Absence of Phase-Fluctuation Contribution to the Low-Frequency Optical Conductivity in d-Wave Superconductors at Zero Temperature

The behavior of the low-frequency optical conductivity _reg() in superconducting cuprates is, at the present, an open and interesting issue. In particular, since the zero-temperature and zero-frequency limit of _reg() attains a value much larger than the universal value expected within a self-consistent T-matrix calculation, an intriguing possibility is that the collective mode can also contribute to _reg(). By taking into account the effect of dissipation on the collective mode in a d-wave superconductor, we evaluate the phase-fluctuation contribution to _reg(0) within the formalism of the phase-only action. We show that even though the collective mode contributes to _reg() at finite frequencies, approaching the zero-temperature and zero-frequency regime the corrections at _reg() due to phase fluctuations vanish.     

On the measurement and physical meaning of the cleavage fracture stress in steel

Elastic-plastic two-dimensional (2D) and three-dimensional (3D) finite element models (FEM) are used to analyze the stress distributions ahead of notches of four-point bending (4PB) and three-point bending (3PB) specimens with various sizes of a C-Mn steel. By accurately measuring the location of the cleavage initiation sites, the local cleavage fracture stress _f and the macroscopic cleavage fracture stress _F is accurately measured. The _f and _F measured by 2D FEM are higher than that by 3D FEM. _f values are lower than the _F, and the _f values could be predicted by _f=(0.8––1.0)_F. With increasing specimen sizes (W,B and a) and specimen widths (B) and changing loading methods (4PB and 3PB), the fracture load P _f changes considerably, but the _F and _f remain nearly constant. The stable lower boundary _F and _f values could be obtained by using notched specimens with sizes larger than the Griffiths–Owen specimen. The local cleavage fracture stress _f could be accurately used in the analysis of fracture micromechanism, and to characterize intrinsic toughness of steel. The macroscopic cleavage fracture stress _F is suggested to be a potential engineering parameter which can be used to assess fracture toughness of steel and to design engineering structure.     

Fracture stress difference of notched polycarbonate between atmospheric pressure and a hydrostatic pressure

Experimental data on fracture stress of polycarbonate (PC) with and without various artificial notches have been obtained at atmospheric pressure and a high hydrostatic pressure (400 MPa). The difference in fracture stress, _F, between both pressures was directly proportional to the intensity of pressure,P, and was inversely proportional to the stress concentration factor of the notch,K _n such that _F following the form of the Kaieda-Oguchi formula, _F. By using the combined stress concentration factor,K _nc, of superposed notch and craze, and by considering the change in elastic modulus due to pressure, the experimental data agreed with the modified Kaieda-Oguchi formula. The stress concentration factor of the craze was calculated by using the Dugdale model.     

The influence of grain size on the creep of uranium dioxide

The creep of uranium dioxide has been investigated as a function of grain size. At high stresses, when creep is controlled by dislocation movement, grain boundaries exert a strengthening effect and this strengthening is correlated with the Hall-Petch equation. The degree of strengthening diminishes with increases in temperature. At lower stresses, when creep is controlled by mass transport, grain boundaries exert a weakening effect owing to the reduction in diffusion path length as grain size is reduced. In this range behaviour is correlated with the Nabarro-Herring equation with stress replaced by an effective stress _E=–₀ where ₀ is a threshold stress for diffusional creep associated with the limitation of the ability of boundaries to emit and absorb vacancies. ₀ appears to decrease as grain size is increased.     

Shock response of stainless steel at high temperature

Measurements of the dynamic tensile strength of HR-2 (Cr-Ni-Mn-N) stainless steel have been carried out over the initial temperature range of 300 K–1000 K at shock stress of 8 GPa, the corresponding spall strength _f and Hugoniot elastic limit _HEL are determined from the wave profiles. In the temperature range of 300 K–806 K, _f and _HEL decrease linearly with increasing temperature T, i.e., _f = 5.63-4.32 × 10^–3T, _HEL = 2.08-1.54 × 10^–3T, but when heated to 980 K, _HEL increases from 0.84 GPa at 806 K to 0.93 GPa at 980 K and _f keeps at an almost fixed value of 2.15 GPa. The TEM analysis on recovery samples identified the existence of intermatallic compound Ni₃Al and the carbide Cr₂₃C₆ in the sample of 806 K, another intermatallic compound Ni₃Ti was found in the sample of 980 K. All these products emerge along crystal boundary. While no such products were found in the samples of 300 K and 650 K.     

Stress relaxation in hot-drawn low density polyethylene

The tensile stress relaxation behaviour of hot-drawn low density polyethylene, (LDPE), has been investigated at room temperature at various draw ratios. The drawing was performed at 85° C. The main result was an increase in relaxation rate in the draw direction, especially at low draw ratios when compared to the relaxation behaviour of the isotropic material. This is attributed to a lowering of the internal stress. The position of the relaxation curves along the log time axis was also changed as a result of the drawing, corresponding to a shift to shorter times. The activation volume, , varied with the initial effective stress ₀ ^* according to ₀ ^* 10kT, where ₀ ^* =₀ — _i, is the difference between the applied initial stress, ₀, and the internal stress _i. This result supports earlier findings relating to similarities in the stress relaxation behaviour of different solids.     

On internal stress and activation volume in polymers

The two-site model is developed for the analysis of stress relaxation data. It is shown that the product of d In (– )/d and (- _i) is constant where is the applied stress, _i is the (deformation-induced) internal stress and = d/dt. The quantity d In ( )/d is often presented in the literature as the (experimental) activation volume, and there are many examples in which the above relationship with (- _i) holds true. This is in apparent contradiction to the arguments that lead to the association of the quantity d In (– )/d with the activation volume, since these normally start with the premise that the activation volume is independent of stress. In the modified theory presented here the source of this anomaly is apparent. Similar anomalies arise in the estimation of activation volume from creep or constant strain rate tests and these are also examined from the standpoint of the site model theory. In the derivation presented here full account is taken of the site population distribution and this is the major difference compared to most other analyses. The predicted behaviour is identical to that obtained with the standard linear solid. Consideration is also given to the orientation-dependence of stress-aided activation.     

Effective conductivity of a composite of poly-dispered spherical particles in a linear continuum

Rayleigh's method is used to find the electric potentials of a composite of poly-dispered spherical particles in a linear continuum in an external electric field. Based on the solutions of potentials, analytical formula for the effective electric conductivity is derived. Based on the formula, several factors, such as the number of spherical inclusions, the spatial distribution of the spheres, the contrast ratio _i / _h (where, _i and _h are the conductivities of the spherical inclusion and the host medium, respectively) and volume fraction of the inclusions, are discussed. Our results show that at high volume fraction, the effective conductivity is also affected by the spatial distribution of the inclusions.     

Evaluation of biaxial stress failure surfaces for a glass fabric reinforced polyester resin under static and fatigue loading

In an attempt to evaluate failure theories for a glass fabric reinforced polyester resin over 370 tests have been conducted on thin-walled tubes under combined axial loading and internal pressure, both for static and fatigue loading. For plane stress the results are considered in relation to imaginary failure surfaces in ₁, ₂, ₆ space. A limited measure of agreement between theories and results can be obtained after highly subjective selection of data. Only those theories which involve complex stress properties provide a reasonable fit. The behaviour of tubular specimens is strongly influenced by the presence of joints in the reinforcements.Nomenclature _x , _y nominal hoop and axial (principal) stresses in a thin-walled tube - ₁, ₂, ₆ normal and shear stresses in the direction of the principal material axes - F ₁,F ₂,F ₆ strengths in the principal material directions and the in-plane shear strength - F _1t,F _2t tensile strengths in the principal material directions - F _1c,F _2c compressive strengths in the principal material directions - K ₂ a constant evaluated from a combined stress test - H ₁₂ normal stress interaction component of a strength tensor - off-axis angle - S-N curve conventional stress-log life fatigue curve - R principal stress ratio, _y /gs _x      

Electrical conductivity,thermoelectric power and dielectric constant of NiWO4

The a.c. electrical conductivity ( _ac), thermoelectric power () and dielectric constant () of antiferromagnetic NiWO₄ are presented. _ac and have been measured in the temperature range 300 to 1000 K and in the temperature range 600 to 1000 K. Conductivity data are interpreted in the light of band theory of solids. The compound obeys the exponential law of conductivity = ₀ exp (–W/kT). Activation energy has been estimated as 0.75eV. The conductivity result is summarized in the following equation =2.86 exp (–0.75 eV/kT)^–1 cm^–1 in the intrinsic region. The material is p-type below 660 K and above 950 K, and is n-type between 660 and 950 K.     

Distribution characteristics of mechanical properties and correlation between the respective properties on S35C carbon steel

Mechanical properties of tensile strength, , upper yield stress, _SU, lower yield stress, _SL elongation, , area reduction, , Vickers hardness, H _v, and impact absorbed energy, E, were examined using 50 specimens of S35C carbon steel, which were machined from two bars supplied from the same charged and heat-treated material. Distribution characteristics of these properties are discussed, and the correlation between each pair of them is investigated from a statistical viewpoint. The main conclusions obtained are summarized as follows; distribution characteristics of _B, _SL, , , H _v and E are well approximated by a normal distribution, but those of asu are not approximated as well by this type of distribution. In the latter case, a Weibull distribution is preferable to represent the distribution pattern. No significant correlation was observed between each pair of the above mechanical properties. Consequently, individual properties have the inherent distribution characteristics independent of the other properties.     

Strength of the pyroceram So115M under lengthy static loading

The article presents the results of long-time strength tests of the casting pyroceram SO115M at room temperature by the method of three-point bending. We obtained the power dependence of the time to failure _f on the applied load : _f ^–11.7.Translated from Problemy Prochnosti, No. 1, pp. 102–104, January, 1996.     

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.

     

Effect of magnesium content on the stress exponent and effective stress in the steady state creep of Al-Mg alloys

The stress exponent of steady state creep,n, and the internal ( _i) and effective stresses ( _e) have been determined using the strain transient dip test for a series of polycrystalline Al-Mg alloys creep tested at 300° C and compared with previously published data. The internal or dislocation back stress, _i, varied with applied stress,, but was insensitive to magnesium content of the alloy, being represented by the empirical equation _i=1.084 ^1.802. Such an applied stress dependence of _i can be explained by using an equation for _i of the form _i (dislocation density)^1/2 and published values for the stress dependence of dislocation density. Values of the friction stress, _f, derived using the equation _e/=(1–c) (1– _f/), indicate that _f is not dependent on the magnesium content. A constant value of _f can best be rationalized by postulating that the creep dislocation structure is relatively insensitive to the magnesium content of the alloy.On leave from Engineering Materials Department, University of Windsor, Windsor, Ontario N9B 3P4, Canada.     

Frequency and temperature dependence of the millimeter wave conductivity of epitaxial YBa2Cu3O7 films

A millimeter wave spectrometer for frequencies between 100 and 350 GHz consisting of continuously tunable backward wave oscillators as sources and a quasioptical interferometer in the Mach-Zehnder configuration was used to measure the transmittivity in phase and amplitude of YBa₂Cu₃O₇ thin films on NdGaO₃ substrates. From the measured spectra we derived the real and imaginary part of the dynamic conductivity= ₁+i ₂ in the superconducting state as a function of temperature. The ₁(T) and ₂(T) values at 300 GHz were compared to corresponding values at 19 GHz determined by surface impedance measurements of the same films using a shielded dielectric resonator. Our observed frequency dependence of both ₁(T) and ₂(T) is consistent with a strong reduction of the quasiparticle scattering rate ^–1(T) with decreasing temperature belowT _c .     

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     

Charged defects-controlled conductivity in Ge-ln-Se glasses

The variation of the d.c. electrical conductivity, , with composition and temperature was investigated for glasses of the Ge-In-Se system. The results indicate a decrease in the activation energy for electrical conductivity, E, and an increase in on introduction of indium into Ge-Se glasses. The changes in E and with composition (selenium content in the glasses) are identical for the Ge_x In₅ Se_95–x and Ge_x In₈Se_92–x families. The results have been traced to the conduction controlled by charged defects in these chalcogenide glasses. The changes in E and have been explained by a shift in the Fermi level, being brought by the introduction of indium.     

Residual thermal stresses in the pull-out specimen: A finite element calculation

The residual thermal stress field in the pull-out specimen is calculated in the case of a high properties thermoset system (carbon-bismaleimide). The calculation is performed within the framework of the linear theory of elasticity by means of a finite element method. The specimen is modelled as a three-phase composite (holder-fibre-matrix). The meniscus which forms at the fibre entry is taken into account in order to provide a realistic stress concentration. The latter is far higher than the matrix strength. Evidence that fibre debonding propagates from the fibre end during cooling is then produced.Nomenclature T thermal load - L _e embedded length - r _f fibre radius - _c curvature radius of the meniscus (fibre entry) - r _c radial dimension of the finite element mesh - E _m,E _h matrix and holder moduli - E _A,E _T fibre axial and transverse moduli - _m, _h matrix and holder thermal expansion coefficients - _A, _T fibre axial and transverse thermal expansion coefficients - _rr, , _zz, _rz non-zero components of the residual stress field - _rr ⁱ , ^im , _zz ^im , _rz ⁱ stresses at the interface in the matrix (r=r _f + ) - _rr ⁱ , ^if , _zz ^if , _rz ⁱ stresses at the interface in the fibre (r=r _f–) - _p1 maximum principal stress - _zz ^f mean axial stress over the fibre section - _rupt ^m matrix strength - u _r ,u _z non-zero components of the displacement field     

Carbon fibre-reinforced cement

This review describes fabrication processes for aligned fibre and random fibre carbonreinforced cement and links important process parameters with composite theory. The way in which the material fits into the general framework of crack constraint and matrix cracking theories is discussed. A broad survey is made of the mechanical properties, durability and dimensional stability of a variety of carbon-reinforced cement composites, and economic constraints on potential applications are considered.List of symbols b breadth of three-point bend specimen - d depth of three-point bend specimen - E _c composite Young's modulus - E _f fibre Young's modulus - E _m matrix Young's modulus - l fibre length - l _c fibre critical transfer length - l _s specimen span in three-point bend test - m Weibull modulus - r fibre radius - P applied load - V _f fibre volume fraction - V _m matrix volume fraction - x length of fibre needed to transfer load _mu V _m - x _d crack spacing in a composite with short, aligned fibres - _fu fibre ultimate strain - _mu matrix ultimate strain - _fu fibre ultimate strength - _mu matrix ultimate strength - _cu composite ultimate strength - _MOR modulus of rupture - _T tensile strength - interlaminar shear strength - _i interfacial shear strength - _m matrix work of fracture - _F work of fracture