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.     

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)     

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.     

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.     

Fatigue properties of pure metals

The report considers the stress fatigue limit _D, the fatigue to tensile strength ratio _D/R_m, and the deformation fatigue limit _D of pure metals. For FCC and some HCP metals there exists a linear correlation between fatigue limit and tensile strength. The fatigue limit and the fatigue ratio _D/R_m depend upon the homologous temperature. The fatigue ratios of BCC metals are higher than the fatigue ratios of other metals at room temperature as is also true for metals with nearly the same homologous temperature. The deformation fatigue limit decreases rapidly when T_h - 0.5 and T_h 1. When T_h = 1 then _D = 0, _D = 0 and _D/R_m = 0. The deformation fatigue limits of metals belonging to the same subgroup of the periodic element system and having, similar homologous temperatures at room temperature are very similar.

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Zusammenfassung Der Artikel behandelt die Dauerfestigkeit _D des Ermuedungskoeffizienten _D/R_m und die Deformation _D, die der Dauerfestigkeit entspricht. Fuer kubisch flachzentrierte Metalle und einige hexagonal Metalle existiert eine lineare Korrelation zwischen Dauerfestigkeit _D and Zugfestigkeit R_m. Die Dauerfestigkeit _D und der Ermuedungskoeffizient _D/R_m koennen als Funktion der homologischen Temperatur betrachtet werden. Der Ermuedungskoeffizient der kubisch raumzentrierten Metalle ist groesser als der der anderen in Raumtemperatur. Dieses stimmt auch fuer Metalle mit aehnlichen hornologischen Temperaturen. Die Deformation _D, die der Dauerfestigkeit entspricht, faellt schnell ab, wenn T_h > 0.5 und T_h 1, Wenn T_h = 1, darn ist _D = 0, _D = 0 und _D/R_m = 0. Die Metalle, die zu derselben Gruppe des periodischen Systems der Elemente gehoeren and aehnliche homologische Temperaturen (in Raumtemperatur) besitzen, haben sehr aehnliche Werte der Deformation _D.

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Résumé L'article parle de la contrainte de la limite de fatigue _D le facteur de fatigue _D/R_m et la déformation _D correspondante à la limite de fatigue des métaux pures. Pour les métaux du système cubique aux faces centrées (c. f. c.) et quelques uns du système hexagonale (hex. c.) il existe une correlation linéare entre la limite de fatigue _D et de la résistance a la fraction R_m. La limite de fatigue _D et le facteur de fatigue _D/R_m dependent de la température homologue. Les facteurs de la fatigue des métaux du système cubique centre (c.c.) sort plus grands que ceux des autres métaux dans la température de chambre, aussi por des métaux qui ont presque la même température homologue. La déformation correspondante à la limite de fatigue tombe rapidement quand T_h > 0.5 et T_h 1. Quand T_h = 1, alors _D = 0, _D = 0 et _D/R_m = 0. Cettes déformations _D pour les métaux du même groupe du système périodique des éléments sont proches si les températures homologues dans la température de chambre sont analogues.

     

Determining the error in reproducing continuous random signals by means of pulsed frequency transducers

Conclusions Owing to the application of the suggested parameters , , _ge, and m which characterize the PFM system, it is possible to approximate the error relationships by means of linear functions convenient for calculations. The suggested technique can be applied for determining modulation and restoration errors of voltage-to-frequency and frequency-to-code transducers for various signal distribution laws and correlation functions.Translated from Izmeritel'naya Tekhnika, Vol. 19, No. 6, pp. 25–26, June, 1976.     

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.     

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.     

Mixed state odd Hall effect in YBa2Cu3O7−δ with unidirectional twins

We have studied the mixed state Hall effect in YBa₂Cu₃O_7– single crystals with unidirectional twins: _xy=f(H). An analysis of the Hall conductivity _xy in free flux flow regime reveals that _xy can be successfully described by two terms which are related to the quasiparticle excitations and the motion of free vortices respectively. We have discovered the strong twin dependence of the Hall conductivity _xy in the pinning (TAFF) regime and detected the sign reverse of the Hall conductivity _xy at changing angle (+45 °–45 °) between current and twin plane that indicated the presence of backflow of vortices. We have observed that _xy tends to - while approaching the melting line. These results provide the evidence of strong planar pinning influence on the Hall conductivity.     

Calculated evaluations of the threshold characteristics for ferritic-pearlitic steels

On the basis of modern physical concepts of the process of formation of the prefailure zone and rules of the change in microcleavage resistance during deformation within the limits of the deformation theory of plasticity relationships of the threshold criteria of fracture to the standard mechanical properties _t, _0.2, and _K were obtained analytically. It was shown that the resistance to crack advance in ferritic-pearlitic steels is determined by the strain hardening exponent. A method of analytical determination of the constant relating the two basic threshold characteristics K_th and _th was developed.Translated from Problemy Prochnosti, No. 4, pp. 50–56, April, 1992.     

Ageing characteristics of Al-2.5% Li rapidly solidified alloy

The present work was performed on an Al–2.5 wt% Li alloy produced by melt spinning. The ribbons were aged in the temperature range 180 to 310° C for times between 1 min and 120h. The kinetics of coarsening of (Al₃Li) and (AlLi) phase particles were investigated using transmission electron microscopy and hardness measurements. The results show that coarsening of (Al₃Li) follows a simple linear relation with the cube root of Me, whereas coarsening of (AlLi) does not follow the same trend. We believe that the (AlLi) phase nucleates at the (Al₃Li)/matrix interface and grows by the dissolution of the nearby (Al₃Li) particles. The mechanical properties of the powder metallurgy alloy show that a large volume fraction of PFZ contributes to the alloy ductility, 11 %, in the aged condition. Also, the yield strength is greatly improved due to refinement effects enhanced by rapid solidification.     

Electrical conductivity of α-H x V2O5 (X=0.00-0.27): Dependence of hydrogen concentration and orientation

The dependence of the electrical conductivity, , on the hydrogen concentration and crystallographic orientation has been investigated using single crystals of H_xV₂O₅, which were grown by the Bridgeman method and doped with hydrogen within the solid solubility in the phase by the spillover technique. The temperature dependence of showed the feature of diffusive hopping of thermally activated electrons above 180 K and variable range hopping below 180 K. The dependence of on the crystallographic orientation was little different from that of V₂O₅. The change in with the hydrogen concentration was not monotonic; increases withx up tox }-0.06, but decreases abovex }-0.06. This behaviour can be explained based on the competition between the increase in the carrier density and the depression of the mobility of carriers with increasingx.     

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.     

Transport results from untwinned YBCO: Washboard frequency of the vortex lattice,and the quasiparticle mean free path

The washboard frequency of the moving vortex lattice in untwinned YBa₂ Cu₃ O_6.93 may be observed through mode-locking to an externally applied ac current of frequency _ext. The interference between and _ext results in jumps in the dc current-voltage characteristics when and _ext are harmonically related¹. The interference effect disappears in the vortex liquid state. The Hall conductivity _xy below T_c in YBCO contains contributions² from a positive quasiparticle (qp) term (H) and a negative vortex term (1/H). The qp term is surprisingly large well below T_c and implies a large gap anisotropy and a long qp mean free path (mfp). The thermal Hall effect³ _xy is closely related to the qp _xy; _xy is produced by asymmetric scattering of qp by pinned vortices. The qp mfp at H = 0, extracted from _xy and extended to low T by _xy, increases remarkably from 90 Å at T_c to more than 0.5m at 22 K.     

The Tc Amplification by “Shape Resonance” in Doped MgB2

We have applied two channels ( and ) superconductivity model to the Al_1–x Mg _x B₂. Using the experimental data, we have calculated the strength of the interchannel pairing due to quantum interference effects, probed by the interband coupling parameter, and the two gaps as a function of the x. While in MgB₂ the quantum interference effects gives an amplification of T _c by factor 1.5 in comparison with the dominant intra band single channel pairing, in AlMgB₄ the amplification is about 100, in comparison with the dominant intra band single channel pairing.     

Evaluation of the true activation enthalpy of superplastic flow including a threshold stress

A new method is suggested for the evaluation of the true activation enthalpy for alloys where the strain rate of the superplastic flow varies with a power of an effective stress _e = -_o, where and _o are the applied stress and a threshold stress, respectively. Some earlier results concerning superplastic AlMgZnCu alloys containing chromium and in which a strongly temperature-dependent threshold stress can be revealed, are reanalysed. The results are in good agreement with the previous ones. It has been shown further that for the alloys investigated the true activation energy increases with increasing chromium content.     

Effect of swelling in the extrusion of an elastic liquid from a capillary

The article demonstrates that the profile of a jet extruded from a capillary can be constructed from the data of uniform stretching.Notation V₁ mean velocity of the liquid in the capillary - r₁ radius of the capillary - q flow rate - P pressure in the preinlet zone of the capillary - F₁ and F₀ extrusion force - ₁ and ₂ elastic deformations in parallel Maxwellian elements upon stretching - p, z radial and longitudinal coordinates, respectively - v_z(p) velocity of steady-state flow in the capillary - _i modulus of elasticity in the i-th Maxwellian element - swelling coefficient - t time - l length of the specimen at instant t - elastic deformation - x rate of deformation - ₀, ₁ deformation - L length from which onward the radius of the jet extruded from the capillary practically does not change - r radius of the jet - t₁ time from which onward the radius of the specimen in retardation practically does not change - d₀,l ₀ diameter and length, respectively, of the undeformed specimen - l _r length to which the extended specimen tends after removal of the load - b length of the capillary - ₁ tensile stress (for uniform stretching at t = 0, for extrusion from the capillary at z = 0) - l _H length of the stretched specimen at the instant of beginning reading of t Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 39, No. 2, pp. 343–350, August, 1980.     

Transport properties of CdGa2Se4 thin films

The d.c. conductivity was determined for CdGa₂Se₄ thin films in the temperature range 300–625 K for as-deposited and heat-treated films. The conductivity at room temperature of films of thickness 326 nm prepared at a temperature of 573 K was found to be 10^–12 ( cm)^–1. The dependence of the electrical conductivity on the annealing temperature in a vacuum of 1 Pa for a thin film of thickness 140 nm, shows that the electrical conductivity increases with increasing annealing temperature. However, the activation energies E and E decrease with increasing annealing temperature. The data of these annealed films are in agreement with the Meyer–Neldel rule. The thermoelectric power measurements indicate p-type conduction in the as-deposited films as well as for the heat-treated films. The p-type conduction is due to the cadmium deficiency as indicated by EDX. The difference between the value of the activation energy calculated from the thermoelectric power E _S and that obtained from the conductivity E indicates the presence of long-range potential fluctuations.     

Methodology for the determination of the interfacial properties of brittle matrix composites

The interfacial properties of a glass-ceramic matrix composite (SiC/CAS) were determined from single-fibre push-out tests using the interfacial test system. The coefficient of friction, , the residual clamping stress, _c, and fibre axial residual stress, _z , were extracted by fitting the experimental stress versus fibre-end displacement curves using the models of Hsueh, and Kerans and Parthasarathy. Using Hsueh's model, the intrinsic interfacial frictional stress (=_c) was found to be 11.1±3.2 MPa, whereas by using Kerans-Parthasarathy's model it was found to be 8.2±1.5 MPa. Comparisons between these models are included, together with a discussion of data analysis techniques.Nomenclature _z Axial fibre residual stress (Pa) - ^* Effective clamping stress (Pa) - _c Residual clamping stress (Pa) - _p Poisson's effect-induced clamping stress (Pa) - _d ⁰ Debond stress in the absence of residual stresses (Pa) - _d Experimental debond stress (Pa) - Compressive applied stress (Pa) - Interfacial shear stress (Pa) - u Fibre-end displacement (m) - h Debond length (m) - r Fibre radius (m) - E _f Fibre Young's modulus (Pa) - E _m Matrix Young's modulus (Pa) - v _f Fibre Poisson's ratio (dimensionless) - v _m Matrix Poisson's ratio (dimensionless) - f Fibre volume fraction (dimensionless) - k Parameter (dimensionless) - D Parameter (dimensionless) - Interfacial coefficient of friction (dimensionless) - G _i Interface toughness (J m^–2) - C _m Load-train compliance (m N^–1)