Sliding wigner solid on liquid4He

We report a systematic experimental study of the anomalous nonlinear magnetotransport in the Wigner solid (WS) trapped on a liquid⁴He surface. The ac Corbino conductivity _xx exhibits an abrupt jump at a certain driving voltage. The threshold input voltage V_th for the _xx jump varies as V_th B^{–0.8 –1} n^1.5 _sE, where B, , n_s, and E are magnetic field, frequency, electron density and pressing electric field, respectively. We interpret the _xx jump as the transition between the WS accompanied with the periodic 4He surface deformation and the WS which decouples from the surface, due to the large driving force. A simple model is presented: The rigid-potential sliding model qualitatively explains the above mentioned behaviors of V_th. We have found that the dependences of _xx on B and E are different from those of the liquid phase. The anomalous behaviors of _xx play a crucial role on the sliding model.     

The spin diffusion coefficient of3He in3He-4He solutions

The transport properties of³He in³He-⁴He solutions with molar concentrations of 5, 9, 14, and 24% have been studied for 0.9 KT2.5 K. The spin diffusion coefficientD _s and the longitudinal relaxation timeT ₁ were measured by the spin-echo method for temperatures both above and below the solution lambda temperatureT . The spin-echo method measures the diffusion coefficient for magnetizationD _s, which differs from the usual diffusion coefficient for particlesD belowT .D _s depends on the³He-³He scattering cross section _FF and the³He-roton/phonon cross section _FB, whileD depends only on _FB. The distinction betweenD _s andD is elaborated in terms of a simple mutual-friction model for diffusion. The two scattering mechanisms are clearly evident in the behavior ofD _s as a function of concentrationx and temperature. The contribution due to the³He-³He scattering is inversely proportional tox, indicating that the³He can be treated in first approximation as a classical gas (the Pomeranchuk model). The predictions of various theoretical models are compared with the results, where possible, but most of the previous theoretical work is not applicable to the concentration range and temperatures of these measurements.Supported in part by the National Science Foundation and the U.S. Office of Naval Research.     

Singular behavior of the mass conductivity of liquid4He in a superleak at the superfluid transition

From measurements of the damping of sound in liquid ⁴ He caused by fluid penetration into a porous medium at the superfluid transitionT _s, we have deduced a critical temperature dependence of the mass conductivity of the form (T–T _s ) ^{–0.31±0.03}      

Theory of the Surface Tension Maximum of Liquid 3He

Recently Matsumoto et al. performed very precise measurements of the surface tension of liquid ³He, (T), at low temperatures and found that (T) exhibits a small maximum at about 100 mK. Existing theories are unable to explain this anomaly. On the basis of a local approximation for the entropy in which the Fermi liquid effect is included, we can evaluate the variation of (T), (T)=(T)–(0), as a function of T and of the number density (or the interaction strength). It is found that (T) consists of two terms; a T² term and T⁴ ln T term. We predict that, for the density of real liquid ³He, exhibits a tiny minimum and a small but relatively larger maximum. This prediction explains qualitatively and quantitatively all salient features of the observed (T).     

Studies of Both Intralayer and Interlayer Electrodynamics of Bi2Sr2CaCu2O y in the Superconducting State

We studied the intralayer and interlayer electrodynamics of the superconducting Bi ₂ Sr ₂ CaCu ₂ O _y single crystals in the microwave and millimeter wave regions. The c-axis superfluid density fraction _s ^c shows a more rapid growth than _s ^ab just below T _c and a weaker temperature dependence at low temperatures. These features suggest the anisotropic growth mechanism of _s . The temperature dependence of _ab ¹ shows a large increase below T _c with a broad peak around 20 K. However, to discuss the detailed behavior of ₁ ^c or the anisotropy of conductivity _ab ¹ / _c ¹ in the superconducting state, much more careful measurement is required in the c-axis direction.     

Surface Cracking of Steels in the Process of Cyclic Deformation in Aqueous Media

We study the conditions of surface cracking of steels for pipelines of power-generating equipment subjected to cyclic deformation in aqueous media. A new parameter is proposed for the evaluation of the corrosion fatigue of materials, namely, a certain characteristic level of stresses within the limits of the loading cycle ( = _s ) above which the deformed surface undergoes noticeable electrochemical activation. A model scheme of the surface corrosion-fatigue cracking regarded as a result of the synergetic action of cyclic stresses and electrochemical processes is proposed. A criterion of initiation of a surface crack of length a relating the characteristic stress _s to the maximum stress in a cycle _max, the strength of corrosion current I _cor, and the constants of electrochemical dissolution of the metal on the deformed surface is established. On this basis, we propose an engineering estimate for the period of initiation of surface corrosion-fatigue cracks.     

Interfacial Tension of Phase-Separated 3He-4He Mixtures Under Pressure

We measured the interfacial tension of ³ He- ⁴ He mixtures at several pressures up to 21 atm down to 0.18 K as well as the number densities of the ⁴ He-rich and ³ He-rich liquids at the phase-separation line. The interfacial tension was proportional to T² up to about 0.4 K without respect to pressure, and the coefficient of this T² -dependence did not depend strongly on pressure. The extrapolated values of the interfacial tension at absolute zero decreased drastically up to 3 atm, and at this level the decrease about 15% of the value at the saturated vapor pressure _i = 23.9 ± 1.0 N/m. However, this tension was unchanged above 7 atm. From the pressure dependence of the absolute value the adsorption of ³ He relative to the Gibbs dividing surface of ⁴ He component was calculated.     

Energetics, Surface Tension, and Prewetting Jump of Superfluid 4He Films Adsorbed on Planar Substrates

The energetics of superfluid films of ⁴ He adsorbed on planar substrates is studied. For this purpose, the energy per particle of the systems e is expressed as a polynomial expansion in powers of the inverse of coverage. The surface energy _A and the chemical potential , defined according to basic equations of thermodynamics, are written in terms of the proposed expansion. From the expression derived for the third-sound velocity c₃ it is possible to estimate the prewetting jump. This formalism is applied for analyzing films adsorbed on alkali metal and H ₂ surfaces. For all the examined substrates the determined energy per particle in the limit of films with infinite width, e , is in agreement with the value for bulk liquid ⁴ He. The surface tensions and prewetting jumps obtained in the present work are compared with results of previous studies. Also at the     

Fatigue of metallic glasses

The fatigue behaviour of Ni₄₉Fe₂₉P₁₄B₆Si₂, Ni₄₈Fe₂₉P₁₄B₆Al₃ and Pd_77.5Cu₆Si_16.5 metallic glasses is examined. In the finite lifetime regime the relationship between stress amplitude ( _a), fracture stress ( _f), mean stress ( _m) and cycles to failure (N _f) is _a=A( _f–_m) (2N _f) ^b , whereA andb are 16.9 and –0.40 respectively for reduced gauge section Ni₄₉ strips (for _m 140 kg mm^–2) and 27.0 and –0.44 for Pd base wires. These results are unusual in thatA 1. Consequently, a sharp discontinuity exists near _a( _f– _m) –1. In a simple tensile test failure occurs at _f(=_y) and 2N_f=1; for peak stresses only a percent or so less than _f the sample will withstand hundreds of cycles of stress. For uniform cross-section glassy metal filaments, a fatigue limit is observed at stress ratios ( _a/ _f) in the vicinity of 0.07 to 0.15. The fatigue limit for reduced section specimens is a factor of 2 higher. Fatigue failure of the Ni-Fe strips may occur under partially or fully plane stress or plane strain conditions, depending on sample thickness and stress. Final failure of the Pd_77.5Cu₆Si_16.5 wires always occurs by general yielding of the remaining section.     

The transverse acoustic impedance of dilute solutions of3He in superfluid4He

The transverse acoustic impedanceZ=R–iX of dilute solutions of³He in superfluid⁴He has been measured at a frequency (/2) of 20.5 MHz at temperaturesT from 30 mK to the transition at T. The³He concentrations studied werec=0.014, 0.031, 0.053, 0.060, and 0.092 below 1 K, thoughc decreased slightly near the point. The impedance was found from the temperature dependence of the quality factor and the resonant frequency of anAT-cut quartz crystal resonator immersed in the liquid. Below 1 K,Z is due to the Fermi gas of³He quasiparticles, and in the collisionless limit, 1 ( is a relaxation time),R remains constant whileX goes to zero. Measurements ofR(c, T) andX(c, T) were analyzed to determine the momentum accommodation coefficient (c, T) and (c, T). The relaxation times were in good agreement with previous work, while (c, T) was independent ofc, but increased from 0.29±0.03 below 0.1 K to 1.0±0.1 above 0.8 K. Various mechanisms are suggested to explain this. Between 1.0 and 1.5 K the³He quasiparticles and the thermally excited rotons are in the hydrodynamic region, 1. Values of the total viscosity (c, T) were obtained and analyzed to give the³He gas viscosity and the³He-³He and roton-³He scattering rates, both of which were energy-dependent. The superfluid healing length a was also measured. Near the point we founda=(0.1±0.03)^–2/3 nm, where =1–T/T, proportional to the phase coherence length . Our data are consistent with the hypothesis that _s/T is a universal constant for superfluid dilute solutions, where _s is the superfluid density. Between 1.0 and 1.8 K we found thata(c, T) was comparable to measurements in³He-⁴He films.     

Steady-state creep in a 25 wt % Cr-20 wt % Ni austenitic stainless steel

Steady-state creep behaviour of a 25 wt % Cr-20 wt % Ni stainless steel without precipitates was studied in the stress range 9.8 to 39.2 MPa at temperatures between 1133 and 1193 K. The results of stress-drop tests indicate that, in the steady-state creep region, diffusion-controlled recovery creep is dominant. Such recovery creep can be accounted for in terms of the composition of the internal stress, _i=_s+_c, except in the case of fine-grained specimens where d<80 m, whered is the mean grain diameter, _s is possible to reduce easily and is comparable to the driving stress for creep, and _c is the persistent stress field due to metastable substructure. In the fine-grained specimens, it is suggested that the steady-state creep is dominantly controlled by grain boundaries.     

Superfluidity of Pure 3He and Mixtures of 3He and 4He in Aerogel

We describe new torsional oscillator experiments on ³ He confined in 98.2% open aerogel. In one, we monitored the superfluid fraction of pure ³ He at T << T _c while we gradually changed the sample pressure. The resulting change in density alters ₀ of the superfluid ³ He relative to the distribution of the length scales (correlations) of silica in the aerogel. We observed a T = 0 normal-to-super fluid transition at a pressure of about 6.5 bar, in marked contrast to the bulk where liquid ³ He is superfluid at all pressures. In the second experiment, we measured the temperature dependence of the ³ He _s at a pressure of 21.6 bar with different amounts of ⁴ He present in the cell. Adding 2-3% ⁴ He slightly increases both T _c and _s . We found that for ⁴ He concentrations between 2% and 34%, the ³ He T _c increases by a very small amount. However, _s , which for pure ³ He in aerogel at 0.5T _c is no more than 11%, falls by another factor of 7. This behavior (constant T _c , reduced _s ) is similar to that observed in granular superconducting films where the long-range order is controlled by phase coherence between adjacent grains.     

The critical tensile stress criterion for cleavage

An elastic-plastic analysis was used to accurately measure the microscopic cleavage strength _inff ^sup* of notched bars of high nitrogen steel in bending. It was found that _inff ^sup* increases as the root radius of the notch decreases. For > 0.010, the variation of _inff ^sup* with , and the difference between _ingff ^sup* and the cleavage fracture strength of a plane tensile specimen, _f, may result from a statistical effect, due to differences in the volume of highly stressed material in the plastic zone. For < .010, the primary reason for the apparent increase in _inff ^sup* with decreasing , is the steep stress gradient at the notch tip, which forces the critical plastic zone size to extend further to insure that unstable microcraek propagation can occur. Both the statistical and stress gradient effects have been quantitatively evaluated and found to be in good agreement with the experimental data.

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Zusammenfassung Man benutzte eine elastisch-plastische Analyse, um die miktoskopische Spaltungssärke _inff ^sup* von eingekerbten Stahlbarren mit hohem Stickstoffgehalt im Krümmen sorgfältig zu messen. Man stellte fest, dass _inff ^sup* sich vergrösserte während der Wurzelradius der Kerbe sick verringerte. Da > 0.010 ist, ist die Variation von _inff ^sup* mit der Spaitungsfrakturstärke einer planaren Spannungsprobe, _f durch einen statistischen Effekt und durch Unterschiede in dem Volumen von stark angespanntem Material in der plastischen Zone verursacht. Da > 0.010 ist, ist der ursprüngliche Grund für die augenscheinliche Zunahme in _f mit verringertem , der tiefe Anspannungsgradiente an der Kerbenspitze, durch welche sich die kritische, plastische Zonengrösse weiter ausdehnen muss, um die labile Mikrorissausbreitung zu sichern. Die statistischen und Anspannungsgradienteffekte wurden quantitative ausgewertet und man fand gute Übereinstimmung mit den experimentellen werten.

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Résumé Une analyse élastoplastique a été utilisée pour la mesure précise de la résistance microscopique du clivage _inff ^sup* de barreaux entaillées d'acier à haute teneur en azote et soumis à flexion.On a trouvé que _inff ^sup* s'accroit lorsque diminue le rayon d'arrondi à la racine de fentaille. Lorsque est supérieur à 0,25 mm, la variation de _inff ^sup* en fonction de est due à un effet statistique du aux différences de volume de matière soumis, dans la zone de déformation plastique, à des contraintes; élevées. Il en est également de même l'écart entre _inff ^sup* et la contrainte de rupture par clivage dune éprouvette de traction sans entaille.Lorsque est inférieur à 0,25 mm, la raison principale de l'accroissement de _inff ^sup* avec des valeurs de décroissantes réside dans l'existence d'un gradient aigu des contraintes à la pointe de la fissure. Un tel gradient force à s'accrcître lesdimensions de la zone critique de deformation plastique, pour que puisse se produire une propagation instable dune microfissure.On a pu évaluer quantitativement ces effets statistiques d'une part et de gradients de contraintes d'autre part, et l'on s'est trouvé en accord satisfaisant avec les données expérimenetales.

     

Dynamic emission of dislocations from a crack tip — a computer simulation

The behavior of dynamic emission of dislocations from the tip of a stationary crack under mode II or mode III loading is examined. A critical stress intensity factor, K _D is assumed for dislocation emission. After emission, the dislocation moves with a velocity which varies with the effective shear stress to the third power. The effective shear stress is due to the applied stress , modified by the presence of the crack and all other dislocations minus the lattice friction stress, ^F. The effects of K _D, , and ^F on the rate of emission, the plastic zone strain rate, the plastic zone size, the dislocation distribution, and the dislocation-free zone are reported.

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Résumé On a examiné le comportement d'une émission dynamique de dislocations depuis l'extrémité d'une fissure stationnaire. On suppose que l'émission de dislocations est caractérisée par un facteur critique d'intensité de constrainte K _D. Après son émission, la dislocation se meut avec une vitesse qui varie avec le cube de la constrainte effective de cisaillement. Celle-ci résulte de la constrainte appliquée , modifiée par la présence de la fissure et de toutes les autres dislocations, et sous déduction de la contrainte de friction du réseau, ^F.On étudie les effects de K _D, et ^F sur la vitesse d'émission des dislocations, la vitesse de déformation plastique, la taille de la zone plastique, la distribution des dislocations, et sur la zone qui en est dépourvue.

     

The nucleation of vortices in superfluid4He: Answers and questions

The velocity threshold for phase slips, v_c, has been measured in ultra-pure⁴He and ultra-dilute³He-⁴He mixtures, down to a temperature of 15 mK. These experiments have revealed a crossover from a temperature-dependent régime above 150 mK to a plateau of v _c below in ultra-pure⁴He. Concentrations of³He impurities as low as a few parts in 10^–9 greatly affect the plateau régime, causing v _c to decrease markedly at low temperature. These observations are interpreted in the framework of the nucleation, either thermally activated or by quantum tunnelling, of vortices in the approximate shape of half-rings. These vortices form on wall asperities at local velocities u_s estimated to be 22 m/s in these experiments. The half-ring model is shown to yield a critical velocity of the same magnitude but leaves many basic questions unanswered.     

Influence of threshold parameters on cleavage fracture predictions using the Weibull stress model

This study explores applications of three-parameter Weibull stress models to predict cleavage fracture behavior in ferritic structural steels tested in the transition region. The work emphasizes the role of the threshold parameters (_th and _{w – min}) in cleavage fracture predictions of a surface crack specimen loaded predominantly in tension for an A515-70 pressure vessel steel. A recently proposed procedure based upon a toughness scaling methodology using a modified Weibull stress (^* _w) extends the calibration scheme for the Weibull modulus, m, to include the threshold parameters. The methodology is applied to calibrate the Weibull stress parameter for the tested material and then to predict the toughness distribution for the surface crack specimen. While the functional relationship between ^* _w and m suggests a strong effect of the threshold stress, _th, on the calibrated m-parameter, the results show a remarkably weak dependence of fracture predictions on _th as does the dependence of fracture predictions on _w–min for this specimen.     

Crack-tip damaged zones in rubber-toughened amorphous polymers: a micromechanical model

The various stages of crack propagation in rubber-toughened amorphous polymers (onset and arrest, stable and unstable growth) are governed by the rate of energy dissipation in the cracktip damaged zone; hence the relationship between the applied stress intensity factorK ₁ and the damaged zone size is of utmost importance. The size of the crack-tip damaged zone has been related toK ₁ via a parameter which is characteristic of the material in given conditions: this factor is proportional to the threshold stress for damage initiation in a triaxial stress field, and has been denoted by ^*. Theoretical values of ^* have been calculated by means of a micromechanical model involving the derivation of the stresses near the particles and the application of damage initiation criteria. The morphology, average size and volume fraction of the rubbery particles have been taken into account together with the nature of the matrix. The calculated values of ^* have been successfully compared with the experimental ones, for a wide set of high-impact polystyrenes (HIPS) and rubber-toughened poly(methyl methacrylate) (RTPMMA).Nomenclature PS; HIPS polystyrene; high-impact polystyrene - PMMA; RTPMMA poly(methyl methacrylate); rubber-toughened PMMA - MI; CS/H; CS/R particle morphologies (multiple inclusion; hard core - rubber shell; rubber core - rigid shell) - K _r;K _g bulk moduli of rubber and glassy materials - G _r;G _g shear moduli of the same materials - v _p particle volume fraction - L mean centre-to-centre distance between neighbouring particles - B; H; W standard names for the dimensions of the compact tension specimen - R _y size of the crack-tip plastic zone in a homogeneous material - h half thickness of the crack-tip damaged zone - r; polar coordinates around the crack tip (Fig. 1) - r;r _p distance from particle centre; particle radius - p normalized distance from the particle (Equation 5) - K ₁;K _1c;K _1p stress intensity factor; critical values ofK ₁ at the onset of and during crack growth - G _1c plane strain energy release rate - _y yield stress in uniaxial tension - _th macroscopic threshold stress for the onset of local damage initiation in a composite material - ^* characteristic parameter (Equation 3) - ⁰; ₁ ⁰ ; ₂ ⁰ ; ₃ ⁰ applied stress tensor and its three principal stresses - ⁰ uniaxial applied stress - ; ₁; ₂; ₃ local stress tensor and its three principal stresses - A tensor which elements are the ratios of those of over those of ⁰ (Equation 4) - v Poisson's coefficient of the matrix - g triaxiality factor of the crack-tip stress field - _e; _p Mises equivalent stress; dilatational stress (negative pressure) - I ₁;I ₂ invariants of the stress tensor - U ₁;U ₂ material parameters for argon and Hannoosh's craze initiation criterion (Equation 12)     

Multilayer adsorption of3He and4He on zeolite from 4 K to 80 K

The adsorption isotherms of⁴He, N₂, and argon have been measured on synthetic zeolite (Linde Molecular Sieve 13X) at 78 K, and of³He and⁴He, also on zeolite 13X, in the temperature range 4 K to 20 K. The results are presented in tabular and graphical form. The N₂ isotherms, which showed characteristic step-like behavior, served to assess the specific surface area, which was 527 m² g^–1 based on a standard N₂ molecular area of 16.2 Ų. It also provided a value ofE ₁ equal to 2530 cal mole^–1. The argon isotherm at 78 K yielded a specific surface area for the zeolite 13X in fair agreement with that from the N₂ data. Nine isotherms were taken for⁴He between 4 K and 20 K and four for³He in the same temperature range. These isotherms permitted good evaluations of the isosteric heats of adsorption to be made and plotted as a function of coverage, yielding, for⁴He,Q _st =1580 j mole^–1 at zero coverage,Q _st =1030 j mole^–1 at monolayer coverage andE ₂=480 j mole^–1 at two-layer coverage. For³He, which showed everywhere smaller Q _st values. Q _st =1420 at zero coverage. By use of the Steele equation applied to⁴He, we found that the monolayer coverageV _m1 0.29 cm³ (STP) m^–2, and the second-layer coverage,V _m2 0.10 cm³ (STP) m^–2.Supported in part by a grant from the National Science Foundation and by contracts with ONR and the Department of Defense (Themis).     

Liquid4He as a cryogenic coating for gaseous spin polarized3He

Nuclear spin polarization can be produced by optical pumping of³He gas in sealed samples at a temperature below 1 K. The wall relaxation can be minimized by covering the container walls with a⁴He film, and relaxation timesT ₁ longer than 1000 sec have been observed. A study of the residual relaxation is reported here. Spin relaxation appears to take place near the helium-substrate boundary. This situation is characterized by an adsorption energy very different from the known binding energies of³He quasiparticles on and in bulk liquid⁴He. Models are presented which reproduce the observedT ₁ variations on temperature and on other experimental parameters.Laboratoire associé au Centre National de la Recherche Scientifique et à l'Université Paris VI.     

Notch tip stress distribution in strain hardening materials

Using the results of elastic-plastic stress analyses for notched bars, it is shown that a modified form of slip-line field solution can satisfactorily explain the variation of longitudinal stress ahead of notch tips in strain hardening materials.

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Résumé En utilisant les résultats d'analyses de contrainte élastoplastique dans le cas de barres entaillées, on montre qu'il est possible d'utiliser une forme simplifiée de solution du champ des lignes de glissement pour expliquer de façon satisfaisante la variation des contraintes longitudinales en avant d'extrémités d'entaille dans des matériaux susceptibles d'un écrouissage.

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Nomenclature _yy longitudinal tensile stress in the notch tip plastic zone - _xx transverse stress in the x-direction - _zz transverse stress in the z-direction - k yield stress in shear - ₀ yield stress in tension - ₀ ^* strain hardened yield stress (flow stress) - _{0/* c} flow stress at notch tip - _total total strain _pl plastic strain _l principal strain - _{1 c} maximum principal strain at notch tip - _1pl plastic strain in they-direction - _{1 cp1} E_{1 pl} at notch tip - _eff effective plastic strain - _{c eff eff} at notch tip - ₀ yield strainC Stress decay constant in the notch tip region - /e_pl linear strain hardening rate - n strain hardening exponent in power hardening law - 2 flank angle of notch - distance from notch tip - p notch tip radius - k _I applied stress intensity for Mode I loading - E Young's modulus - V _c crack tip opening displacement