the effect of density of Diamond-Like Carbon (DLC) thin film on tribological property. DLC thin films having the density of 1.9-2.5 g/cm3 were prepared on (111) of Silicon surface by the hot cathode PIG Discharge Type Plasma-Enhanced CVD method. Ball on disk friction experiments were carried out using diamond ball. The friction coefficients increased from 0.05 to 0.20 with increasing film density in the density range below 2.185 g/cm3, but those did not change above 2.185 g/cm3. The specific wear value also increased with increasing film density. The effect of film density on tribological property between DLC thin film and diamond pin was examined by molecular dynamics simulation. The diamond pin slides to uniaxial direction on DLC thin film having the density 1.5-3.0 g/cm3. Atoms of DLC thin film having the density of 1.5 g/cm3 were moved most actively. Atomic movement being like to shear deformation was observed during frictional processes. The effect of film density on friction coefficients in MD simulations was similar to that in experiments qualitatively.

Thermal Fluctuations of Freely Suspended Smectic-A Films from Mesoscopic to Molecular Length Scales

Smectic-A (SmA) liquid crystals are uniaxial systems possessing long-range orientational order. In addition, the molecules are on average arranged in equidistant layers, while the translational order is liquidlike in the other two directions. The positional order along the uniaxial direction is not truly long range but decays algebraically with position as r 2h . This absence of true long-range

Phase transition in strongly stretched polymer gels

We will present the volume phase transition of cylindrical poly N-isopropylacrylamide (NIPA) gels under large distortion along the uniaxial direction. The neutral and slightly ionized NIPA gels were synthesized in submillimeter diameter. The gel was stretched along the uniaxial direction with the longitudinal deformation ratio, α∥ (the ratio of the stretched length to the equilibrium length in the swollen state at 30 °C). By keeping the longitudinal length constant, we have simultaneously measured the equilibrium diameter and the force as functions of temperature. The swelling curves of the neutral NIPA gels including the volume phase transition temperature were obtained for several deformations in the range between α∥=1 and α∥=6. With increasing α∥, the transition temperature increased up to 1 °C in the small deformation below α∥∼3.5, and it saturated and slightly decreased in the large α∥ above α∥∼4. At the transition temperature on heating, the force to keep the length constant increased discontinuously in the smaller region below α∥∼3.5. The magnitude of this steplike change in the force began to decrease in the vicinity of α∥∼3.5, and hereafter exhibited negative change above α∥∼4. In the case of the slightly ionized NIPA gels, these effects came into play at an earlier stage, that is, at smaller α∥, because of the prestretching due to the ionic pressure. The present observations, especially for the strong deformation above α∥∼4, were discussed by the equation of states of strongly stretched hydrophobic gels on the basis of the extended Flory-type free energy taken into account the non-Gaussian effect.

Morphology andIn Situ Deformation of Polyamide Films Investigated with Scanning Force Microscopy

The structure and the mechanical properties of polyamide (PA) films were studied with scanning force microscopy (SFM). The spherulitic structure of the films was resolved and the amorphous and crystalline regions could be identified without heavy metal staining or evaporation. The identification was achieved with different contrast mechanisms in SFM, such as the force modulation technique and phase imaging. The mechanical properties of the PA films were investigated with a tensile tester. The films were drawn in a uniaxial direction. The deformation of the spherulite structures could be imaged by in situ SFM. A schematic model for the interpretation of the observed structures is presented.

Determination of exchange anisotropy by means of ac susceptometry in Co/CoO bilayers

The strength, or stiffness Kud, of the magnetic unidirectional anisotropy in Co (90–300 Å)/CoO bilayers has been studied with ac susceptometry. We develop a model to describe the ac susceptibility which indicates that the stiffness Kud decreases linearly with temperature as (1−T/Tcrit). As the temperature increases and Kud decreases, a rotation of the magnetization to a uniaxial easy direction is observed. The values of Kud determined by this small amplitude field oscillation technique are found to be larger than the values obtained when evaluated by the usual hysteresis loop technique. We observe that the exchange field, Hex, could be greatly enhanced and its deterioration after repeated magnetization reversals significantly reduced by partially capping the Co surface prior to oxygen exposure.

Stress‐induced azimuthal anisotropy in borehole flexural waves

A nonlinear elastic model for acoustic waves in a stressed medium is used to calculate tectonic stress‐induced changes in borehole flexural dispersions. Our theoretical analysis shows that a horizontal uniaxial stress in the formation causes a crossover in flexural dispersions for the radial polarization aligned parallel and normal to the stress direction. This crossover in flexural dispersions is caused by stress‐induced radial heterogeneities in acoustic wave velocities that are different in the two principal stress directions. Other sources of borehole flexural anisotropy caused by finely layered dipping beds, aligned fractures, or microstructures found in shales, exhibit neither such radial heterogeneities nor flexural dispersion crossovers. Consequently, a crossover in flexural dispersion can be used as an indicator of stress‐induced anistropy. In this situation, the fast shear polarization direction coincides with the far‐field uniaxial stress direction. The analysis also yields an expression for the largest shear stress parameter in terms of the fast and slow seismic shear‐wave velocities with shear polarization parallel and perpendicular to the far‐field stress direction.

Strain-Direction-Dependent Growth Morphology of Vicinal Si(001) Surface.

The shape dependence of vicinal Si(001) surfaces on the applied uniaxial strain direction is studied. This dependence is intimately related to the anisotropic nature of the intrinsic strain field which originates from the surface dimerization. The strain relief mechanism is shown to be different in two orthogonal directions. Normal to the steps, step-pair bunching and waving lead to formation of hillocks and pits. Along the step direction, bending of step pairs forms a cusp which later develops into a deep groove.

Borehole propagation in berea sandstone: Stress-induced dipole anisotropy

Stress-induced dipole anisotropy exhibits a crossover in the principal flexural wave slowness dispersions oriented parallel and perpendicular to the farfield uniaxial compressive stress direction. This crossover phenomena is a result of borehole stress concentrations and can be used as a new technique for distinguishing stress-induced anisotropy from intrinsic anisotropy. Theoretical modeling (based on third-order elasticity) and laboratory measurements (from 10 to 60 kHz) have been made on a large block of berea sandstone subject to a uniaxial stress of up to 5 MPa. The two flexural mode dispersions are obtained by Prony’s processing of an array of measured waveforms for dipole orientations parallel and perpendicular to the stress direction. The theoretical dispersions in the presence of biasing stresses are obtained from the solution of equations of motion for small dynamic fields superimposed on a static bias. Good agreement has been obtained between theory and experiment including a dispersion crossover phenomena unique to stress-induced anisotropy.

Effective Tensile Stiffness of Finite-Width Quasi-Isotropic Laminates

Finite width laminate tensile specimens cut out of a plate with a quasiisotropic [I 30'/900]s-layup are considered in regard to their effective axial stiffness properties. Due to the existing free-edge effect, the effective coupon stiffness can be clearly reduced in comparison to the predictions of classical laminated plate theory. The respective stiffness reduction depends on both the width of the given laminate specimen and also the applied uniaxial tensile direction. This means that the quasi-isotropic laminate as a finite width specimen actually does not show an isotropic uniaxial tensile stiffness. For the description of this effect a closed-form analysis is presented and the respective stiffness reduction is derived in a consistent way. For comparison and validation purposes detailed finite element analyses have also been performed showing good agreement with the closed-form results.

Hamiltonian descriptions of the micromagnetics of finite continuous media in the effective field formalism

The equations of motion describing the response of a ferromagnetic system in the micromagnetic approximation can be developed by applying Hamilton's principle to an action functional constructed from a Lagrangian density composed of the usual free energy expression, together with a suitably chosen expression for the kinetic energy density so as to produce the characteristic Larmour precession associated with magnetic spin systems under the influence of an external field. If a Hamiltonian density functional is introduced through the usual Legendre transformation, it can be shown that by a suitable choice of the form of the generalised momenta and the standard form of the Rayleigh dissipation energy density functional, equations of motion are derived which are formally equivalent to the Landau-Lifshitz-Gilbert form. However, the form of the equations of motion permits a substantial increase in the time step. An alternative description which avoids the introduction of a kinetic energy term, but which retains all the essential attributes of the former approach, is described. The model is applied to small rectangular polycrystalline cobalt samples with in-plane randomly oriented uniaxial anisotropy directions.

Phase transition in constrained polymer gels

Equilibrium diameter and stress change of cylindrical N-isopropylacrylamide gels of submillimeter diameter were measured as a function of temperature under mechanical constraint along the uniaxial direction. As a result, the transition temperature increased up to 1 °C with increasing the strain, which indicates the possibility of the uniaxial strain-induced phase transition of gels at selected temperatures. The diameter and stress change were quite successfully described by the phenomenological model on the basis of the extended Flory-type free energy. The time course of the change in diameter and stress was also studied after the large steplike elongation was rapidly applied. A gradual increase in diameter during decrease in stress was detected clearly in the swollen phase, which is due to the relaxation of polymer segmental units to approach the favored conformation under the strong deformation.

Third-Order Piezoresistance Coefficients in p-Type Si

By tensor property and crystallographic symmetry considerations, it has been shown that there are 20 independent nonzero components of the third-order piezoresistance (TOPR) coefficients (π3) for cubic semiconductors which crystallize in diamond ( Oh) or zincblende ( Td) structures. Piezoresistances for p-type Si obtained from conductivities calculated for sixfold degenerate valence bands with and without stress are highly nonlinear so that they are more suitably expressed with up to the fifth-order (π5) stress terms than merely with up to the π3 term for the odd-order stress terms. For several current and uniaxial stress directions, TOPRs are given in terms of 20 components of TOPR coefficients, and calculated π3's which are obtained using up to the third-order stress term correspond in the order of magnitude with experimental ones.

Computer experiments of fracture of two-dimensional Henley-Elser type quasicrystals

Fracture processes in two-dimensional Henley-Elser type quasicrystals were studied by the molecular dynamics method (MD). The model structures were ternary systems and were prerelaxed for the subsequent deformation process. The deformation process was simulated by structural relaxation by MD while the calculation cell was extended along a uniaxial direction by a certain factor after a certain time interval. The system temperature was kept constant by the scaling method during the calculation. The system temperature and potential energy were monitored to detect microscopic bond breaking in the sample. We found a large variance in the change in potential energy when the deformation process moved into plasticity from elasticity, corresponding to the creation of voids in the specimen. By changing the strength of the model potential, different types of deformation process were observed. The stability of the pentagonal clusters, which are considered to be the unit structure of the quasicrystal, was changed drastically according to the strength of the heterogenous interaction in the pentagonal cluster. Consequently, different types of fracture, i.e. interpentagonal and intrapentagonal cluster, occured depending on the strength of the heterogeneous interaction.

Anisotropic crack damage and stress-memory effects in rocks under triaxial loading

We present and analyse the results from tests performed on sandstone cubes in which the bulk acoustic emission (AE) and the ultrasonic P and S wave velocity changes in the three principal directions were monitored and measured simultaneously with the stress and strain as samples were deformed in three orthogonal directions independently. This combination of measurements has enabled us to directly link AE to the formation of new crack damage. From uniaxial cyclic stressing experiments the existence of the Kaiser stress-memory effect was confirmed based on the AE data and the velocity data from directions normal to the uniaxial stress direction. Furthermore, the effect was found to occur in each principal stress direction independently, regardless of whether or not stresses were applied along orthogonal axes. This suggests that the crack damage formed during stressing is highly anisotropic, with the new microcracks formed during each stress cycle having minor axes parallel to the minor principal compressive stress direction. The observed effects have been modelled in terms of the elastic closure of preexisting cracks and the formation of new highly-oriented dilatant cracks. The dilatant cracks produce AE. By modelling these two separate processes we have demonstrated a clear quantitative correlation between the AE output and the new dilatant crack damage formed during deviatoric stressing. We believe our results confirm the potential of the use of the Kaiser effect to determine crustal stresses from measurements on oriented core plugs taken from borehole cores.

Alloying design of oxide dispersion strengthened ferritic steel for long life FBRs core materials

Oxide dispersion strengthened (ODS) ferritic steels with excellent swelling resistance and superior high temperature strength are prospective cladding materials for advanced fast breeder reactors. The addition of Ti in 13Cr-3W ODS ferritic steels improved the high temperature strength remarkably by the formation of uniformly distributed ultra-fine oxide particles. ODS ferritic steels have a bamboo-like grain structure and a strong deformation texture. The decrease of creep rupture strength in the bi-axial direction compared to the uni-axial direction is attributed mainly to this unique bamboo grain structure. Nearly equivalent creep rupture strength for both bi-axial and uni-axial direction was successfully attained by introducing the α to γ transformation in ODS martensitic steel.

Tube manufacturing and mechanical properties of oxide dispersion strengthened ferritic steel

In order to apply the ODS ferritic steels for the prospective cladding materials of advanced fast breeder reactors, fabrication tests of thin-walled cladding tubes were carried out from a viewpoint of future industrial manufacturing. The manufactured claddings within the specification limit exhibited a superior high temperature strength and sufficient Charpy impact properties. The degradation of creep rupture strength in the bi-axial direction, as compared with the uni-axial direction, is mainly attributed to the grain boundary fracture mode within the elongated bamboo grain structure.

S/N ratio of thin-film media estimated by computer simulation (abstract)

This paper derives a formula that shows a direct relationship between the noise spectrum and the magnetization configuration of a thin-film medium. This formula was obtained by Fourier transforming the down-track autocorrelation function of jMx-Mz, where Mx and Mz are the longitudinal and vertical components of magnetization. The formula takes into account not only the variance of the magnetization deviation, which is generally used to estimate media noise, but also the correlation between deviations in magnetization. This lets us calculate the noise power spectrum as well as the signal power, once the magnetization configuration in a medium is obtained, which means that the magnetic microstructures such as vortex, zig-zag domain, etc., are automatically included in noise calculation. To investigate the relationships between the noise properties and the magnetic properties of media with various anisotropy fields (Hk), axis orientation of easy magnetization, and stiffness constants (A), recorded magnetization configurations were simulated using the Landau–Lifshitz–Gilbert equation.1,2 The easy axis was oriented from uniaxial direction to random direction in plane. The simulated noise power spectra were maximized at a relatively long wavelength, which was varied by changing Hk, or the easy axis orientation. The noise power was strongly influenced by Hk and the axis orientation of magnetization. This confirms that preferential orientation along the longitudinal direction and a large Hk are effective for improving the S/N ratio. On the other hand, the stiffness constant less improved media noise than Hk and axis orientation, suggesting that the preferential orientation of the easy axis is more effective to reduce media noise than isolating magnetic grains.1

Changes in orientation and intensity of remanent magnetization of magnetite samples induced by uniaxial pressure

The effect of uniaxial pressure on the remanent magnetization of artificial samples containing synthetic or natural magnetite of various grain sizes was examined. Changes in intensity, declination and inclination of initial remanent magnetization were determined while the sample was being compressed. It was shown that the applied uniaxial pressure caused a decrease in inclination except when the initial inclination was either 0° or 90° with respect to the stress axis. The remanent magnetization rotated towards the plane perpendicular to the stress direction. This effect is most pronounced when the initial inclination is 30° with respect to the uniaxial pressure direction.